Anyone know about Television Broadcast? AM for video, FM for audio

[ski]

Shameless homework question that might be answered by knowledge greater than mine:

Why is FM used for audio, and AM for video?

I thought AM can only go up to 10kHz, which can't cover half the audible range (which is why AM only has talk radio shows), so that might be why FM is used for sound... but why is AM used for video? :confused:

[ialsohaveadream]

Ummmm....I majored in TV production, and unless my memory is TERRIBLE, video doesn't go out over the AM band. It's a whole separate band: Ultra High Frequency (UHF) or Very High Frequency (VHF).

[hapoo]

Maybe he means the modulation is AM?

TV uses vestitigial sideband AM for modulation, as to why... google it :)


It was probably used cause AM compresses information much more than FM and since video needs a lot of spectrum, this was the only way to make it all fit.

[DaFunkyUnit]

hint hint:

the receivers.

;)

[hapoo]

hint hint:

the receivers.

;)

Would that be refering to the fact that it has to be decoded by a sweeping CRT?

[ialsohaveadream]

Maybe he means the modulation is AM?


D'oh! I was just thinking spectrum. :)

[InfiniteNothing]

I had no idea so I looked it up.


A typical TV signal as described above requires 4 MHz of bandwidth. By the time you add in sound, something called a vestigial sideband and a little buffer space, a TV signal requires 6 MHz of bandwidth. Therefore, the FCC allocated three bands of frequencies in the radio spectrum, chopped into 6-MHz slices, to accommodate TV channels:
54 to 88 MHz for channels 2 to 6
174 to 216 MHz for channels 7 through 13
470 to 890 MHz for UHF channels 14 through 83
The composite TV signal described in the previous sections can be broadcast to your house on any available channel. The composite video signal is amplitude-modulated into the appropriate frequency, and then the sound is frequency-modulated (+/- 25 KHz) as a separate signal, like this: How stuff works (http://stuffo.howstuffworks.com/tv12.htm)

[Kevster]

I had no idea so I looked it up.

How stuff works (http://stuffo.howstuffworks.com/tv12.htm)

Thanks for that. It has been a while for me since I worked on those kinds of transmission systems. Most of the FDM transmission gear I work with now is in the ~190-197 THz range.

[dsuds]

AM simply mixes the carrier frequency and the signal together to form the broadcasted signal. The bandwidth range is +/- maximum signal frequency (IIRC). The signal is greatly influenced by atmospheric noise, lightning, etc. as this noise changes the amplitude of the signal.

FM changes the frequency of the carrier signal. I can't remember all of the specifics but FM requires much more bandwidth. 200KHz is required for each FM radio station to broadcast resonable fidelity audio. FM is not as affected by noise because noise changes the amplitude, not the frequency of the signal. FM chops the top & bottom off the signal to eliminate most of the noise.

TV is a mad conglomeration of technologies designed to eliminate backwards compatibility issues. Originally TV was mono, B&W, and defined in a 6MHz bandwidth. When color came around, it was determined that to keep the same bandwidth and still work with older sets, the video signal would be separated into luminance & chrominance signals and phase mixed (I'm not sure of the exacts here as it's been a LONG time since I went over this in college electronics). When closed captioning was added, it was again do to provide the most backwards compatibility. This signal was placed in the horizontal blanking - the dead time when the electron beam returns to the left-hand side to start another scan line. And I don't even know where they put the stereo & 5.1 signals in broadcast TV as they did not have those technologies when I was in college.

[WhiskeyPapa]

We've answered "how", but have we answered "why?" That was Ski's question.

FM certainly can be used for video, even within the 6MHz bandwidth restriction (I believe frequency modulation is used for many fiber optic video transmission systems.) I would say the reason amplitude modulation is used for broadcast video is because of the cost of components back in the 20's and 30's when television was being developed. FM broadcast systems almost always use more complex circuitry than AM systems. Combine that with the need for backward compatibility, and you have your answer.

[dsuds]

I seriously doubt 6MHz would be even a fraction of the bandwidth needed for video using a frequency modulated signal. Think about it... 200KHz bandwidth is needed to transmit a simple 10kHz signal (I know it's stereo now, but mono takes the same bandwidth). Video is easily more than 300kHz worth of signal (6MHz / 200KHz = 30, 30 x 10KHz = 300Khz).

In my opinion, it took the advent of digital TV broadcasts (DirecTV, Dish, etc.) to redefine how TV should be done. All of the old stuff was just an awful mess of patches and workarounds.

[DaFunkyUnit]

We've answered "how", but have we answered "why?" That was Ski's question.

FM certainly can be used for video, even within the 6MHz bandwidth restriction (I believe frequency modulation is used for many fiber optic video transmission systems.) I would say the reason amplitude modulation is used for broadcast video is because of the cost of components back in the 20's and 30's when television was being developed. FM broadcast systems almost always use more complex circuitry than AM systems. Combine that with the need for backward compatibility, and you have your answer.

bingo. from a marketing standpoint, you want cheap receivers, so that everyone will jump on your platform bandwagon.

[dsuds]

Some of this is also related to when the different technologies were developed. The first TV sets were already in operation in the late 1920 & early 1930's. The first test of FM radio did not occur until 1934. By then the basic standards for TV had already shaken themselves out. By 1939, when the 1st FM radio stations went on-air, there were already over 20 TV stations.

The backwards compatibility issue is one the FCC has been fighting for a LONG time. This is something that was not taken lightly as the costs of televisions back then was very high. So it was decided that all of the upgrades to TV would have to be backwards compatible. We still see those compatibilities today. You can still use an old tube B&W TV, although why anyone would want to is beyond me.

[WhiskeyPapa]

I seriously doubt 6MHz would be even a fraction of the bandwidth needed for video using a frequency modulated signal. Think about it... 200KHz bandwidth is needed to transmit a simple 10kHz signal Actually, some fiber optic video systems use only 7MHz of bandwith for full-duplex frequency modulated video and audio. Simplex video-only could easily use less than 6MHz.

Also, television audio (as well as VHF and UHF radio communications) use only 5KHz of bandwidth, not 200KHz.

[dsuds]

Analog on fiber? I thought all of the fiber optic stuff was digital using either PCM or PWM. Digital would be a different fish altogether because of compression.

200KHz is the bandwidth allotment for FM radio. I was talking about using FM techniques to modulate the entire TV broadcast. 5KHz sounds about right for the original TV spec. If I remember correctly, the stereo TV sound (L-R) is not in the same place as the mono (L+R).

[welfareloser]

wow. i drool on in idiocy as this thread flies farther and farther over my head...

[special ed] YAY MY FRIENDS ARE SMART YAY SMART FRIENDS YAAAAY! [/special ed]

[WhiskeyPapa]

Hey, Ski! Did you get the right answer?

[ski]

They extended the due date till tomorrow at 8am, so I'm gathering my info right now :) THANKS! :D

[WhiskeyPapa]

Don't forget to tell us what the "right" answer is!