Maintaining ATSC compliance
Nov 1, 2008 12:00 PM, By John Wilkie
Keeping up with the A/53, A/52 and A/65 standards will ensure proper signal reception.
With the dawn of digital-only TV broadcasting just a few months away, full-service broadcasters and digital transmission plants are finally drawing the scrutiny and attention they deserve. Many are finding that encoders and PSIP generators that once were in compliance with all applicable ATSC standards and FCC regulations are now out of compliance, if for no other reason than the underlying standard(s) changed but the equipment did not.
Digital transmission will be your station's sole bread and butter within a few months. As a result, complying with all current ATSC standards and applicable FCC rules is the best, if not only, way to ensure that broadcast signals are usable by all receivers.
Transport stream multiplex
We need to first start with what a transport stream multiplex really is. Whether operating one or three TV stations, broadcasters use one or more transport stream multiplexes to send compressed (retail) signals to viewers in their homes and businesses.
Unlike the limited options permitted with analog broadcasting and its single service per broadcast channel, the multiplex offers a nice toolkit, with just a few rules on how to use it. The ability to place a content stream on any packet ID (PID) carries with it the responsibility to provide viewers with a way to find and process that content stream.
Standards and constraints
The ATSC has published three standards that build upon the MPEG-2 work to create the digital television system used in the United States, Canada, Mexico and other countries. These are the Digital Television, Digital Audio and PSIP standards, known respectively as A/53, A/52 and A/65, each of which the FCC has adopted into its regulations.
Figure 1. The program map table (PMTMT) is comprised of sections for each program number represented in a transport stream, each section of which contains the packet ID (PID) and characteristics of each elementary stream in the program service. Image courtesy EtherGuide Systems.
Select image to enlarge.
MPEG-2 transport streams can include one or more packetized video and audio data elementary streams, which can make up one or more MPEG-2 program services. For receivers to render the elementary streams into a usable TV channel, the transport stream multiplex must include metadata describing where to find elementary streams as well as simple combining directions. Broadcast metadata can generally be thought of as having two classes: signaling metadata (useful mostly to receivers) and announcement metadata (useful mostly to viewers).
Fundamentally, ATSC standards used in broadcasting impose constraints on MPEG-2 transport streams. The best-known constraint is “Table 3,” which attempted to limit ATSC broadcasters to a handful of video formats. While this is still an A/53 provision, the ATSC has no enforcement powers, and the FCC, which does, has pointedly chosen not to adopt Table 3. So broadcasters are free to use any screen dimension that is evenly divisible by 16.
One ATSC constraint is that each program element is the sole user of each PID. Another key constraint is the encapsulating of analog (CEA-608) and digital (CEA-708) closed-captioning into video elementary streams.
The MPEG-2 specification provides for a program association table (PAT), found in a specific location that signals where to find each of the program map table (PMT) sections that describe the individual virtual channels in the multiplex. While MPEG doesn't specify how often these should be transmitted, A/53 requires that a PAT appear in a transport stream at least once every 100ms, while each PMT section should appear at least once every 400ms. (See Figure 1 on page 58.)
PAT and PMT descriptors
It is important that all entries in the PAT and PMT sections be accurate; otherwise, some receivers may not render a virtual channel or even fail to tune in a multiplex. Each program_number value may be only used once.
Likewise, each PMT section must include the program_number value signaled in the PAT for that virtual channel. Each elementary stream in the transport stream must be listed in at least one PMT section or that stream is unusable.
ATSC A/53 imposes a few often overlooked requirements on the PMT. First, each must include a program smoothing buffer descriptor. There are two fields in the descriptor: sb_leak_rate, which is the maximum rate for bits leaving the buffer, and sb_size, which signals the size in bytes of the smoothing buffer. While A/53 seems to imply that a value of zero is permitted for both of these fields, such settings would specify a buffer with zero length and no purpose. Typical values for sb_leak_rate are 1000 (signaling 400,000b/s) up to the maximum bit rate for the program, and a typical value for sb_size is 512.
Additionally, when an MPEG-2 video stream is described in the PMT, there must be a data_stream_alignment_descriptor in the element's descriptor loop, with descriptor_length equal to one and alignment_type equal to two (for video_access_unit).
PSIP and MPEG-2
Now, we turn to making sure that the PSIP properly describes the MPEG-2 arrangement. PSIP has a broader context and scope than MPEG-2 PSI (including analog channels), extending up to 128 hours (16 days) into the future, whereas MPEG-2 PSI is about the “here and now.”
It is important to note that there is some overlap between MPEG-2 PATs and PMTs and ATSC PSIP, and compliance requires the overlapped data to be consistent. Perhaps most important here is that the MPEG-2 PAT and the PSIP terrestrial virtual channel table (TVCT) both have the same value for transport_stream_id. Otherwise, receiving equipment in homes and at cable TV headends will be fouled up. Of course, the transport_stream_id value must be the one assigned to your station by the FCC.
| Want to use this article? Click here for options! |  |
