[PD] Control over audio

Chris McCormick chris at mccormick.cx
Tue Mar 8 04:47:26 CET 2022


Hi all,

Replying to my own thread again.


On 7/3/22 8:43 pm, Chris McCormick wrote:
> On 7/3/22 7:34 pm, Chris McCormick wrote:
>> 1. Make the signal was somewhat robust to volume changes (e.g. if the 
>> source volume was not at max it can still be read).
>> 2. Make the signal robust o quite a saturated spectrum (e.g. if the 
>> source is playing high bandwidth music the signal is still recoverable).
>> 3. Make the signal inaudible such that if it goes into a regular 
>> mixing desk it doesn't make it sound bad.
> 
> 
> Sorry to reply to my own thread but it seems like a "lock-in amplifier" 
> might be what I want:
> 
> https://en.wikipedia.org/wiki/Lock-in_amplifier


I've attached a basic implementation of this. The toggle state is 
transmitted over a noisy audio channel using an 18kHz carrier at 10% 
volume (inaudible to me but my 11 year old might find it irritating - 
will test). At the moment there is both lag (10s of ms) and jitter (10s 
of ms).

I think this can be improved by adding a second detector 90 degrees out 
of phase:

 > These two quantities represent the signal as a vector relative to the 
lock-in reference oscillator. By computing the magnitude (R) of the 
signal vector, the phase dependency is removed.

So I'll try that next and see if I can reduce the jitter. If I can get 
reasonably constant lag it will be good enough for a sync pulse as that 
can be compensated for on the receiving end.

After building this test I'm optimistic this silly in-band sync 
messaging can be achieved without interfering with the music itself. \o/

Cheers,

Chris.


-- 
https://mccormick.cx/
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