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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 10000 times)
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K1JJ
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« Reply #100 on: February 26, 2020, 09:07:43 PM »

Thanks for the info and circuit, Clark -

What you say makes perfect logical sense to me. But what I don't understand is if I sweep the filter frequency up and down from 2 KHz to 8 KHz, I see no evidence of the transmitted waveform changing. I use my own  asymmetrical voice and sock an extended yaaaaallo and see no change at all... ?


While on the subject:

I made some careful two-tone IMD and THD tests tonight with both filters on the rig.  My normal hi-fi FT-1000D IMD is about -33 DB 3rd order, and the harmonic distortion is about -40DB.

I tried both filters on AM and SSB and could not see any signs whatsoever of IMD or THD changing with the filters in or out. They are apparently cleaner than the power amplifiers in the rig and below my measuring floor, which is understandable.

I tried many combinations of signal change and filter frequency changes with good, smooth results with both.  No signs of clock crud or strange signals at all. This is with the longer pot leads too.

T

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« Reply #101 on: February 26, 2020, 09:24:02 PM »

It actually takes a bit of practice to isolate the opposing vocal frequencies since normal speech or even a hearty yaaaaallo involve a complex mix of sounds. That circuit, or the level/polarity indicator on the MAX board, makes it easier to visualize and experiment. Everyone is different, but I've found that "aaaaaahhhhh" (lower frequencies) and "eeeeeeeee" (higher frequencies) generate completely opposite polarities for most folks. This is best done off the air of course unless you're really comfortable with yourself and the inevitable "What the hell are you doing?" that you'll hear when you unkey.

The main point is that when someone's mic is "phased correctly" it's really just phased for the majority of their speech.

This video demonstrates the effect:

http://dw.convertfiles.com/files/0490507001582771942/6af520b2-1c73-4e42-a89d-e6d79421b709.mp4
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« Reply #102 on: February 27, 2020, 01:09:16 AM »

Almost finished with this phase of the job - a good working filter, sharp as a razor, adjustable on the fly, no hum and a great waveform!

The last problem was some 120AC hum in the audio line but Rick told me how to borrow a buffer stage on the MAX board to isolate the input. Worked like a charm and there is absolutely no 60 Hz or 120 Hz on my signal.


What is pictured below is the Max board Maxim 295 SCAF filter  set ON THE FLY in real time to  +- 4.5 KHz,  +-6KHz and +-8 KHz  sssss's of my voice.  I can switch in narrow, medium and wide filtering and then vary its bandwidth even more with the panel pots. Overlapping coverage from 2.1 KHz to 10KHz.  Notice how sharp the skirts are. They are very close to this sharpness in normal voice speech, thought not as densely packed.

The only critique is I see a faint bit of blue wisp indicating very low energy high freqs in the form of ssss's leaking out when the waterfall sensitivity is increased. Most of this stuff is invisible under normal band conditions, but on a BIG local condition day running the big mawl, I'm sure some sibilance will be heard past the obvious filter limits.


This is why the next step is to try four Maxim chips in series rather than the present two. Rick offered to send me some parts so I can experiment away. As said, he has been very helpful on this project.  I was telling him that my "technical highlight" of the year was seeing my transmit AM signal (on the Butterworth filter) waterfall with tight sidebands, almost like an SDR rig. I've always had issues containing my sssss highs, but for the first time I have total control.  Amazing technology.

Now lets see how far we can push these chips in cascade.  The "QUAD SCAFs"  as coined by Rick.

T


Below -  *switched on the fly, real time*   4.5KHz, 6Khz and 8KHz transmit audio bandwidth on the 4-1000A plate modulated rig:



* DSCF0002.JPG (342.57 KB, 1280x960 - viewed 143 times.)

* DSCF0003 (2).JPG (337.89 KB, 1280x960 - viewed 139 times.)
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« Reply #103 on: February 27, 2020, 09:29:33 AM »

... one's mic is "phased correctly" it's really just phased for the majority of their speech.

This video demonstrates the effect:

http://dw.convertfiles.com/files/0490507001582771942/6af520b2-1c73-4e42-a89d-e6d79421b709.mp4
[/quote

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« Reply #104 on: February 27, 2020, 10:30:20 AM »

Yeah, sorry. This forum doesn't allow videos to be uploaded, so I converted it to mp4 and provided a link. The problem is that these converter links expire.

