Dwayne Reid writes:
>I'm much more an analog guy rather than digital and have fairly limited DSP
>experience.  But analog mixers work in a similar fashion.  Look at it this
>way: you want to maximize your Signal to Noise Ratio.  So: you choose your
>components such that when all inputs have their maximum input signal
>applied, the mixer stage output is just below clip.  For a 2 input mixer,
>each input contributes 50% of the output.  If one input goes away, the
>output drops by half.
>
>The problem with clipping the signal is that it sounds just awful.

        Some types of digital systems accentuate this problem.  Many
digital telephones use Mu-Law encoding in North America and A-Law
encoding everywhere else to break up 8-bit PCM in to a peace-wise
logarithmic function.  This gives you really good and clear audio when
things are working right, but Heaven help you if
A.  It is a conference call and two people talk loudly at once or
B.  The call is from an outside trunk and the audio is a bit too hot.

        The effect is a sort of kazoo-like sound in which both voices
break up as if the mixing process is clipping.

        I think it messes up the logarithmic function when it sees 0's
or 0xFF's but I haven't sat down and calculated exactly what is
happening because I don't know enough of the details.  All I know is
that you don't have to have golden ears to hear the result of the
clipping when it happens.  It's not just sort of distorted, but
unintelligible during the peaks.

        I think the closest one can come to having a multi-channel
mixer that appears to not attenuate all the audio streams is to devise
some sort of peak limiter like one might find in a sound recorder or
broadcasting station to "listen" to the output of the mixer and bring
it down when several of the inputs have high audio levels
simultaneously.

        The peak limiter then decays back to normal when the peak
passes.  The effect of that can be audible, but it is a lot more
pleasant than the clipping.

Martin McCormick WB5AGZ  Stillwater, OK
OSU Information Technology Division Network Operations Group

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