From: gerry@bluemoon.use.com (Gerard M. Foley)
Subject: Modulation and SCPC
Organization: Blue Moon BBS ((614) 868-998[024])
Date: Sun, 15 Nov 92 08:05:28 EST

Sorry if this is a second posting of this material, but I
can't find a record of sending it:
    This may be old hat to many, and may have mistakes, but some

of those who want to tune in odd things on satellites may be

helped by it.

    What we receive from satellites, or from ordinary local TV

and radio stations is electromagnetic waves.  When these hit

an antenna, whether rabbit ears or the antenna part of an LNB,

they cause the electrons in the antenna to move, and the motions

of the electrons can be amplified, that is, made to cause much

larger quantities of electrons to move in the same patterns as

the original tiny motions in the antenna.

    These motions are called "currents", and the motive force

for the motions is called "voltage".  In many cases the voltage

and current are proportional to one another.  One way we use

the voltage or current is as "signals" (we also use them to 

light our electric lamps and turn our electric motors).

The earliest case of using electric currents as signals was in

the wired telegraph, and the way it was done was by causing current

to flow in short or long pulses, or to cease to flow in such pulses,

and agreeing that if we had a short pulse followed by a short gap

followed by a long pulse followed by a long gap it would mean the

letter "A", while if we had a short pulse followed by a long gap

it would mean "E" and so on.

    The next step was to recognize that the vibrations of the air

that we hear as sounds could be made to move a metal diaphragm

which could be connected mechanically to something that would

cause a current to change at the roughly the same rate and 

magnitude as the air pressure, and that this current could cause

another distant something to make a distant diaphragm move the air 

near someone else's ear to make a sound similar to the one at

the beginning.  Alexander Graham Bell said "Watson, come here,

I need you".

    Another step was to recognize that the brightness of a scene

in the focal plane of a camera could be encoded into variations of

current which could cause the brightness to be reproduced at a

distant place.

    The first use of radio waves to carry signals was to use them

as the telegrapher used current, to turn the wave on and off in

a coded fashion (and in fact to use the very same code as was used

in Europe for the telegraph.   Later we generated very pure waves

(that is, of one and only one wavelength and frequency) and caused

their strength to vary in synchrony with the currents we used for

telephony.   To recover the signal, we threw half the signal away

in a rectifier and "filtered" the bits that were left by using them

to charge a capacitor.  The mode is called amplitude modulation, and

is used for most radio communication on frequencies below 30MHz,

and above 30MHz for the video part of all broadcast television.

The process of using a signal of one kind to affect a medium of

another kind to send a recoverable message (including picture or

what have you) is "modulation".  In amplitude modulation, the

strength of the signal is used to change the strength of the

radio wave, also called the "carrier wave", and the signal is

recovered by some gadgetry that will change the strength of the

radio wave back to current or voltage that can be used to move

the cone of a loudspeaker or to control the brightness of the

light on the screen of a television tube.

     There are now lots of other ways which are sometimes used

to modulate radio waves, but one of the most popular is "frequency

modulation".  Here, instead of having the carrier wave constant

in frequency and variable in strength, the strength is constant

and the frequency is varied.  If the signal frequency is high,

the carrier frequency is made to go up and down at the same high

rate.  If the signal is loud, the carrier frequency is changed a lot;

if the signal is soft, the carrier frequency is not changed very

much.  An immediate advantage of this is that if loud bursts of

radio wave, "static" from thunderstorms for instance, get mixed

in with the radio signal between the transmitter and the receiver,

at the receiver you can clip off everything above some level and

still be able to recover the speech or music or picture signal by

interpreting the variations in frequency properly.  In fact, 

because of phenomena too complicated to go into here, you can

reduce the importance of all extraneous noise, weak or strong, 

added into the radio signal between transmitter and receiver 

if you choose the method of frequency modulation wisely.

     This is not done for free.  Even though the carrier wave

used in amplitude modulation radio is very clean and stable,

the very process of modulating it to carry audio or video

makes the resulting radio signal have many frequencies above

and below that of the carrier wave, so that the space taken

up by the radio or TV signal on the dial is approximately

twice the highest frequency in the modulating signal that 

contains the audio or video you are trying to transmit.

To get the noise reduction of frequency modulation, you 

have to change the carrier frequency, when sending the 

loudest audio or strongest video, several times as much 

as the highest audio or video frequency.

      A trick that works pretty well for amplitude modulation is,

after forming the signal that is twice as wide as it needs to be, 

cut off half of it and also cut off the carrier wave.  Then send

that, a single sideband signal, and at the receiver put back a

carrier wave close enough to the frequency of the original carrier

so that you can reconstruct the original signal.  If that won't

do, cut off most of one half of the original modulated signal

and send the remainder, the carrier and the other half.  If

you're reasonably careful, you won't miss the part that wasn't 

transmitted.  This is the way amplitude modulated television is

transmitted.

      To put numbers on its, most AM entertainment radio signals 

take up five or ten kilohertz on the dial.  FM entertainment audio 

takes up most of 200 kilohertz, which is the channel spacing.      

The AM part of the broadcast video signal is nearly 4000 kilohertz

wide, but you want sound too, so the whole NTSC with FM sound 

uses almost all of the 6000 kilohertz (6 MHz) channel it is given.

      Because satellite transponders were allowed to use a pretty

big bandwidth, and because they needed a pretty big bandwidth to

make up for the low power that the satellite downlink transponders

can use (less than 20 watts, compared with the tens of thousands

of watts TV broadcast stations use) and the 20000 mile distance

to and from the satellite, each satellite gets 400 MHz, which it

can use twice, once in each polarization, so the (C-band) channels

are over 30 MHz wide.  This means that they can use a modulating

signal a lot like the one that is used for broadcasting (including

an FM audio channel or two or three ..) and use that to frequency

modulate the uplink and downlink video channels.  Your TVRO

receiver converts the downlink frequency modulated signal into

a video signal (part of the "baseband" signal out of the set) 

that is just right for your monitor, or which can amplitude 

modulate a channel 2, 3 or 4 carrier to go to any TV set.

 

    


gerry@bluemoon.rn  My amateur radio callsign is K8EF, and my
packet radio address is K8EF@W8CQK.OH.USA.NA
Newsgroups: rec.video.satellite