Selection of a modulation technique for ATSC DTV was a contested process between Zenith VSB and GI QAM. VSB emerged as the winner only to be challenged by proponents of COFDM because the rollout of OTA DTV was plagued by multipath reception problems.
Broadcasters' requirements
Since then, efforts have been underway to improve and extend the ATSC VSB modulation tool kit. Two methods have been considered: Enhanced VSB (E-VSB) that has been standardized and Advanced VSB (A-VSB) that is now a standardization candidate.
ATSC president Mark Richer has said that the development of E-VSB and A-VSB is in response to broadcasters' requests for greater flexibility in the DTV transmission system, wider over-the-air reception coverage and mobile DTV services. Richer has said that he is confident that an ATSC standard to support mobile and handheld DTV reception will eventually come to pass.
Enhanced technology
Developed by Zenith and ATI Technologies, E-VSB provides operation modes that trade payload data rate for a lower carrier-to-noise threshold and improved receiver performance. The operation modes are one-half and one-fourth of the data rates.
Prior to the usual 8-VSB processing of 188B MPEG-2 transport packets (data randomizing, Reed-Solomon (RS) encoding and interleaving), normal, one-half and one-fourth data rate packets are identified and then multiplexed. Information in the transport stream indicates whether a packet is to be processed as main, one-half or one-fourth rate by the VSB processor.
E-VSB processing techniques reduce the transport stream data payload to 164B. This facilitates an additional stage of data randomizing, RS encoding and interleaving. Twenty additional FEC bits are appended to the 164B payload. An SMPTE 310 stream interfaces the output of the enhanced processing chain with main 8-VSB processing.
E-VSB in the field
Tests conducted in the Baltimore area in fall 2005 by Sinclair Broadcast Group and Zenith Electronics confirmed the predicted performance of the ATSC E-VSB standard. The Association for Maximum Service Television worked with Sinclair and Zenith to analyze test results.
The study found that the E-VSB bit stream delivered a 6dB improvement compared to the normal stream, and that it did not negatively affect the normal stream reception of legacy receivers.
But, as DTV receiver front ends continue to improve, E-VSB may not be necessary. In a paper presented at NAB2006, Wayne Bretl, Zenith R&D manager, reported that other field tests have shown that the addition of E-VSB packets when processed by fifth-generation E-VSB receivers from LG Electronics showed minimal performance gains when compared to the same receivers processing normal VSB signals. Adding E-VSB packets, however, did improve equalizer convergence.
An amendment to the ATSC A/53C DTV Standard added E-VSB as an optional mode of transmission. The ATSC Technology and Standards Group is considering the possibility of specifying the advanced AVC and VC-1 video codecs for use with E-VSB.
Mobile DTV
Developing technology that supports mobile DTV capability is the design goal of A-VSB. The method uses a supplementary reference signal (SRS) and turbo codes to enable reception at speeds, theoretically, up to 150mph. Tubo coding replaces the data protection and corretion processing of VSB and E-VSB.
The SRS enables A-VSB receivers to remain locked to the transmission. This ability facilitates reliable reception in mobile applications and in environments with high interference. The technique constructs a transport stream packet that includes a stuffing region for the insertion of supplementary reference signal (SRS) data.
Turbo coding techniques are another way that A-VSB differentiates itself from other ATSC modulation techniques. Turbo codes approach the theoretical limit of maximum data transfer over a noisy channel.
A-VSB on the road
A demonstration at KVMY-TV in Las Vegas showed that one-fourth rate coding (3Mb/s) was needed to transmit a 750Kb/s media stream. An additional 2.8Mb/s was used by the SRS. The total of 5.8Mb/s left 13.89Mb/s in the transport stream for an HDTV program.
In another series of tests last September, Rohde & Schwarz adapted WUTV's digital transmitter in Buffalo, NY, to emit two 1.5Mb/s A-VSB turbo-coded streams and an 8-VSB HD signal. Both signals were received in a van outfitted with DTV antennas on the roof and a prototype Samsung A-VSB receiver. The A-VSB broadcast turbo-coded pictures were received at up to 80mph.
The prototype receiver, a Samsung YEPP portable digital media player decoded and displayed the MPEG-4 video from the turbo-coded streams. An ATSC receiver and LCD display demonstrated the inferior mobile-reception capabilities of standard 8-VSB broadcasts, compared to A-VSB.
The Communications Research Centre in Ottawa, Ontario, is conducting formal tests for the ATSC of A-VSB. The technology is in the draft stage of consideration as an amendment to A/53.
The real world
Both E-VSB and A-VSB are compatible with VSB receivers and provide improved data protection by reducing the data rate. How will they support HDTV? Use of AVC and VC-1 compression may be necessary to enable E-VSB and A-VSB to reach their full potential and support high-quality HD as well as provide a solution to ATSC 8-VSB transmission issues.
E-VSB received a lot of publicity when it was announced and standardized. Commercial deployment was expected to begin in early 2006; yet to date, broadcasters have not actively deployed it, and E-VSB functionality has not been implemented in DTV receivers.
There are three other, incompatible, mobile TV delivery technologies currently being touted: Qualcomm's MediaFLO,Crown Castle's implementation of DVB-H and a proposal from IPWireless to use existing 3G networks to “multicast” TV signals to subscribers.
Before the broadcast industry gets too excited about the opportunities A-VSB might bring, just remember that adoption of any mobile TV broadcast standard will not be determined by a standards committee. It will be determined by the consumer electronics market. Compare 1400 TV stations against tens of millions of potential mobile TV receivers, (and tens of billions of dollars to be made) who do you think carries the most weight in the decision process?
E-mail your thoughts, experiences and suggestions for future topics to editor@broadcastengineering.com with the subject line T2D.
