Tom Barry wrote: > But I'm still not sure at all of the other side. That is, how much > FEC overhead is needed to get similar M/H reception quality on > DVB-T. Is it truly 3 or more times better at mobile or does > DVB-T also have to use massive extra error correction to get > quality M/H reception. If so, how much? > > That is, how many bits / second can reliable emerge from a > moving handheld receiver using DVB-T in a 6 mhz channel, > after subtracting FEC overhead? The results from tests are a good thing, of course, and Mark will have some available soon. But this doesn't preclude considering what the underlying principles are. And among these is that DVB-T does HM by addressing two components: the FEC (and GI) of the robust channel AND the constellation. HM uses QPSK, which is analogous to 2-VSB. However, in 8-VSB, if M/H were to use a different constellation, the entire multiplex would have to use that same constellation. Since only the 8-VSB one is used, that kind of limits what the M/H channel has to be built on. So you can't *directly* compare the FEC mode used in the robust channel, between the two systems. But you CAN compare the bit rates with the C/N margin requirements, which ultimately is all that matters. For DVB-T, the document that shows what one can expect is ETSI 300 EN 744. It doesn't give all results for 6 MHz channels, but you can draw the same comparisons using the 8 MHz results. What that document shows, for 8 MHz channels, is that in the mode that favors the HM channel most, you can reduce the wide channel's capacity from 24.13 to 12.06 Mb/s, to retain the same robustness you had in non-HM mode. Doing so, you will get an HM channel that gives you as much as 6.03 Mb/s of capacity, with a C/N margin of 6.5 dB of in a gaussian channel (this is with the same 1/32 GI as the wide channel). So, I suppose you could conclude that this would be the same overall effect as 8-VSB M/H with a 1/2 rate FEC, right? The 12 Mb/s taken away from the wide channel becomes 6 Mb/s in the HM channel. So the next question is, what C/N margin is required in the MPH or A-VSB, when the 1/2 rate option is used? IIRC, A-VSB gives you 4 dB of C/N for a 1/4 rate channel and a single receive antenna, and something like 1.9 dB of C/N if a diversity receive antenna is used with 1/4 rate. But I do not remember, or maybe the numbers were not given, for the 1/2 rate option. The other consideration is that when putting COFDM into HM, you are going to get a fairly big robust channel. Whereas with MPH or A-VSB, it's more like DVB-H. The robust channel can take up less of the wide channel (and of course provide less M/H capacity as a consequence). This may be desirable, though, for stations that want to retain the ability to transmit an HD and SD multiplex along with M/H. In short, using a simple 6/8 ratio: COFDM HM in a 6 MHz channel could provide you with 9 Mb/s of wide channel (16.5 dB of C/N margin required) and 4.5 Mb/s in the H/M channel (6.5 dB of C/N). Echo tolerance would not be impressive, though, because we are assuming 1/32 GI. The way MPH was being used, the 8T-VSB wide channel was 14.9 Mb/s (15.2 dB of C/N) and the narrow channel should be something less than 2.2 Mb/s if 1/2 rate were used, at an undisclosed C/N margin. Or something less than 1.1 Mb/s (~4 dB C/N) if 1/4 rate is used. The "something less" is to account for the extra overhead needed for the training and sync sequences, but that shouldn't be a big number. Bert _________________________________________________________________ Get Windows Live and get whatever you need, wherever you are. Start here. http://www.windowslive.com/default.html?ocid=TXT_TAGLM_WL_Home_082008 ---------------------------------------------------------------------- You can UNSUBSCRIBE from the OpenDTV list in two ways: - Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word unsubscribe in the subject line.