switching at zero cross/noiseless switching

Buck Buchanan voltagecontrolled at home.com
Wed Nov 10 18:03:26 CET 1999


Hello all - I spent some time searching the archives for this but
probably wasn't using a good search spec....

I'm wondering if there's anyone out there who can point me towards a
resource on switching audio at the zero crossing.  Or for that matter,
other techniques for noiseless switching at speed.  I'm open to using
specialized ICs.  Because of high speed, parts like the SSM2402 and
other "slow on/off" techniques won't work.

I'm trying to come up with a 4 channel "hard panning" circuit (actually
a 1-to-4 switch) that will switch at speeds up to several KHz or
higher.  In a 4 channel system, I want to make sounds instantly jump to
other channels with no click or pop.  

Am I correct in thinking that even IF the switch itself adds no pop,
there will still be a pop since an instantanious switch at some
arbitrary point in the wave will cause a sharp transistion (fast rise
time) which itself will be the pop?  And furthermore, if I did manage to
make it switch at the zero crossing, wouldn't this be eliminated?

So far I've tried running the audio through a 4052 (2x 4ch mux) running
+/-12VDC supplies.  I tried this part first since it will switch
bi-polar signals with 0-Vcc logic.  There's a definate pop during
switching at any speed.  Next I tried running the audio differentially
through the 4052 thinking that maybe the pop was both caused by the 4052
and common mode.  No good - still a pop.  Note that when I switch the
4052 with no audio input, there is no pop or glitch at all. 
Unfortunately, my 200MHz scope w/delayed sweep just went belly-up
yesterday so I'm down to a cheap 20MHz job and am having trouble
"seeing" the glitch.  I hope to remedy this situation soon.

My thought so far was to try and use some kind of clocked latch to
switch the 4052 only during zero crossings of the audio.  The clock
would of course be based on the zero crossing of the audio.  Yes, this
falls appart when switching speed is greater than the audio frequency
but for most apps this might work???

Don Lancaster (or was it Bob Pease?) says an hour of research beats 10
in the lab - so here my research starts.  I'm thinking maybe this
technique has some industry standard name and/or specialized parts that
make the design easier. Maybe some video switching IC or something might
help?

Any tips, suggestions, or input is greatly appreciated!

Buck



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