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Custom Audio Filter designs for Ham AM - Dynamic Bandwidth Control




 
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Author Topic: Custom Audio Filter designs for Ham AM - Dynamic Bandwidth Control  (Read 5416 times)
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KK4YY
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« Reply #75 on: February 20, 2020, 10:29:12 PM »

I'd say the "Courtesy Filter" is a success.  (Thanks Don) T
Wow, It seems that I've named a piece of JJ equipment. My place in AM history is now secure. Wink
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« Reply #76 on: February 21, 2020, 01:05:48 AM »

I'd say the "Courtesy Filter" is a success.  (Thanks Don) T
Wow, It seems that I've named a piece of JJ equipment. My place in AM history is now secure. Wink

Yep, that's a great honor, Don.  

We just received some additional help from the JJ marketing dept. The brilliant man (who wants to remain nameless) running the logos dept came up with this today;

The new APF "Courtesy", by Trojan:  


* Trojan Audio Filters, LLC.jpeg (35.29 KB, 500x351 - viewed 76 times.)
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz,  +-6.0 KHz or  +-8.0 KHz when needed. 

"You know my name"  https://youtu.be/noGjJyEDm5s?t=135

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« Reply #77 on: February 21, 2020, 01:19:09 AM »

UPDATE:  I tried the Courtesy Filter  on the 4-1000A plate modulated rig.  I found the insertion loss was higher than I thought and had to add some more gain before the GFZ audio driver.

Running some voice audio tests I found the bandwidth to be exactly like earlier tests on the FT-1000D and 3-500Z amplifier. (about + -  6.5 KHz bandwidth)  The IMD appears to be superb. I ran some tone tests (with the filter in) and saw -45DB 3rd order !!!  when the 4X1 rig was run conservatively at 1/2 power.  That is better than the Ft-1000D and amplifier lash-up by about -10 to -12 DB.  This pleased  me greatly because the 4X1 is louder and needs to be cleaner. This means that the filter is not adversely affecting the IMD of the rig.

I tried adding in a low level NPL. (Negative peak limiter) and found when the modulation hit the preset -95%, the IMD went to hell. (inter-modulation distortion)   It was down to -15DB 3rd which is abominable. That word means it really sucked.  So I'm back to simplicity and find as long as I stay away from the baseline, the big rig has great fidelity, is narrow, clean,  and bad ass with great APF Courtesy to all on the band.  Thank you. Thank you very much. I'll be in Vegas all week.

T

Here is a picture of Tron when he first found out about the APF Courtesy:


* timtronization.jpg (44.06 KB, 499x769 - viewed 100 times.)
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz,  +-6.0 KHz or  +-8.0 KHz when needed. 

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« Reply #78 on: February 21, 2020, 01:15:18 PM »



5.5 KHz Filter



Love the idea of the all-in-one AM processor. Looks like a brilliant piece of work.

But I hate to see all this narrowing up of formerly excellent-sounding signals. Maybe people with severe hearing loss or old unmodified stock receivers can't tell the difference, but I sure can.

My default audio bandwidth is 7.5 kHz. Anything less than that doesn't sound high fidelity to me. If I can do it without causing interference, I'll go 10 kHz.

Anything under 6 kHz starts to sound pretty bad to my ears; just getting off the air until things clear up starts to look attractive. Maybe as a stopgap, during a temporary interference situation, it's tolerable -- for a few minutes.

It's not hard with a good SDR (especially at 48 kHz sampling rate, where the filters are truly awesomely sharp) to keep on receiving 7 kHz audio or better from AM signals, even in crowded conditions. Just narrow up the sideband (as little as needed, of course) that's getting the most interference, and keep the clearer sideband as wide as you can. This works 75 per cent. or more of the time for me.

I'd rather listen to 7.5 kHz audio with a few sibilants of interference from a nearby station than listen to 5 kHz of audio interference-free. The narrower audio bandwidth is not only less natural -- it's also less intelligible. Contrary to what the ARRL and ham lore tell us, narrower audio is always less intelligible, all things being equal, not the reverse.

No one needs to explain crowding to me, or the rationale behind going narrower. I understand it completely. I just disagree with it. If AM operators would stop thinking that 5 kHz spacing is acceptable, and never do that, the problem would be 99 per cent. solved without ruining the AM fidelity that many have spent years achieving.

73,

Kevin, WB4AIO.
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« Reply #79 on: February 21, 2020, 02:17:36 PM »

. If AM operators would stop thinking that 5 kHz spacing is acceptable, and never do that, the problem would be 99 per cent. solved without ruining the AM fidelity that many have spent years achieving.
73,
Kevin, WB4AIO.

Hi Kevin,

That's the problem. AM stations do come in within 5 KHz spacing quite frequently. Maybe we should be addressing that problem more aggressively. We can either have an RF battle with them or tighten up our bandwidth or sign out. When it happens, I prefer to tighten up with a transmit filter. It's so easy to do.  This will probably never happen down on 3730, but on 3870 to 3885 there is often a crowd at prime times.  The AM rallies can be bedlam with no spacing at all.  People like to flock together.

The filter idea is all about having the option to run full-boat 7.5 KHz audio at anytime...or narrower when we choose. Flexibility.

