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Topic: extracting mix minus from mixed signal  (Read 1319 times) previous topic - next topic
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extracting mix minus from mixed signal

Is there a box - audio DSP which would do the following

signal A -> in 1
signal B- > in 2 = A+some other signal

output = generated mix minus = signal B - A

Question is put like this since separate signal B could not be taken from closed loop system.
It has to work near realtime, for conferencing application.

Regards.

Re: extracting mix minus from mixed signal

Reply #1 –
Not sure whether your signals are analog or digital.  If digital, and the signal B was summed digitally, you should be able to completely remove A from B with anything that will sum A and B with the ability to reverse the polarity of the A source.  Plenty of hardware or DSP mixers can do this.  If analog, you can do the same thing in a mixer, but the residual A left in the mix minus will depend on how flat the frequency and phase response of the paths are: The original mixer that summed the signals, the analog path since summing, and the analog path for the "A" signal.  Good systems can generate analog rejection down to -40, -50 dB if everything is clean and stable.  Analog broadcast TV consoles started having this capability by subtraction, as opposed to summed mix minuses, in the late 1980's, 1990 time frame, my former employer installed several and they were fine for feeding mix minuses to news correspondents in the field who can't be fed a full mix because of the delay in their ears.  Digital consoles have always had this capability, some by summing, some by subtracting out the undesired audio.

You can also do this with a hybrid, with varying degrees of null, the mixed signal is connected to the "two wire" port, the "a" signal goes in the four wire input, the resulting "b only" comes out the four wire output.  Auto null units, expensive, as made by Telos and others work pretty well and are used extensively to terminate telephone "POTS" lines in broadcasting.  You can actually make a moderately decent analog passive hybrid with two transformers, a variable resistor and a capacitor decade box, or just a collection of appropriate capacitors.  There needs to be some way to adjust the level of at least one of the audio signals feeding the transformers also to get a reasonable null.  These passive hybrids are in about every analog telephone and were extensively used in carrier telephony equipment, as well as broadcasting and related equipment before the era of digital audio.

My former employer had a 50 channel conference bridge for POTS telephone calls made from 50 Telos hybrids, (actually their less expensive model) and a 50 port broadcast intercom for summing/splitting the audio.  By turning on/off crosspoints in the intercom matrix, it could be configured as one 50 port bridge, or several smaller simultaneous bridges.  It worked very well, was in service for many years while I was there, and for all I know, may still be in service (I've been retired a couple of years now).

Re: extracting mix minus from mixed signal

Reply #2 –
A rather old, but mostly effective analog solution was the MixMinus Plus from Henry Engineering.  Out of production now, but there's one on ebay...., cheap enough to play with.