On 2/17/2016 10:17 PM, wrote:
On Wed, 17 Feb 2016 16:45:01 -0500, "Mr. Luddite"
wrote:

On 2/17/2016 3:21 PM, Justan Olphart wrote:
On 2/17/2016 1:21 PM, Mr. Luddite wrote:
On 2/17/2016 1:01 PM, wrote:
On Wed, 17 Feb 2016 12:36:05 -0500, "Mr. Luddite"
wrote:

Many people don't realize
that AM radio's bandwidth is limited to 10Khz

That is why most records made in the 60s and earlier sound like they
do. They were mixed to play on the radio. It wasn't until "hi fi" LPs
came around that you started getting decent sound. Even those got
"remastered" for CDs. When you play old tunes on a good system that
limitation becomes immediately apparent. I doubt the original source
material even exists to remaster them in a lot of cases. I suppose
they can try to expand the sound digitally but it will just be a guess
about what it was supposed to be.



I don't think the old recordings were purposely mixed to play on the
radio. High quality recordings were being made in the 30's and 40's.
They just don't sound very good on AM. Also .. that 10Khz bandwidth
is really only 5Khz available for audio modulation. The 10Khz is the
total of what the allocated spectrum is above and below the carrier
frequency. So, you are only hearing up to 5Khz.


Amplitude modulation

AM is "amplitude modulation" however what I was referring to is what
the "modulation" is. AM has a 5khz band for audio, 5Khz on each side of
the carrier freq for a total of 10Khz. For example:

WBZ in Boston transmits on a carrier frequency of 1030Khz. If they
modulate that carrier with a 1hz test tone (audio) the frequencies
received by the AM radio are 1031Khz and 1029Khz. In the old days the
1029Khz would be discarded but I think they use that side of the carrier
freq for station ID info and attempts at AM stereo.

The AM receiver has a "beat frequency oscillator" which is tuned to the
carrier freq of 1030Khz. The oscillator freq (1030Khz) and the received
transmitted freq (with the test tone) are "beat" together in a
superheterodyne circuit which yields the original freqs, the sum of the
freqs and the difference between the freqs. The difference is what is
used, being the 1Khz tone which is sent to the amplifier section and
then to the speaker. But, the maximum *audio* modulation can only be 5Khz.

===

Richard, with all due respect, that's not quite the way it works. The
classic AM receivers of our youth, and many of the current generation,
were super hetrodyne receivers. They used something called a local
oscillator to down convert the signal to a fixed Intermediate
Frequency (IF), typically 455 KHz. From there the signal was sent to
the Detector circuit which rectifed it and thus recovered the audio
portion.

The circuit you describe with a Beat Frequency Oscilator (BFO) is the
way CW and SSB transmissions are received. Typically the BFO
frequency would be at or near the IF, e.g., 455 KHz.

You're absolutely correct that the audio maximum frequency of a
standard AM radio is 5 KHz. That's an arbitrary limit imposed by
broadcasting standards used to keep stations from interfering with
other nearby frequencies.


We are saying the same thing. I just left out the the 455Khz
intermediate freq for simplicity. The BFO (controlled by your tuning
dial or pushbutton) is tuned to the carrier freq however, as I recall
and not to the fixed, IF frequency. If that were the case you couldn't
select any particular broadcast frequency.

On Thu, 18 Feb 2016 00:08:51 -0500, "Mr. Luddite"
wrote:

We are saying the same thing. I just left out the the 455Khz
intermediate freq for simplicity. The BFO (controlled by your tuning
dial or pushbutton) is tuned to the carrier freq however, as I recall
and not to the fixed, IF frequency. If that were the case you couldn't
select any particular broadcast frequency.

===

It's a matter of standard terminology.

The local oscilator (LO) is what allows you to select stations by
frequency. The BFO is specialized for CW and SSB reception and many
(most) radios do not have a BFO.

https://en.wikipedia.org/wiki/File:Superheterodyne_receiver_block_diagram_2.svg

https://en.wikipedia.org/wiki/Beat_frequency_oscillator

On Thu, 18 Feb 2016 01:04:50 -0500,
wrote:

On Thu, 18 Feb 2016 00:08:51 -0500, "Mr. Luddite"
wrote:

We are saying the same thing. I just left out the the 455Khz
intermediate freq for simplicity. The BFO (controlled by your tuning
dial or pushbutton) is tuned to the carrier freq however, as I recall
and not to the fixed, IF frequency. If that were the case you couldn't
select any particular broadcast frequency.

===

It's a matter of standard terminology.

The local oscilator (LO) is what allows you to select stations by
frequency. The BFO is specialized for CW and SSB reception and many
(most) radios do not have a BFO.

https://en.wikipedia.org/wiki/File:Superheterodyne_receiver_block_diagram_2.svg

https://en.wikipedia.org/wiki/Beat_frequency_oscillator

Where does the cats whisker fit in there? ;-)

On Thu, 18 Feb 2016 02:27:53 -0500, wrote:

On Thu, 18 Feb 2016 01:04:50 -0500,
wrote:

On Thu, 18 Feb 2016 00:08:51 -0500, "Mr. Luddite"
wrote:

We are saying the same thing. I just left out the the 455Khz
intermediate freq for simplicity. The BFO (controlled by your tuning
dial or pushbutton) is tuned to the carrier freq however, as I recall
and not to the fixed, IF frequency. If that were the case you couldn't
select any particular broadcast frequency.

===

It's a matter of standard terminology.

The local oscilator (LO) is what allows you to select stations by
frequency. The BFO is specialized for CW and SSB reception and many
(most) radios do not have a BFO.

https://en.wikipedia.org/wiki/File:Superheterodyne_receiver_block_diagram_2.svg

https://en.wikipedia.org/wiki/Beat_frequency_oscillator

Where does the cats whisker fit in there? ;-)


===

That's easy, the cats whisker is basically a semiconductor diode that
demodulates the AM signal. Of course the availability of vacuum tube
diodes and packaged semiconductors made the "whisker" obsolete. :-)

On Thu, 18 Feb 2016 09:51:14 -0500,
wrote:


Where does the cats whisker fit in there? ;-)


===

That's easy, the cats whisker is basically a semiconductor diode that
demodulates the AM signal. Of course the availability of vacuum tube
diodes and packaged semiconductors made the "whisker" obsolete. :-)

I was just screwing with you guys. I did make a very simple radio when
I was a kid using the brand new germanium diode for the detector.
There really wasn't much to it since it was basically the old crystal
set design.
The next radio I made was a 3 tube deal that was one of the projects
in a 12 week (by mail) electronics course I took. You got a box of
parts every week and a new project building on what you did the week
before along with a bunch of training material and a self checked
quiz.
It was pretty good training for a kid and the radio worked pretty well
for what it was. It got me interested in DXing