how to decode a fm subcarrier

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n9mfk

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Hi All,
i need some help I like to decode a fm radio station subcarrier
i try look and this video but can not sen to get the setting correct
thanks for any help
 

bill4long

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It might be hard to get help since it's not legal to decode SCA in the US.

Not true. It is not illegal to merely build or possess a device that decodes SCA in the USA. Moreover, it is perfectly legal to decode SCA in the USA if you are listening content for which you have a license to receive, or to received unlicensed content. Such examples of unlicensed content are reading for the blind services, educational services, church related content, and student experimental broadcasts from college radio stations, etc. For example, the Mormon church used to broadcast SCA in California in the 80s. No license was required to listen and was perfectly legal.

For license requirements for particular SCA content, contact the broadcaster.
 
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KK4JUG

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Not true. It is not illegal to merely build or possess a device that decodes SCA in the USA. Moreover, it is perfectly legal to decode SCA in the USA if you are listening content for which you have a license to receive, or to received unlicensed content. Such examples of unlicensed content are reading for the blind services, educational services, church related content, and student experimental broadcasts from college radio stations, etc. For example, the Mormon church used to broadcast SCA in California in the 80s. No license was required to listen and was perfectly legal.

For license requirements for particular SCA content, contact the broadcaster.
That's a very lucid response, considering your avatar:
1559943669080.png :)
 

dlwtrunked

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Hi All,
i need some help I like to decode a fm radio station subcarrier
i try look and this video but can not sen to get the setting correct
thanks for any help

I use free VBCable as I found it better than pay VAC. Once installed, set it up to allow 192 kHz sampling.
(Note that very few FM broadcast stations now have subcarriers. They were more common in the past.)

1. Use HDSDR to tune the FM broadcast station in WFM mode ( and set the rate to 192 kHz in . Set the output to be VBCable input. Be sure to set the Bandwidth sampling rate to 192 kHz.
2. Set the input for SDR# to be the output of VBCable.
3. Use SDR# in NFM mode to tune the 67 or 92 kHz subcarrier. (A good signal is required.)
4. Set the output of SDR# to be your speakers.

You can often improve the signal by
As others mentioned, the U.S. ECPA of 1986 made it illegal to receive subcarriers of FM broadcast stations if not authorized by them. At that time pay services include PRN (Physicians Radio Network, Muzak, Farm news, etc. You will not find those there now. Today, sometimes subcarriers will carry another broadcast station but most of what is around today in some area of the US appears to be reading services for the blind. It did the same for studio-transmitter links and of course cell phones.
 

ko6jw_2

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Back in the early 70's I was a broadcast engineer for a couple of FM stations. Stereo was just coming into the market. I learned all about subcarriers.

The most common subcarrier is at 19Khz which is used in all FM stereo broadcasts. The spectrum image clearly shows that sub carrier. However, you don't need any special equipment to decode it. That's what FM receivers are designed to do. The SCA was used for Muzak etc. It was limited bandwidth and not stereo. Broadcast FM signals roll off sharply above 15Khz so as not to interfere with the 19Khz subcarrier. The stereo effect is based on a sum and difference signal. The sum being the mono signal and the difference is used to create the stereo effect. The use of the 67 and 92Khz subcarriers was never very common. In my area there was only one FM station that provided it for Muzak. Elevator music. Now a days there are specialized streaming services that will tailor background music to your business. All digital. Again, not something you would want to listen to.

So I guess if the point is to try to hack some technology just because you can, it is a valid area of study. If your intent is to get to worthwhile programming for listening enjoyment - not worth the effort. Besides with all the streaming services around now you can do it for yourself on the internet.
 

KK4JUG

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Some 50+ years ago, I worked at an FM MOR radio station (that will remain anonymous because they're still on the air). We also provided background music using a subcarrier. It was a 24-hour station but management didn't want to pay for an overnight jock so we devised a completely illegal system to provide music and station IDs.

First, we recorded about 75 PSAs, station IDs and station promos on a 30-minute tape cartridge. Back then, stations used an NAB Type II cartridge for commercials, etc. (It's the same configuration as the old 4-track music cartridges that got beat out by 8 track.) Wider carts could hold as much as 30+ minutes of content. For a timer, we mounted a copper wire to the minute hand of a clock and a piece of metallic "cue" tape at 30 and 60 on the face. Every 30 minutes, the minute hand would make contact with the metal tape and activate the cart machine and play the PSAs and promos. When the copper wire lost contact with the tape, the cart machine would play to the end of the current message and quit. The music played on the air was the same as what went out on the SCA. It used a "juke box" configuration of records playing at 16 2/3 rpm. The juke box was configured so that it was able to detect when and if the record hung up and the cart machine was loud enough to override the music.

I can say all this now because the statute of limitations has run out and the cheapskate manager has passed away.
 
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