die@hydra.UUCP (Dave Emery) (03/14/85)
--- Following is retransmission of an item sent a few weeks ago that may
___ not have be posted correctly to the net due to system problem.
Some comments on SCA monitoring.
Equipment
First, I have had good luck with a SCA adapter made by
a company called Communications Poly Services, Box 3251, Westford
Mass, 01886. Their model is a SCA-3.
By the way, FM stations also use 91 khz
subcarriers (the FCC currently allows anything between about 60 and
100 khz, I forget the exact limits), so a tunable means of looking at
subcarriers with some selectivity so it can handle multiple subcarriers on
the same main carrier is very desirable.
Main Channel Audio Bleed into SCA channel
Unless the tuner any SCA adapter is used with has very good
IF filters with nearly constant group delay across their bandpass, the
main carrier modulation will cause phase modulation of the subcarrier to some
degree. (This is in fact the principle behind many group delay measuring
systems). This causes some unaviodable bleed of main channel audio
into the SCA audio. The only cure for the problem is better IF filters
(or better alignment of the ones you have)
I am told that the professional SCA tuners
built for Muzak and other audio services use special ceramic IF filters
that have almost an order of magnitude flatter group delay characteristic
than the cheap Japanese filters that almost all (even high end) FM tuners
seem to use.
I suppose that someone who is good at designing filters could design
a phase compensating network given measured characteristics of an IF system,
but for demodulating data it isn't really necessary to suppress main
channel audio that many db. (I, for one, cannot stand Muzak so being
able to listen to it completely free of main channel bleed is not a priority).
I have heard that many Muzak services use compressor/expandor systems
a la Dolby or DBX which have the effect of suppressing perceived main audio
bleed much further. (If you intend to actually listen to SCA programming
you might try to find out if it is companded and if so re-expand it's
dynamic range at your end - it will probably sound much better)
Actual SCA signals.
I have observed the following in the Boston area:
More or less standard Audio:
1. Muzak (some 10-15 channels of supermarket, store, and restaurant
music, some almost listenable, some obnoxious)
At least two stations carry the same exact feed (I think of real
MUZAK) in very close time synchronism, I suspect that
they get it from a satellite feed.
2. Readings for the blind of daily newpapers and magazines. (Three
different stations carry this)
3. Audio of sister AM stations (a technique used to piggyback
audio over a studio-transmitter link to a common transmitter site).
4. Physician's Radio Network (Will Martin's comments apply, I can't
even interest my medical student wife).
5. NBC radio network feed (seen some years ago) right off the wire
cue beeps and all.
6. Educational programing from local community colleges.
7. Audio of carrier current college AM station being distributed to
power lines in dorms (seen some years ago) on sister FM station.
Non Audio signals:
1. Transmitter remote control and monitoring (in use for many years)
Older systems transmit continuous .5-3 khz tone as FM on 67 khz
subcarrier, tone's exact frequency represents meter reading.
Newer systems transmit sequences of ASCII characters using
start stop transmission on an FSK'd tone FM modulated on a
67 or 91 khz subcarrier (103 modem tones are
sometimes used I believe since the chips are so widely available).
What one sees on the FM transmitter output is usually telemetry
reporting transmitter parameters (plate current, VSWR etc) back
to the studio, by using the FM signal to do this the station
can aviod having to pay for a telephone line. The command link
to the transmitter is completed through a subcarrier on the
studio transmitter link (which usually gets filtered out
before the signal is fed to the transmitter)
2. Dow Jones Pocket Quote stock qoutes. This signal is 4800 baud
asynchronous (start stop) transmission direct keyed (FSK) onto
a 67 khz subcarrier with a shift of about 10 khz from mark to
space. Mark is low and space high in frequency, the signal
uses 8 information bits and a parity bit, and a stop bit. The
qoute data is in a binary format that can be deciphered with
a little effort, a variant of standard ticker symbols are used
with prices in packed binary. I understand this service is
or was offered in more than 100 cities. I am not sure whether
the prices are delayed 15 minutes as is the standard for broadcast
tickers or are live.
3. A direct 67 khz FSK keyed 9600 baud data stream carried by a
local university radio station, that I am told contains a
dial up bulletin board. Students can call the radio station
and post notices which get broadcast every so often on the
9600 baud feed.
4. Paging services. I have been told that there are at least
a couple of SCA subcarriers used for paging. I do not know
what exact format the paging is in (whether it's start stop
transmission of characters (using a UART or equivalent)
or long strings of bits (using dedicated LSI)).
5. Remote control of electric utility loads. The local power
companies are experimenting with controlling their off-peak
loads (such as cheap-rate water heaters) with SCA subcarriers.
Currently two stations carry the 1070/1270 hz FSK 300 baud
control commands as audio tones on a 67 khz subcarrier.
So far unidentified (I'm lazy)
6. FSK audio signal (known to be such from spectrum plot) carried
as fm modulation on 67 khz subcarrier. Apparently frequency
shifted from about 3khz to about 5.5 khz this carries 1 to
5 second bursts of what seems to be 1200 baud keying. I don't
know what format the data is in or what information is carried.
