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My favorite Telarc CD
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. |
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