This might be a better question for the Gnuradio list, but for years I've known that some broadcast FM stations actually transmit a second audio program which is somehow modulating a subcarrier in the main transmission. It probably carries mostly elevator music type junk but is rumored to be commercial-free (a rare thing in the US!). I remember seeing dedicated receivers advertised for this, but never actually bought one because I wasn't sure it was active in my area. These date back to Lafayette Electronics days I think and ever since so 40 years or more.
I've noticed (in SdrSharp) that on some FM stations I can see what look like they might be subcarriers on either side of the main signal. Anyone decoded those?
Alan
----- Radio Astronomy - the ultimate DX
Alan,
I believe the subcarriers you mention are the HDRadio sideband. I've tried decoding this band but have been unable so far - it seems that iBiquity holds their proprietary standards close and I guess the community hasn't yet figured out how to decode it. If anyone is aware of an approach to decoding HDRadio, I'd love to know.
http://en.wikipedia.org/wiki/HD_Radio
Ethan
On Sun, Mar 17, 2013 at 10:48 AM, Alan Corey alancorey@yahoo.com wrote:
This might be a better question for the Gnuradio list, but for years I've known that some broadcast FM stations actually transmit a second audio program which is somehow modulating a subcarrier in the main transmission. It probably carries mostly elevator music type junk but is rumored to be commercial-free (a rare thing in the US!). I remember seeing dedicated receivers advertised for this, but never actually bought one because I wasn't sure it was active in my area. These date back to Lafayette Electronics days I think and ever since so 40 years or more.
I've noticed (in SdrSharp) that on some FM stations I can see what look like they might be subcarriers on either side of the main signal. Anyone decoded those?
Alan
Radio Astronomy - the ultimate DX
On 3/17/2013 9:48 AM, Alan Corey wrote:
I can see what look like they might be subcarriers on either side of the main signal. Anyone decoded those?
Hi Alan,
Depending on where you live, there could be several subcarriers present. From your description I'm pretty sure you're referring to Sub Carrier Authorization (SCA) which has been mainly used for background music and an audio book reading service for the blind as well as other voice and data services. SCA uses 67 or 92 KHz subcarriers which can be seen on the composite FM signal. If you have a soundcard with sufficient bandwidth you can send the output of SDR# (in NBFM, 150 KHz mode) to another instance of SDR# *(sound card, DSB mode) using Virtual Audio Cable. My soundcard won't go high enough but here's a set of screen images from a system that can, where the various elements on the upper side of the FM channel center are annotated: http://img208.imageshack.us/img208/5922/fmspectrum.png
The RDS (Radio Data System) or RDBS in the US digital data channel is easily seen at 57 KHz, The level of all these subcarriers is much lower than the stereo pilot so you'll need a strong signal for decoding.
73, Bob W9RAN
On 3/17/2013 11:32 AM, Robert Nickels wrote:
On 3/17/2013 9:48 AM, Alan Corey wrote:
I can see what look like they might be subcarriers on either side of the main signal. Anyone decoded those?
Hi Alan,
Depending on where you live, there could be several subcarriers present in addition to the prominent 19 KHz stereo pilot. From your description I'm pretty sure you're referring to Sub Carrier Authorization (SCA) which has been mainly used for background music and an audio book reading service for the blind as well as other voice and data services. SCA uses 67 or 92 KHz subcarriers which can be seen on the composite FM signal. If you have a soundcard with sufficient bandwidth you can send the output of SDR# (in NBFM, 150 KHz mode) to another instance of SDR# *(sound card, DSB mode) using Virtual Audio Cable. My soundcard won't go high enough but here's a set of screen images from a system that can, where the various elements on the upper side of the FM channel center are annotated: http://img208.imageshack.us/img208/5922/fmspectrum.png
Another subcarrier is used for RDS (Radio Data System) or RDBS in the US digital data channel which can easily seen at 57 KHz on stations utilizing this service. In the US The arithmetic sum of all multiplex subcarriers may not exceed 20% modulation and different rules apply than for the primary broadcast channel.
73, Bob W9RAN
Yes, SCA is what I was thinking of. I used to be an electronics technician in consumer electronics back in the 70s and this was on licensing exams, aside from seeing them in catalogs. Being from a rural area it was just something I read about, I never actually saw one. I had forgotten the L-R signal was on a 19 KHz subcarrier, I was looking for a 1.9 KHz, which makes no sense since it would be in the audio passband.
If I run into it again I'll grab a screen shot, but what I was probably seeing was an overload condition on my poor little NooElec dongle. There's a 2 meter repeater about 20 miles away, but line of sight. When that keys up I see peaks all over the 2 MHz range that SDRSharp shows me for 2 meters, but they all go away when it unkeys. The FM broadcast station I was thinking of is a little farther away but of course a lot more powerful. When I look for them now I can't find them. I remember checking one to see if it was 1.9 KHz away from the main peak, but it was several times that, maybe around 8 KHz.
