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Author Topic: 5KC Active Audio Filter for Brick Wall AM Use  (Read 20235 times)
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Steve - WB3HUZ
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« Reply #25 on: February 08, 2010, 12:22:52 PM »

Further, most people's receiver aren't going to hear audio past about 5 kHz. To run frequency response much greater than this is just plain stupid.
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N2DTS
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« Reply #26 on: February 08, 2010, 01:00:42 PM »

Being an ass because someone else is an ass is great fun.
Get those really wide signals on the band and take care of the guys running ssb, that will show them!

I was in a qso yesterday on 3885, and one of the usual very wide guys was on 3875 with his 25 to 30 KHz wide strapping signal, so I joined in and opened the EQ up to 20,000 Hz and leaned into the audio.
The guy I was talking to thought it sounded very good, his passband might go out to 3100 Hz on the pro 3.

It was great fun being 6 years old again!


The way we need to work it is have one qso on the lower part of the band, say 3810, and the other qso can be on 3900, if you want to start another qso, go to 40 meters.

If we can get the signals 60 or more KHz wide, there will not be anyplace for the ssb guys to go until they get the 3cx15000 amplifier working, which will give us time to get our 3cx30,000 finals warmed up.

If anyone listens to ssb on a good modern rig, they would find out that even a very strapping AM signal does not bother ssb much at all. They can get VERY close and have a fine qso if they want, and going out to 30 KHz is not going to bother them one bit unless they live in town....

During the 'contest' there were 4 or 5 qso's with about 5 KHz spacing and it seemed to work great at my qth.
Most people had quite narrow signals, even if the passband was wide, because they don't mulitband compress without limiting the highs, so the extreme high stuff is way down and does not seem to cause trouble.

Brett





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ke7trp
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« Reply #27 on: February 09, 2010, 02:13:12 PM »

Most wont implement the filter.  The big audio sound is going to be killed.  I would make or buy one.  I think its needed.  But I really cant see others using it.

C
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AB2EZ
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« Reply #28 on: February 09, 2010, 03:55:18 PM »

Dan

I don't remember which MAXIM switched capacitor filter is standard in the Idiom Press product. I'm pretty sure it wasn't an elliptical filter. I think it was a Butterworth filter, but I'm not sure.

MAXIM has a large variety of switched capacitor filters (I think much a larger variety than what they offered at the time I bought the Idiom Press SCAF-1 kit... several years ago).

Since I already have an audio chain that can insert a low-pass brick-wall filter with a cutoff of 4.5 kHz or greater...  I only used the switched capacitor filter, on-the-air, when I wanted to reduce my single-sided bandwidth below 4.5 kHz. So I don't think anyone ever head me on-the-air with it set to 5 or 6 kHz.

Like anything involving audio... you need to try it to see if you like the way it sounds with your voice and your microphone,..., and your ears.

It should be easy to substitute a different switched capacitor filter (e.g. elliptic) ... and they only cost about $5.00 in quantities of 1.

In my shack, I am now using digital filters... which were not available in the past... in the form I wanted, and at the price I was willing to pay. I didn't want to build a box from individual components to implement digital filtering... and I couldn't find anything that met my requirements commercially.

As pointed out, these days you should be able to implement any kind of audio filter you want, using a pc with any plain vanilla sound card... if you can find the application software that meets your needs (free is good).

Each passing month there are more free applications available to download that can be used... and the user interfaces keep getting simpler and more versatile. I haven't yet found a freeware application that can implement brick wall filters... but there may be one out there.

Keep in mind that the delay through any filter (LC or digital or whatever) is lower-bounded by (roughly) 1/w; where w is the change in frequency that corresponds to the transition from no attenuation to high attenuation. So if a brick wall filter is very steep (as opposed to very deep), the delay through it will be relatively large. For example, if you design the filter to go from no attenuation to high attenuation when the frequency changes from 5000Hz to 5050 Hz... then the delay through the filter will be at least 1/50th of a second = 20 milliseconds. This follows from mathematics... and is independent of how the filter is constructed.

Stu
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« Reply #29 on: February 09, 2010, 09:49:40 PM »

As pointed out, these days you should be able to implement any kind of audio filter you want, using a pc with any plain vanilla sound card... if you can find the application software that meets you needs (free is good).

Each passing month there are more free applications available to download that can be used... and the user interfaces keep getting simpler and more versatile. I haven't yet found a freeware application that can implement brick wall filters... but there may be one out there.
Stu

Stu,

I use http://www.reaper.fm/  It's capable of using just about any plugin available, and it comes with some that do brick wall.  I use it as my audio chain, with a few commercial plugins as well as freeware / free-to-use ones.

IIRC, Reaper is free for non commercial use....  Or it used to be, the one I have IS.  If you want it for commercial use, pony up the cash.  It's EVERY bit as good as Adobe Audition, but much less in the wallet area.


--Shane
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AB2EZ
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« Reply #30 on: February 10, 2010, 07:56:26 AM »

Dan

I checked the schematic of the Idiom Press SCAF-1 when I got home yesterday. The filter component is a MAX 295 (Butterworth, 8 pole)

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1370
http://datasheets.maxim-ic.com/en/ds/MAX291-MAX296.pdf

Also, the schematic of the unit, as I modified it, is attached below

Stu


* SCAF-1.jpg (61.14 KB, 960x720 - viewed 1012 times.)
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« Reply #31 on: February 15, 2010, 07:11:33 PM »

Hi Stu:

Thanks for the schematic and information. I knew that as the filter gets sharper, the delay (and phase shift) gets longer.

