Decoding FM subcarrier SCA with GNURadio

Related: FMeXtra vs. HD Radio digital FM SCA

Decode SCA broadcast in the 88-108 MHz VHF FM broadcast band with GNU Radio Companion WBFM_SCA.grc along with numerous other GNU Radio examples.

This article explains SCA decoding as well as modern uses of spectrum adjacent to the main stereo VHF FM broadcast band signal.

How does SCA decoding work?

Consider the baseband FM diagram below. Do not confuse the abscissa with RF spectrum–this is baseband spectrum that is FM modulated before being transmitted on air. Unfortunately even some technically-oriented websites get this basic and critical fact wrong.

Plain monaural broadcast FM as used prior to 1955/1961, and used wisely by some talk stations today instead of stereo to improve geographic coverage area by reduction of multipath interference, is frequency modulated audio from 50 Hz - 15 kHz. FM multiplexing was employed in the early FM experiments by Armstrong, back to Nov. 1934 to send additional audio channels, faxes, etc. and SCA became generally legal for broadcasters in 1955. The April 1961 selection of the GE/Zenith FM stereo system placed a 19 kHz stereo pilot, high enough above legacy monaural receivers low-pass filter response to not bother them. Of course, circuitry and speakers available in June 1961 along with human hearing response would render the 19 kHz pilot inaudible in any use case.

year mono % SCA % stereo L-R % stereo pilot %
1948-1955 100      
1955-1961 90 10    
1961-1984 40 10 2 * 20 10
1984- (typ.) 45 10 2 * 22.5 10
1984- (max.) 40 20 2 * 20 10
  • stereo + SCA permits 110% modulation, deviation: 75 kHz → 82.5 kHz
  • SCA % allowed includes 57 kHz RDS
  • by taking 10 % away from main audio (5% L+R, 5% L-R) a total of 20 % can be SCA
WBFM baseband spectrum elements
WBFM baseband spectra with SCA, RDS, stereo elements.

SCA decode steps

This is a quantitative description of what’s needed to decode SCA audio

  1. 88-108 MHz receiver, perhaps direct conversion RTL-SDR, assuming 960 ks/s
  2. bandpass filter say ± 180 kHz before FM demodulation, to block adjacent stations while not making too much distortion. Remember the infinite sidebands of FM–don’t just clip to ± 75 kHz, it will sound badly distorted. OK to decimate down to 480 ks/s.
  3. plain FM demodulation (differentiator) – do not use de-emphasis at this point.
  4. frequency translate and low-pass filter SCA carrier e.g. -67 kHz shift, then low-pass filter 10 kHz or so.
  5. FM demodulate this signal with 150 µs de-emphasis and decimate to sound card rate e.g. 48 ks/s
  6. A normal baseband audio signal is emitted, if one was broadcast at this SCA frequency.

GNU Radio Companion SCA Decode Block Diagram

GNURadio WBFM SCA block diagram
GNU Radio WBFM SCA block diagram.

SCA broadcast: Boston, MA

RF Freq. (MHz) SCA Freq. (kHz) Content
90.9 67 Tonal Asian spoken + music
92.9 67 carrier only
104.1 67 Spanish catholic
104.1 92 carrier

Digital radio receiver in smartphone

A DAB+ smartphone: LG Stylus 2 K520 finally arrived in 2016, but like analog FM receivers in smartphones, requires headphones to be plugged in for the antenna. If HD Radio could likewise be brought to smartphones, this would be helpful–but there’s challenge enough getting carriers and Apple to enable the analog FM chips.

Future of FM broadcasting

Unlike the partially forced shutdown of analog FM in Europe, due to the different economy for Europe vs. North America, I foresee a continued gradual transition to digital AM/FM. A key question is when will AM/FM stations start to go full digital? Will it be in 5 years or 10 years? I think the first full digital HD Radio AM/FM station could occur in 7-10 years as I also foresee a massive turnover to electric and hybrid automobiles. These automobiles should include HD Radio receivers.

Yes, some electric cars currently don’t have HD Radio. I think that’s an legacy aberration caused by a few factors, including

  • regulatory enabling of higher HD Radio injection power (-14 dBc and higher, up from -20 dBc)
  • poor timing sync on some early HD Radio broadcasts giving bad analog/digital blend experience
  • too-high expectation of digital only coverage from -20 dBc. Mitigated by move to -14 dBc for most cases.


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