Simple Audio Limiter ?

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XXL
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Simple Audio Limiter ?

Post by XXL » Tue Mar 16, 2021 3:08 pm

Looking for a simple audio limiter for FM Tx driver. im going to design it on to the board.

Thanks

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Re: Simple Audio Limiter ?

Post by sinus trouble » Wed Mar 17, 2021 12:07 am

Probably the simplest limiter you could get!
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Re: Simple Audio Limiter ?

Post by sinus trouble » Wed Mar 17, 2021 12:20 am

For such a simple circuit it is clever the way it works, Taking advantage of the diodes inability to conduct below the 0.7V threshold. (Dependent on the type of diode ofcourse)

I must stress it does work and is very easy to experiment with? But dont expect any HIFI dynamics.

If you wanna get more complicated? There are loads of circuits available! Im sure Albert posted some a while back!

Good Luck! :)
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Re: Simple Audio Limiter ?

Post by Albert H » Wed Mar 17, 2021 1:47 am

Take a look at viewtopic.php?f=11&t=1501&hilit=Audio+Limiter

That stereo limiter is easy and cheap to build, and has low noise and distortion (it doesn't have the Transconductance Amps in the direct audio path).
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Re: Simple Audio Limiter ?

Post by OldskoolPirate » Wed Mar 17, 2021 9:38 am

@sinus i totally forgot about using 2 diodes like that. They use them in guitar pedals for overdrive distortion but obviously I wouldn’t be going anywhere near that loud 😛.

I’m literally looking for something to chop the odd clipping here and there off to keep it within range. Atm I’m processing the audio before the stream but sometimes it still overshoots. And thanks Albert I will have a read up.
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Re: Simple Audio Limiter ?

Post by 1608cc » Wed Mar 17, 2021 4:22 pm

I tested circuit with photoresistor-LED feedback. I made it on breadboard and it looked like it work - but I didn't found right values of components to make it fine, and left project. 220uF/47k RC network is responsible for attac/decay time (soft limiter / compressor) and 4,7V zeners cut the signal above ~5V (hard limiter / clipper)
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Re: Simple Audio Limiter ?

Post by jvok » Wed Mar 17, 2021 11:47 pm

IME diode-connected transistors work better than actual diodes in that clipper circuit. Link the base and collector together as one pin, and emitter as the other. Use pnp for D1 and npn for D2.

The bjts give you much harder limiting than the diodes. The threshold is lower (0.45v vs 0.7v for the diodes) but the onset is much more sudden. Diodes have a higher clipping level but a really gradual onset which means they're causing distortion way before they actually clip.

If 0.45v is too weak to drive your transmitter then put an opamp gain stage with a pot after the clipper. Then you can dial the limiting level to whatever you want.

Oh and I found somewhere around 10k was the best resistor to use, at least with bc546/556. (Might be different with different transistors but probably not much).

Edit: one other thing. You should put the 15khz low pass filter AFTER the clipper. Otherwise when its clipping hard you'll get harmonics feeding through (might cause stereo light flickering).

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Re: Simple Audio Limiter ?

Post by Albert H » Thu Mar 18, 2021 2:41 am

NO!!!!!! :o :stop

Everything we're trying to do is about good quality. Clippers just add distortion. You need a proper fast attack limiter. The circuit is rather more complicated than the resistor and two diodes, but it doesn't add gross distortion!

The ideal limiter for broadcast includes a delay line, so that the gain cell is turned down before the peak arrives. You have to get a delay just long enough to eliminate the "attack" time of the limiter. I've used digital delay lines, analogue delay lines (using a whole heap of "all-pass" filters), and BBD delay lines. One commercial product I designed used a pair of the Panasonic MN3007 BBD ICs (one for each channel) with a really fast bi-phase clock, to get a delay of 720µs. This was enough time for the gain control to activate, and the delay through the limiter was imperceptible - it was just like being a few centimetres further from the 'speakers!

The best gain control ICs I've used are the "Blackmer" chips from THAT. Their 2181 SIL chips can't be beaten - incredibly low noise and distortion, and they're only around $6 each. The limiter requires a dual supply (+/- 15V), and I included LED bargraph indicators for input levels, output levels and gain reduction, all calibrated in dB, with (very close to) PPM characteristics, so that the operation of the limiter was obvious, but largely inaudible! The advantage of the delay-line approach is that the limiter tends not to overshoot, so there's no "pumping" or
"breathing" artefacts.

My commercial limiters are built in 19" rack boxes, usually 2U high. A useful trick is to power the LED chains off their own supply, so that the switching artefacts as the LED currents change don't get on to the supply rails for the audio part of the device.

My units have balanced audio in and out, to prevent hum and noise on the cabling, and can be set up for several working levels from -12dB up to +24dB, and have a minimum of 12 dB headroom at all settings. They have found use in broadcasting (of course), but I have several clients who use them in auditorium public address systems to prevent over-enthusiastic live sound "engineers" from blowing up amplifiers and speakers!
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Re: Simple Audio Limiter ?

