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1994-09-28
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FAQ: SINGLE CHANNEL PER CARRIER AUDIO
Date: Mon, 8 Nov 1993 22:39:00 EST
Reply-To: HOMESAT - Home Satellite Technology
Sender: HOMESAT - Home Satellite Technology
From: Gary Bourgois
Subject: Re: scpc faq sheets
SCPC, or Single Channel Per Carrier audio reception is considered by some to
be arcane voodoo, the reception of which requires a Frankenstein type
laboratory of spinning wheels and spark gap devices. Indeed satellite audio
of the more "normal" subcarrier variety is virtually unknown and certainly
unappreciated in many circles, so something that requires a certain amount of
tinkering and skill gets left by the wayside, and thus many people lose out
on some fascinating and valuable audio program opportunities.
To this end, this SCPC FAQ will deal with as much of the why and how of SCPC
reception as is possible, and will include a regularly updated listing of
signals available to the SCPC listener.
SCPC: HOW AND WHY
SCPC audio signals exist because they meet a number of specific needs to
their intended users: the Radio Broadcasters. At one time, SCPC was the
primary satellite distribution method for network audio feeds. In recent
years, the larger networks such as CBS, Mututal, NBC, Westwood One and
others, have gone to a DIGITAL system called DATS and SEDATS (Digital Audio
Transmission System and Spectrum Efficient Digital Audio Transmission System)
At this time, there is no hobby or consumer receivers for these digital
transmissions, but due to the high costs involved, most regional networks, as
well as smaller national networks continue to use the more economical SCPC
method and for good reason.
SCPC has one very large advantage over standard subcarrier audio, as well as
FM^2 audio. In both later cases, the audio signals MUST be uplinked from a
common point. Thus if a broadcaster uses standard subcarrier, he has to
deliver his signal to the uplink, which may be hundreds of miles away. To do
this with any signal quality requires costly digital phone lines. And indeed
SOME subcarrier audio services are actually turned around from SCPC signals!
The advantage of SCPC can be derived from its very name. SINGLE CHANNEL PER
CARRIER. Each audio channel is uplinked separately, and each from its own
location. One transponder can have over 100 different SCPC carriers each
uplinked from its own location anywhere in the country. This is a big
tactical advantage.
This is an approximate spectrum plot of the 70 MHz IF output of a
receiver tuned to a transponder carrying video. (This ASCII drawing was done
by Bob Hoffman of Pitt Computer Services a contributer to the
rec.video.satellite / HOMESAT newsgroup):
|
|
| ---------
| / \
| / \
| / \
| / \
| / \
| / \
| / \
| / \
| / \
| / \
|-------------- ----------------
|
+-----------------------------|-----------------------------|
50 MHz 70 Mhz 90 MHz
There is one very-wide-band signal that contains the video information
as well as all of the subcarriers.
Now here is that same 70 MHz IF output when tuned to a transponder with
SCPC signals:
|
|
| I I I I
| I I I I I
| I I I I I I I I
| I I I I I I I I I
| I I I I I I I I I I I I
| I I I I I I I I I I I I
| I I I I I I I I I I I I I
| I I I I I I I I I I I I I I
| I I I I I I I I I I I I I I
| I I I I I I I I I I I I I I
| I I I I I I I I I I I I I I
|___I___I___I___I___I___I___I___I___I___I___I___I___I___I___
|
+-----------------------------|-----------------------------|
50 MHz 70 Mhz 90 MHz
Each of the columns of "I"s is a separate narrow-band signal. They
are not necessarily evenly spaced nor are they all the same strength
or bandwidth.
In the broadcast environment, the radio station uses a commercial SCPC
receiver, which either has a fixed frequency, or is frequency agile and tuned
with thumbwheels or other simple tuning methods.
Broadcast SCPC receivers are referenced to 70 mhz, since the original SCPC
receivers (and most still in use) operate from a 70 mhz block. This goes
back to the late 1970's when LNA technology was all we had.
