Broadcast FM subcarrier decoding?

Alan Corey alancorey at yahoo.com
Tue Mar 19 03:07:43 UTC 2013


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 at gmail.com>
>To: 
>Cc: osmocom-sdr at 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
>
>
>
>
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