(Sorry to break the threading, I just subscribed. I've been a long time reader of this list but this is my first post.)
I'm pretty interested to see d-star being discussed here, since this has been a topic of interest for me for a while. It is my position that in the US AMBE is already prohibited on amateur bands under the FCC part 97.113 (a) (4) prohibition against encryption, and that any codec which is incompletely documented, patented, incredibly expensive, prohibited for software implementation could be described no more accurately than a "codes or ciphers intended to obscure the meaning". I've drafted a complaint to the FCC to that effect, but have not yet fired it off because I do not yet have a have a good alternative. To that end I've been working on creating a fairly narrowband OFDM based modem for ham use using gnuradio. The intention was to define a technique which used OFDM, QAM16, LDPC, and Speex to deliver better than better than typical narrow FM audio quality under signal conditions which would not otherwise grant copyable speech. But I've run into issues with managing peak power: Naive OFDM has a very significant difference between peak and average power and since much of the interesting target equipment is power limited that is a serious issue. The significant gap between peak and average power also makes level adjustment for users using soundcard-based software modems much harder. There are a number of approaches to controlling OFDM peak power, but everything I've tried that works well has been computationally expensive, or not especially robust against weak signal conditions. I had not previously considered forking d-star's signaling and simply using Speex instead of ambe, as has been suggested here. Speex is a truly excellent codec for speech. At its target bitrates it is arguably one of the the best performing voice codec available. It is also fully open, not patent encumbered, and the freely licensed reference implementation has been ported to a number of DSPs and low power CPUs. At 8-32kbit/sec (32kbit for wide band) speex is hard to argue with... For higher bitrates and more general purpose audio Xiph now has a new ultra low latency free codec called CELT in development (http://www.celt-codec.org/). *However* at the bitrate that AMBE is used at in d-star speex's delivered quality is not good at all. The only reason speex has any support for those bitrates is to encode background noise during quiet times in VBR applications. This may present a challenge to anyone trying to stuff speex into d-star. The free world may yet have a speech codec for AMBE-like bitrates coming, but one does not publicly exist yet. If anyone takes any action on this front I'd be interested in helping out. If something picks up off this list please keep me in the loop. Thanks! _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
