This tutorial covers the key ATSC documents and practices to maintain DTV transmission compliance.
For many years, compliance in the broadcast industry has meant conducting a successful FCC proof of performance, and the technical requirements for compliance were clearly spelled out in the FCC's rules and regulations. With the advent of digital broadcasting, the rules have been revised and augmented, and while some video transmission requirements have become obsolete, the new requirements for the transmission of ATSC-encoded video are more complex and can be found across many documents. Here, we'll take a look at where much of that information is found.
Out-of-channel spectrum emissions
The FCC rules were developed to ensure various approved operating conditions — basically, protection of stations from interference and compatibility with receivers. In order to prevent harmful interference to other stations, a broadcast station must carefully control its emission spectrum. Several documents are available to assist in the measurement of in-band and out-of-band emitted power. One such document is ATSC A/64B, “Recommended practice: Transmission measurement and compliance for digital television.” The FCC rules for NTSC service allow for an average power variation ranging between 80 percent (-1dB) and 110 percent (+0.4dB) of total authorized power. In order to not compromise the DTV noise threshold for fringe area reception, A/64B recommends that the DTV transmitted signal power should be kept within 95 percent and 105 percent of authorized power (+/-0.2dB). Because of the variations possible with different measuring equipment, it's recommended that stations calibrate their power meters using a calorimeter-type power meter.
The allowed limits for out-of-band DTV emissions are specified in Section 73.622(h) “Digital television table of allotments” of the FCC rules for full-service DTV stations and in Section 74.794 for low-power stations. This DTV emission mask is illustrated in Figure 1 on page 24. Note that the “in channel” power spectral density is -10.6 dB when measured in a 500kHz bandwidth. (This figure is derived by accounting for the 8-VSB signal effective bandwidth of 5.38MHz and by including the pilot signal power.)
It has been recognized that the techniques required to measure the spectrum of a digitally-modulated transmitter are generally unfamiliar to the broadcast industry, resulting in significant variations in the results obtained by various observers. To remedy this, techniques have been developed to produce accurate and FCC-compliant measurements. The IEEE Standard 1631-2008, “Recommended practice for measurement of 8-VSB digital television transmission mask compliance for the USA,” covers the theory, techniques and procedures for measuring the spectral characteristics of 8-VSB transmitters in frequencies near assigned channels. The standard explains characteristics and measurement procedures that ensure measurement uniformity across different users. Hopefully, this will form the basis for a new generation of measurement equipment, perhaps even integrated in the transmitters themselves. And while broadcast stations may opt to have contractors carry out the detailed measurements described in these documents, a familiarity with the procedures is invaluable to an understanding of where potential problems can occur.
ATSC bit stream compliance
In addition to the RF emission characteristics, the FCC rules require digital television broadcasters to comply with the specifications set forth in three key ATSC standards:
ATSC A/52: “Digital audio compression (AC-3);”
ATSC A/53: “Digital television standard” (except for compression format constraints); and
ATSC A/65C: “Program and system information protocol for terrestrial broadcast and cable.”
The commission also encourages broadcasters to consult:
ATSC A/54A: “Recommended practice: Guide to use of the ATSC digital television standard;” and
ATSC A/69, “Recommended practice PSIP implementation guidelines for broadcasters.”
While compliance with A/52 and A/53 will ordinarily be met by using compliant equipment, there are sometimes outlying circumstances or parameters that can be problematic. At the transport level, some of these conditions can be appreciated by consulting ATSC A/78A: “Recommended practice: Transport stream verification.” This document categorizes various errors that may result in any of the following: transport stream off-air, program off-air, component missing, poor quality of service or technically nonconformant stream. Stress bit streams are available from various sources to assist in identifying and rectifying many of these conditions.
Compliance with A/65C — commonly called PSIP — is required by the FCC rules, and details are not specified in the latter, but the commission's Report and Order (FCC 07-228) describing the rulemaking does spell out some specifics about compliance. For one, broadcasters must populate the required PSIP tables and descriptors with the correct information to help receivers assemble functioning guides. The standard also mandates completing tables and descriptors that require one-time setup to be set correctly, including the transport stream identifier (TSID), short channel names, service type, modulation mode source ID and service location descriptor. The TSID information should also be consistent in the terrestrial virtual channel table (TVCT) and the program association table (PAT).
Broadcasters must populate the event information tables (EITs) with accurate information about each event and update the EIT if more accurate information becomes available. Although no update rate is specified in the ATSC standard or in the rules, the commission has encouraged stations to update the EIT as soon as possible when overages or other circumstances result in changes to scheduled programs. ATSC A/65C also requires that the EITs contain the caption service descriptor to facilitate a DTV receiver's search for closed-caption information.
ATSC A/69 spells out errors that are common in many PSIP implementations, such as:
missing tables, particularly the system time table (STT) and EIT;
a major channel number set to the DTV RF channel number, rather than the associated (legacy) NTSC channel number;
TSID set to zero or one, the NTSC TSID, or another station's TSID, or not set the same in the three required places; and
the system time missing or set to 00:00:00 on 1/6/1980.
In order to facilitate receiver handling of different aspect ratios, the broadcast community has developed the active format description (AFD), which is described in A/53 and elsewhere. The FCC does not require broadcasters to use AFD, but broadcasters that choose to use it must adhere to A/53. (The commission does, however, encourage television manufacturers to implement the CEA-CEB16 “Active format description (AFD) & Bar Data Recommended Practice” into their TV sets.) The broadcaster's handling of AFD in the plant is defined in SMPTE 2016-1, “Format for active format description and bar data,” which specifies the carriage of the AFD code in the facility SDI links and provides guidance on how the code should be set in encoders and transcoders.
To evaluate how well an ATSC-compliant encoder performs, complex suites containing specialized and artificial test patterns have been designed to stress various aspects of processing and to force failure modes. On the receiver side, transport streams are available from various sources that exercise different modes of AFC, closed captions, audio/video synchronization and ratings systems. These are often used by receiver manufacturers in developing and testing their products.
The ATSC standard (and the FCC rules) have no specific compliance requirements on audio/video synchronization, in part because the problem is quite complex and is still not completely understood. Recently, EBU and SMPTE have been finalizing a set of recommendations to help update synchronizing and time labeling standards for broadcast and other content. This work describes detailed standardization needs and suggests practical, cost-effective timing and synchronization approaches in media creation and broadcast technologies that will be useful for the foreseeable future. At press time, a report was due at IBC. Expect more work in this area as solutions emerge.
This discussion is not intended to replace an understanding of the legal requirements for compliant broadcasting, but serves as a technical summary of some of the relevant issues. Make sure you consult the right authorities to ensure FCC compliance!
Aldo Cugnini is a consultant in the digital television industry.
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