Doc. IS-191
26 June 2003
ATSC Implementation Subcommittee Finding:
Relative Timing of Sound and Vision for Broadcast Operations
Advanced Television Systems Committee
1750 K Street, N.W.
Suite 1200
Washington, D.C. 20006
The Advanced Television Systems Committee, Inc., is an international,
non-profit organization developing voluntary standards for digital television.
The ATSC member organizations represent the broadcast, broadcast equipment,
motion picture, consumer electronics, computer, cable, satellite, and
semiconductor industries.
Specifically, ATSC is working to coordinate television standards among
different communications media focusing on digital television, interactive
systems, and broadband multimedia communications. ATSC is also developing
digital television implementation strategies and presenting educational
seminars on the ATSC standards.
ATSC was formed in 1982 by the member organizations of the Joint Committee
on InterSociety Coordination (JCIC): the Electronic Industries Association
(EIA), the Institute of Electrical and Electronic Engineers (IEEE), the
National Association of Broadcasters (NAB), the National Cable Television
Association (NCTA), and the Society of Motion Picture and Television Engineers
(SMPTE). Currently, there are approximately 160 members representing the
broadcast, broadcast equipment, motion picture, consumer electronics,
computer, cable, satellite, and semiconductor industries.
ATSC Digital TV Standards include digital high definition television (HDTV),
standard definition television (SDTV), data broadcasting, multi-channel
surround-sound audio, and satellite direct-to-home broadcasting.
ATSC Implementation Subcommittee Finding:
Relative Timing of Sound and Vision for Broadcast Operations
The end-to-end DTV audio-video production, distribution, and broadcast
system is a complex array of digital processing, compression, decompression,
and storage devices. Each component in the system imposes latency on the
audio and/or video signals flowing through it. System design goals often
call for the relative audio-video latency through each component to be
in the sub-millisecond range. Operationally, unequal delays can be imposed
on the audio and video signals respectively, and these delays compromise
audio-video synchronization.
One of the overarching goals of the DTV broadcasting system is to deliver
audio and video in proper synchronization to the viewer. Because each
digital audio and video component in the chain from production to reception
imposes some degree of latency on the signals passing through it, and
the delays imposed on the audio and video signals are typically unequal,
each component harbors the potential to cause an audio-video synchronization
error at its output. The overall audio-video synchronization error is
the algebraic sum of the individual synchronization errors encountered
in the chain. While a given synchronization error may cause either a positive
or negative differential shift in audio/video timing, the video signal
is typically subjected to greater delay than the audio signal, and the
tendency is therefore toward video lagging behind audio. Thus, there is
a requirement to monitor audio-video synchronization at various points
within in the system and to make corrections, where required, in order
to deliver to the viewer audio-video synchronization within the required
tolerance.
In addition, there are points within the end-to-end chain that require
A/V synchronization to be maintained, such as switching points, monitoring
points, and transmission/encoding points.
For the purposes of this discussion, the end-to-end DTV system may be
divided into four segments: acquisition and production/post production
(contribution system), release facility and distribution system, local
broadcast station, and home receiver. IS finds that steps must be taken
to ensure that the audio and video signals delivered at the output stage
of each of the four segments are synchronized within a tight tolerance
(see below).
At the production-post production stage, audio-video synchronization errors
can occur in the capture stage, in film-to-video transfer, and in editing.
The product may be delivered on video tape or by various electronic means,
but whatever the delivery medium, IS finds that steps must be taken to
ensure that audio-video synchronization in the delivered product falls
within the required tolerance.
The release facility segment contains a number of devices through which
the DTV audio and video signals are passed, which variously impose compression
and de-compression, processing, and storage and their attendant differential
delays on the signals. The process of distributing the signals to affiliate
stations typically requires compression and decompression steps. IS finds
that it is incumbent on the release facility to correct the differential
audio/video delays that the signals experience within the plant so that
the initial timing relationship is restored to a tight tolerance before
the signals reach the distribution encoder. IS also finds that synchronization
to a tight tolerance should be maintained in any encode/decode process
that is involved in delivering the signals to the affiliate station, so
that the tight A/V synchronization can be monitored at switching and other
points.
The affiliate station segment contains a number of devices that are similar
to those encountered in the release facility segment and that generate
the same types of differential audio-video delays, including switching
and monitoring points. IS finds that audio-video synchronization should
be restored to a tight tolerance before the signals are input to the broadcast
station’s ATSC audio and video encoding devices to assure that the
presentation time stamps placed on the audio and video access units by
the encoder faithfully represent correct synchronization.
IS finds that under all operational situations, at the inputs to the DTV
encoding devices, the sound program should be tightly synchronized to
the video program. The sound program should never lead the video program
by more than 15 milliseconds, and should never lag the video program by
more than 45 milliseconds.
MPEG-2 models the end-to-end delay from an encoder’s signal input
to a decoder’s signal output as constant. This end-to-end delay
is the sum of the delays from encoding, encoder buffering, multiplexing,
transmission, de-multiplexing, decoder buffering, decoding, and presentation.
Presentation time stamps are required in the MPEG bit stream at intervals
not exceeding 700 milliseconds. The MPEG System Target Decoder model allows
a maximum decoder buffer delay of one second. Audio and video presentation
units that represent sound and pictures that are to be presented simultaneously
may be separated in time within the transport stream by as much as one
second. In order to produce synchronized output, IS finds that the receiver
must recover the encoder’s System Time Clock (STC) and use the Presentation
Time Stamps (PTS) to present the audio-video content to the viewer with
a tolerance of +/-15 milliseconds of the time indicated by PTS.
Although real aural and visual presentation devices typically have finite
and different inherent delays, and may have additional delays imposed
by post-processing or output functions, the System Target Decoder models
these delays as zero. IS finds that such delays must be corrected before
the audio and video signals are presented to the viewer.
The IS has undertaken additional work to develop tolerances for system
design. Pending that finding, designers should strive for zero differential
offset throughout the system.
Note: ITU R BT.1359-1 (1998) was carefully considered and found
inadequate for purposes of audio and video synchronization for DTV broadcasting.