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz,  +-6.0 KHz or  +-8.0 KHz when needed. 

"You know my name"  https://youtu.be/noGjJyEDm5s?t=135

There's nothing like an old dog... a puppy... a dog in its prime... or ANY dog!
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« Reply #80 on: February 21, 2020, 11:25:11 PM »

. If AM operators would stop thinking that 5 kHz spacing is acceptable, and never do that, the problem would be 99 per cent. solved without ruining the AM fidelity that many have spent years achieving.
73,
Kevin, WB4AIO.

Hi Kevin,

That's the problem. AM stations do come in within 5 KHz spacing quite frequently. Maybe we should be addressing that problem more aggressively. We can either have an RF battle with them or tighten up our bandwidth or sign out. When it happens, I prefer to tighten up with a transmit filter. It's so easy to do.  This will probably never happen down on 3730, but on 3870 to 3885 there is often a crowd at prime times.  The AM rallies can be bedlam with no spacing at all.  People like to flock together.

The filter idea is all about having the option to run full-boat 7.5 KHz audio at anytime...or narrower when we choose. Flexibility.

T


Yes, Tom, I agree. It's good to have that option to go narrower when absolutely necessary; it's something I do myself when the situation demands it. I do think that we should run 7 to 10 kHz of audio normally, though. It sounds better, amazes anyone who takes the trouble to widen his receive passband and listen through a decent sound system, and potentially sparks interest in AM (or eSSB) experimentation. It's also a rebuke to that tired old lie that narrower is always better and more effective. It isn't.

73,

Kevin, WB4AIO.
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« Reply #81 on: February 22, 2020, 01:54:26 AM »

Kevin,

It's good we have this discussion...  I consider you one of the sophisticated audio guys on AM, so I am listening closely...  Wink

In regards to the unique 5 KHz spacing problem between 3870 to 3885, I know, I know....we should all spread out beyond these walls, but few have for the last 40+ years.  I think it all begins when the first station starts up on 3880. This is a mistake. The next station then starts up on 3885 because they want to stay away from the 3890 ssb gang.  The next station is at 3875 to stay away from the 3870 and lower ssb groups. And there we have it... 5 KHz spacing and QRM.  (unless we switch in 5 KHz filters)  

The better alternative is for the first station to grab 3885, then the next station grabs 3878 and the next station hits 3872.  (or 3885 and 3875 for 10 KHz on quiet days)  This is more reasonable spacing and I've seen it work many times.  Especially when someone has sharp receiver skirts like you described using your SDR.

You make a good point about using a very hi-fi receiver that can actually hear + - 10 KHz.  I must admit before 2 weeks ago my receiver was limited to about + - 7.5 KHz so I paid little attention to higher audio. But with the new SDR I have going into a stereo and big speakers, I can hear the difference when someone really opens it up. The subtle breathing, lip and tongue noise are brighter and more articulated. And I can hear the difference in my own transmit monitor when I go from ~6.5 KHz to no filter at all. (+- 10 KHz)   It is worth it for sure, like a treat.

There are some stations that run 10 KHz audio at quiet times and it sounds great. As long as the highs terminate in wispy sibilance and are not real artifacts and splatter we are golden. There are a few stations on with some splatter mixed in and I have hesitated to say anything to avoid being a complainer. I sometimes think they will figure this out themselves. I have tried hard to increase AMer awareness of running IMD tests, triangle / sinewave tests and being self-aware of our own signals.   With today's cheap SDR spectrum scopes and a simple 2-tone test, we have all the tools to monitor ourselves. This was not available in the past. The SDR competition to older rigs is intense and has made a new standard. We need to be on top of it all with our older rigs. My whole station consists of five analog, plate modulated and linear amps so I'm in that boat.

There is nothing worse than another AMer telling us we are splattering all over the band... nothing worse... Shocked   Lately I just love the feeling of confidence - getting on with the new filter and watching the SDR spectrum and waterfall scope with my sibilance just kissing the edge of an adjacent QSO knowing it is not bothering them. I just hate the feeling in the past when I would unkey and hear the adjacent QSO AMer say to his friend, "there is someone up the band taking you out OM."  Being able to forget about myself and focus on my QSO is what it's about. It's akin to going to a party dressed like a bum. We focus on ourselves and worry about what we look like. But wear a nice suit and we forget about ourselves and just have fun.

For the most part AMers are very accommodating to one another. I think it would be great if AMers in general were looked upon by the ham world as being considerate and aware operators that were always willing to slide up or down the band a  wee bit when congested, run less bandwidth when the conditions suggested and generally did not dig in our heels when a little QRM starts up. I look at it like giving up my seat on the subway to anyone who asked. But it's rare, and it's more about having an easy-going "turf" attitude I am talking about.

I've been having more fun in the last two months then ever before on the air and working on new projects.  It's all about the friends we have made over the years and getting excited about improving our stations.  Helping newer Amers who we may never meet through this AMFone BB is a legacy I hope to leave for years to come.. Just think if we had this great BB resource of like-minded AMers years ago, Kevin...

Take care, OM.

T
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"You know my name"  https://youtu.be/noGjJyEDm5s?t=135

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« Reply #82 on: February 22, 2020, 11:12:46 AM »

We're making more progress...