The screenshots below show the polarity and level indicator on the MAX processor boards. The red LED shows predominant negative polarity, the green LED shows predominant positive polarity, and the yellow LED is a normal level indicator. The righthand red LED flashes on with peak levels.

Symmetric tone (Red and Green LEDs are lit):



Positive polarity with "eeeeee" sound (Green LED is lit):



Negative polarity with "aaaaahhhh" sound (Red LED is lit):



With normal complex speech, the red and green polarity LEDs flicker alternately but the green is on most of the time, indicating the predominant polarity. The negative polarity LED is usually blue, but for some reason my phone shows that color flushed, so I replaced it with red for more clear display.
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« Reply #105 on: February 27, 2020, 11:24:39 AM »

"...normal complex speech..."  Huh


* blinkenlichten.jpg (79.38 KB, 675x168 - viewed 149 times.)
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« Reply #106 on: February 27, 2020, 07:44:21 PM »


Yes dear friends,

When you want that syrupy smooth audio, just open the top, and pour it on!



* MRS Butterworths.jpg (284.38 KB, 786x1000 - viewed 136 times.)
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« Reply #107 on: February 27, 2020, 08:48:37 PM »

ON AIR:

I got some filter audio reports from the locals on 3885 AM today. I switched filters, switched bandwidths, fished for SDR spectrum and ear reports.  Here's what I came up with...

Most said it sounded FB using either filter. W1IA said the bandwidth was well contained on the spec anayzer and sounded great. Jay/ N3WWL liked what he heard. Bear had me try the SCAF filter at 8KHz and then out of line. He said he could hear some faint spectral artifacts with the SCAF filter in the extreme highs.  I then tried the discrete Butterworth filter and he thought it was cleaner, but was not sure.

The +- 2.5 KHz bandwidth sounded too restrictive, while most said they could live with +-4 KHz no problem.  +-6 KHz was a good compromise while others thought there was a big difference in audio quality going to 8 KHz. Much of the opinions depended much on the receiver bandwidth, of course.

I plan to get more listening reports over the next few days to get a consensus. I didn't hear the Tron on today who is my main man for audio reports.


SQUAREWAVE BENCH TESTS:

Bear later suggested I run a square wave thru the two filters to compare them.  I hooked the dual trace scope to both filters and fed them together to get an overlay. The old scope actually sync'd.  Amazingly, except for the low freq tilt,  they were almost identical from about 80 Hz up until about 3000 Hz when the square wave turns into a sine wave as the filter starts to affect the signal at 5.5 KHz.  Again, both filters tracked closely.  

The first two pictures below were taken at 500 Hz using a squarewave.   However, the third picture taken at 20 Hz shows the SCAF  has a tilt slope while the discrete is a pretty good squarewave. I would attribute this to using smaller coupling caps in the SCAF. I am using 150 uF couplings in the discrete, simply because I had them. I think Rick said he was using 10 uF coupling. Bigger caps would even out the response I think.... assuming that's what they want to do with Max.

The swept response of the discrete Butterworth goes down to 2 Hz.  They both have close to the same ripple and slope but there is a slight bit more overshoot and ringing in the SCAF filter, but probably not of concern..

I'm very pleased so far with both filter's transparency and peformance.

T

Below: The two filters independently compared at 500 Hz, 500 Hz and 20 Hz  - (TWO TRACES TOGETHER):





* DSCF0001 (3).JPG (330.62 KB, 1280x960 - viewed 84 times.)

* DSCF0002 (3).JPG (331.62 KB, 1280x960 - viewed 86 times.)

* DSCF0007.JPG (325.53 KB, 1280x960 - viewed 84 times.)
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« Reply #108 on: February 27, 2020, 10:06:35 PM »

Here is a clearer view of the SCAF Filter  (top) and the Discrete Butterworth (bottom) using a 300 Hz square wave signal.

T



Below: Notice the SCAF has a little more tilt and the ripple and overshoot is slightly higher.  My guess is once the tilt is fixed it will be hard to hear the difference in the tiny ripple vs: the added versatility of the SCAF.  Opinions?


* DSCF0003 (4).JPG (336.18 KB, 1280x960 - viewed 145 times.)
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« Reply #109 on: February 27, 2020, 10:36:53 PM »


The overshoot amplitude and the number of cycles of ringing does tell a story about
the sound. While not unexpected, the fact that the two filters differ on overshoot indicates
that they may sound different.

Ideally there would be little or no overshoot.

