Broadcast audio processors

[KQ6F]

I've been looking at commercial audio processors designed for the broadcast industry and am wondering how much one would contribute to my AM signal.  It appears to me that the main features of these boxes are aimed primarily at improving the sound of transmitted music.  OTOH, I've heard a few ham stations that employ one of these processors and they sounded great.  But they were also using clean transmitters and studio mics.  So how much glory was from the processor..

[k7pp]

You can find lot's written on the CBS "volumemax and automax broadcast processors on the net.
In the late sixties and early seventies these units appeared on the scene in the AM broadcast industry and provided a twist on the standard peak compressors by providing asymmetrical modulation.   Their circuit was potted and unable to be copied.  Their circuit made sure all audio sent to the AM transmitter was rich in positive peaks. The other unit kept the average audio very high using a fairly convention compressor.   Pretty smart.
Modulation peaks of up to 130 percent positive were possible thereby allowing the transmitter much more sideband power while still not exceeding 100 percent modulation on the negative peaks.

Every once and a while they show up on the used market and are snapped up as fast as they appear.
The two units also provided higher average program power and were a real hit right off the bat.

Today's processors used in the broadcast business are pretty expensive but there are still some that are somewhat affordable and do a fairly good job.
I really am "into" the AM scene and have spent considerable time improving the audio of my Viking Valiant and have not had to break the bank to do it.

I use an inexpensive Behringer "C1" Condenser Microphone driving an ART single channel audio processor.  I take the output of the ART and apply it to the phone patch jack on the back of the Valiant.  I have bypassed all the audio circuitry in the transmitter and re-wired the driver tube as a phase splitter.
I have also rewired the clipping pot to allow me to re-balance the phase splitter to asymmetrically modulate the Valiant allowing fairly undistorted peaks to 120 percent.  The only limitation is the Modulation transformer which could be replaced for much better low end audio.   I'm working on that.

Many have mods that reconfigure the modulation scheme in the Valiant but
I went this route.  I would think mine would compare favorably to many modified AM stations on the air today and I didn't spend a fortune.

Take a few minutes and do a search on the internet.  There is much published info available.

73s
Pete, K7PP

[KQ6F]

Pete - thanks for the thoughtful reply.

I've spent some internet time comparing various broadcast processors.  Their approaches vary but it appears to me that the best ones have a couple things in common:  multi-band limiters and ability to handle asymmetric modulation (in a few cases, even the use of 'phase flippers' that sense asymmetry and flip the phase in order to point the largest peaks in the positive direction).  But all have one thing in common: they make audio sound LOUDER.

It seems to me that the multi-band limiters are mostly aimed at improving the sound of music and would have little effect upon voice.  I may be wrong here and would welcome feedback from anybody who has experience with one of these boxes.

And as far as asymmetric modulation is concerned, some manufacturers actually eschew it and run the audio through a phase scrambler stage to kill asymmetry.  They then run it through stages that add "density" (which I think is just another word for 'compressor').  These folks argue that the best way to sound louder is to maintain symmetrical modulation, but at a higher average modulation level.

IMO, the worst kind of processor is the one that simply clips positive peaks at 125% and negative ones at 95-98%.  Those just add distortion.  I've seen claims of "distortion-free clipping" which is an oxymoron if I've ever heard one.

I've played around with various Behringer boxes that provide compression and EQ and claim to generate  other exotic effects.  But I've always been suspicious and have rarely trusted my ability to adjust them correctly.  At the moment am running a Symetrix 528E which seems to be a more down-to-earth box.

[Steve - K4HX]

If the turn over frequencies of the multiband processors are set properly, a big difference can be noticed. The 8 and 10 band units are most certainly over kill. But 2 and three band units like the Dorroughs and CRL and very usable for amateur radio applications and won't cost your very much.