We've decided to advance the  passive 10th order Butterworth design.  It is working well.   The only change is to add buffer stages to the input and output making it an active Butterworth filter. So don't build anything yet.

Frank / GFZ is modeling simple 2N2222 transistor stages that requires only 13.8VDC to work.   This will permit us to put the filter anywhere in the audio chain without worrying about disturbing the filter's design characteristics.

I have the parts and hope to test it soon.

John/ JSW has already made good progress on a professional PC board holding three different filters.  We expect to use the same buffers for all three to reduce parts count..

I will be testing Rick/KHK's SCAF (switched capacitor audio filter)  in a few days too, so stand by for more...

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz,  +-6.0 KHz or  +-8.0 KHz when needed. 

"You know my name"  https://youtu.be/noGjJyEDm5s?t=135

There's nothing like an old dog... a puppy... a dog in its prime... or ANY dog!
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« Reply #83 on: February 23, 2020, 10:31:55 AM »

Kevin,

It's good we have this discussion...  I consider you one of the sophisticated audio guys on AM, so I am listening closely...  ;)



Thank you, Tom. I mess around with audio a lot (my latest fun and learning experience is using VST plugins in my all-digital audio chain) but am not 100 per cent. satisfied with my signal. You are one of the Renaissance men of radio in my opinion, far beyond just audio. Been following your career since 1973, sonny!



In regards to the unique 5 KHz spacing problem between 3870 to 3885, I know, I know....we should all spread out beyond these walls, but few have for the last 40+ years.  I think it all begins when the first station starts up on 3880. This is a mistake. The next station then starts up on 3885 because they want to stay away from the 3890 ssb gang.  The next station is at 3875 to stay away from the 3870 and lower ssb groups. And there we have it... 5 KHz spacing and QRM.  (unless we switch in 5 KHz filters)  

The better alternative is for the first station to grab 3885, then the next station grabs 3878 and the next station hits 3872.  (or 3885 and 3875 for 10 KHz on quiet days)  This is more reasonable spacing and I've seen it work many times.  Especially when someone has sharp receiver skirts like you described using your SDR.



True.

The AM window interference problem is complex, partly technical and partly psychological.

Some people live and listen in a bubble in which only "their gang" matters and those people "over there" in the other part of the country don't matter (even though they're propagating in just fine), so parking 5 kHz away from them and narrowing the (probably stock and horribly narrow) receiver even more is justified -- because, I suppose, tuning to another part of the band is too much trouble. Sometimes this psychological bubble is so impregnable that they'll park ONE or even ZERO kHz away and just fire up.

Then there's the very real technical bubble -- the tiny high-angle reception bubble created by people running inverted vees at 20 feet and the like, and exacerbated by high local noise levels. Such folks have a hard time hearing anything other than NVIS semi-locals and falsely assume that others 500-plus miles away can't hear them -- wrong! But they plop 5 kHz away anyway, and evidently think that's perfectly fine.

In some ways, channelized operation would be better -- but, then again, amateur radio is experimental and wild and free, so let everyone be free, even with their wobbly VFOs, bizarre ideas of how close one can get to another QSO, distorted audio in which the IMD-hash is almost as strong as the desired audio, 300-3000 Hz audio that was a stupid idea to begin with, QSOs in which participants are spread out over several kHz, and all the other things I don't like too much. That freedom to do almost anything is also what allows _us_ to experiment with high fidelity wideband audio, AM, DSB, eSSB, and all the things I DO like very much. (If we did have channelized operation, I would want AM/DSB channels to be spaced 16 kHz, SSB channels 8 kHz. This would allow 7.5 kHz audio with a small guard band. Wow, would that be nice!)

To totally avoid interference with 5 kHz spacing, we'd need to chop our audio off at 2.5 kHz -- 2 kHz to account for BA drifting. Sheesh! Might as well run one of those awful 2.1 kHz SSB rigs -- or, better yet, just sign off and spend more time with my wife and my collie.


You make a good point about using a very hi-fi receiver that can actually hear + - 10 KHz.  I must admit before 2 weeks ago my receiver was limited to about + - 7.5 KHz so I paid little attention to higher audio. But with the new SDR I have going into a stereo and big speakers, I can hear the difference when someone really opens it up. The subtle breathing, lip and tongue noise are brighter and more articulated. And I can hear the difference in my own transmit monitor when I go from ~6.5 KHz to no filter at all. (+- 10 KHz)   It is worth it for sure, like a treat.



Studies have shown that limiting speech bandwidth to less than 8 kHz reduces intelligibility. The old canard that "3 kHz is more intelligible" was nonsense from the beginning. Three kHz was chosen because it's as narrow as you can get without _severely_ reducing intelligibility, and the telephone engineers that came up with it (actually, they chose 3.5 kHz with a 500 Hz guard band) were counting beans (dollars) to save on landline equalizer coils and, later, get as many channels as possible in their carrier multiplex systems. New phones with HD Voice (finally!) now open that up to more than 7 kHz and it's _astoundingly_ better. The ham radio establishment of the 1950s, unfortunately, followed their lead, and eventually even refused advertising space to transmitters that had broadcast quality, of which there were quite a few before the moronic "narrow is better" idea took hold.

