Re: Using an FM-Carrier for the Y [Luminance] Signal -- how to relieve the bandwidth issue?

On Sep 23, 3:02 pm, Brendan Gillatt wrote in

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Okay now you're actualy irritating me quite a LOT.

How so? I am just asking extremely interesting questions about video technology.

Please, do yourself a favour: buy an Amazon book voucher and shut up.

I?ve tried painfully hard to answer my questions doing my own research

-- including reading information from books. I still haven?t found answers to my questions. That is why I am turning to NGs for assistance.

Is Quadrature Frequency Modulation used? What for?

AFAIK, Quadrature Amplitude Modulation uses a combo of AM and Phase-Shift-Keying.

So QFM would use a combination of FM and PSK. Would this decrease the bandwidth required for broadcasting FM luminance video signals?

I did a Google Search for ?Quadrature Frequency Modulation? on

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None of those pages made any sense to me.

One major reason the luminance signals of television are broadcasted on an AM-carrier instead of an FM-carrier is because FM requires large amounts of bandwidth. Is there a way to use FM video without hogging so much bandwidth using QFM?

Quotes from

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Multiple-level quadrature modulation, > "constellation modulation", > is most common for packing > lots of bits per Hz of bandwidth. > The more you pack, > the better the s/n ratio has to be. >
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Does this mean that the FM-equivalents-of-QAM and Constellation Modulation can -- at least in theory -- be applied to FM video so that excessive bandwidth is not needed? If so, then what would be the minimum radio-frequency required to transmit the video signal? Is a 150 kHz radio-frequency sufficient to be an FM carrier for this video signal?

Constellation modulation is a type of AM. What?s the FM-equivalent of constellation modulation?

Has multiple-level QFM ever been used for video before?

Reply to
Green Xenon [Radium]
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I had a quick look at a few of them.

Some guy seems to be trying to patent something he calls Quadrature Frequency Modulation. I see this as similar to the many perpetual motion machines that get patented.

If it works at all I can't see it offering anything different from QPSK (aka QAM4) in performance or bandwidth requirements.

Brian Gregory. (In the UK) 
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Reply to
Brian Gregory [UK]

When I looked into various modulation schemes (for digital signals, or at least conveying bits) about twenty years ago, I found many wonderful techniques claimed for bandwidth reduction. They all boiled down to filtering - either the individual bit stream(s) before modulation, or the modulated (subcarrier) signal afterwards, or combinations of both. Abbreviations often included an N or an M for "near" or "minimum".

Quite apart from the theoretical Shannon limits someone has already mentioned, there was (and still is) the matter of clock recovery: many of these papers often had "assuming perfect timing" hidden away somewhere (or even just implied). In a real practical system, where you're trying to convey information from one place to a distinct other, with no cheat wire round the back of the bench carrying the clock signal, you have to recover your clock timing - i. e. where the symbols start, end, and are to be sampled - from the actual received signal. With many of these filtered wonders, this is not practical!

Of course, digital signal processing has come on enormously since then (even twenty years ago I had to just about admit, though it pained me to do so, that it was possible to do better than Morse code, with _very_ complex algorithms!), such that it _is_ possible to do a lot better at clock recovery than it used to be, but the basic problem does still remain.

J. P. Gilliver                  |  Tel: VNET 791 3298 (+44 1634 203298) 
BAE SYSTEMS Electronics and     |  Fax: VNET 791 4831 (+44 1634 204831) 
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J. P. Gilliver

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