"tony sayer" wrote in message
...
In article , David Looser
scribeth thus
"tony sayer" wrote in message
...

Its only in recent times that they have had an opportunity to
downgrade
it owing to the bitrates they can set it at.. You'd never hear
someone
from the pre digital times saying that they could -downgrade- the
transmission system in use.


Rubbish!. AM radio was downgraded by reducing it's bandwidth from 9kHz
to
4.5kHz betwen the 1930s and the 1960s.


Could you qualify that please?
AM radio, AFAIK, has always has an audio frequency response limit of
4KHz which means it needs an RF bandwidth of 8KHz. The station spacing
is thus set at 9KHz to (theoretically) leave a guard band between
stations. In practice for most domestic radios this is of little
consequence as it would be nigh impossible to hear two stations on
adjacent frequencies - something that would be possible on a
commercial/professional receiver with a much more closely controlled
passband.


--
Woody

harrogate three at ntlworld dot com

In article ,
Woody wrote:
Rubbish!. AM radio was downgraded by reducing it's bandwidth from 9kHz
to
4.5kHz betwen the 1930s and the 1960s.


Could you qualify that please?
AM radio, AFAIK, has always has an audio frequency response limit of
4KHz which means it needs an RF bandwidth of 8KHz. The station spacing
is thus set at 9KHz to (theoretically) leave a guard band between
stations. In practice for most domestic radios this is of little
consequence as it would be nigh impossible to hear two stations on
adjacent frequencies - something that would be possible on a
commercial/professional receiver with a much more closely controlled
passband.

Not so - the changes making the 9 kHz an international standard came in on
IIRC the early '70s. When R1 started up in the London area the 247 metre
transmissions had a bandwidth exceeding 12 kHz - the landline feeding that
transmitter was also wide band. With a good AM receiver the frequency
response didn't sound much different to FM when R1&2 did simulcasts.

--
*Nostalgia isn't what is used to be.

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" wrote in message
...
In article ,
Woody wrote:

AM radio, AFAIK, has always has an audio frequency response limit of
4KHz which means it needs an RF bandwidth of 8KHz. The station spacing
is thus set at 9KHz to (theoretically) leave a guard band between
stations. In practice for most domestic radios this is of little
consequence as it would be nigh impossible to hear two stations on
adjacent frequencies - something that would be possible on a
commercial/professional receiver with a much more closely controlled
passband.

Not so - the changes making the 9 kHz an international standard came in on
IIRC the early '70s. When R1 started up in the London area the 247 metre
transmissions had a bandwidth exceeding 12 kHz - the landline feeding that
transmitter was also wide band. With a good AM receiver the frequency
response didn't sound much different to FM when R1&2 did simulcasts.

--

Dead right! Woody has got this story back to front.

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.

When Baird broadcast his 30-line TV on MW between 1930 and 1935 his
transmissions included baseband frequencies up to 13kHz.

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. By then, of course, the "quality"
brigade had moved to FM. Eventually it became apparent that all the
extended transmitted sidebands were doing was to increase interference, so
international agreements were made to limit transmitted bandwidth to 4.5kHz.

David.

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

--
Change 'noise' to 'jcgl' if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

"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

--
Change 'noise' to 'jcgl' if you wish to email me.
Electronics
http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html



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.

--
http://audiopages.googlepages.com

In message , Jim Lesurf
writes


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

Out of interest, which region is 8kHz? I thought it was only 9 or 10.
I've had a quick look, but can't find 8.
--
Ian

"Ian Jackson" wrote in message
...
In message , Jim Lesurf
writes


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

Out of interest, which region is 8kHz? I thought it was only 9 or 10. I've
had a quick look, but can't find 8.

Yup, it's 9kHz or 10kHz dependiing on region. There's no 8.

David.

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


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

Out of interest, which region is 8kHz? I thought it was only 9 or 10.
I've had a quick look, but can't find 8.

Yup, it's 9kHz or 10kHz dependiing on region. There's no 8.

In that case, my error. I assume I was confusing that with 8kHz. Memory
fault.

Slainte,

Jim

--
Change 'noise' to 'jcgl' if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

In article , Ian Jackson
wrote:
In message , Jim Lesurf
writes


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

Out of interest, which region is 8kHz? I thought it was only 9 or 10.
I've had a quick look, but can't find 8.

Afraid I've forgotten! :-) It may be either Africa or the USA, but I
can't recall. I do remember that the World Conferences established some
regions as 8kHz and others 9kHz. Think this was when the BBC on long wave
went from 200kHz to 198kHz to suit. But alas my memory of this is hazy.

IIRC one of the Quad AM tuners also had a choice of notch filter frequency
to cater for the region.

Slainte,

Jim

--
Change 'noise' to 'jcgl' if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html