49
\$\begingroup\$

While desoldering useful components off of old computer hardware, I found quite a large number of 14.31818 MHz crystals.

This seemed odd to me. Why use such an irregular frequency with a very nontrivial conversion to human time units?

At first I thought that it must be a multiple of another frequency with a certain dedicated use (such as 44.1 kHz commonly used as an audio sampling frequency), but my guessing only led to two numbers pretty close to it: 1/7*10⁸ Hz and π/22*10⁸ Hz, both to about 2‰, and I can't seem to deduce what any of these would be useful for.

\$\endgroup\$
4
  • \$\begingroup\$ In fact, 14318180 has the prime number 715909 among its prime factors... (14318180 = 2 * 2 * 5 * 715909) \$\endgroup\$ Commented Jul 31, 2016 at 15:20
  • 1
    \$\begingroup\$ Though the exactly frequency is probably 14.318181818181 MHz ("81" repeats). \$\endgroup\$ Commented Jul 31, 2016 at 15:28
  • \$\begingroup\$ LMGTFY: en.wikipedia.org/wiki/Crystal_oscillator_frequencies \$\endgroup\$ Commented Aug 1, 2016 at 16:46
  • \$\begingroup\$ also worth noting: vogons.org/viewtopic.php?t=45255&p=451355 ; it's a direct result of being used for NTSC output on xGAs, though. \$\endgroup\$ Commented Aug 1, 2016 at 16:50

2 Answers 2

Reset to default
55
\$\begingroup\$

It is exactly 4× the NTSC color-burst frequency of 3.579545 MHz. Since it is (well, used to be) used in huge quantities in commercial color TV sets, it is both commonly available, and particularly useful when you want to generate a signal to be displayed on such a TV.

The exact frequency for the NTSC color burst is 30 frames/sec x 525 lines/frame x 455/2 cycles/line / 1.001 (a correction factor that avoids a problem1) = 3579545.4545... cycles/second.

1 The specific problem is that intermodulation between the color and audio subcarriers could produce a signal at around 917 kHz, producing very visible moving diagonal lines on the screen. The adjustment factor of 1.001 makes the audio subcarrier an integer multiple (286) of the horizontal scan frequency. This means that the dots produced by this signal are in alternating positions on alternating lines, and more importantly, they stand still. This is visually the least objectionable.

\$\endgroup\$
5
  • 3
    \$\begingroup\$ Yes, that's reasonable. So in the parts which do not output a video signal, the only reason is availability? The answer does also still beg the question of why that particular frequency was chosen for the NTSC standard itself. \$\endgroup\$ Commented Jul 31, 2016 at 14:15
  • 27
    \$\begingroup\$ The exact frequency for the NTSC color burst is 30 frames/sec x 525 lines/frame x 455/2 cycles/line / 1.001 (a correction factor that avoids a problem) = 3579545.4545... cycles/second. I don't remember the precise reason the correction factor is needed, but you can probably look it up. I think it had something to do with having the field rate (59.94 Hz) NOT precisely locked to the power line frequency. \$\endgroup\$ Commented Jul 31, 2016 at 14:21
  • 29
    \$\begingroup\$ For anybody else looking, the factor of 1.001 looks like it is present to prevent the sound carrier and video carrier from having similar frequencies, which could cause video artifacts. en.wikipedia.org/wiki/NTSC#Color_encoding \$\endgroup\$ Commented Jul 31, 2016 at 15:00
  • 4
    \$\begingroup\$ +1: Decades ago (and with a European bias) we referred to NTSC as 'never twice same colour'. \$\endgroup\$ Commented Aug 1, 2016 at 16:30
  • 6
    \$\begingroup\$ @copper.hat: Sure, and PAL = "Pay for Added Luxury" (re: cost of delay line) and SECAM = "System Essentially Contrary to American Method". \$\endgroup\$ Commented Aug 1, 2016 at 16:43
1
\$\begingroup\$

Not only could the color burst be derived from that frequency, but all other clocks used to generate an NTSC signal, including horizontal and vertical sync, blanking, color burst gate, and pixel clock. Lots of smart folks must have hashed out the NTSC specs.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.