ALL SCPC signals that can be received exist for broadcast reasons. One major
reason that stations such as WJR Detroit, WGN Chicago, etc are on SCPC 24
hours a day is not to provide a listening service, but because the stations
are key stations for SPORTS TEAMS. Football, Baseball, etc. on state
networks use these home stations as the source for the games that are carried
through out the state. When the games are not being played, the regular
station continues to be broadcast on the SCPC as a convienience to make sure
the radio station is tuned to the proper frequency and that their reception
equipment is working. The bonus for the TVRO SCPC listener is that we get to
hear local radio from all over the country, much like the old time AM RADIO
listeners used to hunch over their radios late at night to hear distant
stations. The advantage WE dish owners have is that we can listen Day or
Night, and we get FM broadcast quality.
SCPC signals are found on specific satellite locations. These are mainly:
GALAXY 2 - Sports and talk networks, some music networks. Many Major
Market radio stations up there for sports broadcasts.
GALAXY 4 - National Public Radio (Many channels) Various state
public radio networks. BBC, Reading services for the
sight impaired, Some sports and talk networks, Religious
networks, 2 Channels of Chinese radio, La Fiesta Mexicana.
SATCOM C5 - Music services, some Sports. SCPC on C5 is rather
scattered. C5 is the location of all the major DATS
and SEDATS digital radio services.
TEACHING YOUR SATELLITE RECEIVER NEW TRICKS
There are a number of ways you can receive SCPC with your home dish. The
first time I did it, the total investment was $3 that I spent on a TV band
radio that I bought at a garage sale.
If your receiver has a 70 mhz loop (It will be clearly marked in the back of
your receiver, and will have a short length of wire going from one F
connector to another) you can use the cheapo method to tinker with SCPC and
get a feel for what is up there.
All you need to do is split the loop with an F splitter (such as used to
connect two TV sets to one cable) and run one of the outputs to a TV band
radio (Such as the Radio Shack Portavision 40) that tunes the AUDIO portion
of the VHF television band.
Next you must put your Satellite receiver on a channel carrying SCPC signals.
Two good ones to try are: G4/03 or G2/03 as these are the most
populated SCPC channels there are.
Put the TV band radio on the TV-1 Band which has channels 2 to 6 on it. Use
the analogue tuning dial to sweep slowly from channel 2 to channel 6. You
should start hearing audio signals right away. If you live where there are a
LOT of FM stations, and your TV radio is a cheap one, it is possible you
might get some "Bleedover" from the FM band, but the SCPC signals will be
easy to identify since their audio quality will be different, and the volume
will also be a bit lower than regular FM stations. You may also notice some
hum on the SCPC signals. This is a function of your Satellite Receiver's
Power Supply, and there is not much you can do about it. It will be more
noticable on narrower signals with lower volume because you will have to turn
the audio up on the TV radio, and this will increase the hum level. Thus
this is not a perfect method, but will provide some interesting listening
within its limitations.
70 Mhz Loop type receivers are still made, but most manufacturers have gone
to higher loop frequencies. Drake uses 140 mhz, Trackers use something in
the 400 mhz range. These will not work with a TV band radio.
Receivers that work well on the 70 mhz loop include older units like the
UNIDEN UST-5000, 6000, 7000 series. These receivers are available on the
used market for $70-$100, and if you buy one of these receivers AND a TV band
radio, you have a very inexpensive SCPC setup.
THE SCANNER METHOD
Another method that works, though does not provide perfect audio quality is
to use a scanner radio that will tune the IF loop of your particular
receiver. One thing you must make note of though is that the scanner (public
service police/fire type receiver) has WIDEBAND FM capability. Otherwise the
signals you receive will be terribly distorted. Scanners that work well
include the Radio Shack PRO-2006, AOR and YPITERU scanners. To tune the
audio you must use the scanning up and down mode which is not as handy as
using a tuning knob, but will work.
DEDICATED SCPC RECEIVERS
If you get serious about SCPC, you might want to consider purchasing a
dedicated receiver made for SCPC reception. Here you will spend a bit of
money, over $400, but if you have a particular type of programming you enjoy,
such as a favorite out of state sports team, talk network, or State Public
Radio network, the cost will be worth it. All these units hook directly to
your LNB, and the first three have a built in splitter. In the case of the
ICOM R-100, you will need a 950-1450 block splitter with a DC BLOCK (You MUST
use a DC block or you will damage your receiver).