Then there are also the subtleties of the human voice that come across in wideband audio, but are lost with "communications quality." These have a role in conveying tones of voice and psychological cues of personality and expression and shades of meaning that should not be disregarded. Another factor is the aesthetic factor, the happiness that beautiful audio can bring -- just as valid a human goal as a beautiful building or oil painting or sculpture.


There are some stations that run 10 KHz audio at quiet times and it sounds great. As long as the highs terminate in wispy sibilance and are not real artifacts and splatter we are golden. There are a few stations on with some splatter mixed in and I have hesitated to say anything to avoid being a complainer. I sometimes think they will figure this out themselves. I have tried hard to increase AMer awareness of running IMD tests, triangle / sinewave tests and being self-aware of our own signals.   With today's cheap SDR spectrum scopes and a simple 2-tone test, we have all the tools to monitor ourselves. This was not available in the past. The SDR competition to older rigs is intense and has made a new standard. We need to be on top of it all with our older rigs. My whole station consists of five analog, plate modulated and linear amps so I'm in that boat.


I couldn't agree more. As W1AEX says, in many ways we are living in the best age of amateur radio that has ever existed.

73,

Kevin, WB4AIO.

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« Reply #84 on: February 23, 2020, 11:31:20 AM »

The bandwidth issue needs to be looked at from microphone to speaker while considering the factors in between.

AM is unique in that everything from 3kc to 10kc frequency response (not bandwidth) in both transmission and reception has its place. When the bands are quiet and lightly occupied, nothing sounds smoother than a cleanly modulated broadband signal. But since this is not always the case, particularly after the sun sets, ops are faced with a decision.

I agree with Tom about narrowing up when things get crowded, but not just to be neighborly. Even if a transmission remains "Hi-Fi", it's unlikely that other ops on frequency are going to leave their receivers set wide and tolerate the artifacts from adjacent signals. This leaves the question about being "wide" when nobody can appreciate it.

Also, concentrating modulated energy into a narrower spectrum contributes to intelligibility, a point not lost on equipment manufacturers who want customers raving about piercing pileups and scoring difficult DX contacts. That's not going to happen with 20-20kc audio.

SEE? YOU AMers NEED TO TIGHTEN IT UP!!!

That's the popular argument, but it makes no sense. AM's strength is not in DXing. AM is the "QSO Mode" where ops don't need to constantly twiddle with a RIT control each time someone keys up and we can recognize each other at hamfests by voice.

Bottom line: Hi-Fi AM is fantastic and it's the hallmark of the mode, but it's only good if others can appreciate it too. There's no need to be a part of adjacent QSOs if nobody can hear those highs.

Side bar: For Pete's sake (and everyone else's) if you have to operate in the 3875-3880-3885 segment of 75M, 3880 is NOT the first choice, it's the last. Think of it as the "middle seat" on an aircraft. Choose the window or aisle first. Then again, there is no "AM Window" any more so spread it out folks...
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« Reply #85 on: February 23, 2020, 09:41:10 PM »

Very good on all....

Lots of interesting points regarding the psychological and technical features of the AM Window, audio bandwidths, etc.  An important point mentioned by Kevin is that if two AM stations are spaced 5 KHz apart, then both need to run +- 2.5 Khz audio JUST to barely keep out of each others' way. That is an eye-opener to guys who get within 5 KHz and then complain of splattering neighbors....  That's why broadcast stations are spaced 10 KHz apart and run +- 5KHz audio.

I've been having a lot of fun lately with the SDR spectrum and waterfall scopes as I get involved in a QSO.  I've noticed a few things as I get better at it...  (I'm pretty good at pattern recognition due to my job.)  I have a new appreciation for clean, well contained and managed AM signals, no matter the bandwidth..

Yesterday, an old friend Dave K2IJY came into our QSO using a BC transmitter and was very loud. I noticed his unique spectrum of exactly + - 10KHz. It was a beautiful spectrum like it was highly intentional and showed an "intelligent being" was controlling it. The audio was dense thru-out the full spectrum and cut at 10 KHz like a crewcut. I asked him how was he doing this?? He said he was using an analog Orban processor, something like a 9xx?  IIRC.  He knew what he was doing and it showed.

I later heard a 100 watt stock DX-100 and could see it was struggling. The bandwidth was somewhat random... wide, then narrow then some splatter, then not. It was not an "intelligent alien" controlling his signal...  Grin

An analogy is when there's a thunderstorm and a lightning bolt strikes. The spectrum will show many megacycles of garbage, no rhyme or reason.  Then compare this to the commercial AM broadcast band and there is absolute order... +-5 KHz with highly intelligent data. And we  clearly see it stand out on the spectrum scope and waterfall.

I was looking at two stations - one talking on essb the other day. One was 2.4 KHz wide and the other was 4 KHz. Both had sharp skirts. It was perfectly easy to see the difference. The 4 KHz station sounded like a million bux. The 2.4 KHz ssb station sounded like a regular ricebox with a stock ssb filter. Who had the best contained and controlled signal? I suppose both did but the 4 KHz guy was stretching himself to go the extra mile to sound spectacular and he did.

