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NTSC Legacy Formats



Quoting Jim Mendrala:

<< I don't see any problem with the TV sets locking on to 30 vs 29.97 because
in the early days of television (before color) that is exactly what was done
between the network color originations and local black & white transmissions.
The local stations had their 30 fps sync generators and the networks had their
29.97 sync generators. No one then experienced any difficulties. Todays phase
locked loops will lock on with no problem. >>

Hi Jim,

I can't think of any reason why a monochrome receiver would not run equally
well at 29.97 or 30 FPS; they don't have to care about subcarrier.  But until
all broadcasters switched over to 29.97 FPS permanently in the late '60s,
people just took it for granted that you sometimes had to adjust the
horizontal and vertical hold of their receivers when switching between color
and monochrome broadcasts.  Ten years later, I was working as a TV repairman
to help pay for college, and a few customers still thought you were supposed
to do that whenever you saw a b&w picture!  It wasn't considered a big deal;
on most sets then, the controls were readily accessible because the circuitry
wasn't exactly stable to begin with!

Unfortunately, there is no simple adjustment for color TV receivers.  As some
other TIGers pointed out earlier on, the subcarrier frequency of
NTSC--3.579545 MHz--was specifically chosen to be 227.5 times the line
rate--15,734 Hz--so as to interleave.  Interleave (e.g. the reversal of chroma
phase on the same line numbers in alternate frames) cancels out the picture
interference that would otherwise result because the chroma subcarrier
frequency is inside the video frequency range.  If you divide 15,734 by 525,
you get our old friend 29.97 FPS.

Now if you wished to run 525 at 30 FPS, the line rate would have to revert to
its original pre-color value of 15,750 Hz.  And that would put subcarrier at
3.583125 MHz, assuming we're still using the same interleaving factor.   But
the tolerance on the existing subcarrier standard is plus or minus 10 (ten)
Hz, and all existing sets are built for that narrow a lock range.  As our new
subcarrier ends up 3,580 Hz away, the sets would not recognize it and would
revert to monochrome mode.

<< Play a tape back from your VCR the picture syncs to the 60 Hz power line if
no
video signal is present. If a video signal is present some but not all VCRs
will sync to the incoming video at 29.97 Hz. >>

I'd like to know what kind of VCR you're talking about here!  All videotape
recorders must lock to incoming video in order to record it.   They must play
it back at the proper rate, as was pointed out above, in order to see a color
picture.  Consumer VCRs, and most broadcast ones as well, contain color burst
crystals so color playback is possible even if there is no input signal.  If
one attempts to play back a blank tape (or one with bad or missing control
track), the machines will generally free-run at some arbitrary speed, which
can't really be called a frame rate, since there are no coherent "frames." 

Best regards,
Christopher Bacon

---
Thanks to Blake Jones for support of the TIG in 1998.
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924 subscribers in 36 countries on Tue Feb 17 08:40:02 PST 1998 
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