[time-nuts] Re: World's most accurate PC clock!
brooke at pacific.net
Sun Jul 3 20:08:16 EDT 2005
Thanks for the corrections and additions, they've been made to the web page.
Brooke Clarke, N6GCE
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
Magnus Danielson wrote:
> From: Brooke Clarke <brooke at pacific.net>
> Subject: Re: [time-nuts] Re: World's most accurate PC clock!
> Date: Sun, 03 Jul 2005 16:09:15 -0700
> Message-ID: <42C8701B.3070409 at pacific.net>
>>I've factored a number of the common crystal frequencies and have added
>>their common applications, see:
> A few notes:
> 27 MHz is "magic" since it is a common multiple to the NTSC and PAL line
> systems. It is used in ITU-R BT.601 compatible video systems, the base
> frequency for MPEG and MPEG-2 timing (that used a 90 kHz clock but went to full
> 27 MHz when they found they needed it, a bit kludgy but it works).
> The DCF should be 77,5 kHz sharp. I found one ETSI document listing it at
> 77,5 MHz but that was a little to high. They got an editorial comment from me
> on that one.
> The 32,768 kHz is used in computers all over the place for their real-time
> hardware clock. We had it since the IBM AT, so that was a definit fix for the
> kludgy counter trick. Ah well.
> As for your music references, there have existed a number of frequencies that
> have been used, not all crystal oscillators. None of them is really correct.
> The 4,33619 MHz "PAL" frequency is incorrect. It should be 4,43361875 MHZ
> (+/- 5 Hz). Actually, that is for PAL B, D, G, H, I and N. Typo I'd assume.
> 1,544 MHz, 2,048 MHz, 19,44 MHz, 51,84 MHz and 155,52 MHz is standard telecom
> frequencies. The two first ones are the traditional PDH rates (and thus
> synchronisation frequencies), the third is the normal SDH/SONET reference
> oscillator frequency, but 51,84 MHz and 155,52 MHz is more commonly used for
> bit-clock reference.
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