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Author Topic: FM broadcast SCA audio from SDR software?  (Read 453 times)

Offline ThaDood

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FM broadcast SCA audio from SDR software?
« on: June 27, 2020, 1909 UTC »
Ya know... I've been reminded of a kit that ya add to the IF of an FM broadcast RX to decode the SCA audio. I've thought for years of doing this, but in our ever encroaching digital age, I'm wondering if any major FM stations from major metro areas are doing it anymore. But, if any still are, can SDR software decode that? Am I wayyyyyyyyyyyyyy to late to even ask this?
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Offline redhat

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Re: FM broadcast SCA audio from SDR software?
« Reply #1 on: June 29, 2020, 0036 UTC »
...I'm wondering if any major FM stations from major metro areas are doing it anymore.

https://youtu.be/fdCkFrde4jQ

When I started in broadcast engineering, some of the local public stations did "books for the blind" using SCA equipment installed at the transmitter sites.

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« Last Edit: June 29, 2020, 0043 UTC by redhat »
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Offline ThaDood

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Re: FM broadcast SCA audio from SDR software?
« Reply #2 on: June 29, 2020, 1759 UTC »
Ya know, with SCA MPX'ing around for decades, how come no one seemed to have the foresight to have those MPX'ed channels to broadcast various news, talk, NOAA WX, other MX formats, or even paid programming, to the SCA service, but to only have an incredibly myopic use of this service? The idea of extra channels, as with HD Radio today, could have been way more utilized years ago.   
I can't decide upon what's worst, young and stupid, or old and chemically dumbed down.

Offline redhat

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Re: FM broadcast SCA audio from SDR software?
« Reply #3 on: June 29, 2020, 1846 UTC »
Primarily because receivers were not widely available and adding subcarriers does cause degradation to the host stations program audio, as well as loudness loss.  SCA injection needed to run around 20%, and this meant the host stations program audio would have to be reduced by that amount to keep the peaks under 100%.  The commission later added an exception stating that for every percent of subcarrier injection, the peak modulation could be increased by 0.5%.  This means if you are running 20% injection of SCA (or RDS interestingly) your new peak ceiling is 110%.

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Offline ChrisSmolinski

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Re: FM broadcast SCA audio from SDR software?
« Reply #4 on: June 29, 2020, 1917 UTC »
I used to have several local FM stations transmitting SCA, but it had been several years since I checked, so I just did. I did not find any, except for 107.7 which is transmitting an unmodulated SCA carrier at 67 kHz.
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Offline RobRich

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Re: FM broadcast SCA audio from SDR software?
« Reply #5 on: August 14, 2020, 2202 UTC »
AFAIK, there were some dedicated SCA broadcasters in South Florida years ago. Seems I remember Metrosonix being mentioned for modified portables.

http://www.metrosonix.com/

Looks like Metrosonix still offers some models. Around $40 to $90 from a quick skim of stock. o.0
« Last Edit: August 15, 2020, 0011 UTC by RobRich »
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