Why is NTSC frame rate 29.97 Hz?

[snip]

Err - isn't that kind of what said (or was trying to say?) The fact that
the PAL 4.43 line/field relationship to subcarrier relationship is different
to that of NTSC 3.58 means that the requirement to change the field/frame
rate to avoid interference with the audio subcarrier is removed? (Or are
you saying that the different vision and sound channel spacings are not
important?)

Steve

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--
znark

ISTR that the frame rate was adjusted to reduce possible interference
between the vision and audio components in the NTSC M RF modulated signal
when the colour subcarrier was added. Given that the colour subcarrier has
a directly locked relationship to the line and field scanning rates one of
the simplest ways of altering the subcarrier frequency to reduce the chances
of audio interference was to subtly alter the frame rate.

The different line/field scan to subcarrier relationship chosen for PAL,
coupled with the different vision/sound carrier spacings meant that it was
not required as the PAL subcarrier was less likely to cause interference to
the sound carrier?

This is all from memory - and dim memory at that - but a Google search will
no doubt find a concise explanation.

Steve

Ah - I must have written ambiguously!

Indeed - though NTSC M has a universal vision / sound spacing given the
common RF transmission standards, the various PAL B/G/I/D/K/H etc. standards
don't - with the audio being sited at various spacings WRT to video - as
you say (and for that matter added multiple audio carriers in the case of
analogue stereo or NICAM broadcasts, or SAP and stereo in the US even...)

Not how I read it ... the precision offset and the small change in the
field rate amount to the same thing - the implication (as I read it) was
that they weren't

Vision and sound channel spacings are totally irrelevant to PAL or NTSC
(or for that matter SECAM), transmission standards are another thing
entirely. After all there is 5 Meg bandwidth PAL as well as UK's 5.5 Meg
- does that have any influence on the frequency of the subcarrier?

Tony Quinn --- The Voice Of Insanity
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.
Not at all. 625/50 PAL CSC is 283.75 x line frequency (F NAT) + a 25 Hz
offset. (which achieves the same purpose).

Tony Quinn --- The Voice Of Insanity
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