PAL timing - part 1

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< Digital Electronic ~ PAL timing - part 1

eric

Posted: Thu Oct 05, 2006 11:45 pm Post subject: PAL timing - part 1

Site Admin Joined: 01 Jan 1970 Posts: 311

Design: T. Giesberts
This circuit is intended more as a design idea than as a finished
project, in the light of the number of standard ICs that are
employed.
In the PAL television system, the CCIR B and G standards specify
that the colour carrier is directly coupled to the line rate, with a
25 Hz offset. The frequency ratio and offset are chosen to suppress
interference patterns, according to the formula
fcolour = 283.75 fline + 25 Hz
At a line rate of 15,625 Hz, this means that the PAL colour carrier
frequency is 4.43361875 MHz. Single-sideband modulation is frequently
used to obtain the correct relationship with the line rate. For
example, the frequency of a crystal oscillator can be offset by
25 Hz, divided by 1135 and then multiplied by 8 to obtain twice the
actual line rate. This is a rather complicated procedure, which we
think could be made a lot simpler.
There is a fixed ratio between the 25 Hz frame rate and four times
the colour carrier frequency. You can calculate this yourself —-
four times the colour carrier frequency is exactly equal to 709,379
times the frame rate! An obvious approach is to use a crystal oscillator
running at four times the colour carrier frequency and divide
its output by 709,379 to obtain the frame rate. The line rate can
then be derived from the frame rate with

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