"SFTV_troy" wrote in message
oups.com...

Earl Kiosterud wrote:

Synchronous AM demodulation uses a locally regenerated carrier, fed along with the AM
signal
(upper or lower set of sidebands) to a multiplier (modulator). The result is the audio.
It
replaces the envelope (diode) detector usually used. You can think of it as another
superhet
stage where the result, instead of another IF frequency, is the baseband audio. That's
because the local oscillator is the same frequency as the carrier of the (IF) signal, so
the
difference is zero. The sidebands wind up translated to baseband audio instead of to
another IF frequency.

There are advantages. Since one set of sidebands or the other can be used, if there's a
distant station 10KHz away, causing that AM whistle, you just switch to the other set of
sidebands, whichever comes in the cleanest. Also, it doesn't depend on proper amplitude
and
phase of both sets of sidebands to work properly, as does the regular envelope detector,
so
it works better with impaired signals.



I only understood about 75% of what your wrote, but if I understand
your meaning, this new receiving technique would not improve the sound
(it would still be limited from 100-6000 hertz), but would only reduce
interference.


Troy,

Well, the 6 KHz limit is due to the narrow bandwidth of the receivers, not the detector
used, or the stations. I think most AM radios actually do much worse than that. AM radios
are designed with a limited bandpass because it gets noisy as the bandwidth goes up. The AM
band is a soup of distant stations, particularly at night, and that's the source of much of
the noise. AM radio stations in the US are allowed up to 10 KHz audio. That's pretty
listenable -- there's only a little over a half octave to the 15 KHz limit of FM.

The synchronous detector, in addition to being able to use one set of sidebands or the
other, whichever is the best under the conditions, is not subject to distortion from
asymmetrical sidebands, such as when there is fading, multipath, etc. There may be a
non-flat audio bandpass from those conditions, but a conventional detector will also have
distortion.
--
Regards from Virginia Beach,

Earl Kiosterud
www.smokeylake.com