The point I'm making is years ago we got on AM and hoped for the best. We hoped our signals were clean and orderly. We depended on the LACK of complaints to tell us we were OK. Except for an oscilloscope that's all I had to go on.

Nowadays with the spectrum tools we have, we can take control of our transmitters whether they are 60 year old boatanchors or newer SDR rigs. I just love doing the detective work and training myself to pick out what a station is doing with their rig by the order or disorder of their signals.  This has nothing to do with absolute bandwidth, rather how are they managing their signals to achieve the bandwidth they choose to use and keeping it clean.

In general I'd say the majority of the hi-fi experienced AM stations who operate between 3870 to 3890 are running about +-  6.5 KHz audio and most signals are well contained. They are very self-aware. Then there are a few who are not sure what bandwidth their rigs are transmitting. But nowadays, when a rig breaks and starts splattering, it ain't long before they are informed.

All good.

T
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« Reply #86 on: February 25, 2020, 12:09:57 AM »

Today I finished the 5.5 KHz prototype.  I added a 2N2222 emitter follower at the input and a 2N2222 buffer at the output of the 10th order Butterworth passive LP filter, making it an active filter.

Frank did the modeling changes as posted below.

It worked right away, with no mistakes - which is unusual for me... Wink   I was pleased with the initial bench test. It was perfectly flat from 2 cycles up to about 4.8 KHz. It then started dropping off and got faster beyond 5.5 KHz and was way down at  6.5 KHz.  Hardly a residual trace at 7.2 KHz.

I noticed a few improvements as a result of the filter being better isolated from the external world. The previous passive filter had a slight ripple hesitation at about 6.7 KHz as I reported last week. This slight ripple/hesitation was gone.  The flatness and fall of the skirt was near perfect in shape. I noticed on the bench the drop off had moved out about 500 Hz farther than before, but on the air, it appeared more behaved and very sharp with the spectrum waterfall cutoff of 6.5KHz.  This is a 5.5 KHz filter, but the brick wall effect is around 6.5 KHz in practice it seems.

I ran it into a dummy load.  Without the filter it was the usual + - 10-12 KHz. The filter in-line actually looks cleaner and sharper than before at a razor 6.5 KHz.  Another thing I noticed is the highs seem slightly cleaner in the monitor.. they sounded similar in quality when the filter is off line.  The filter is near invisible below cutoff as far as I can tell.

Butterworth filters have a great reputation for several reasons. I am glad I went this route.  We are looking at doing 3 or 4 filters with buffers on one board. At the present moment I have plans for a 2.5 KHz ssb filter for DXing when very crowed, a 4.0 for AM or ssb  (+- 5KHz)     a 5.1 (+- 6.2 KHZ)  a 7.0 (+- 8.5 KHz)  and a 10 KHz (+- 12 KHz)

The option will be either an ssb filter or the 10KHz since only four filters will fit on the board.  I plan on the ssb filter rather than the 10 KHz. We will have to see what the final decision is for the number of filters in total..

Oh, another benefit is the slight residual 120Hz hum I had using the passive filter is totally gone. The audio line did not like the passive filter alone, but with it isolated, it appears cleaner, less noisy and works like a champ with no observable ripple or ringing.

It uses 13.8 DC now and 600 ohms in and out. Now on to the good looking PCB version.  Frank wants to normalize the filters so that they have the same gain for ease of fast A/B switching in and out without readjusting the audio gain. Oughta be a slick box when finished.

One of my goals with this project is to get the 4.0 filter to be brickwall at +- 5.0 KHz, just like a BC station... Wink  I will add on some more poles just to see how far I can take it at 5.0 KHz.

T

 
The finished 5.5 KHz prototype:


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* DSCF0005.JPG (319.91 KB, 1280x960 - viewed 52 times.)

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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz,  +-6.0 KHz or  +-8.0 KHz when needed. 

"You know my name"  https://youtu.be/noGjJyEDm5s?t=135

There's nothing like an old dog... a puppy... a dog in its prime... or ANY dog!
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« Reply #87 on: February 25, 2020, 08:19:14 AM »

Very good on all....

Lots of interesting points regarding the psychological and technical features of the AM Window, audio bandwidths, etc.  An important point mentioned by Kevin is that if two AM stations are spaced 5 KHz apart, then both need to run +- 2.5 Khz audio JUST to barely keep out of each others' way. That is an eye-opener to guys who get within 5 KHz and then complain of splattering neighbors....  That's why broadcast stations are spaced 10 KHz apart and run +- 5KHz audio.

[...]

I just love doing the detective work and training myself to pick out what a station is doing with their rig by the order or disorder of their signals.  This has nothing to do with absolute bandwidth, rather how are they managing their signals to achieve the bandwidth they choose to use and keeping it clean.

[...]


Very interesting points on order versus disorder. Disorder is entropy, the most powerful force in the Universe so far as we know. And Life in general, and we in particular, are entropy-fighting creatures. So, by ordering our transmitted intelligence and energy well, we are serving the Life Force. Bet George Bernard Shaw never thought of that!

