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Dolby FM

Dolby Labs introduced this technique in June 1971 in an attempt
to solve one of the fundamental problems of FM transmissions,
that of high frequency overload coupled with background hiss.
The FM system has a triangular noise spectrum, rising at 6 dB per octave.
This makes it quite a noisy system, so the idea of pre-emphasis
and subsequent de-emphasis was introduced,
thus making the noise constant above the turnover point of the de-emphasis.

This worked quite well in the late 1940s when the spectral distribution of the programmes broadcast were fairly limited in high frequencies.
However it caused a problem as sources become richer in high frequencies;
the US pre-emphasis time constant of 75 us gives a 14 dB boost at 10 kHz,
and the European one of 50 us gives 10 dB. To avoid over-modulation,
the broadcaster has to either lower the average level
(this making the apparent noise worse) or has to employ pretty drastic limiters
which degrade the sound.

The situation was then made even worse by the introduction of stereo;
the GE-Zenith system used almost everywhere in the world
introduces over 20 dB more noise.

It was realized that the characteristic of the M noise was quite similar
to that on cassette tape, and so thought that the use of Dolby B-type noise reduction
might have some good effects. Indeed, measurements showed
a reduction in perceived noise of some 9 dB (weighted measurement).
Instead of using all of this possible noise reduction,
Dolby Labs suggested that the FM time constant should
at the same time be changed to 25 us,
thus considerably improving the high frequency overload performance
and yet retaining a significant measure of noise reduction.
In fact Dolby Labs had little success in introducing these ideas into Europe;
the BBC showed interest but never did anything.

A number of stations in the USA did adopt the idea,
and for that market quite a few manufacturers introduced tunes with noise reduction
and the time constant change. Ultimately the technique foundered
on the compatibility issue; the signal did not sound quite right
on conventional receivers - the time constant change helped to tame
the extra brightness of the Dolby-encoded signal as heard on standard receivers,
but the ruler-flat frequency response of the FM system showed up the differences.
This was in contrast to the situation in pre-recorded cassettes, where
this added brilliance went a long way to restoring the correct frequency response
of the typical poor cassette deck of that time. So the idea faded away.

A Dolby FM T-shirt

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