In the abstract, the leading edge is a high frequency, and the goal of the filter is to
roll off high frequencies. If the filter could do this perfectly, only the rise time of that
leading edge and the squareness of the corner would be rounded (here it shows
overshoot and ringing).

I don't recall if it is possible for such a high order filter to not exhibit the ringing,
and/or if it is possible to control/damp it. Would have to pull out a text...

I thought they did sound somewhat different.

Subjectively on my R-390A @16kHz all of the filter widths sounded different, as did
both filter circuits...

It's certainly an interesting experiment!

 
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« Reply #110 on: February 28, 2020, 10:08:33 PM »

I changed the coupling caps from 1 uF to 150 uF and the tilt is almost gone and the filter response looks like a square wave down to 10 Hz or so.  The ripple is smaller.  Big improvement.


I'm going thru voice tests now but this should bring back the lows and maybe make the performance more transparant.

With this change I think we can now tolerate a bit more ripple with four chips in cascade.

* BTW, I just found that part of the slight tilt is caused by the signal generator and the scope.  So this SCAF filter is really performing better than pictured.

T


Both Pics show squarewave at 16 Hz -   SCAF Filter:



* DSCF0005.JPG (323.78 KB, 1280x960 - viewed 137 times.)

* DSCF0007.JPG (327.07 KB, 1280x960 - viewed 130 times.)
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« Reply #111 on: March 01, 2020, 12:15:00 AM »

After more on-air reports and bench tests, I have decided to go with the MAXIM 295 SCAF chip approach to the LP filter. It was a tough decision because  both filters had advantages and disadvantages. I am not convinced that the SCAF has any artifacts enough to hear. It's such a close toss up in performance. I think going with the SCAF is best. It has the best skirts and has more control of the frequency changes.  The tilt has been optimized and the ripples are about the same in both filters. I have no transmit hum at all using either now that buffers have been used.

I ended up with just ONE panel pot that controls the overall filter cutoff frequency. With four pre-marked dots I can set the filter quickly to  +- 3 KHz,  +-4.5 KHz,  +-6.5 KHz and +-8 KHz.  These settings are where the haircut starts -  where the actual sidebands end.

After a week of actual on-air practice I am able to see band conditions on the spectrum waterfall and set the appropriate bandwidth in seconds.  I can see if my sidebands are hitting the adjacent QSO or are clear. In fact today I was demonstrating the +- 8KHz position to Bruce/W2XR and I could see it was too wide and hitting the close QSO down the band. So I quickly switched back to 6.5 KHz and I was clear. This is a new double-check way of operating for me and I feel more self-aware of what's happening on the band around me.

The only thing left is to build up the extra two Maxim 295 SCAF chips and add them in in cascade, doubling the size of the filter.  I still see a slight bit of energy that leaks out past the skirts... everyone has it due to IMD or simply not a sharp enough audio roll off. If the four chips kill this extra fuzz, the extra ripple added to the filter may be worth it.  I should know sometime next week.

*** As a perfectionist failsafe, I have a filter in/out switch that will completely take the filter out of the system for those days I feel like letting the freak flag fly at at +-10KHz, using the natural bandwidth of the transmitter I'm using.  

All in all, I'd say the W8KHK / N1BCG Max implementation and MAXIM SCAF chip, when set up right, is a good design and will do the job quite transparently. My thanks to  Rick/KHK for his email suggestions and parts, Frank/GFZ for related filter modeling and the many guys on the air who took the time to run tests with me from time to time.

T
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« Reply #112 on: March 02, 2020, 11:11:48 PM »

Tom, after you finish testing with two additional Maxim Max295 Butterworth filter chips in the chain, you might like to substitute a quad of Max296 Bessel filter chips and compare the results.  The cutoff slope is not quite as steep with the Bessel chips, but that may no longer be an issue with four in cascade.  

The  Bessel devices do not have the overshoot issues exhibited by the Butterworth, so impulse performance (as demonstrated with your square wave test) might be significantly cleaner.  I will provide four 296's if you think you would like to try them.

Depending upon the results of your testing, we may consider adding two more Maxim devices to the artwork before the next rev PCB of the Max Processor goes to fab.  

Your efforts testing, as well as making suggestions for improvements, are very much appreciated!
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« Reply #113 on: March 03, 2020, 12:31:16 AM »

Sounds good Rick!  Glad to help.

I think the 296 chip swap test - sacrificing some skirt steepness for the chance of a cleaner filter response is well worth a try.

It makes sense and we could end up with a really sharp squarewave with little to no ripple. Who could ask for more?