One correction, though: It's mostly in a few big radio markets, especially in the Great Eastern Megalopolis, that _some_ AM broadcasters have adopted the execrable +-5 kHz audio standard, something that is not required and that they never should have done (the REAL AM interference problem is not from adjacents or second adjacents -- it's from a zillion stations co-channel on every channel, and reducing bandwidth helps not at all with that). The vast majority of AM broadcast stations (and ALL of them in my area) are, thank goodness, +-10 kHz.

73,

Kevin, WB4AIO.

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« Reply #88 on: February 25, 2020, 09:17:43 AM »

[...]
I agree with Tom about narrowing up when things get crowded, but not just to be neighborly. Even if a transmission remains "Hi-Fi", it's unlikely that other ops on frequency are going to leave their receivers set wide and tolerate the artifacts from adjacent signals. This leaves the question about being "wide" when nobody can appreciate it.


Understood. But even if you feel you cannot open up your receiver much, and find it hard to believe that your QSO partner could either, there may be other people listening in who can. I sometimes find that an op has made a choice to use a narrow transmit bandwidth when I could have easily heard 7 or 10 kHz from him in my location. Disappointment!

Also, concentrating modulated energy into a narrower spectrum contributes to intelligibility, a point not lost on equipment manufacturers who want customers raving about piercing pileups and scoring difficult DX contacts. That's not going to happen with 20-20kc audio.

SEE? YOU AMers NEED TO TIGHTEN IT UP!!!

That's the popular argument, but it makes no sense. AM's strength is not in DXing. AM is the "QSO Mode" where ops don't need to constantly twiddle with a RIT control each time someone keys up and we can recognize each other at hamfests by voice. [...]



Actually, concentrating your energy into a narrower passband does improve the signal to noise ratio (a LITTLE), but that does not always translate into increased intelligibility. One day, I'll post one of the studies here that show wider audio is more intelligible than narrow audio.

I agree 100 per cent. about AM (and I would add eSSB) being the QSO mode, with the sound tailored for enjoyability, naturalness, and conveying subtleties of character, meaning, and feeling in the sound.

I think about it this way: If you cut out all your bass, and heavily clip your audio, it will probably help your intelligibility when your signal is barely detectable, below the noise. So, for people who seek out such situations, such as snagging the rare one using a manpack and a whip on the far side of the Himalayas, that kind of audio is preferred.

But 95 per cent. of my operating involves nice, healthy signal-to-noise ratios where space shuttle audio does not help at all -- in fact it hurts in more ways than one.

Many stations run space shuttle audio all the time. But if I do that, and cater to the 5 per cent. of stations on super-weak paths just so they can understand half of my words if I repeat them three times, then I am simultaneously torturing 95 per cent. of the people I talk to by making them listen to wretched audio -- for no reason at all.

Side bar: For Pete's sake (and everyone else's) if you have to operate in the 3875-3880-3885 segment of 75M, 3880 is NOT the first choice, it's the last. Think of it as the "middle seat" on an aircraft. Choose the window or aisle first. Then again, there is no "AM Window" any more so spread it out folks...

Agreed! I'd go even further. There are only two seats, aisle and window. 3880 is sitting on the arm rests between the seats.

73,

Kevin, WB4AIO.
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« Reply #89 on: February 25, 2020, 07:10:03 PM »

Yesterday I received  Rick/W8KHK's SCAF, Switched Capacitor Audio Filter,   (contained on the Max board)  and I started testing it on the bench today. It initially had some residual clock feedthrough crud, but a simple low pass filter fixed that and it is now clean. This is an older version board and that has been fixed and Rick has been on top of it..

It swept just like the Butterworth on the bench. Very close in smooth response and sharp skirts.  After all, two Maxim SCAF chips in cascade is pretty bad-ass.

I then hooked the SCAF up to the rig and ran some more tests. It was easy to adjust quickly to the desired filter freq and it sounded as good as the Butterworth in the monitor.   I ran it on ssb at 2.1 and worked fine.  I tried it on AM at +- 4.0, 5.0 and 8 KHz and I was able to adjust it on the fly to give perfect skirts at those freqs. I figured +- 8 KHz is as high as I will ever run my bandwidth. It did sound sweet at 8 KHz!  And it sounded like crap on ssb at 2.1 KHz due to the bandwidth... Wink   The difference between my FT-1000D on ssb running 3KHz and 2.1 is unbelievable, so I am going to stay with 3.0 on ssb..

For my use, the easiest way to quickly adjust the filter wud be to modify the board and mount three larger 1-turn pots on the front panel with a switch for narrow, medium and wide. This combo would give continuous control from ~ 2-12 KHz. (or more if needed)

I am impressed but first I need to run some IMD tests with the rig and check it more carefully against the Butterworth. Also, I want some on air A/B tests to see if Tron hears any difference or artifacts.

 On the air while watching the SDR spectrum analyzer and waterfall, I notice it is definately in better control of the skirts than the Butterworth. There are no rouge spikes when hitting sssís.  It actually resembles SDR DSP on the waterfall when using the FT-1000D on hi-fi AM which is only -30DB 3rd order.

We shall see.  Our Butterworth is a fine filter and a lot of work, but it will be a simple matter of which one works and sounds the best in the end.

Rick has been very helpful getting me up to speed on the filter - so it went very smoothly with no errors so far. No smoke or board arc-overs so far...

