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From:	Brian Inglis <Brian DOT Inglis AT SystematicSW DOT Invalid>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Performance enhancement for gettimeofday()?
Organization:	Systematic Software
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Date:	Sat, 13 Jan 2007 18:09:46 GMT
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On Wed, 10 Jan 2007 13:25:42 -0500 in comp.os.msdos.djgpp, "Rod
Pemberton" <do_not_have AT bitfoad DOT cmm> wrote:

>
><Gordon DOT Schumacher AT seagate DOT com> wrote in message
>news:OF77475379 DOT 7BA371D5-ON8725725F DOT 00598107-8725725F DOT 0059B5ED AT seagate DOT com...
>> Rod Pemberton wrote on Tue, 9 Jan 2007 at 03:46:03 -0500:
>>
>> # I believe this it the math you'll need:
>> #
>> # 14.318Mhz=4*3.58Mhz=4*(4.5Mhz*455/572)
>> #     (4.5Mhz US TV bandwith/channel, 455 colorburst phase changes/line,
>> 572
>> # total lines/frame including sync)
>> # 14.318Mhz/12=1.93182Mhz
>>
>> Aha, this is the one that's why our numbers don't agree:
>> 14.318MHz divided by 12 is actually 1.193666... MHz.
>>
>
>Sorry, it appears I failed to type a 1 following the decimal.  It's not
>14.318000MHz, but 14.318181MHz.  You really need to enter
>4*4.5*(10^6)*455/572 to compute the 14.318MHz and work from there.  IIRC
>('twas 25+ years ago), it's 4 times the colorburst as calculated by the
>original engineer who designed the US color TV standard.  That way you won't
>loose precision.  Of course, a real crystal usually has a tolerance range,
>but that range is usually small compared to the frequency, like +/- 100Hz or
>+/-10KHz.  Of course, you could go to Mouser or another electronic supplier,
>and look for a crystal if you think the range would help.
>
>Like you, I'll use ... for repeating digits.  The 1 and 8 repeat for both.
>I was using more decimals but rounded/truncated.
>
>14.318181818181...  Mhz  / 12 = 1.193181818181... Mhz.
>1.193181818181...Mhz / 65536 = 18.206509676846 Hz

IIRC crystal frequency 157.5MHz = 9/2*7*5*1E6, /11 colour burst
14318181.8..Hz, /3 for PC clock 4772727.27..Hz, /4 for timer
1193181.8..Hz, /65536 for tick 18.2065096768465909..Hz, giving a period
of 54925.4095238..us with the last six digits repeating.  
So the relevant factors here are 7*5*3/11/2^19.

-- 
Thanks. Take care, Brian Inglis 	Calgary, Alberta, Canada

Brian DOT Inglis AT CSi DOT com 	(Brian[dot]Inglis{at}SystematicSW[dot]ab[dot]ca)
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