Enjoyed the QST article, my copy came about a week after my dongle. I built one of the upconverter kits from Hayseed Hamfest, and it works, but it's deaf as a post. Even on my 150 foot longwire I can only hear the stronger stations. AM broadcast stations and the strong bible beaters are about it. Tuning the HF ham bands, I've rarely heard hams and then only 1 side of each QSO.
The SA612 data sheet I've got (Philips) is skimpy on specifics but I wonder about the oscillator injection level. I've been trying to decide whether to put a pot in there to adjust or a dual gate mosfet broadband preamp on it. It seems like the oscillator level should be close to the signal level. I've got some BF998 FETs I was going to use because I can put a pot on the gate 2 voltage to control the gain.
73, Alan, AB1JX
----- Radio Astronomy - the ultimate DX
From: Robert Nickels ranickels@gmail.com To: Cc: osmocom-sdr@lists.osmocom.org Sent: Monday, March 18, 2013 11:31 AM Subject: Re: Broadcast FM subcarrier decoding?
On 3/17/2013 11:32 AM, Robert Nickels wrote:
On 3/17/2013 9:48 AM, Alan Corey wrote:
I can see what look like they might be subcarriers on either side of the main signal. Anyone decoded those?
Hi Alan,
Depending on where you live, there could be several subcarriers present in addition to the prominent 19 KHz stereo pilot. From your description I'm pretty sure you're referring to Sub Carrier Authorization (SCA) which has been mainly used for background music and an audio book reading service for the blind as well as other voice and data services. SCA uses 67 or 92 KHz subcarriers which can be seen on the composite FM signal. If you have a soundcard with sufficient bandwidth you can send the output of SDR# (in NBFM, 150 KHz mode) to another instance of SDR# *(sound card, DSB mode) using Virtual Audio Cable. My soundcard won't go high enough but here's a set of screen images from a system that can, where the various elements on the upper side of the FM channel center are annotated: http://img208.imageshack.us/img208/5922/fmspectrum.png
Another subcarrier is used for RDS (Radio Data System) or RDBS in the US digital data channel which can easily seen at 57 KHz on stations utilizing this service. In the US The arithmetic sum of all multiplex subcarriers may not exceed 20% modulation and different rules apply than for the primary broadcast channel.
73, Bob W9RAN
SCA is what I was thinking of, but this sounds more like overload. From my raw notes:
If I tune to 88.500 MHz I see signals about 88.3367 and 88.662 The upper one is 162 KHz away, the lower one 163. If I click on one I hear nothing.
The same is true of WAMC(?) at 90.300 with 90.138 and 90.465 so 162 and 165.
----- I don't have a screen shot and I haven't seen it happen lately.
Alan
----- Radio Astronomy - the ultimate DX
----- Original Message -----
From: Robert Nickels ranickel@comcast.net To: osmocom-sdr@lists.osmocom.org Cc: Sent: Sunday, March 17, 2013 12:32 PM Subject: Re: Broadcast FM subcarrier decoding?
On 3/17/2013 9:48 AM, Alan Corey wrote:
I can see what look like they might be subcarriers on either side of the
main signal. Anyone decoded those?
Hi Alan,
Depending on where you live, there could be several subcarriers present. From your description I'm pretty sure you're referring to Sub Carrier Authorization (SCA) which has been mainly used for background music and an audio book reading service for the blind as well as other voice and data services. SCA uses 67 or 92 KHz subcarriers which can be seen on the composite FM signal. If you have a soundcard with sufficient bandwidth you can send the output of SDR# (in NBFM, 150 KHz mode) to another instance of SDR# *(sound card, DSB mode) using Virtual Audio Cable. My soundcard won't go high enough but here's a set of screen images from a system that can, where the various elements on the upper side of the FM channel center are annotated: http://img208.imageshack.us/img208/5922/fmspectrum.png
The RDS (Radio Data System) or RDBS in the US digital data channel is easily seen at 57 KHz, The level of all these subcarriers is much lower than the stereo pilot so you'll need a strong signal for decoding.
73, Bob W9RAN
On 3/17/2013 9:48 AM, Alan Corey wrote:
This might be a better question for the Gnuradio list, but for years I've known that some broadcast FM stations actually transmit a second audio program which is somehow modulating a subcarrier in the main transmission. It probably carries mostly elevator music type junk but is rumored to be commercial-free (a rare thing in the US!). I remember seeing dedicated receivers advertised for this, but never actually bought one because I wasn't sure it was active in my area. These date back to Lafayette Electronics days I think and ever since so 40 years or more.
I've noticed (in SdrSharp) that on some FM stations I can see what look like they might be subcarriers on either side of the main signal. Anyone decoded those?
Alan
Radio Astronomy - the ultimate DX
Many, if not most, FM stations US have subcarriers, if only to carry the stereo (L-R) signal and the bit that turns the "stereo" indication on. On the two SCA channels for alternate programming, you will find "elevator music," news broadcasts, audio books for the blind, and alternate language programming on them. Since they're usually done up as a subscription service, they don't have commercials. I've never decoded them, but I used to work at the transmitter site for an FM radio station. Our SCA's carried "ambient music" (for which I had to change the tape if one ran out) and some sort of agricultural news broadcast.
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