Oh, one last thing...  FWIW, when the filter was adjusted to 3.5 KHz or lower, the mic polarity switch needed to be flipped 180 degrees to give the best positive peaks.  Above 4.0 Khz, the mic polarity remained the same as when the filter is out of the path. My Butterworth is fixed on 5.5 KHz, so I cannot compare the two on this basis. No big deal - just wanted to mention it.  My voice is different from yours, so you may or may not see the same thing. I do not use an all-pass filter for symetrical audio.


More soon.

T
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« Reply #90 on: February 25, 2020, 09:03:33 PM »

The tandem Maxim 295 chips are Butterworth filters, known for their sharp cut-off. The Butterworth downside is overshoot which is why they are located prior to the clipper stage. We tried Maxim 296 chips with Bessel filters to address the overshoot but the sacrifice in slope wasn't worth it so we reverted back to the 295s.

The MAX processor has abundant customization notes. Most might build and use it as-is, but the AM crowd in particular are experimenters and like things their way.

Plans for purple chaser lights and chrome hubs are in the works.

FWIW, when the filter was adjusted to 3.5 KHz or lower, the mic polarity switch needed to be flipped 180 degrees to give the best positive peaks.  Above 4.0 Khz, the mic polarity remained the same as when the filter is out of the path. My Butterworth is fixed on 5.5 KHz, so I cannot compare the two on this basis. No big deal - just wanted to mention it.  My voice is different from yours, so you may or may not see the same thing. I do not use an all-pass filter for symetrical audio

This happens because vocal asymmetry is not consistent across the vocal range. It's quite common for the dominant polarity of lower frequencies to conflict with upper frequencies. Using a low-cut filter or limiting higher frequencies will both reduce the opposing polarities. The use of an all-pass filter eliminates this issue.
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« Reply #91 on: February 25, 2020, 10:02:58 PM »


FWIW, when the filter was adjusted to 3.5 KHz or lower, the mic polarity switch needed to be flipped 180 degrees to give the best positive peaks.  Above 4.0 Khz, the mic polarity remained the same as when the filter is out of the path. My Butterworth is fixed on 5.5 KHz, so I cannot compare the two on this basis. No big deal - just wanted to mention it.  My voice is different from yours, so you may or may not see the same thing. I do not use an all-pass filter for symetrical audio

This happens because vocal asymmetry is not consistent across the vocal range. It's quite common for the dominant polarity of lower frequencies to conflict with upper frequencies. Using a low-cut filter or limiting higher frequencies will both reduce the opposing polarities. The use of an all-pass filter eliminates this issue.
Split highs and lows to separate channels, flip the phase of one channel and recombine. Positive peaks galore!
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« Reply #92 on: February 25, 2020, 10:19:54 PM »

Butterworth filters best characteristic is passband flatness. They are called maximally flat filters. If you want a sharp cutoff, go with a Chebychev. If you want good impulse response go with a Bessel. I'd go with a Bessel to avoid overshoot. A lowpass filter at the tail end of the processing chain can put back in peaks that you've tried so hard to remove.
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« Reply #93 on: February 26, 2020, 02:32:45 AM »

This nice aluminum box has been sitting around for years waiting for a job.  I usually don't waste time. I received the old version 3 MAX with the SCAF MAXIM chips and got it working today.  I asked Rick about my adding filter frequency control pots and toggle switches on my front panel rather than trimmers on the board. He thought it was a cool idea and told me to solder right to the header pins and add a switch and pot in series for the three trimmers.. Easy enuff.  He warned me about potential instability with RF, but all is stable which is a sign of quality design.

I first did a lot of tests before the mods to make sure everything was stable in an RF field and it was. The clock can be sensitive to RF so I kept the chip and connections close to he front panel. The pot wires are the brown Teflon wires you see at the front of the cabinet.  This is obviously a custom SCAF implementation and as Clark suggested, shows the versatility and modification potential of the overall MAX unit.  The board has in and out headers all around and you can add or subtract whatever you need. In my case the MAXIM SCAF and clock are activated.

I added in a LP filter to kill any remaining clock crud and it was completely gone.  I will drill some heat vent holes tmw.

I ran some tests with the panel pots mods. Bottom line is the SCAF roll off is about as sharp as the discrete Butterworth, but the ease of changing filter frequencies is amazing. With a flip of one of the three toggle switches I can go to preset filter frequencies of +- 4.5, 6.0 KHz or 8 KHz.  (or whatever you want preset. The three knobs on the front panel allow me to overlap the three filters and tune them up or down in bandwidth.  So I have it set right now to cover CONTINUOSLY +- 4.0 to +- 8.0 Khz transmit bandwidth. What a difference between 4.0  and 8.0 KHz in audio quality!

The isolation is good - I hear nor see any hum when the mic is unplugged and on the air.... none at all. The 120VAC isolation power transformer did the trick for the 60 Hz hum and the MAXIM chip appears to have good buffering internally according to Rick. So no 120 VAC hum either.

I have the knobs set at the middle top as the presets - and can swing the filter freq monkey back and forth just by eyeballing the knob position. I will probably label the knob points to signify frequency.
 
