Audio limiter - UK Vintage Radio Repair and Restoration Discussion Forum

arjoll · 14th Mar 2007, 8:04 am

Hopefully this coming weekend I'll be working on a project I've been putting off for a while. I'm installing a backup programme unit at a remote broadcast site - one which receives a processed composite signal over an STL so no audio processing onsite.

I've got a similar EPU at our Gore site, but that is between the Comstream satellite rx and the Aphex 2020 processor so its just fed with minimally processed audio.

For this current one I was going to pre-process the material (multiband compression and peak limiting) during recording, but wanted to incorporate some final imiting on-site. I've found a circuit here which I've copied below which seems to have the right kind of specs, and is certainly better than other ones I've found which tend to be as crude as a couple of 1N4148's in the feedback loop of an op-amp to just clip the signal.

Anyone here have any whiz-bang guaranteed-to-work circuits? The ideal would be a dual band limiter more designed for FM, but its only for backup programming when the link goes down so anything other than dead air would be good. I might try making this one more frequency sensitive, probably by putting the preemphasis before the limiter - just got to find *that* circuit now!

Also the notes on the site suggest a tantalum for the 1uF cap C5 - given more recent experience with exploding tants would some kind of high capacitance ceramic or film cap be better here?

Thanks
Andrew

peter_sol · 14th Mar 2007, 9:35 am

Hi the circuit you have there states that it is a peak limiter which means that it will only limit if the signal goes over the peak

By the sound of it you will not be going over because you have already done your compressing and limiting at the studio before feeding the link.
Most transmitters have compressors built in anyway unless yours is home made .
Possibly this is not the circuit you seek
All the best
Peter.

arjoll · 14th Mar 2007, 9:53 am

Thanks Peter

The exciter is an RVR TEX30LCD, currently fed composite direct from the STL receiver. There is no processing - limiting etc, and ours was before the stereo coder was standard so there is just composite and mono in (why pay for a stereo coder that's nowhere near as good as the one on the Aphex).

I was thinking of the limiter solely being a protection limiter in case something goes wrong with the emergency programme, maybe driving it slightly into limiting to cover a couple of dB variation in levels.

Denis G4DWC · 14th Mar 2007, 10:25 am

I've had a fair bit of experience with designing compressors, limiters, companders etc. The snag with simple circuits, such as that given, is the overdrive has to have occured at the output before the gain can be reduced. A bit like closing the door after the horse has bolted. In your circuit the large signal has to be there at the output before the FET can turn on and increase attenuation at the input. Attack and decay times are set by CR delays and in the case of the circuit shown the 1uF and 3M9 in parallel will set the decay. I make that a time constant of 3.9 seconds. (I can't see the circuit whilst I'm typing).

IMHO The best way to properly limit and or compress an audio signal is using a RF compressor. This converts the audio to a single sideband signal at a convenient IF, in my case 1.4MHz, Limit the signal at 1.4MHz, filter out the harmonics generated by the clipping process and then convert back down to audio using the same local oscillator used for the up conversion to 1.4MHz. As you have only clipped a 1,4MHz signal you will have limited, and compressed if you want, audio free of harmonics and undesireable time delays. The down side is the circuitry is a fair bit more complicated than a single OP amp. I suspect that these days a similar function could be achieved with DSP. I would be interested to know if this is the case. A DSP programmer I knew some years ago shuddered at the thought of what was involved to achieve the same result.

oldeurope · 15th Mar 2007, 9:08 am

Hi, in my 405 line converter I am using this one:
https://www.vintage-radio.net/forum/...8&d=1156952285

If you use an audio delay ccd, you don't have the trouble with the "horse".

Kind regards,
Darius

peter_sol · 15th Mar 2007, 4:10 pm

Hi
How does this circuit work?

Regards

Peter.

oldeurope · 15th Mar 2007, 6:07 pm

Quote:

Originally Posted by peter_sol

Hi
How does this circuit work?

Regards

Peter.

Hi Peter,
please have a look here:

https://www.vintage-radio.net/forum/...ead.php?t=3723

Kind regards,
Darius

Andy Green · 27th Jul 2007, 7:46 pm

HI Andrew,
I may be way too late for this project, but I have done exactly this sort of thing before, and looked at it on a modulation analyser.

If you record the backup audio onto CD or some form of solid state backup then preprocessing as you say will work just fine - don't forget to use a processor with pre-emphasis in the side chain, or alternatively record the backup audio with pre emphasis. Then use your back to back diode circuit in an op-amp, and adjust the levels so that it barely clips only the overshoots caused by the anti aliasing filters in the recording/ playback devices, and it wil sound fine, and won't overdeviate the transmitter.

arjoll · 28th Jul 2007, 12:03 am

Hi Andy - thanks for replying.

Not too late, we ran out of time earlier in the year to complete this project - after that post we had several events at one of our forward linking sites (eventually traced to bad power - the entire site is going on to batteries in the net month or so) and so the Mid Dome EPU went onto the back burner. We also had some work to do on-site with adjusting the aerial array to better cope with ice buildup (it worked - we haven't gone off-air because of reflected power this winter!)

