At 11:56 PM 6/11/2010, you wrote: >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.
It is a problem across much of the world. Here, the situation is slightly different, but again one where narrow bandwidth helps. We're pretty much our of 2m repeater pairs here in Melbourne. However, despite there being a bit of spectrum for simplex, as while we have 4MHz on 2m, unlike the US, we don't have any repeaters below 146 MHz. This leaves quite a bit of simplex room from around 145.2 (top of the digital/packet sub band) to 145.8 (bottom of the satellite segment), minus a couple of "special purpose" frequencies (ARDF/foxhunting, Morse practice beacon, RTTY). However, this simplex room is quite full, with several IRLP/Echolink nodes, as well as various clubs and groups, who regular use simplex frequencies. In fact, the simplex part of the band is often busier than the repeaters down here. Even with D-STAR's (effective) 12.5 kHz bandwidth, we're able to slip D-STAR in between the 25 kHz spaced FM channels. 6.5 kHz bandwidth would make this even neater. >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 could be particularly valuable for repeaters, especially as sites become more limited in availability, so if one site can carry more than one stream of traffic, that would work to our advantage. >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 li >ke to use a PC, soundcard & software to do the >job of a GMSK modem chip and a PIC. That's a good enough reason for me to play. In this part of the world, we'd also be looking at playing with methods to increase the coverage area of TDM, such as optimising the duration of the time slices and pre-compensating for propagation delay. This was one of the well known limitations of GSM, the 30-35km cell radius limit (of the basic GSM system, without any cell extenders). Open terrain and low population densities meant this became a real issue in rural areas. >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. That would be a good starting point. Hmm, maybe we need to move to another digital voice experimenters group. ;) >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. I agree. I can't see Codec2 and D-STAR having any significant roles together (because of the amount of D-STAR gear already out in the field), but I do see Codec2 as having a lot of potential for experimentation, particularly in new VHF/UHF modes, as well as HF DV, if the developers can get the bitrate into the 1000bps and below region (to make Shannon's Law work in our favour). Codec2 will allow the next generation of DV modes to be developed on a PC, which is a low cost way of getting a lot of people into the mode. Look at what PCs did for PSK-31 and SSTV years ago. 73 de VK3JED / VK3IRL http://vkradio.com