Now that the Butterworth and SCAF are both in a box with plugs, I get to try them in cascade or compare them separately for tests. That will be fun.  I don't hear or see any issues at all with the SCAF so far. The SCAF parts count is greatly reduced compared to building four Butterworth filters.

I'd like to push the envelope and try a 3 or 4 MAXIM chip SCAF in cascade experiment.  There is surely a limit that I want to explore to see how close to DSP skirts I can get. Right now the RF bandwidth on the waterfall is pretty sharp and probably limited by the small IMD of the amplifiers.


Bottom line is it appears that the Max designers, Rick and Clark did a great job getting the filter to function well.  It oughta be an interesting box in the future if this is any indication.

T


Latest SCAF implementation:
  


* DSCF0005.JPG (323.1 KB, 1280x960 - viewed 63 times.)

* DSCF0006.JPG (321.9 KB, 1280x960 - viewed 56 times.)

* DSCF0008.JPG (325.68 KB, 1280x960 - viewed 51 times.)
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« Reply #94 on: February 26, 2020, 02:00:08 PM »

I tried something I always wanted to do.  

First of all the two filters are about the same in performance when run alone.

I wanted to test the discrete Butterworth filter with the SCAF filter in cascade.

On the bench using a sig gen and scope - the discrete filter working alone,  I swept  the filter from 5.0 KHz to 7.4 KHz and got a certain DB drop.  I then optimized the SCAF MAXIM filter for the same frequency coverage. Both in cascade,  I swept them over the same frequency and compared results of the two skirts.

The results were quite good!  Dropping the same skirt depth, it  finished at 6.5 KHz.  IE the same skirt drop was sharper by almost 1 KHz.  This is significant.
The roll off looks closer to a DSP skirt. Later I will try the same lashup on the air and give it a good audio listen.

This tells me there is more to go in performance. I want to try four MAXIM SCAF chips in series next.   The Max board will let me insert an outboard pair on a perf board.

A general rule I've observed: The lower the cut off frequency of the filter, (3.5 KHz)  the more the asymmetry is affected in the audio lows.  When using high cutoffs, the lows asymmetry is hardly affected. (6.5 KHz)

T


The two filters in cascade:



* DSCF0001.JPG (318.29 KB, 1280x960 - viewed 36 times.)

* DSCF0003 (2).JPG (313.21 KB, 1280x960 - viewed 31 times.)

* DSCF0002 (2).JPG (311.7 KB, 1280x960 - viewed 34 times.)
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« Reply #95 on: February 26, 2020, 04:20:19 PM »

Tom,

IF the design impedance for the "passive" filter is 600 ohms, you don't have a 600 ohm load on either end
that I could see. Maybe if you set it for 600 ohms the frequencies will magically fall into place? Just an idea.

                _-_-

PS, if you want more of a brickwall effect, turn one section into an elliptic filter by putting a cap
across the inductor. Yes, it needs to be simulated and tweaked in situ to get the proper "dip" at
the right frequency.
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« Reply #96 on: February 26, 2020, 05:02:49 PM »

Hi Bear,

The discrete Butterworth now has 2N2222 buffers going in and out so the loads are not as important as when passive before.  The two in cascade are really unreal.

But before I fool around any further, I want to see what FOUR MAXIM 295 SCAF chips look like in cascade.   We could wrap this project up quickly and reduce the parts count by a barrel-full ... :-)

Before we spend more time on it, has anyone modeled or tried four chips?

T
 
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« Reply #97 on: February 26, 2020, 07:56:38 PM »

you're going fast enuf.....hard to keep up....glad you're having fun.....Steve
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« Reply #98 on: February 26, 2020, 08:18:23 PM »

BTW, my idea that lowering the filter cutoff below 3.5 KHz is affecting the lows or phase shift affecting the lows adversely seems invalid. I swept the filter cutoff up and down from 2 KHz to 8KHz and could NOT see any change in the positive peaks or the asymmetry.  So looks like filters in general are not the villains we make them out to be.


I could not hear any difference between the SCAF and Butterworth during tests tonight into the dummy load.

We will be needing a buffer for the in /out SCAF, however. There is a little 120 VAC hum. It was the same with the passive filter too and the 2N2222s buffers fixed it.  Rick already uses buffers on the MAX board so it is not an issue for the MAX.   Maybe I can use those myself. I was just trying to get by without them... a form of laziness.


T
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« Reply #99 on: February 26, 2020, 08:33:15 PM »

BTW, my idea that lowering the filter cutoff below 3.5 KHz is affecting the lows or phase shift affecting the lows adversely seems invalid. I swept the filter cutoff up and down from 2 KHz to 8KHz and could NOT see any change in the positive peaks or the asymmetry.  So looks like filters in general are not the villains we make them out to be.

You were originally correct. Lowering the cutoff frequency will likely affect asymmetry because you are filtering out the predominant polarity vocal frequencies leaving the opposing lower frequencies alone. The opposite will be true when switching in a low-cut filter. In this case, the opposing polarity lower vocal frequencies will be filtered out and your positive polarity should increase significantly.

None of this will be apparent when using a sinewave of equal positive and negative amplitude. This simple circuit will demonstrate this effect. You'll never think of asymmetry the same way again:

http://www.internetwork.com/radio/n1bcg/RMS_Ratio_Monitor.htm

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