While there is no perfect SCPC receiver for the TVRO home user, these units
come close:
HEIL SC-ONE
The Heil is the first home SCPC receiver ever made. Designed and built by
noted audio engineer Bob Heil, the SC-One is a simple but useful receiver for
general SCPC listening. It has ONE bandwidth of 180khz. Tuning is
accomplished by a gear driven mechanical knob. The unit has a very home made
amateur project look to it, but it works well. Cost: $400.
UNIVERSAL 100
This unit has a digital readout, and tunes with up and down buttons. It
has 50 memories for storing signal locations. HOWEVER it must be noted that
the digital readout is NOT a frequency readout, but merely a four digit
reference number. The memories are not useful for actually storing
individual stations, because the AFC is so robust it grabs whatever station
it wants to. This also causes problems tuning weaker signals near strong
ones. The unit we tested had a hum problem and a microphonic problem (it
made a thunk when you tapped the top of it). Cost: $450
UNIVERSAL 300
A much improved version of the model 100, the 300 has actual frequency
readout, multiple bandwidths and signal processing to yield very professional
results. We have not been able to test this unit, but the specs look good.
The price is a bit out of range for most people though, at $1200.
ICOM R-100
My personal favorite from a cost/benefit standpoint. In addition to
receiving SCPC, the ICOM also has other uses. It tunes continuously from
100kHz to 1800MHz, which means it will tune ANY loop frequency, or direct
from the LNB. It also tunes SHORTWAVE, Public Service Scanner frequencies,
Cellular and cordless phones, Local AM/FM, VHF/UHF TV, and everything else.
ON SCPC the unit has very good audio. It has direct frequency readout. It
has 121 memories, and WILL return to the exact station you have programmed
in, taking into account the fact that you will need a stable LNB or stable
outside temperatures. Otherwise you may have to slightly retune, which is
not a major problem. The Icom is the smallest of all the units, and requires
a 12 volt power supply. It has two bandwidths, but only the WIDE one is
really suitable for SCPC reception. The bandwidth of 150 mhz, is slightly
less than the Heil or Universal, and thus is a bit quieter. In practice,
reception of most SCPC signals on G2 and G4 is excellent. The cost is
reasonable if you take into account that it receives a lot more than SCPC (It
can also tune FM^2 and regular Subcarrier audio, in addition to the entire RF
spectrum) Cost: $600.
SCPC ANOMOLIES
SCPC reception is, as we said, not perfect. And there are a few problems you
will notice. The major one is LNB DRIFT. LNB's were made for video carrier
reception, where a drift of a couple hundred kiloHerz is no big deal and will
not be noticed. On SCPC, such drift will put the signal right out of the
bandpass, and it will vanish. In actual listening, in warmer climates or in
the summer months, this can be a problem requiring constant retuning of the
signal. This can be a real deterent to enjoyable reception.
There are a number of solutions. The best is to purchase a Phase Lock Loop
LNB. These are not cheap. Figure on paying three times for such an LNB as
you would pay for a non phase lock loop unit. Again, if you REALLY want to
listen to SCPC, this might be worth saving for.
If you are fortunate enough to live in an area where temperatures do not vary
much over the day, or remain below freezing during much of the year (such as
the land that time forgot, where I am listening to SCPC as I write this), or
if you tend to tune around a lot, you won't notice the drift.
The signals on SCPC channels are audio processed with various methods of
noise reduction, and when listening to the "raw" signal it appears to be a
bit "bright" and also is compressed. The effect is much like listening to a
DOLBY C encoded cassette on a NON DOLBY playback machine. This effect can be
minimized by adjusting your tone controls to eliminate the brightness and
hiss. Real audio purists may actually add a DBX expander (you may have to
modify the time constant to make it sound right), but for most listening this
is not really necessary.
SCPC LOG. Available stations on SCPC.
--
/ Gary Bourgois, WB8EOH, The Birdwatcher: Marquette Michigan USA \
([-o ~ Tune FRIDAY NIGHT LIVE! S3/21 5.8 Fridays from 9-midnight ~ o-])
\ tvro@lopez.marquette.mi.us or flash@lopez.marquette.MI.US OR: /
(GENIE: BIRDWATCHER)*(MCIMAIL: STUDIOB)*(FAX: [906] 228-7477)*(BBS: 228-4399)