I'm hoping to look at it in sping once we get 4WD access to the site again - its under 2m of snow at the moment (1478m ASL) and helicopters are too expensive. I'm also looking at a homebrew stereo coder - I've got a circuit here with a handful of TL074's and a few 4000B CMOS which I hope to breadboard up. More about that later!

Thanks
Andrew

arjoll · 28th Jul 2007, 12:06 am

Forgot to mention - I did build that circuit with an old FET I had lying around and it doesn't appear to work too well. I haven't checked it on a 'scope yet to see what's happening yet though....its been shoved in the 'later' pile!

Andy Green · 28th Jul 2007, 2:14 pm

Looking at your circuit, it is a very similar circuit to one I used to use back in the early 80's - I found it worked very well, in that it increased the audio density and sounded quite musical in it's attack release and limiting 'knee' characteristics. As a hard limiter, however, it wasn't fast enough in it's attack time. It can be made to work, though the original FM Optimod 8000 used FETs as VCAs. I have also built stereo coders myself in the past - both switching type and matrix - nowadays stuff like this is so cheap that if your time costs money, then it isn't worth building. If it is for pleasure or experimentation however, that is an entirely different matter - PM me or ask in this thread if you need any specific help, and I can dig out some circuits and scan them when I am back in the office

plumbweiss · 4th Aug 2007, 12:11 am

Why not just get an off-the-shelf limiter unit? They are available for between 50-150 quid in a rackmount form, like this:

arjoll · 4th Aug 2007, 11:45 am

Wouldn't touch a Behringer with a bargepole!

I'm looking at something for emergency/backup programme so was wanting to do it as economically as possible, so I will go down the route of pre-processing with a 2020, dbmax or similar and then using a basic clipper as protection - basically what Andy Green has suggested. It will be Oct/Nov before we can get 4WD access to the site so I'll probably look at this a bit more in a month or so.

14th Mar 2007, 8:04 am	#1
arjoll Dekatron Join Date: May 2006 Location: Invercargill, New Zealand Posts: 3,454	Audio limiter Hopefully this coming weekend I'll be working on a project I've been putting off for a while. I'm installing a backup programme unit at a remote broadcast site - one which receives a processed composite signal over an STL so no audio processing onsite. I've got a similar EPU at our Gore site, but that is between the Comstream satellite rx and the Aphex 2020 processor so its just fed with minimally processed audio. For this current one I was going to pre-process the material (multiband compression and peak limiting) during recording, but wanted to incorporate some final imiting on-site. I've found a circuit here which I've copied below which seems to have the right kind of specs, and is certainly better than other ones I've found which tend to be as crude as a couple of 1N4148's in the feedback loop of an op-amp to just clip the signal. Anyone here have any whiz-bang guaranteed-to-work circuits? The ideal would be a dual band limiter more designed for FM, but its only for backup programming when the link goes down so anything other than dead air would be good. I might try making this one more frequency sensitive, probably by putting the preemphasis before the limiter - just got to find that circuit now! Also the notes on the site suggest a tantalum for the 1uF cap C5 - given more recent experience with exploding tants would some kind of high capacitance ceramic or film cap be better here? Thanks Andrew Attached Thumbnails

14th Mar 2007, 9:35 am	#2
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	Re: Audio limiter Hi the circuit you have there states that it is a peak limiter which means that it will only limit if the signal goes over the peak By the sound of it you will not be going over because you have already done your compressing and limiting at the studio before feeding the link. Most transmitters have compressors built in anyway unless yours is home made . Possibly this is not the circuit you seek All the best Peter.

14th Mar 2007, 9:53 am	#3
arjoll Dekatron Join Date: May 2006 Location: Invercargill, New Zealand Posts: 3,454	Re: Audio limiter Thanks Peter The exciter is an RVR TEX30LCD, currently fed composite direct from the STL receiver. There is no processing - limiting etc, and ours was before the stereo coder was standard so there is just composite and mono in (why pay for a stereo coder that's nowhere near as good as the one on the Aphex). I was thinking of the limiter solely being a protection limiter in case something goes wrong with the emergency programme, maybe driving it slightly into limiting to cover a couple of dB variation in levels.

