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Grex > Radio > #1: Welcome to the 2nd edition of Radio | |
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| 25 new of 123 responses total. |
rcurl
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response 75 of 123:
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Jan 28 17:47 UTC 2000 |
Get two of them... :) (actually - how does stereo FM work?)
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prp
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response 76 of 123:
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Jan 28 21:13 UTC 2000 |
The main signal carries A+B, which mono receivers use. There is a
subcarrier which has A-B. Stero receivers add the two signals for
one channel, and subtract them for the other.
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gull
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response 77 of 123:
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Jan 28 21:21 UTC 2000 |
It's a bit complicated. As I recall, the monaural signal (Left+Right) is
present as usual. Combined with this, however, is a Left-Right signal
that's single-sideband amplitude modulated on a 19 kHz "pilot tone." The
whole schpiel is then mixed together and transmitted as an FM signal. Since
normal FM only carries audio out to 15 kHz, the pilot tone and L-R signal
aren't audible on a monaural receiver. When a stereo receiver senses the
pilot tone is present in the demodulated audio, it seperates it out and
demodulates the L-R signal from it. By adding and subtracting that signal
from the monaural signal, it recovers the two channels. (Since
(L+R)-(L-R)=2R, and (L+R)+(L-R)=2L.) I know this isn't very clear...someone
else might be able to explain it better. I could also scan the basic block
diagram of the process from some class notes I have from a few terms ago,
if you'd like.
This system seems a bit Rube Goldberg-ish, but in practice it works pretty
well, and keeps compatibility with monaural FM receivers. (Another scheme
that was tried and thankfully rejected was transmitting the left signal on
the FM broadcast band and the right signal on the AM broadcast band. One of
the obvious problems with this was that the fidelity of the two channels
just wasn't equal.)
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gull
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response 78 of 123:
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Jan 28 21:21 UTC 2000 |
#76 slipped in, and pretty much summarizes my response. ;>
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rcurl
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response 79 of 123:
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Jan 28 21:29 UTC 2000 |
Many thanks. I'm wondering why I hadn't thought to find out a long time
ago how this works. I guess two monaural receiver chips just won't do
it..... :)
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jerome
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response 80 of 123:
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Jan 29 15:08 UTC 2000 |
The L-R signal is sent using double-sideband suppressed carrier centered
around 38 kHz with a bandwidth of 15 kHz. The 19 kHz pilot tone (which
is also transmitted) is used by the receiver to demodulate the L-R channel.
To make things even more interesting there is something called "Subsidiary
Communications Authorization" (SCA) that allows yet another signal to
be sent on this composite WFM signal. This signal is centered on a
subcarrier of 67 kHz and is FM modulated with a maximum deviation of 7.5 kHz.
SCA is often used for commercial-free broadcasting of Muzak, commodity
reporting, the Physicians' Network, and services of interested to the
handicapped.
The August and September 1987 issues of Radio-Electronics magazine had
a two-part "Build This" article for a SCA/FM stereo receiver. I purchased
the kit and built it. The performance is actually quite good. The kit
was made by North Country Radio (P.O. Box 53, Wykagyl Station, NY 11804),
which (I believe) is still in business and last I knew still sells the kit.
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devnull
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response 81 of 123:
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Jan 30 03:00 UTC 2000 |
So do the majority of FM stations transmit SCA?
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gull
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response 82 of 123:
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Jan 30 03:29 UTC 2000 |
Re #80: I forgot that the 19 kHz tone gets doubled to get the actual
subcarrier frequency. Thanks.
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jerome
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response 83 of 123:
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Jan 30 18:09 UTC 2000 |
resp:81 A lot of them don't, actually. One possible reason for this,
according to the article, is "In the case of SCA, modulating one
subcarrier of a stereo signal uses up about 10% maximum of the total
75 kHz deviation (100% modulation). In practice, that reduces the
main channel's signal strength by about 1 dB. Normally, such a drop
in signal level would not be noticeable. However in areas with
crowded FM bands, every dB counts in the race for ratings, and
revenue." The article also mentions that there is sometimes a second
SCA subcarrier at 92 kHz, which (if used) would take even more power
away from the main channel.
