Why isn't FM video used?

Better look into how the L-R signal is modulated at any FM station in the USA.

"Lord Garth"

It still frequency modulates the carrier, but above the audio band.

........ Phil

It would use too much bandwidth/spectrum.

John

What comes down from the birds? QPSK? QAM? DPSK? Thats the same stuff they send down the cable right? MPEG compressed digital video and audio, 4 or 5 megabits a sec?

In the old days, it was FM.

All those "build your own satellite receivers" of 20 years ago had wideband discriminators of some kind, it was kind of amusing that they could have an IF of 70MHz and still have suitable selectivity.

Some people playing with amateur radio tv have played with FM for the video. Though, nobody even considered it until about 15 or 20 years ago, after monitors were common so you'd just build up a receiver from scratch. When regular TVs were the only game in town, they never looked beyond AM for the video.

Michael

Simple answer: FM is wasteful of bandwidth with other things equal. The video signal needs about 3-1/2Mhz of bandwidh. It would be difficult to get this with FM unless the channel spacing was maybe 10 to 20MHz depending on modulation index.

Also note that digital modulation is also AM where the phase also carries information. There are many modulation schemes such as QAM, etc. but they all use AM as the backbone. The idea is to cram as many bits into a Hertz of bandwidth as possible. This is done by utilizing both the amplitude and phase in the modulation scheme. Bob

What if the WMV [Windows Media Video] digital compression is used and the color resolution is decrease sufficiently?

Is it possible for digital modulation to use FM as a backbone? Has this ever been done?

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You can use one form of Modulation (digital) to modulate another form of modulation (fm)

I think you have not digested what the person said about QAM above.

Depends on what you want to do will determine what kind of backbone stystem you would use.

By "digital" you mean PCM?

QAM uses two AM carriers that are 90 degrees out-of-phase with each other.

Does PCM have to have an AM component? Can't PCM have an FM component instead?

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I used digital as you used digital above.

It went over your head. You asked if you can use digital modulation on a frequemcy modulation system. QAM can be considered as a form of digital modulation. So can FSK as someone else has pointed out. FSK answers your question about FM being used.

As for video and FM, FM is often used in fiber optics for video signals, and there are products that I know of for security cameras that transmit video via FM links. I suggest you try GOOGLE.

stystem

PCM is a digital representation of an ANALOG signal. You are confusing your terms here.

It doesn't need to be.

Graham

So, in your words, what are the various "components" of AM, FM, and PCM ?

Now, what is the degree of orthogonality of those components?

Later...

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Most digital modulation schemes use phase as a component of the modulation as well as amplitude. Phase Modulation and Frequency Modulation, FM are related. One is the differential of the other. Think of it this way: If you change the phase of a wave, you instantaneously compress or expand the wave which is a short term change in frequency. So, in a sense, Frequency Modulation is a part of many digital schemes. Howerver, except in rudimentary forms like FSK (frequency shift keying), true FM is not used. Again it comes down to cramming as many bits of information into each Hertz of bandwidth as possible and Phase Modulation has a defintie edge over FM. There are complicated mathematical reasons for this.

To answer your question, it would be possible to combine AM and FM to form a digital constellation of bit values but I don't know of any commercial scheme that does this. I suspect that communication engineers have tried everything and picked what works best. Bob

Beleive it or not, the French use AM for the TV's audio carrier, while using FM for the video carrier. Broadcast System L

I think they are phasing this system-L out

Not so; FM is used only for the chroma components, not the luminance (Y) signal (which is what lives on the "video" carrier), and only one chroma signal is sent per line (that's the whole SECAM thing). FM's still a bandwidth hog, and can't be used for the full-bandwidth Y signal in any analog TV broadcast system.

Bob M.