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FM Broadcast and TV Broadcast Aural Subcarriers

Revision History
Original 08 November 2008
08 June 2009 Corrected reference to IBOQ to IBOC, made several small corrections and clarifications

I recently acquired a Watkins-Johnson 8617A VHF/UHF surveillance receiver and thought it would be interesting to look at the "hidden" signals on FM broadcast and TV broadcast signals.

These signals are multiplexed along with the station's normal audio and modulate the stations main carrier (aural carrier in the case of TV stations.) These are not the relatively recent IBOC digital signals, but rather represent older technology, going back 50 years or more in the case of analog subcarriers. (These signals are called "subcarriers" because they are modulated on the main carrier.)

The test setup is relatively simple; the 8617A's wideband FM detector output connects to an HP3562A dynamic signal analyzer. For our purposes, the 3562A can be considered as a 0-100 KHz spectrum analyzer. The WJ-8617A is operated in FM mode, 300 KHz bandwidth.

For convienence, we can group the subcarriers into three functional categories:

  • Stereo broadcasting related
  • Digital data related
  • Secondary audio services

The figure below, from Wikipedia's article on FM Broadcast, shows the most common FM broadcast subcarrier signals.

Fitting these into our three categories:

Stereo broadcast related—Transmitting stereo requires separate left and right channels. At the time FM stereo broadcasting was developed (approved by the FCC in 1961) compatibility with existing monaural receivers was essential (and remains so today). The approach accepted by the FCC combined the left and right channels for transmission on the main carrier for reception by monaural receivers. The difference between the left and right channels (L-R) is  transmitted as a double-sideband supressed carrier subcarrier signal at 38 KHz. In order to insert the 38 KHz carrier with the correct phase for demodulation in stereo receivers, a phase locked pilot at 19 KHz (one-half the suppressed carrier frequency) is also transmitted. The stereo receiver detects the pilot, doubles its frequency and uses it as the BFO to demodulate the DSB L-R signal. The L+R main and L-R demodulated subcarrier signals are then summed into two paths to provide independent L and R signals. All in all, quite ingenious, considering the available vacuum-tube centered technology when the system was developed in the late 1950's.

Digital data related—The FCC allows FM broadcast stations to also transmit data signals over subcarriers. Two systems are commonly found:

  • RBDS or radio broadcast data system as it's known in the US or RDS as it's called elsewhere, is a relatively slow speed data system transmitted as ±90 degree phase shift keyed modulation centered at 57 KHz (the third harmonic of the 19 KHz stereo pilot tone). The data rate is 1187.5 bits/sec and it is used to provide, amongst other things, the station's call letters and information about the program content to RBDS-enabled receivers.
  • DirectBand is a higher speed data service operated by Microsoft over subcarriers leased from FM broadcast stations. Various data, such as stock quotes, sports scores, news and traffic information can be delivered to subscribers through DirectBand signals. The service uses a complex error detecting and correcting protocol (with encryption) providing a net throughput of about 10.5 kb/s. More information can be found at http://www.msndirect.com/RDS_MSNDirect_Comparison.pdf

Secondary Audio Services—The oldest subcarrier service uses narrow band FM modulated carriers to provide additional low-fidelity audio services. This was known as "SCA" or subsidiary communications authority when it became authorized by the FCC in the 1950's. Historically, these subcarriers carried MUZAK and talking books for the blind, although other information, such as foreign language programming, is also provided by some stations. (MUZAK is now satellite delivered.) The most common SCA carrier is 67 KHz, and some stations also use 92 KHz. 5 KHz deviation is used, and the maximum modulating frequency is limited to keep the SCA sidebands from interfering with other subcarrier signals. 67 KHz SCA is compatible with RBDS, but not with DirectBand.

Let's look at WAMU first. WAMU is an educational station operating on 88.5 MHz in Washington DC. As the figure below shows, it has a number of subcarriers multiplexed onto its main carrier:

  • Stereo pilot
  • Stereo signal
  • RBDS data
  • Analog SCA at 67 KHz
  • Second analog SCA at 92 KHz

At the time I captured the spectrum analyzer image below, the program content was talk, with no significant stereo content. Hence, the L-R signal shows only a bit of residual carrier. Likewise, the 67 KHz subcarrier has no program content at the time of capture but the 92 KHz subcarrier does.

By the way, if you are interested in calibrating these signal levels (in terms of "percent injection" as it is called in the trade), the image below shows a 1 KHz audio tone modulated  on a 88.1 MHz signal with 75 KHz deviation, representing 100% injection. The recovered audio is almost -7 dBVrms. Looking at WAMU's stereo pilot, we see it is around -27 dBVrms, or 20 dB below 100% modulation. Since deviation corresponds to recovered voltage, -20 db represents 10% injection. This is exactly where it should be, as the 19 KHz pilot is normally maintained between 9 and 11% injection.
The RBDS spectrum is interesting enough to deserve a more detailed examination, as pictured below. Note that there is little energy at the center frequency, 57 KHz. This is an interference prevention measure and is characteristic of the modulation method chosen. (The modulation method can be described as either amplitude modulation with suppressed carrier or phase shift keying with ±90° shift. Both have identical spectral signatures.) Essentially all the energy is contained within a 4 KHz bandwidth, so the modulation efficiency is 1187.5 bits/sec / 4 KHz, or 0.3 bits/Hz. (The 1187.5 rate is the raw data rate and includes error correcting and detecting bits, so the real payload delivery is lower, around 673 bits/sec usable.)
The image below is WETA-FM, 90.9 MHz in Washington DC. Like WAMU, WETA-FM is an educational station and during the image capture was transmitting a classical music program.

The most noticeable difference comparing the image to WAMU is  that the L-R subcarrier has significant energy over a wide frequency range, a product of the program content having both left and right channel non-identical information. WETA-FM's analog subcarrier at 67 KHz is also in use at the time I captured the image, as is evidenced by its modulation sidebands.

I found two stations carrying DirectBand modulation, one being WGTB, the subject of the spectrum capture below. (I didn't bother to annotate the other subcarriers.)
Taking a closer look at DirectBand, we see it spreads the energy rather uniformly over its 18 KHz bandwidth. The spectral efficiency is 11500 bits/sec / 18 KHz, or 0.64 bits/Hz, more than twice as efficient as RBDS. (In both cases, I'm using raw data rates. DirectBand's coding is more efficient, so it delivers a larger proportion of the raw data data as usable data.)
I mentioned at the outset of this page that the images are of subcarriers and not the IBOC digital transmisison system, which uses signals outside the normal FM analog bandwidth.

The image below is a normal spectrum analyzer view of WETA-FM illustrating its IBOC digital signals, located above and below its main signal. Each digital subcarrier is about 75-80 KHz wide. WETA-FM's normal analog signal is seen as confined to 200 KHz (±100 KHz) from its 90.9 MHz assigned frequency.
 


Television stations also use an analog FM system for aural transmission, or at least they do until the conversion to digital occurs in early 2009.

There is a slight difference in the frequency arrangement, as subcarrier frequencies are tied to the horizontal sweep rate of approximately 15750 Hz. The pilot is 15750 Hz, and the DSB L-R carrier is twice that or 31500 Hz. The "secondary audio program" is like the FM broadcast SCA, but at 78670 Hz, being 5x the 15750 line rate. (The frequencies are all tied to line rates to avoid interference between the various audio signals and the video information.)