I used the elliptical filter as it is a brick wall version (actually two in series with a negative clipper), and is sonically a little brighter than other filters with the same -3db point. I also used it as the analog Orban units use elliptical filters.

What digital filters do you use? I do understand the digital Orban units have somewhat adjustable digital filters.

Shane:

How is Reaper different in features than Adobe Audition (which I use)? I may consider using the Breakaway Broadcast PC based radio processor in the future.

73,
Dan
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K1JJ
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« Reply #32 on: February 15, 2010, 08:43:16 PM »

I've had Stu's modified filter in use for about a week now on the transmit audio chain. At -96db/octave roll off, it's a keeper.

For example, I can set the TX roll off to start at 5,000 hz and put a tone thru the transmitter to set it at 100% modulation.  (The filter is on the output of the audio chain after the 6-band processor.)  Starting at 5K audio, 100% modulation,  the 5500hz point is down to around 5% modulation. At 6kc there is nothing getting out at all.

In practice I find starting the roll off at 6kc is a good all-around audio bandwidth.  Though 4500hz is tolerable when band cornditions get crowded.  There is definately a difference!

BTW, after a year's hiatus, I just re-installed my SP-600 receiver and tapped off the detector into an audio 12 band EQ. The EQ makes the SP-600 flat into the stereo audio system. The performance is superb - flat from 30hz to about 7kc.   Recently I was running my FT-1000D off its detector, etc. I thought the receive audio was decent until I swept it with my modulated hi-fi transmitter. It actually sucked. Now with the SP-600 system, I can hear who is truly HI-FI on the air and who is more communications audio. Beforehand I was fooling myself. Also, I think I have the recording system ironed out and ready to give out some on-the-air playbacks. The recording gets re-transmitted using the FT-1000D's transmitter balanced modulator which is quite transparent.

Back to the receiver - Bottom line is it's vitally important to sweep our whole receiver system from antenna to audio output to see how flat it really is. IF strips can alter things. Even the SP-600 off the detector needed an EQ to flatten things out.  And remember, the 16 khz receiver filter position passes ~8kc audio, not 16k audio, ( 16kc divided by 2)  unless we tune off the center frequency somewhat.


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.  Easily done in DSP.

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ke7trp
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« Reply #33 on: February 15, 2010, 09:49:42 PM »

Where is information on STu's filter?  Got a picture?

I run the Sp600 JX14.  Its fantastic. I tapped off the back also in a Marantz amp and then a nice speaker. Its great. This makes sense for the 16K filter position. I can roll around slightly and alter the tone.

I have been doing all kinds of tests here.  I got a Real nice Eico Audio Gen off Fleabay. It works great and I can sweep my system and recievers.

C
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K1JJ
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« Reply #34 on: February 15, 2010, 11:45:07 PM »

Where is information on STu's filter?  Got a picture?

I run the Sp600 JX14.  Its fantastic. I tapped off the back also in a Marantz amp and then a nice speaker. Its great. This makes sense for the 16K filter position. I can roll around slightly and alter the tone.

I have been doing all kinds of tests here.  I got a Real nice Eico Audio Gen off Fleabay. It works great and I can sweep my system and recievers.

C

Clark,

Go back a few posts for Stu /AB2EZ's schematics and information. His modified schematic is there too. You wud need to buy the unit and modify it. Works as well as I imagined the best filter I cud possibly build. Two chips in cascade are really something else for a brick wall.


BTW, if your SP-600 is like mine, you may need some slight EQ boost betwwen 20-70hz and again between 2kc-7kc to flatten it out. It didn't take much to get a perfectly flat line from 30hz - 7kc.  Without it the extreme highs and extreme lows lacked slightly. The EQ goes between the detector and audio amp, or wherever is easiest in the post receiver audio chain.

T

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.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
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« Reply #35 on: February 16, 2010, 12:01:22 AM »

Mine can hear out to 10K.  I have not measured it though..   I got the Mute working on mine tonight. I am going to make a post. It was a simple Relay and socket.  I used an old wall wart to power the relay coil with teh T368s contacts out the back.  I tacked on two wires on the send recieve and ran it out the back of the 600.  My 600 had no relay socket.  Its a real pleasure to have this mute when I flip the remote switch at the desk. 

C
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K1JJ
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« Reply #36 on: February 16, 2010, 12:11:02 AM »

Clark,

To have 10kc audio, did you modify the RX to have a wider filter position than the stock 13kc?   Even tuning off freq a little, I found 7kc is all I could squeeze out in the 13kc position. It needed some EQing to pull it out.  Though, I find 13kc is about all I can tolerate on the bands most of the time anyway.   I mean, I can still hear 8-9kc audio, but it starts rolling off up there.

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.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
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« Reply #37 on: February 16, 2010, 12:20:41 AM »

Tom,

I'm curious about the 1000D prod. det. audio.  Any idea why it was so poor coming out of the tap?  I don't know much at all about the D.  Perhaps Yaesu used a 455 KHz IF filter for AM that's too narrow?   They used a 6 KHz murata ceramic AM filter in the 455 IF of my 1000MP Mk V.  I had to pull that sucker out.  3 KHz audio--much too narrow.

Rob
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