14th Mar 2007, 10:25 am	#4
Denis G4DWC Pentode Join Date: Oct 2006 Location: Brackley Northamptonshire, UK. Posts: 240	Re: Audio limiter I've had a fair bit of experience with designing compressors, limiters, companders etc. The snag with simple circuits, such as that given, is the overdrive has to have occured at the output before the gain can be reduced. A bit like closing the door after the horse has bolted. In your circuit the large signal has to be there at the output before the FET can turn on and increase attenuation at the input. Attack and decay times are set by CR delays and in the case of the circuit shown the 1uF and 3M9 in parallel will set the decay. I make that a time constant of 3.9 seconds. (I can't see the circuit whilst I'm typing). IMHO The best way to properly limit and or compress an audio signal is using a RF compressor. This converts the audio to a single sideband signal at a convenient IF, in my case 1.4MHz, Limit the signal at 1.4MHz, filter out the harmonics generated by the clipping process and then convert back down to audio using the same local oscillator used for the up conversion to 1.4MHz. As you have only clipped a 1,4MHz signal you will have limited, and compressed if you want, audio free of harmonics and undesireable time delays. The down side is the circuitry is a fair bit more complicated than a single OP amp. I suspect that these days a similar function could be achieved with DSP. I would be interested to know if this is the case. A DSP programmer I knew some years ago shuddered at the thought of what was involved to achieve the same result. __________________ If you take something apart often enough, eventually you'll have two of them....

15th Mar 2007, 9:08 am	#5
oldeurope Retired Dormant Member Join Date: Dec 2003 Location: Solingen, Germany Posts: 727	Audio limiter some suggestiones Hi, in my 405 line converter I am using this one: https://www.vintage-radio.net/forum/...8&d=1156952285 If you use an audio delay ccd, you don't have the trouble with the "horse". Kind regards, Darius

15th Mar 2007, 4:10 pm	#6
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	Re: Audio limiter Hi How does this circuit work? Regards Peter.

27th Jul 2007, 7:46 pm	#8
Andy Green Hexode Join Date: Jan 2005 Location: Waterford, Republic of Ireland Posts: 259	Re: Audio limiter HI Andrew, I may be way too late for this project, but I have done exactly this sort of thing before, and looked at it on a modulation analyser. If you record the backup audio onto CD or some form of solid state backup then preprocessing as you say will work just fine - don't forget to use a processor with pre-emphasis in the side chain, or alternatively record the backup audio with pre emphasis. Then use your back to back diode circuit in an op-amp, and adjust the levels so that it barely clips only the overshoots caused by the anti aliasing filters in the recording/ playback devices, and it wil sound fine, and won't overdeviate the transmitter.

28th Jul 2007, 12:03 am	#9
arjoll Dekatron Join Date: May 2006 Location: Invercargill, New Zealand Posts: 3,454	Re: Audio limiter Hi Andy - thanks for replying. Not too late, we ran out of time earlier in the year to complete this project - after that post we had several events at one of our forward linking sites (eventually traced to bad power - the entire site is going on to batteries in the net month or so) and so the Mid Dome EPU went onto the back burner. We also had some work to do on-site with adjusting the aerial array to better cope with ice buildup (it worked - we haven't gone off-air because of reflected power this winter!) I'm hoping to look at it in sping once we get 4WD access to the site again - its under 2m of snow at the moment (1478m ASL) and helicopters are too expensive. I'm also looking at a homebrew stereo coder - I've got a circuit here with a handful of TL074's and a few 4000B CMOS which I hope to breadboard up. More about that later! Thanks Andrew

28th Jul 2007, 12:06 am	#10
arjoll Dekatron Join Date: May 2006 Location: Invercargill, New Zealand Posts: 3,454	Re: Audio limiter Forgot to mention - I did build that circuit with an old FET I had lying around and it doesn't appear to work too well. I haven't checked it on a 'scope yet to see what's happening yet though....its been shoved in the 'later' pile!

28th Jul 2007, 2:14 pm	#11
Andy Green Hexode Join Date: Jan 2005 Location: Waterford, Republic of Ireland Posts: 259	Re: Audio limiter Looking at your circuit, it is a very similar circuit to one I used to use back in the early 80's - I found it worked very well, in that it increased the audio density and sounded quite musical in it's attack release and limiting 'knee' characteristics. As a hard limiter, however, it wasn't fast enough in it's attack time. It can be made to work, though the original FM Optimod 8000 used FETs as VCAs. I have also built stereo coders myself in the past - both switching type and matrix - nowadays stuff like this is so cheap that if your time costs money, then it isn't worth building. If it is for pleasure or experimentation however, that is an entirely different matter - PM me or ask in this thread if you need any specific help, and I can dig out some circuits and scan them when I am back in the office

4th Aug 2007, 12:11 am	#12
plumbweiss Retired Dormant Member Join Date: Nov 2006 Location: Manchester, UK. Posts: 157	Re: Audio limiter Why not just get an off-the-shelf limiter unit? They are available for between 50-150 quid in a rackmount form, like this: Attached Thumbnails

4th Aug 2007, 11:45 am	#13
arjoll Dekatron Join Date: May 2006 Location: Invercargill, New Zealand Posts: 3,454	Re: Audio limiter Wouldn't touch a Behringer with a bargepole! I'm looking at something for emergency/backup programme so was wanting to do it as economically as possible, so I will go down the route of pre-processing with a 2020, dbmax or similar and then using a basic clipper as protection - basically what Andy Green has suggested. It will be Oct/Nov before we can get 4WD access to the site so I'll probably look at this a bit more in a month or so.