The CRC comparison report I was referring to was published in the IEEE Transactions on Broadcasting, Vol. 49, No. 3, dated September 2003. This test is devoted entirely to performance in high multipath environments. But it was based on results with a 2nd gen 8-VSB receiver, echo tolerance of which was only -1 to +38 usec. So, without the benefit of the much better echo tolerance specs available with the upcoming Linx and Zenith receivers, and possibly Broadcom as well (will wait for the complete test). It compared 8-VSB with COFDM in four modes, with and without on-channel repeaters (OCRs). The four COFDM modes ranged from 64-QAM, 3/4 FEC, 1/16 GI down to QPSK, 1/2 FEC, 1/16 GI (i.e. 19.76 to 4.39 Mb/s). One interesting sidebar was that with a 0 dB echo, the COFDM receiver used for the test required a 31 dB C/N, whereas with an echo 1 dB lower, 22 dB C/N was needed. Neither of these profiles is as difficult as Brazil E. But the 8-VSB receiver required 31 dB of C/N for a -2 to -3 dB echo, so it was not as good as the COFDM receiver. But flash forward, the newest 8-VSB receivers from Linx and Zenith can operate in Brazil E profile, with reacquisition, with about 25 dB C/N, and echo tolerance is now more like +/- 38 usec or even +/-50 usec. So these are the improvements that 1999 results can't possibly account for. These upcoming 8-VSB receivers are better than the COFDM receivers of 1999 or of 2002 (at the 19+ Mb/s level), at least in some multipath scenarios, and we just don't know how they compare with the more current COFDM receivers. The executive summary is that without OCRs, the 19+ Mb/s COFDM signal and the 8-VSB signal were roughly matched in terms of reception success. Adding the OCRs improved 8-VSB reception about as much as, or more than, reducing COFDM to 16-QAM. But due to the limited 8-VSB echo tolerance, adding OCRs in a few cases killed 8-VSB reception entirely. OCRs were also beneficial to COFDM reception, and in fact there was never a case where OCRs reduced COFDM performance (thanks to the much better echo tolerance). Below is a quote from the end of the article, which I think tells the story pretty well. Bert --------------------------------------- For 8-VSB, with turning the OCR "ON", the reception sector in 9 sites (53% of the total 17 sites) became larger than those obtained by using DVB-T without OCR at the bit rate of 11.71 Mbps (mode 3) [this would be the 16-QAM mode]. It should be pointed out that as shown by parts of these tests, the reception sectors for 8-VSB and DVB-T mode 1 [19.76 Mb/s] were almost the same when the repeater was "OFF". V. FUTURE WORKS A new prototype 8-VSB receiver was evaluated in the laboratory at CRC [3] [the Linx test published October 2002]. This prototype performed much better in the presence of multi-path than previous receivers. The equalizer operated in the range between -29 to +40 us, and in some parts of that range, could handle a zero dB echo [that was in the range -17 to +17 us]. This receiver should be evaluated in field tests similar to the ones presented in this paper to determine if, as one can expect, similar improvements to those obtained with DVB-T mode 1 (19.8 Mbps) with OCR, could also be obtained with 8-VSB. VI. CONCLUSION In this study, an on-channel repeater (OCR) was used to cover a small part of the coverage area of the main transmitter. The results showed that when the delay between the signals coming from the 8-VSB main transmitter and the repeater was very short and within the pre-ghost capability of the receiver, a significant improvement of reception sector was obtained in the majority of the sites in the test area. In order to have a very low delay situation, the internal delay of the OCR must be very short, and there should also be no or very short delay due to path difference between the signals coming from the two sources to the target area. These requirements put some constraints on the configuration of the OCR, as well as on its location with respect to the main transmitter and the target area. However, even under these conditions, a few locations could still be found in which the 8-VSB reception failed when the repeater was turned "ON". The cause of failure was believed to be the reflected OCR signal with a delay outside the pre-ghost range of the receiver. With an 8-VSB receiver more robust against pre-ghost, the constraints on the configuration and location of the repeater could be relaxed to some extent. In this case, it is expected that even if the OCR signal delay at the reception points is longer (but still within the delay range of the equalizer), the OCR can provide reception improvements similar to those obtained in phase II of this study, and comparable to those that can be achieved by more robust modes of transmission. [Note: this was in fact tested, also in Ottawa, with the predictable improvements in performance, in the Linx OCR test of April 8, 2003, results available at the Linx site.] It is recommended that broadcasters consider OCR as an alternative for reception improvement in their technical planning. ---------------------------------------------------------------------- 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.