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Softrock Lite 6.2
Adventures in Electronics and Radio
Elecraft K2 and K3 Transceivers
FM Broadcast and TV
Broadcast Aural Subcarriers
Original 08 November 2008
08 June 2009 Corrected reference to IBOQ to IBOC, made several small corrections
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
- 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
- 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
- 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
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.
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.
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.)