Signals which I have heard or read are transmitted on SCA (but haven't
actually seen or yet demodulated)
1. Commodity news service (CNS) frequency division multiplexed
multi-channel low speed baudot commodity wires. This
signal, which is in a format similar to that used to transmit
low speed newswires to radio stations and newspapers for many
years (since the mid 60's) consists of about 24 subcarriers
at 120 hz intervals starting at a c.f. of 420 hz. Each subcarrier
is FSK'd with a frequency shift of 60 hz from mark to space.
Most of the CNS wires (there are different ones for different
products) are transmitted at 56.88 baud in 5 unit baudot code.
There are both 45.45 baud baudot and 74.2 baud baudot circuits
on the CNS mux however.
2. Commodity pocket quote system. This is a similar system to
the stock pocket qoute. A calculator size device picks up
a SCA subcarrier and displays selected qoutes on a LCD
display. I do not know if the format is similar to the
stock qoute system (4800 baud direct FSK of a SCA subcarrier).
3. Dow Jones news stories. Some years back I saw ads for a
Dow Jones service that would supply your home computer with
late breaking financial news in various catagories (you could
select). I was never able to actually see the SCA transmission
for this service in the Boston area, but from the text of the
ad it was fairly obvious that they were using SCA. More
recently this kind of service has been supplied on a dial
up basis by Dow Jones, and by Rueters through a spread
spectrum transmission on a satellite that allows use of 2-3 foot
dishes.
4. UPI and other wire services. A recent story about UPI's data
communications said that they were having trouble with their
satellite (they also use spread spectrum) distribution system
in certain large cities where there are many 3.7-4.2 ghz
terrestrial systems in use, and that they were going to use
SCA to distribute wire services. I do not know what format they
are using (could be similar to CNS) and would greatly appreciate
hearing from anyone who knows of any actual wire service
SCA transmission anywhere (I'm writing a usenet story on picking
up wire services). [I know of a local FM station which has been
thinking about using a 91 khz subcarrier to relay their AP
service from their transmitter and studio site to a remote
news studio to save money on telephone lines.]
5. Nationwide paging. There has been much talk of setting up a
nationwide paging system that would use SCA subcarriers fed
from a satellite downlink to simultaniously page people over
very wide areas. Paging receivers would look through their
local FM spectrum to find the station carrying the paging. At
one time PBS was going to rescue themselves from Reagan's
financial oblivion by selling this service through all the
National Public Radio outlets. I believe I have read that
while this fell through, someone else has tried to
set this network up.
6. Software distribution. There has been talk on and off about
broadcasting home computer software over SCA subcarriers. I have
read about experiments involving transmission of programs
on telephone request (you call up and ask for a copy of
Zorc or whatever and they send it - possibly encrypted -
over the subcarrier.) I think the idea is that it is a
lot more efficiant to send a program at 4800 or even 9600 baud
through cheap SCA hardware than 300 baud over phone lines.
7. Electronic mail. It has also been suggested (and supposedly
tested) that SCA subcarriers could be used as a distribution
medium for electronic mail. I would hope that encryption
would be used .
8. 32 Kb data. The FCC has just recently authorized transmission
of data on SCA subcarriers at 32 kb (the old maximum rate was
9600). I do not know if FSK or QPSK modulation is to be used.
I have heard that the new digital satellite terminals used for
distribution of most of the major radio network feeds explicitly
support several 32 kb data channels, and that there is talk
of a common carrier like data transission service (with all
sorts of things multiplexed together a la SSS on WTBS) which
would carry a variety of feeds for different nationwide user
communities.
Some Audio services heard of but not heard ...
1. Traffic reports broadcast to commuters in their cars.
2. Commodity prices for farmers.
3. Financial news and commentary.
4. A dentist's office background music and information service
And if you thought this was enough, has anybody looked at
the subcarriers on the sound carriers of their local TV stations ?
TV stations are allowed, just as FM stations can, to transmit
various narrowcast services on subcarriers on their sound carriers.
Much of the available spectrum will go to the new stereo sound signal
and the secondary audio (bilingual signal), but there is space
reserved for other uses.
The principle use for TV subcarriers I have observed is cue channel
audio for Electronic News Gathering (ENG) teams. The director in the
studio talka to the "talent" over this link since most of the
microwave systems used to relay the video are one way only and there is
therefore no backchannel (except VHF/UHF radio). Many of these ENG
subcarriers are at 40 khz or 55-60 khz (pre stereo).
Another use of TV station subcarriers is transmitter telemetry.
For years some of the Boston area stations transmitted a 40 khz FM subcarrier
carrying a FSK modem signal modulated by a repeating pattern of
bits (similar to satellite PCM telemetry formats). Yet another use
of such subcarriers is paging of station employees, I beleive that
both analog and digital transmission has been used for this purpose on TV
sound subcarriers.
Tapping directly off the discriminator on the FM audio chip in a VCR
might be good way to pick off TV sound subcarriers - TV's have all that
horizontal sweep energy floating around waiting to get coupled into your
receiver so it might be difficult to get enough signal to sweep harmonic
ratio to reliably receive the transmission. (Purists may note that
getting a good SNR on any intercarrier sound system requires care - the
new stereo TV's use various tricks to solve the problem but a random
tv or vcr may not supress harmonics of 15734 hz very far on it's
discriminator output)
--
David I. Emery Charles River Data Systems 617-626-1102
983 Concord St., Framingham, MA 01701.uucp: decvax!frog!die