I continue to test the two MAXIM 295 chips at various bandwidths on the air and the reports are good.  Tron was listening to the filter and CRL PMC-300A clipper tonight and said he liked the sound very much.

Tim was running hi-fi audio and his articulation was quite pronounced. I could hear the subtle sounds of "THHHH" like in the word "The" and the sounds of his lips, tongue and teeth. It's subtle but very plain when using a good SDR receiver and sound system, even with band conditions. The effort is worth it to have this kind of transmit bandwidth versatility. In this case it's about listening enjoyment rather than raw communications.

T

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« Reply #114 on: March 03, 2020, 01:01:44 AM »

My hearing is not what it was 60+ years ago when the horizontal output on our 21 inch round kinescope color box sang in my ears.  But still the higher frequencies make the difference in getting the message clearly on the first pass, or asking for a repeat. 

Sibilance is SO important in understanding!  If the bandwidth is available, we should use it.  When we are through, someone else can have it.  Fully renewable resource!

OM, you are 5 by 9 here in South Podunkville.  Would you kindly repeat your call, handle and QTH, preferably with phonetics?Huh?  Yeah, Right!
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« Reply #115 on: March 05, 2020, 09:19:43 PM »

The latest SCAF filter test:

I got the four MAXIM 295 chips running in cascade and made some rough tests for now….

The ripple is doubled as we expected.   The skirts definitely got sharper. Before, the TWO chips would start dropping at 5 KHz and be down to a premarked low level at about 7.2 KHz.   With the FOUR chips, the same drop was finished at 6.5 KHz.  

In comparison, the same passive filter drop starting at 5KHz ended at 7.7 KHz.

The four-chip narrower skirts are significant, but the ripple is now starting to get out of hand. I plan to hook it up to the rig and listen carefully.

The plan now is to plug in the MAXIM 296 chips into the same sockets.  They have less ripple and less rolloff - a good compromise.


T
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« Reply #116 on: March 10, 2020, 11:25:46 PM »

Update:

After trying many combinations of sequence using four Maxim 295 and 296 SCAF audio chips I have determined that the original two Maxim  295 chips in cascade are the best. There is a compromise between ripple and skirt selectivity.  Two chips appears to be the point of diminishing returns.


I tested the board with two 295 chips used in the Max processor. (on air)  I found it to have good on-air skirts with no noticeable ripple or clock generated IMD.  Rick tells me they will stick with this design and move on to other testing.

T
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« Reply #117 on: March 16, 2020, 09:39:02 PM »

Am I seeing some overshoot there? This is what I was talking about in my impulse response post.


ON AIR:

I got some filter audio reports from the locals on 3885 AM today. I switched filters, switched bandwidths, fished for SDR spectrum and ear reports.  Here's what I came up with...

Most said it sounded FB using either filter. W1IA said the bandwidth was well contained on the spec anayzer and sounded great. Jay/ N3WWL liked what he heard. Bear had me try the SCAF filter at 8KHz and then out of line. He said he could hear some faint spectral artifacts with the SCAF filter in the extreme highs.  I then tried the discrete Butterworth filter and he thought it was cleaner, but was not sure.

The +- 2.5 KHz bandwidth sounded too restrictive, while most said they could live with +-4 KHz no problem.  +-6 KHz was a good compromise while others thought there was a big difference in audio quality going to 8 KHz. Much of the opinions depended much on the receiver bandwidth, of course.

I plan to get more listening reports over the next few days to get a consensus. I didn't hear the Tron on today who is my main man for audio reports.


SQUAREWAVE BENCH TESTS:

Bear later suggested I run a square wave thru the two filters to compare them.  I hooked the dual trace scope to both filters and fed them together to get an overlay. The old scope actually sync'd.  Amazingly, except for the low freq tilt,  they were almost identical from about 80 Hz up until about 3000 Hz when the square wave turns into a sine wave as the filter starts to affect the signal at 5.5 KHz.  Again, both filters tracked closely.  

The first two pictures below were taken at 500 Hz using a squarewave.   However, the third picture taken at 20 Hz shows the SCAF  has a tilt slope while the discrete is a pretty good squarewave. I would attribute this to using smaller coupling caps in the SCAF. I am using 150 uF couplings in the discrete, simply because I had them. I think Rick said he was using 10 uF coupling. Bigger caps would even out the response I think.... assuming that's what they want to do with Max.

The swept response of the discrete Butterworth goes down to 2 Hz.  They both have close to the same ripple and slope but there is a slight bit more overshoot and ringing in the SCAF filter, but probably not of concern..

