"Jim Lesurf"

Just to let people know that I have now put up a webpage on the effects of
finite bandwidth on the distortion levels for FM audio broadcasts. Can be
found on my audiomisc site.

The page combines the content from an earlier HFN article with some extra
material I produced a while ago when discussing this issue on
uk.rec.audio.

** See: http://www.audiomisc.co.uk/HFN/BandwidthBlues/page.html

1. The claim made that THD figures for FM receivers in mono are speced at
30% modulation is false.

It is usually speced at full +/- 75 kHz deviation. Even the long obsolete
LM3089 ( FM IF system and detector IC ) has a published THD figure of 0.1%
with full 75kHz deviation (with double tuned quadrature detection).

The figure is 100% for real and verified in many commercial tuners.

http://www.national.com/ds/LM/LM3089.pdf


2. There is another, much simpler way to think about how FM stereo operates
than the one presented.

Essentially, FM stereo is transmitted in time division multiplexing - that
is the left and right channels of the original audio are transmitted as
alternate *analogue* samples frequency modulating the carrier.

There are 38,000 *analogue* samples of each channel, per second. The low
level, 19kHz pilot tone simply allows the receiver to know when each sample
is arriving and which channel it is.

With mono audio input, the L and R samples all follow exactly the same
curve, so the derived frequency modulations all match up and there is full
compatibility.

However, when only one channel is sent, the detected audio signal has 38,000
gaps per second - there being no modulation during the silent channel's
allocated sample times.

A direct result of this is the recovered audio level at the detector is only
half that of the mono case, given the same FM deviation.

3. The effect of pre-emphasis.

FM radio uses high frequency pre-emphasis - with a 50uS time constant in
Europe and Australia - so, all FM receivers have a corresponding 50uS
de-emphasis filter installed in the audio path. This helps to reduce HF
noise.

The effect of this filter is such that at 4kHz, the recovered audio is
attenuated by just over 4dB or by a factor of 0.62

Only 50% of the mono signal level is available when a single stereo channel
is transmitted - then the receiver attenuates that by 0.62 if the
frequency is 4kHz. 0.5 times 0.62 = 0.3

So, if one bench tests an FM stereo receiver with 4kHz tone modulation in
only the L or R channels, the recovered audio level will be 30% of the mono
level at 400Hz. Trying to get more will only lead to gross distortion.

This is nothing to worry about here, as the nature of music and speech
signals is such that full level at 4 kHz is completely un-necessary.

Something every tweeter maker has long known and thoroughly exploited.



....... Phil