Tom Barry wrote: > I think you have to measure spectral efficiency as being AFTER > any correction for errors. That is, subtract both the FEC > overhead plus any resulting average number of error bits that > survive FEC. It is only the resulting throughput of good > original data bits that counts (per Mhz). Yes, of course, but that's why I said: "Any minor differences are only caused by error correction mechanisms layered over the symbol stream. If anything, this almost always results in the COFDM stream being somewhat lower in spectral efficiency, to gain a measure of robustness." If the COFDM example uses 2/3 convolutional FEC and 64-QAM, then any amount of GI is going to bring its spectral efficiency lower than that of 8T-VSB. Even a GI of 1/32, which is the minimum possible. The reason is simple: 8T-VSB uses no GI, and the Reed-Solomon code is essentially identical between ATSC and DVB-T (208,188 vs 204,188). And other overhead, such as training sequences, is small enough that it doesn't change things appreciably. For example, the baseline used for the French TNT is 8K, 64-QAM, 2/3 FEC, 1/32 GI, although that won't work in the few SFNs they have in France. Still, as a baseline, big stick only, that works out to 24.13 Mb/s in their 8 MHz channels. To eliminate any bias caused by the more rapid dropoff of the spectrum with 8K COFDM compared with 8-VSB, I'll use the entire channel width in making the comparison. (The actual numbers are a little higher, because you can't use the entire channel width in practice.) 24.13 / 8 = 3.02 b/s/Hz 19.39 / 6 = 3.23 b/s/Hz The C/N margin required, given that the FEC is virtually identical between the two systems, should be very close, and it is. In a gaussian channel, COFDM set this way needs 16.5 dB of margin, and 8T-VSB needs 15.2 dB. I think the difference is probably caused by the greater reliance on active carriers in COFDM, compared with 8-VSB. Bert ---------------------------------------------------------------------- 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.