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.