[KL7OF]

If the turn over frequencies of the multiband processors are set properly, a big difference can be noticed. The 8 and 10 band units are most certainly over kill. But 2 and three band units like the Dorroughs and CRL and very usable for amateur radio applications and won't cost your very much.

Steve...Thanks for the insight...How do you set your "turn over frequencies"..Monitor??  Audio freq meter? scope?  I have been relying on my own monitoring and recently using a freq meter that reads audio but I'm not sure what I'm looking at...The limitations of most Amateur receivers sets some limits...However I like the loudness factor....I'm looking for some details...Thanks...Steve

[KQ6F]

Thanks, Steve -

The processors I've looked at are analog processors and don't have the ability to adjust their multi-band turnover points.  I guess the digital ones are better in that regard.

In any case - where does one find these processors??  I've been watching ebay like a hawk and haven't seen anything come up in weeks....

[Steve - K4HX]

Mine were set based on some WAGs by me and a little necessity. The Dorrough DAP 310 originally had a turn over frequency between the Mid and High bands of 6.5 kHz. Given that in voice there is little energy about 6.5 kHz and that most receivers do not have IF bandwidths capable of passing modulation above 6.5 kHz, I decided to lower it. Over the years, I've learned through on the air reports and my own tests that with my voice, preemphasis starting at around 1-2 kHz was needed. Most of the SSSS sounds showed up at 2.5-3 kHz and above. So I decided to make the crossover frequency 2.5 kHz. You can see this is not based on any formula or any exhaustive testing but more seat of the pant. It works for my voice. YMMV.

The process was similar for changing the turn over frequency between the Low and Mid bands. Originally, it was 100 Hz. Given that there is not much energy below 100 Hz in my voice, I moved the turnover up to 250 Hz or so.

In both cases, the exact frequency was not critical since the filters involved have a very gentle rolloff slope. So a variation of several hundred Hertz (maybe more) on the Mid to High turn over or a variation of several tens of Hz on the Low to Mid turnover was not significant.

[K5IIA]

Thanks, Steve -

The processors I've looked at are analog processors and don't have the ability to adjust their multi-band turnover points.  I guess the digital ones are better in that regard.

In any case - where does one find these processors??  I've been watching ebay like a hawk and haven't seen anything come up in weeks....

If you get a chance try breakaway broadcast. The demo is free and fully functional. It just plays a jingle every 30 minutes or so. But at least you can play with it on the dummy load for a while and see if you like it.

A small preamp with a little eq'ing is all that is needed on front of it. Will give complete control of bandwidth. Great control of density and has a great sound to it. For 200 bucks I sold my crl stuff and paid for the full version.

[Steve - K4HX]

Good point. There are some nice software based processors available now.

I missed this in KQ6F's post:

The processors I've looked at are analog processors and don't have the ability to adjust their multi-band turnover points.  I guess the digital ones are better in that regard.

Don't assume that you cannot change the cross over frequencies in analog processors. Yes, it won't usually be as simple as turning a knob. You'll likely need to modify the internal circuitry. This is what I did with the DAP 310. But it was easy - just a cap and/or resistor change for each cross over.

[WB4AIO]

To get significant benefit from any processor, you should make sure that the transmitter will fully modulate, cleanly, and be as flat as possible in frequency response -- say 50 to 8,000 Hz as a minimum -- even if you aren't going to run your audio that wide. If you don't do that, the dense waveform produced by the processor will be greatly changed by the transmitter and you will lose most of the benefit. Don't boost the highs or lows or anything in the rig -- it should be dead flat. Do the shaping in the processing.

All the best,

Kevin, WB4AIO.

[IN3IEX]

I think that if you have a PC and maybe a cheap USB audio dongle you could try VOICE SHAPER. You can run it in the background with the dongle.

http://www.dxatlas.com/vshaper/

It has bandpass filter;
7-band equalizer;
noise gate;
compressor;
RF envelope clipper.
I tried it and it works as claimed.

Giorgio