A concerted, cross-industry effort has led the ATSC to publish DTV receiver performance guidelines. A/74, "Recommended Practice: Receiver Performance Guidelines," is the result of a collaboration of broadcasters, consumer electronics manufacturers, semiconductor manufacturers and other ATSC members.
This recommended practice provides performance guidelines for receiver sensitivity, multiple signal overload, phase noise, selectivity and multipath. The document also suggests the use of the antenna-control interface CEA-909 developed by the Consumer Electronics Association, which facilitates automatic control of antenna parameters.
Recommended Practice A/74 addresses the front-end portion of a DTV receiver. The performance guidelines in the document are intended to ensure reliable reception. Guidelines for interference rejection are based on the FCC planning factors that were used to analyze coverage and interference for the initial DTV channel allotments. Guidelines for sensitivity and multipath handling reflect field-testing undertaken by ATTC, MSTV, NAB and receiver manufacturers.
| Fig 1.|
As covered within A/74, the front-end includes all circuitry from the antenna through the process of forward error correction (FEC) that is associated with recovery and demodulation of the 8-VSB signal (see Fig. 1). The output of the receiver front-end is the input to the transport layer decoder.
The circuits contributing to meeting the A/74 guidelines include:
- Antenna and antenna control interface (CEA-909);
- Tuner, including RF amplifiers, associated filtering, and the local oscillator (or pair of local oscillators in the case of double conversion tuners), as well as mixers required to bring the incoming RF-channel frequency down to that of the intermediate frequency (IF) amplifier/filter;
- IF amplification (with automatic gain control) and filtering, including the major portion of pre-decoding gain, channel selectivity and at least a portion of the desired-channel band-shaping;
- Digital demodulation, including in-band interference rejection, multipath cancellation and signal recovery;
¥ FEC, wherein errors in the demodulated digital stream caused by transmission impairments are detected and corrected for incoming signals with signal-to-impairment ratios above a threshold. Packets with uncorrectable errors are flagged for possible mitigation in the video and audio decoders.
A/74 does not discuss optional means by which receivers might attempt to conceal or otherwise mitigate the visible or audible consequences of uncorrected bitstream errors. Although most receivers include circuits that accomplish some degree of error concealment, the results are subjective and not quantified as easily as the performance of the other circuits listed above.
The recommended performance guidelines are divided into four general categories:
- Interference rejection
- Multipath handling
The various industry segments that comprise the broadcast-to-reception chain have positively responded to publication of A/74 and supporting bitstreams have been distributed to various organizations for testing and study. In addition, the ATSC Specialist Group on Receivers, T3/S10, continues to look for and document unique and interesting reception sites that may be useful in designing new receiving devices.FOR MORE INFORMATION
Document A/74, as all ATSC Standards, Recommended Practices, and informational documents, can be downloaded free from the ATSC Web site at http://www.atsc.org Jerry Whitaker can be reached via TV Technology.