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(a)gmail.com>
To:
Cc: osmocom-sdr(a)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