On April 20, 2022, NAB PILOT launched TXID.NABPILOT.org, a webpage for assigning Transmitter Identification codes (TxIDs) for ATSC 3.0 transmitters. Here are some questions and answers concerning all things TxID.
What is a TxID?
The ATSC 3.0 standard defines a way to assign a 13-bit number called a Transmitter Identification code (or TxID) to an ATSC 3.0 transmitter. The TxID signal is injected at a low level into the transmitter’s emitted signal and can be decoded by a special professional receiver for transmitter identification purposes.
Figure N.1.1 from ATSC Standard A/322 “Physical Layer Protocol” showing the injection point for the TxID signal.
What is TxID used for?
Providing an unambiguous transmitter identification code can be useful in a number of applications. Principally, though, the TxID code can aid broadcasters in adjusting the relative power and time delay of multiple transmitters in a single frequency network (SFN) configuration for optimized coverage.
Is TxID a requirement for ATSC 3.0 transmissions?
No, sending a TxID is optional in the ATSC 3.0 suite of standards.
What was the rationale for NAB PILOT to produce the TxID table and what stations are included in the table?
The ATSC 3.0 standard defines the structure for emitting TxID codes but does not provide any guidance on how to make the numerical assignments, noting only that the TxID should be unique to each transmitter on a given RF channel within the largest possible geographic region. NAB PILOT used this principle to construct a TxID assignment table for industry use. In addition to full power TV stations, low power TV, TV translators and transmitters in Canada and Mexico are also included. Changes in the location or class of transmitters or the introduction of new transmitter assignments will be updated in the assignment table on a semi-annual basis. The FCC Licensing and Management System (LMS) is used as the station database for constructing the table.
How many TxIDs are assigned to each station in the table?
The TxID is a 13-bit field and therefore there are 8,192 possible TxID codes. According to recent FCC counts, there are 1757 full power TV stations, 383 Class A TV stations, 1,840 LPTV stations and 3,231 TV translator stations, as well as Canadian and Mexican stations in the frequency coordination zone with the U.S., all of which need to be included in the TxID numbering scheme. With anticipation of even a moderate number of SFN deployments, 8,192 codes are clearly not enough for assigning unique numbers on a national basis. Coupling the RF frequency of the transmitter with the 13-bit TxID for a unique identifier frees up the number space considerably and is the basis used for assignment in the NAB PILOT TxID table.
While much of the early discussion about deployment of SFNs has centered around a relatively small number of transmitters, called a “sparse” SFN, longer term plans are also being discussed by some in the industry where potentially dozens of low power transmitter nodes could be employed in an SFN. As an example, “broadcast radio head” equipment from Saankhya Labs recently exhibited at NAB Show is intended for operation as low as five watts in low-power-low-tower networks for direct-to-mobile and indoor applications with expectations that many such units would be included in a “dense” SFN configuration.
Accordingly, the NAB PILOT TxID table was developed with supporting the possibility of future dense SFNs. In the table, each full power station is assigned 128 TxID codes. Due to their smaller geographic service areas, each low power, Class A or translator station is assigned 32 TxID codes.
Is there any downside to using TxID?
For SFNs to function properly, the signal from each node transmitter must be exactly the same. Transmitting different TxID codes on the same channel necessarily breaks this rule. However, the TxID is robustly coded using BPSK modulation and injected only during the first Preamble period following the Bootstrap symbol at one of 13 discrete levels 9 to 45 dB below the Preamble. The Preamble carries the L1-Basic and L1-Detail Information that include fundamental parameters such as time, version, PLP configurations, etc., and is coded much more robustly than the data portion of the frame. Since the TxID is transmitted only during the Preamble period, it only affects the receiving performance of the Preamble, which is determined by the injection level and modulation and error coding of the Preamble. These parameters are selectable within the station’s ATSC 3.0 Scheduler. With proper parameter selection, the performance of the receiver for successfully receiving the data payload should be unaffected.
Figure N.2.1 from ATSC Standard A/322 “Physical Layer Protocol” showing TxID signal placement in an 8K FFT (for 16K and 32K FFTs the TxID signal is repeated 2 and 4 times respectively).
What are the recommended injection levels for TxID?
Generally, the injection level can be higher with higher levels of error coding on the Preamble. There are seven levels of modulation and error coding defined for the Preamble named mode 1 to mode 7, with mode 1 being the most robust. The ATSC A/327 Recommended Practice “Guidelines for the Physical Layer Protocol” recommends generally to use mode 1 to mode 3, which are based on QPSK modulation, and recommends that the Preamble modes always be more robust than the data payload modes. Using mode 3, a TxID injection level of -15 dB will cause less than .2 dB of SNR threshold detection of the Preamble. If mode 1 or 2 is used, the TxID injection level can be increased to -12 dB without exceeding .2 dB of SNR degradation for the Preamble. A complete table of Preamble SNR degradation as a function of TxID injection level for mode 1 to mode 7 Preamble protection is provided in ATSC A/327.
Should you transmit the TxID all the time?
The ATSC A/327 Recommended Practice says, “broadcasters should turn on the TxID only when network design, adjustment, monitoring or measurements are needed in SFNs since otherwise the injected TxID will cause unnecessary interference to the first Preamble symbol.” However, with the injection levels and Preamble protection guidelines identified in the previous question, the downside of leaving the TxID on is negligible and transmission of TxIDs on a set-and-forget basis is a practical solution that may also be helpful to other spectrum band users that need to identify nearby signals.
Where can you get more information about TxID?
Appendix N of the ATSC Physical Layer Standard (A/322) documents the details of the TxID coding scheme. Section 4.2.17 of the ATSC Guidelines for the Physical Layer Protocol (A/327), includes implementation recommendations. Finally, TXID.NABPILOT.org includes the TxID table of code assignments as well as explanatory information about the table.