"Jim Lesurf" wrote in message
...
In article , David Looser
wrote:


In the 1930s "HiFi" enthusiasts could, and did, spend hundreds of pounds
(equivalent to thousands at today's values) on "quality" receivers.
These sets had sideband responses reasonably flat to 12kHz or more, low
distortion detectors and push-pull output stages. Many had switchable
sideband filters for receiving "difficult" stations and switchable 9kHz
notch filters. With almost all broadcasts being live, and volume
compression limited to a man with a level control and a modulation
meter could do, broadcast quality could be very good indeed from a
local station if a decent aerial was used, much better than what was
available from 78rpm records.

Indeed. This was why the 'Armstrong' company sold a large number of models
of radio receiver chassis during the 40s and early 50s. (Many under other
names supplied as OEM items.) The idea being that at that time a good AM
radio could pick up quite a wide bandwidth signal. They were sold as high
quality chassis for those who wanted much better performance than the norm
for mass-produced radios and radiograms.

All must seem weird now... Different times, different ways.

What happened after the war was that increasing demand for MW stations
lead to them being packed together more closely, both in spectral and
geographic terms. So setmakers started to reduce the sideband responses
of their sets to minimise adjacent channel interference.

And also adding 9kHz[1] whistle notch filers. The old Armstrong 200 range
AM tuners also had a bandwidth that varied with input RF level. Wideband
when the signal was strong, narrowing down as the signal level was
reduced.

[1] Or 8kHz for those regions of the world where 8kHz spacing was adopted.
9kHz isn't uniform around the world IIRC.

Slainte,

Jim

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A few years ago when I worked for Harris, I was given a demonstration of AM
stereo at their factory, using one of their new digital modulation AM
transmitters. The quality was excellent, when compared to the CD being
transmitted there was rather less extreme top (10kHz bandwidth only), but
that was all that was obviously missing. The test was being done in a lab,
so there was no interference, but nevertheless, it showed that there was
little wrong with AM as a method of modulation. AM in the Midwest of the USA
seems better than AM in the UK, both in terms of interference and bandwidth,
maybe their very high powers and large distances between stations helps, but
driving cross-country AM listening is quite feasible, whereas here, it's a
painful experience.

S.

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