I live in a different area now than when I built the radio. All of this
SCA talk is giving me the urge to fire up that radio and see what this
city has to offer in terms of SCA.
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gull
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response 84 of 123:
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Jan 30 20:33 UTC 2000 |
SCA used to be popular as a way to broadcast commercial-free background
music for stores and such. I'd imagine it's less popular now that there are
satellite services such as Muzak.
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jerome
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response 85 of 123:
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Jan 31 03:15 UTC 2000 |
I dusted off my SCA receiver. I found three SCA transmissions: One
background music, one data transmission, and one that sounded like some
type of community service thing. I can't remember if I have the receiver
set for the 67 kHz subcarrier or the 92 kHz one (I should have brought
that adjustment out to the front panel), so I there might have a few
others that I missed.
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goose
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response 86 of 123:
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Jan 31 19:12 UTC 2000 |
http://www.northcountryradio.com of course is where you'll find North
Country Radio!
Jerome, do you live in Ann Arbor?
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jerome
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response 87 of 123:
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Jan 31 22:51 UTC 2000 |
resp:86 (second line) Negative.
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jerome
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response 88 of 123:
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Jan 31 22:54 UTC 2000 |
Oh, and if anyone does want to build North Country Radio's SCA/FM Stereo
receiver, be advised that there were a few mistakes in the article,
including on the schematic. I would be happy to supply details to anyone
who wanted to build one.
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abndaddy
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response 89 of 123:
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Oct 12 23:40 UTC 2000 |
Hello Ya'll! My name is Chad and I am a Special Forces Communications Sergeant
in the Army. After going through the school here at Ft. Bragg I have developed
a "slight' interest in radio communications. I am 15/15 in code, and have
experience with HF,UHF,VHF communications......but I really think that I'm
going to pick up QUITE A BIT here; And that's why I'm here! I need some of
you "OLD-TIMERS" to point me in the right direction for when I get to my Unit
in Germany!
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rcurl
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response 90 of 123:
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Oct 13 03:36 UTC 2000 |
This conference is pretty quiet, although there are a lot of hams around
about, and some even lurk here. But, raise any questions you want, and
we'll see what expertise is reading.
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ball
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response 91 of 123:
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Dec 10 11:06 UTC 2000 |
Re #90: I'd noticed it was pretty quiet in here, but I do
check it periodically for signs of life. Perhaps I should
start asking some dumb questions in an attempt to kick-
start some threads of conversation? =o)
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xdiablox
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response 92 of 123:
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Jan 28 15:51 UTC 2001 |
hi y people im new give us some help danieldunford@yahoo.co.uk cheers
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gull
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response 93 of 123:
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Jan 28 16:57 UTC 2001 |
Could you be more specific? ;)
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frzngost
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response 94 of 123:
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Apr 7 04:17 UTC 2001 |
Hello from Jim VE9MHZ /VE3 in Morrisburg, Ontario Canada
My packet HomeBBS is KA2JXI.#NNY.N.USA.NA in Ogdensburg, New York
My AMPRnet Gateway-BBS is mtlgw.ampr.org
73 and hope to chat with you later
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eprom
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response 95 of 123:
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Apr 8 23:37 UTC 2001 |
re#94 Cool callsign! :)
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goose
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response 96 of 123:
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Apr 11 17:32 UTC 2001 |
Welcome Jim.
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sadko
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response 97 of 123:
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Feb 27 22:55 UTC 2002 |
Hi ev erybody. Ham call is kd2nw Tom from NY Can somebody give me some
hints on setting up my computer and rig for PSK?
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goose
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response 98 of 123:
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Mar 20 15:24 UTC 2002 |
Sure, get a good interface (build or buy, I built) don't overdrive the input
of your rig, and use lower power (I use 50w on my 100w rig) due to the high
duty cycle of psk31.
See you on 20M!
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goose
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response 99 of 123:
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Mar 21 19:32 UTC 2002 |
Just a little self serving info: Two weeks ago I passed my Extra Class upgrade
exam! I've also applied for a vanity call and hopefully will be K8MZO in
short order.
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