Appendix; Section 1 Fm Stereo Signals And The Asymmetrical; Charge Coupled Fm Detector; Section 2 Am Stereo Signals And The Ultra High Fidelity Wide Band Am Stereo - Carver TX-11b Manual

Appendix
SECTION 1
FM Stereo Signals and the Asymmetrical
Charge Coupled FM Detector™*
FM radio stations started life broadcasting a carrier frequency
modulated by music. When you listen to "FM 108", what your tuner is
receiving is that frequency —108 MHz (megahertz, or millions of
cycles per second) — modulated by a music spectrum from 30 Hz to
15 kHz (KiloHertz, or thousands of cycles per second). The Federal
Communication Commission ensures (by law and threat of fine) that
the rock music on one station won't interfere with classics on the
next.
When radio stations came under pressure to deliver stereo sound,
some system of broadcasting two signals, short of setting up another
station, became necessary. Early experiments were actually tried with
one channel on FM, the other on AM. Needless to say, some other
solution had to be found. One constraint of the stereo broadcast
system required that table radios get a usable mono signal — not just
the left, or just the right channel. What was finally adopted and
approved by the FCC was a broadcasting system where both
channels (L+R) were available as usual. To supply stereo, a
difference signal (L- R) was developed and used to modulate
another carrier at 38 kHz.
Now for the first complication. It turns out that broadcasting that signal
in stereo required too much frequency deviation (the difference
between the necessary signal and the carrier), so the 38 kHz L- R
carrier is suppressed and a much smaller amplitude 19 kHz pilot is
added.
Mono radios use only the first block of the signal. The SCA, or
Subsidiary Communication Authority signal, is often leased to
companies that specialize in commerical-free background music for
stores, dentist's offices, plus special programming for the visually
impaired — even foreign language broadcasts. To receive an SCA
service, a special tuner is needed. These signals can't be picked up
ontheTX-11b.
The components of the transmitted FM stereo signal that concerns
us are L+R signals, and most importantly, the L- R signals. The
graphic representation of a sample FM signal shows the various
relationships of the different signal components in frequency and
amplitude.
There are two more things you should know: 1) actually, only 15% of
the L- R signal is different than the L+R signal; 2) the L- R portion
of the transmitted signal is very prone to mishaps between the
transmitting tower and your TX-11 b. It's this signal component that,
when degraded, causes noise and distortion problems, usually
induced by multipath reflections off hills, buildings, the ground...
whatever.
The Asymmetrical Charge-Coupled FM Detector operates principally
on the L - R signal components, improving it from 10 dB to 23 dB.
However, it can only improve the L+R component by 1 dB. If you
kHz 15kHz 23kHz
19 kHz (pilot)
FREQUENCY «-
SCA at 67 kHz
receive the L+R signal components well, but suffer from poor L- R
reception, the Asymmetrical Charge-Coupled Detector will drastically
improve the reception. If the L+R signal is poorly received as well,
the circuit can only help a little. Degraded L+R reception is often due
to improper or inadequate signal strength. This could indicate a distant
station, or the need of a better antenna system.
Almost all stereo FM reception problems can be traced to poor L- R
signals. The Asymmetrical Charge-Coupled FM Detector is extremely
effective in reducing the noise and distortion associated with this type
of multipath interference. In short, it can provide stereo reception that
is as noise-free as FM broadcast in mono. Because of the nature of
the components of a transmitted FM signal, the circuit can't
significantly improve a poor mono program.
SECTION 2
AM Stereo Signals and The Ultra High
Fidelity Wide Band AM Stereo
This tuner is absolutely unique in that its basic AM performance
capability is essentially equal to, and in some cases superior to, the
performance that FM can deliver. AM is inherently immune to
multipath distortion, the primary problem of FM. There is no
fundamental limitation to frequency response, distortion or noise in
AM. AM systems historically have fallen by the wayside until very
recently, when manufacturers have typically added them as
afterthoughts. The AM systems in most radios sound terrible because
they have bandwidth that extends to 2 or 3kHz; they have signal to
noise ratios in the 30s or 40s, instead of 60s, 70s, or 80s; they have
distortion that is usually 3-8% - not high fidelity by any means. The
distortion levels in this machine are orders of magnitude below that,
the signal to noise ratio is far superior, and it also has AM stereo.
CQUAM™ AM stereo is a brand new system developed by Motorola.
The way it works basically is that a signal is phase modulated, a sub
carrier is phase modulated on top of the regular carrier, and a phase
detector responsive to the phase deviation of that carrier separates
out the left and the right channel signals.
The way it's done is; the L+R part of the stereo signal is broadcast
as amplitude modulated AM, the L- R is broadcast on that same
carrier as a phase modulated signal. It's actually FM because
whenever you have a phase modulation you have a corresponding
FM deviation and whenever you have an FM deviation, you have a
corresponding phase modulation. So by modulating the phase and
18