Last week in RF Report I mentioned that the Advanced Television Systems Committee (ATSC) had approved the Enhanced VSB (E-VSB) standard. The ATSC issued a news release last Tuesday announcing the approval. ATSC President Mark Richer commented, "E-VSB was developed in response to the desire of broadcasters to add more flexibility to the DTV Standard. E-VSB was chosen from among multiple technologies after a rigorous evaluation process that included laboratory and field tests." As noted in last week's article, the standard allows broadcasters to trade off data rate for a lower carrier to noise ratio and a more robust signal. ATSC Digital Television Standard (A/53), Revision C including Amendment 1 is now available for downloading. Amendment 1 is the section concerning E-VSB.
One major difference between the main service and the enhanced services is the coding rate. The main service uses 2/3-rate coding, whereas the enhanced services can use either 1/2 or 1/4 rate coding. Amendment 1 describes exactly how this is accomplished in a way that does not interfere with reception of the main service on legacy receivers without E-VSB capability. One look at Amendment 1 and it is obvious this is not a simple procedure! Data bandwidth is allocated between enhanced and main service streams in 32 steps, ranging from no enhanced service data to no main service data. For 1/2 rate coding, with all segments allocated to the enhanced service, the maximum data rate is 8.4585 Mbps. At 1/4 rate coding, the data rate drops to 4.2292 Mbps.
Modern video coding and compression techniques allow standard definition TV to be transmitted in data bandwidths as low as 1.5 Mbps or less with acceptable degradation for many types of programming when viewed on the size screen likely to be found on portable and handheld TV sets. The reduced resolution service could also serve as a "fall back" option should the main service carrier to noise ratio drop below the level needed for reliable reception or be used to offer separate programming or data to computers, hand-held devices or portable TV sets.
For 8-VSB modulation, at the enhanced data rate closest to 1.5 Mbps, the remaining main service bandwidth is approximately 12 Mbps if the enhanced service uses 1/4 rate coding. If the enhanced service uses 1/2 rate coding, the data rate available for the main service increases to 15.9 Mbps, which is enough for many HDTV programs.
ATSC has several Candidate Standards that support E-VSB. These include CS/T3-608, which covers AVC and H.264 (MPEG4) coding and CS/T3-609, covering VC-9 (Windows Media 9) coding. Candidate Standard CS/T3-606 defines enhancements to the ATSC PSIP standard required to support E-VSB. Audio is covered in CS/T3-613 for enhanced AC-3 bit streams and in CS/T3-614 for robust mode high efficiency audio system characteristics.
For more information, see Digital Television Standard (A/53), Revision C including Amendment 1. While the standard is complex, it is possible to obtain a good understanding of how E-VSB works, the options available and how it can be used in a DTV transmission system even if the more complex technical details are skipped.
