Companding to reduce noise?

[smoguzbenjamin]

hey all,

I've got a stray SA 571 compander chip here, wondering what I can use it for. I'm working on a little preamp since I have a ratshack wireless system that outputs almost 0. I used a 386-based circuit to boost the signal, but that's so incredibly noisy that even a DS-1 hisses like a lisping snake. So now I'm working on an opamp-based replacement, but I though, what if I comapnd the signal? Would that help, or would it only eat power for nothing?

cheers
Ben

[Mike Burgundy]

you can use the expander bit as a noise gate which works to eliminate hiss when youre not playing. If the hiss is still slightly audiible when youre playing, you should seek the problem elsewhere.
I'm guessing the problem resides in the wireless system, actually.

[Mark Hammer]

First off, compander chips don't eat THAT much power.

Companding is useful for making sure that whatever noise comes out at the output jack is made up more of the noise at the input jack than of the noise acquired in the circuit itself.  It does not filter anything out, but simply adjusts relative amplitude of things.  Generally speaking, if the noise produced within the circuit itself is not all that substantial, companding will not add a great deal.  That's why one rarely sees it except in those cases where risk of internally generated noise is quite high, such as with tape or clocked delay (BBD) circuits.

The other place where companding is used is to compensate for low headroom.  By clamping the maximum signal level pushed through a circuit, distortion can be kept to a minimum.  Since the expansion and compression amounts are yoked, the normal dynamics are restored by the expansion.  Think of it like putting that really tall Chinese NBA player in a wheelchair while you push him down a cramped hall, and then he stands up and stretches again after coming out of the hall.  Original height is restored, and nobody damaged their head.

It's relatively easy to see that dynamic headroom and noise are linked.  If you can't run a hot signal without risking excessive distortion, then it gets hard to improve the S/N ratio by having a signal noticeably higher in amplitude than the noise floor of the circuit.

Where companding MIGHT be useful to you is if there are some serious limits on the amplitude of the transmitted signal.  Indeed that was traditionally the home base of most studio limiters - to respect the amplitude restrictions of AM broadcasting (remember when AM radio was rock and not just reactionary open line shows?).  If you can run the transmitted signal hotter by compressing it, and then expand it to push all the extraneous RFI way into the background, then you're onto something.  I wouldn't see it as a panacea for op-amp noise, though.

[smoguzbenjamin]

Right, so actually companding would be of no use. But using the 571 as a noise gate would be really cool! This is basically the big switching box of my whole pedalboard ( Guitar, Amp, Send, Return, plus the optional amplifier ckt) so to have a noise gate in there would be really useful. Do you have more info on that, Mike?

By the way Mike, the wireless is OK. I pluggd in my guitar directly to the ckt and rolled down the volume to 1 and turned on the amp circuit. Same hiss, same noise, different source. So my guess is: noisy amplifier.

[Ed G.]

I'm surprised that with all the possibilities for the 571 chip, you don't see more projects based around it. I remember some discussion a while back about a dual compressor circuit that splits the signal via a crossover, then compresses the two bands differently.

[smoguzbenjamin]

Right, I've looked through the 571 appnotes and I'm at a loss. I need some help here

[Peter Snow]

For more circuits and a whole load of information on the NE570/571 get Tom Henry's  "Making Music with the NE570 Compander" - available from Midwest Analog Products.  Here are some of the circuits in the book:

Expander
Compressor
Automatic Level Control (ALC)
VCAs
Envelope Follower

Although not covered in his book, Vocoders also spring to mind - but you'll need more than one NE570  

Good luck,

Peter

[smoguzbenjamin]

Thanks, I'll look into that.

Now I've been thhinking. The 571 has a built-in envelope detector, so for noise-gate use you'd want low gain on low input. But that means (if you think logically) that the louder you play, the more you're amplified.... Interesting...

[bioroids]

Actually companding may help, if you think the wireless system introduces the noise, then, you compress before it and expand on the receiver... you'll need two chips though...

You can make a compressor too

Luck

Miguel

[StephenGiles]

Would the Bel noise reduction circuit be of any use, I built this a few years ago to use with my old Teac 3340 and it worked very well. I think it works something like a DBX noise reduction.
Stephen

[Paul Marossy]

Just thinking "out loud" here - could you use a compander in conjunction with an LM386 audio opamp to get the distortion level down lower than what it normally is? In other words, make an LM386 based practice amp that distorts less easily?

[Ed G.]

Just thinking "out loud" here - could you use a compander in conjunction with an LM386 audio opamp to get the distortion level down lower than what it normally is? In other words, make an LM386 based practice amp that distorts less easily?

Paul,
I'm by no means an expert on this, but I'm thinking yes... to a point. The compressor would have to function as a limiter, squashing down the signal somewhere below what it actually is, limiting the peak of the signal that would otherwise cross the distortion threshold. I think another solution would be to use a higher-wattage audio opamp (or find the schematic for the ruby amp that uses 2xLM386 setup.

[Paul Marossy]

Well, I was thinking that an LM380 may work better than a LM386, but I just had the thought. Also, isn't there a distortion trim on the 570 compandor chip? Probably not a worthwhile exercise to try using a compander chip in this application, but in theory...

[RDV]

I'm working on a little preamp since I have a ratshack wireless system that outputs almost 0.

You have Radio Shacks in the Netherlands!?

RDV