Technology Corner: Randy Hoffner

NTSC Signals Have a TSID, Too

MPEG-2 transport streams, of which ATSC transport streams are a subset, can be uniquely identified with a Transport Stream Identifier, also known as a TSID. NTSC signals are not transport streams, but they have TSIDs too, which in their case stands for Transmission Signal Identifier. NTSC TSIDs, which facilitate the integration of NTSC signals into the DTV program guide, may be inserted into the Extended Data Services (XDS) portion of the NTSC Line 21 signals.

The analog TSID is defined in EIA 752, Specification for Transport of Transmission Signal Identifier (TSID) Using Extended Data Service; for further information on Extended Data Services, see EIA 608, Recommended Practice for Line 21 Data Service. The MPEG-2 TSID is defined in ISO/IEC 13818-1, the MPEG-2 system specification.

The carriage of an analog TSID is not required, but it can be beneficial to the viewer and to the broadcaster. Upon initial setup, and at regular intervals when the receiver is in the "off" state, a hybrid DTV/NTSC receiver should scan all channels according to the appropriate channel plan, broadcast or cable, to determine on which channels analog and digital signals are present. As each signal is found, the receiver should take note of its analog or digital TSID.

VIRTUAL CHANNEL TABLE

After this, when the receiver is asked to acquire a specific service, it can use the frequency on which that service was last found, rather than relying on the listing transmitted in the Virtual Channel Table (VCT). If the analog TSID is present, a viewer watching a hybrid DTV/NTSC set in the New York metropolitan area tuned to major-minor Channel 7-0 would know that the set was in fact receiving Channel 7 from New York, and not, in some propagation anomaly, receiving Channel 7 from Washington, D.C., for example.

The PSIP standard, ATSC A/65, specifies that each DTV station be assigned a unique Transport Stream Identifier, a 16-bit unsigned hexadecimal integer in the range from 0x0000 to 0xFFFF – a range that provides a possible 65,536 numbers.

It also provides that the NTSC station paired with each DTV station can be assigned a related unique Transmission Signal Identifier. The ATSC has recommended that DTV TSIDs be assigned only odd numbers, with NTSC TSIDs being assigned even numbers. In this way, a given NTSC station and its related DTV station may use contiguous even and odd numbers.

The FCC does not legally require that TSIDs be transmitted, but they are required by receivers in order to use the advanced channel navigation features that are afforded by PSIP. A full set of proposed digital and analog TSIDs for the United States may be found on the MSTV Web site: www.mstv.org.

These numbers were determined by the assignment of consecutively ascending values to stations in the order in which they appear in the FCC’s DTV Table of Allotments. They begin with number 2 (Hex 0x0002), assigned to Channel 2 in Anchorage, Alaska, and number 3 (0x0003) assigned to Channel 2’s associated DTV station, Channel 18; they end with number 3396 (0x0D44), assigned to NTSC Channel 27 in Christiansted, Virgin Islands, and number 3397 (0x0D45), assigned to its associated DTV station, Channel 5.

DEFAULT TSID

Many manufacturers ship ATSC encoders that are programmed to transmit a default TSID number of either 0 or 1, and the list begins with number 2 so that those defaults do not correspond to any real TSID number. The TSID numbers for our previous example are as follows: Channel 7 in New York is 2162 (0x0872), and its associated DTV station, Channel 45, is number 2163 (0x0873); Channel 7 in Washington, D.C., is 532 (0x214), and its associated DTV station is 533 (0x215).

If all the above stations were transmitting their proper TSID numbers, the viewer in the New York area could enter 7-0 on the remote control and know that the program displayed was from Channel 7 in New York (or get a message that this was not the case), while entering 7-1 would display the first program on the associated DTV station, Channel 45.

The concept of major and minor channel numbers affords great flexibility to the broadcaster and easy navigation to the viewer. It further allows NTSC/DTV broadcasters to associate their DTV programming with the NTSC channel branding that they have established over many years in the marketplace.

HYPOTHETICAL EXAMPLE

To go back to our hypothetical example, NTSC Channel 7 in New York has been assigned DTV Channel 45. Most viewers know where to find Channel 7 on their dial, but most do not know that its associated DTV station is on Channel 45. PSIP provides that both analog Channel 7 and digital Channel 45 carry the major channel number 7.

Minor channel numbers may range from 0 to 99, with 0 corresponding to the NTSC signal. Typically, the program(s) being transmitted in the DTV transport stream are numbered consecutively beginning with 1. Thus, the viewer enters 7-0, and receives analog Channel 7, or, presuming one DTV program, the viewer enters 7-1, and receives DTV Channel 45.

If Channel 45 has three programs in its multiplex, they would typically be numbered 7-1, 7-2 and 7-3. For DTV stations that do not have an associated NTSC station (at least one exists today), their RF channel number is their major channel number.

Major channel numbers for broadcast stations then may range from 2 to 69. Major channel numbers from 70 through 99 may be used to identify groups of digital services carried in a multiplex that the broadcaster wishes for some reason to be identified by a different major channel number. This might be applied if a local TV station wanted to broadcast community college lectures along with its regular programming, to cite an example proposed in the PSIP standard.

Currently, PBS is uplinking a broadcast-ready 19 Mbps transport stream that may be passed directly through a local PBS station’s DTV transmitter, and this transport stream carries the major channel number 80.

The PSIP major and minor channel numbers, along with the facility to uniquely identify each DTV and NTSC signal with a TSID number, afford great flexibility for the broadcaster and ease of operation for the DTV viewer.

Randy Hoffner is manager of technology and strategic planning at ABC, New York, N.Y. The views expressed in his column are his own, and not necessarily those of ABC. Write to him c/o TV Technology.