Understanding signal-to-noise ratios

admin

https://www.soundonsound.com/for ... p?f=15&p=523943

Looking at the specs of various preamps, converters and mics got me wondering. Is the SNR of any given recording system only as good as its worst piece of gear? Or does noise all add up?

For example, the A-weighted SNR of the RME Micstasy is something like 118.5dB, whilst esoteric offerings by Merging go up to 123.5dB.

But what does this really mean when plugging in a Schoeps CCM mic with 79dB SNR?

admin

It is a bloody minefield Mjfe2!

I am sure Hugh will be along shortly and give us the full SP and no doubt a link to an in depth SoS article. In the meantime. Yes, noise through a system does 'add up' but in special way so that the noisiest component in the system is, by far the dominating factor.

In a 'mic - preamp-converter setup the preamp will almost certainly be the main noise source and even a very low specification converter would not add significantly to the overall noise. That is for a dynamic mic. Capacitor microphones have onboard amplifiers and so generate noise but there are many now that are fabulously quiet and often the combination of a good capacitor mic and a less than state of art preamp is the 'least noise' option.

The major noise factor for mic systems these days however is the environment. Even a modest interface and a dynamic mic (e.g. Steinberg UR22+SM57) will have their noise swamped by the background in most domestic situations and if you were recording 'Open Mic Night' at your local an 11 bit setup would be MORE than adequate!

The whole area of noise and specifications is a very confusing one. Most manufacturers quote 'weighted' noise figures but don't TELL you that (or the weighting curve used) . Often noise is specified without reference levels. Bear in mind a set of three figure..

0.22 microV
-131dBu
-133dBV That is the noise generated by a 150 Ohm resistor (mic Z) over a 20kHz bandwidth at 20C. No system can improve on that unless R is substantially lower, e.g. a moving coil gram pickup or you run the kit in liquid Nitrogen!

We are very lucky today. For £200 you can buy a mic/AI combination that has a noise performance unheard of 40 years ago in even the finest studio.

I commend Douglas Self's 'Small Signal Audio Design' to you. Almost no maths and will give you a very good grounding about the limits of noise in audio circuits. Print out the title and leave several copies about if a birthday is near!

Dave.