It's also a path to better compression. Currently Codec 2 applies the same number of bits to each frame, even if it's silence, or very similar to the last frame.
- David On 01/04/17 12:02, Jeroen Vreeken wrote: > 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 >> >> >> >> ------------------------------------------------------------------------------ >> Check out the vibrant tech community on one of the world's most >> engaging tech sites, Slashdot.org! http://sdm.link/slashdot >> >> >> _______________________________________________ >> Freetel-codec2 mailing list >> Freetel-codec2@lists.sourceforge.net >> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > > > > ------------------------------------------------------------------------------ > Check out the vibrant tech community on one of the world's most > engaging tech sites, Slashdot.org! http://sdm.link/slashdot > > > > _______________________________________________ > Freetel-codec2 mailing list > Freetel-codec2@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
