Thread: FM "deviation"
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Old 31st Mar 2010, 4:34 am   #28
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: FM "deviation"

FM has an inherent triangular noise characteristic, with noise increasing at 6 dB/octave with increasing modulating frequency. One way of looking at this is that for any given deviation level, as the modulating frequency increases, the modulation index decreases. And the higher the modulation index, the better the signal-to-noise ratio, and vice versa.

To achieve a rectangular noise characteristic, as for example occurs with AM, then the use of a 6 dB/octave AF roll-off in receivers is necessary. In turn this requires mirror-image boosting (pre-emphasis) at the transmitter.

Some narrowband FM communications systems simply use 6dB/octave slopes across the voice frequency communications band (300 to 3400 Hz). Given that in this case heavy limiting and peak clipping are acceptable, such an approach is workable. However, for broadcasting, program energy distribution does not allow the application of a simple 6 dB/octave constant slope across the whole audio band, say 40 to 15 000 Hz. The compromise is to select a mid-frequency turnover point beyond which the 6 dB/octave slopes apply, and below which, where the response is flat, the noise is not obtrusive anyway. The two standard pre-emphasis/de-emphasis curve numbers are the “European” 50 microseconds (3.18 kHz turnover) and the “American” 75 microseconds (2.12 kHz turnover point). 25 microseconds was also proposed for use with Dolby B noise reduction, but I don’t think its use ever became widespread.

Even so lack of treble headroom could be a problem. In an earlier version of this world, before the Optimod era (optimum for whom, I wonder?) I seem to recall that the BBC used variable pre-emphasis limiting (on Radio 3 at least) as an alternative to compression.

Cheers,
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