Post by Albert H » Thu Mar 18, 2021 10:43 am

Here's a "good enough" limiter that will prevent the worst excesses of over-modulation. It uses just two cheap ICs and you can throw it together in about ½ hr:
570limit.jpg
It's very easy to build - even on Veroboard. The 22k input resistors can be increased to as high as 100k for "line level" inputs. Increasing the supply to 15 or even 18V (maximum) will increase headroom. Reduce the 470Ω resistor to as low as 100Ω for fastest attack. The 47µ timing capacitor can be reduced to 22µ for faster decay. The PNP transistor can be as shown, or BC213L, BC560, 2N3906 etc - just be sure to check the pinout!

It's easy to add a gain reduction bargraph circuit using another couple of LM339 comparators if required (I'll add the circuit if anyone is interested).
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Re: Simple Audio Limiter ?

Post by BriansBrain » Thu Mar 18, 2021 1:47 pm

XXL wrote: Tue Mar 16, 2021 3:08 pm Looking for a simple audio limiter for FM Tx driver. im going to design it on to the board.
What is your audio source :?:
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Re: Simple Audio Limiter ?

Post by radium98 » Thu Mar 18, 2021 8:12 pm

https://pira.cz/enlter.htm

never seen simple but forgettable link.

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Re: Simple Audio Limiter ?

Post by Albert H » Fri Mar 19, 2021 11:02 am

XXL wrote: Tue Mar 16, 2021 3:08 pm Looking for a simple audio limiter for FM TX driver. I'm going to design it on to the board.
What are you trying to achieve? Are you trying to produce a "broadcast sound" or are you just trying to prevent over-modulation?

It's usually not a good idea to have audio processing in the same box as RF stuff - the RF energy can get into parts of the circuit that can wreak havoc with the audio quality!

If you're just trying to prevent over-modulation, a pair of back-to-back diodes around an op-amp will give soft clipping, and if your varicap sensitivity is configured for 75kHz deviation at the level clamped by the clipper, you're on to a winner. The simple diodes across the audio path will work (sort-of) but will introduce gross distortion.

Is your modulation mono or stereo? Is there going to be RDS on the signal? Both of these have implications for the settings of a clipper.

My exciter design includes a level clamp to prevent over-deviation (mostly due to processing overshoots and very rapid peaks). The modulation sensitivity of my board is exactly the same right across the band, so that the clamp circuit is effective whatever frequency the exciter is set to. This eliminates a stage of alignment when changing frequency.

My link receiver includes a squelch circuit to mute the audio output in the event of loss of signal - giving a silent carrier on link loss. This also helps to prevent over-deviation. All the significant audio processing is done at the link transmit end - multi-band delay-line compression and limiting, multiplex generation and RDS insertion all happens in a set of 19" rack modules by the link transmitter. Steps are taken to ensure that the RF doesn't get into the audio gear (mostly involving a long coax feeder to the link transmit aerial!).
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Re: Simple Audio Limiter ?

Post by jvok » Sat Mar 20, 2021 2:31 pm

I'm working on a design atm that uses a clipper like that after the main limiter to catch the peaks. I found as long as the limiter is reasonably fast then the distortion is inaudible, because it's only clipping so briefly.

I've played around with a delay line but decided the number of opamps in the all pass network is way too expensive to get any useful delay. Maybe one day I'll make a "full fat" version with the delay line but for now I'm happy with the clipper. Digital delay could be useful but I kind of think at that point you might as well go full DSP.

Most of the limiters I see online don't have anything to stop overshoot on transients (clipper or delay). IMO you might as well at least put a clipper on the output given how cheap it is. That way you know you're never over modulating even briefly.

My limiter also has a variable preemphasis circuit which gradually turns down the preemphasis (raises the corner frequency) as it limits. Below the threshold it has full preemphasis, the harder you drive it the higher the corner frequency gets, until eventually its completely flat. This means that you can safely put the preemphasis *after* the main limiter. Solves the problem some limiters have where the preemphasis makes them over sensitive to treble.

I'll post a schematic when I get it all fully tested.

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Re: Simple Audio Limiter ?

Post by Krakatoa » Sun Mar 21, 2021 9:33 am

In a strategy to make a very simple (but quite featured) stereo limiter, has anyone ever looked at the FV-1 DSP chip from Spin Semiconductor?
Of course it needs programming, but there I think it could fit a pre-emphasis, and look-ahead compressor limiter, and mabe with some clever coding could fit dual band stereo audio processing.

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Re: Simple Audio Limiter ?

Post by Albert H » Sun Mar 21, 2021 4:58 pm

The FV-1 has rather poor specifications. It's great for guitar effects (its intended purpose), but it's not great for full bandwidth audio.
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Re: Simple Audio Limiter ?

Post by rigmo » Wed Sep 22, 2021 10:31 pm

radium98 wrote: Thu Mar 18, 2021 8:12 pm https://pira.cz/enlter.htm

never seen simple but forgettable link.
fine
mono and stereo
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Re: Simple Audio Limiter ?

Post by rigmo » Sun Oct 10, 2021 5:52 pm

pira stereo limiter
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Re: Simple Audio Limiter ?

Post by rigmo » Wed Jan 26, 2022 9:48 pm


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