[BC] Noise Free Radio

[Harold Hallikainen]

> On Saturday 06 October 2007 12:52 pm, Dale Adkins-WINI wrote:
>>  I've forgotten the details, but twenty or twenty-five years ago
>>  George Yazell was promoting a project he called "Noise Free Radio"
>>  in which you would frequency modulate  your AM carrier.
>>  Can somebody relate what ever happened to the proposal?
>
>  For some reason, it died on the vine.
>  ( too early, I suspect )
>
>  'Twas a simple concept.
>  FM the AM carrier some miniscule deviation.
>  Transmitting is easy.
>
>  At the receiver, multiply the AM carrier up by some large
>  factor, such that the deviation was now about 75 khz, then mix
>  that to produce an IF of 10.7 Mhz, and feed it into the normal
>  FM if strip.
>  Receiving, not so easy.
>
>  Then, the whole idea has some challenges.
>  What happens when the modulating frequency exceeds the deviation ?
>  Part of the concept involved modulating FM an AM carrier with 15 Khz
>  audio, but deviation of the carrier was something like 5 hz.
>
>  To this day, methinks it could be done, but the technology for detecting
>  the instantaneous rate of change of the rate of change of the
> instantaneous
>  carrier phase just didn't exist with any reliability to function in the
> real world.
>

I don't think the multiplying and hetrodyning gets you and andvantage over
just doing FM demodulation at carrier or some IF. I tend to think of
bandwidth and snr as two forms of the same thing, just as energy and
matter could be thought of as two forms of the same thing. You can trade
one for the other, but the sum is the same. If you have a digital channel
that can handle a certain bit rate, you can either run a wide frequency
response audio signal (high sample rate) or a high snr audio signal (more
bits per sample). In RF, you have a channel of a certain width, and you
have a snr for the channel. These two determine how much information you
can shove down the channel. In FM, we get a better demodulated snr because
we are running a wider channel. In the proposed noise free radio, the
channel was not wider, so there would be no demodulated audio snr
advantage. In addition, the narrow deviation required to stay within the
channel made the sideband power pretty low, making the effects of channel
noise worse than on AM.

I recall a station in Florida got an experimental authorization for the
proposed narrow band FM. I never saw the report, though.

Harold


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