I use a similar trick when I mix voice and data in FreeDV mode 2400B.
But the detection is a bit simpler: Just look at the 'energy' component
of the vocoder.
If the energy of a frame is low and there is data to send the data will
be send and the receiver will inject a bit of silence.
Regards,
Jeroen
On 04/01/2017 01:43 AM, Steve wrote:
> I was playing with the pairphone
> <https://github.com/ObjectToolworks/ppmodem> modem, and it dawned on
> me that the software was using this modem to send both vocoder data
> and control data. It did this based on the boolean bit for VAD. Well,
> that's probably not novel, but I asked myself, how the heck could they
> know when the vocoder output was speech or noise??
>
> Well, it turns out that there is an algorithm just for that purpose.
> It looks at each vocoder frame and a little history and bada-boom
> youse gotta VAD bit.
>
> I was trying to figure out how they squeezed all the encryption
> information and voice into 1200 bit/s, if the voice itself was 1200
> bit/s! Aha, they look for breaks in the voice and send control bits
> instead.
>
> I suspect there are quite a few frames with no voice. If a person was
> looking up something, they might not say anything while they were
> otherwise tasked. This probably results in an excess of control
> information. Dumb data just to fill the void.
>
> They almost need something like a duration detector, and then send
> text data. A private SMS below the voice and control.
>
> Have a good weekend,
> 73/steve
>
>
>
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