I'm very pleased so far with both filter's transparency and peformance.

T

Below: The two filters independently compared at 500 Hz, 500 Hz and 20 Hz  - (TWO TRACES TOGETHER):




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« Reply #118 on: March 20, 2020, 09:01:42 PM »


Below -  *switched on the fly, real time*   4.5KHz, 6Khz and 8KHz transmit audio bandwidth on the 4-1000A plate modulated rig:



Does it mute for a few ms (the little gaps) when shifting gears or do you do that in your design?
If so, Can it be 'speed shifted' so muting is not present, or it doesn't work that way?
No one would notice, just curious.
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« Reply #119 on: March 20, 2020, 09:32:59 PM »

Hi Pat:   The gaps resulted from intentionally pausing my voice ssss's to make a clean separation between filters.  

It can make seamless excursions with no carrier interruption if wanted, producing sloping lines during adjustment.

Steve: Yes, there is some overshoot in the SCAF filter. The best filter uses two Max 295s in cascade for skirt and overshoot compromise.

I have been experimenting with some newer DSP audio filters that have the same 8th order Butterworth performance without the overshoot. There are many ways to accomplish a tighter, more contained signal.  Passive, SCAF, DSP, etc.

Presently, I set my AM transmit filter to start roll-off at about +- 4.5 KHz and the normal sibilance sidebands stop at about +- 6 KHz, which is a good compromise for fidelity and reasonable bandwidth. I use it all the time now and occasionally get comments from SDR waterfall users saying even though the signal is BIG, (4-1000A) it is tight and well behaved. That was the goal. It was a lot of work experimenting, but I'm very happy with the final results.

T


I usually run the bottom or center filter curve unless the band is quiet:


* Audio Filter on 4-1000A Rig.jpg (342.57 KB, 1280x960 - viewed 63 times.)
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« Reply #120 on: March 28, 2020, 01:15:27 PM »

Nice spectra there Tom!

Check out Bessel filters for no/minimum overshoot.
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« Reply #121 on: June 14, 2020, 10:55:26 PM »

 Another possible option is to use  DSP  filtering.  Then one can  set the TX  BW  to  anything you want.   Brickwall  cutoff,  no phase shift, and no ripple,  no distortion.   I use this technique when narrowing a  6 khz  ESSB signal  down to  a  narrower TX  BW... like 3.5 or  4 khz... or  5 khz  etc.  DSP can also be used to limit the lowest TX freqs,  like 40 hz,  50, 80, etc.   Most of the  DSP  boxes will  cover  20 hz to  20 khz..and anything between. It can  be used for AM /  FM / ESSB,  cxr and one sideband, etc.

Shane put me onto this thread a long time ago, and I 'forgot'  to offer a comment at the time.

Jim  VE7RF
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« Reply #122 on: June 15, 2020, 12:23:45 AM »

Another possible option is to use  DSP  filtering.  Then one can  set the TX  BW  to  anything you want.   Brickwall  cutoff,  no phase shift, and no ripple,  no distortion.   I use this technique when narrowing a  6 khz  ESSB signal  down to  a  narrower TX  BW... like 3.5 or  4 khz... or  5 khz  etc.  DSP can also be used to limit the lowest TX freqs,  like 40 hz,  50, 80, etc.   Most of the  DSP  boxes will  cover  20 hz to  20 khz..and anything between. It can  be used for AM /  FM / ESSB,  cxr and one sideband, etc.

Shane put me onto this thread a long time ago, and I 'forgot'  to offer a comment at the time.

Jim  VE7RF


Hi Jim,

Yep, that's exactly what I am doing now.  Since this thread was written about analog filters I've changed over to DSP control for bandwidth, limiting and EQing - works FB. Yes, I use mine to make a sharp cut at below 40Hz too.

I am using a DBX DriveRack speaker management system. It lets you input a low level audio signal and tailor it in DSP.  The only limiting factor is hi-level IMD from the final that can cause wider bandwidth than desired.  I've worked hard to make my rigs clean so the low level DSP stuff works well. No pre-distortion here yet... just old school homebrew tube AM transmitters or standard linears for ssb.

I think I've seen some of your stuff around related to hi-fi ESSB and it looks informative.   Thanks for stopping by.   

Tom, K1JJ
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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. 

We die three times; when our body expires,  when we are buried and when our name is uttered for the last time.  All my dogs are named "Yaz."
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