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Trouble_007

Two common frequencies used at the introduction of the PC were 3.579545 MHz. & 4.43361875 MHz

Chosen because of the existing supply of cheap xtals manufactured for NTSC & PAL color television sets.

NTSC : https://en.m.wikipedia.org/wiki/NTSC

PAL : https://en.m.wikipedia.org/wiki/PAL

Edits to fix typos :( And add;

Crystal Oscillator Frequencies : https://en.m.wikipedia.org/wiki/Crystal_oscillator_frequenci...

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That is why the IBM PC has used a 14.31818 MHz oscillator, i.e. 4 times the NTSC frequency of 3.579545 MHz, and which divided by 3 provided a clock frequency for the CPU that was close enough to 5 MHz (the maximum clock frequency of the 1st version of Intel 8088).

While the current computers with Intel/AMD CPUs do no longer care about NTSC, all still have a timer equivalent with the Intel 8253, which uses the clock frequency of 14.31818 MHz, to ensure compatibility with the original IBM PC.

For that reason: cheap available quartz crystals, since the 1970s, the shortwave radio frequency 3.579Mhz has been a spot on the dial where radio hams use low power home made morse code transmitters. At night, a range of a couple of hundred files is usually possible.

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