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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: Mon, 15 Jan 2007 04:28:45 GMT
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To: djgpp AT delorie DOT com
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On Sat, 13 Jan 2007 18:18:48 -0500 in comp.os.msdos.djgpp, "Rod
Pemberton" <do_not_have AT bitfoad DOT cmm> wrote:

>"Brian Inglis" <Brian DOT Inglis AT SystematicSW DOT Invalid> wrote in message
>news:5t3iq2dlv4hoh2sdf9c4dbot50e6ge28mn AT 4ax DOT com...
>> 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
>
>A 157.5Mhz crystal in 1980's?    ROFL!

My assumption was that the frequency is quoted as 157.5/11Mhz various
places and that commercial broadcast equipment would have generated and
divided down that reference oscillator (don't really know if it
could/would have been crystal in the valve/tube era, although WWII army
radios operated in the 30-40MHz range and came with 72-120 crystals) to
get an accurate, stable colour burst frequency for transmission. 

>In a PC, in the 1980's?    ROFL!  (Where's the Kleenex, I've got to wipe
>away the tears...)

I'm well aware that TVs and PCs used a 14...MHz crystal. 

>Unfortunately, I wasn't actively monitoring the advancement in crystal
>oscillator frequencies.  However, if you're interested, this link contains
>an oscillator frequency timeline:
>http://www.npcamerica.com/Datasheets/KEYNOTE2.PDF

Timeline actually shows development of *CMOS* ICs for timebase
generation. 

-- 
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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