Hi Jonathan, fortunately the AMBE chip is so efficient that we don't have to start running QPSK/OFDM and changing over to ultra linear PAs, like we have for Digital ATV. It seems churlish to suggest that everyone change their radios, repeaters and power amplifiers, just to cope with the deficiencies of open source codecs.
The answer to why there's a push for 6.25 KHz, could best be answered during a chat with G4TSN, who will fill you in on how hard it is to get new kit on the air on VHF, especially near conurbations. This problem exists across much of Europe and the US, it's not a local issue. TDM would have the benefit of reusing the precious spectrum (see above) and the valuable hill top sites/equipment, that volunteers spend so much effort financing and maintaining. TDM is also something to experiment with. To say, "Why do TDM?", could encourage others to say, "Why do sound card based GMSK?". Both of the items above are valid for experimental purposes and self training. There are undoubtedly many amateurs that have not worked on TDM systems professionally and some might enjoy learning about TDM, just as some amateurs like to use a PC, soundcard & software to do the job of a GMSK modem chip and a PIC. As it happens, there is already a well developed open source stack for GSM that could be adapted to our needs... but now we're heading rather OT. I agree that Codec2 is a neat project and one to be encouraged. Whether Codec2 has any place within D-Star or even if D-Star will still be the preferred DV mode when Codec2 matures, are entirely different questions altogether. 73 de Darren G7LWT On 11 June 2010 12:59, Jonathan Naylor <[email protected]> wrote: > > > Getting 9600 Bd into 12.5 kHz isn't too difficult as long as you're willing > to sacrifice weak signal performance, Shannons law tells us that. I would > imagine OFDM or even m-ary PSK would be capable of providing the required > throughput. > > The big advantage that GMSK has for manufacturers is that they can use the > same class C output stages as they do for FM. PSK and particularly OFDM > require extremely linear amplifiers. > > If we go over to TDM then we're in the land of GSM or NexEdge > re-implementation and why should we do that? > > I think the development of codec2 should be encouraged as it represents > something fun, rather than just plugging in black boxes which seems all too > prevalent these days. > > BTW why is there a push to 6.25 kHz channel spacing, the 2m band is hardly > that busy these days. A better argument for that could have been made in the > early-80s when the channels were choc-a-bloc but not now. We almost went > 12.5 kHz then (remember S20x?) but we didn't. > > Jonathan G4KLX > > >
