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'[OT] Composite video signal (50Hz, 625 lines)'
1998\01\04@140148 by Mike Keitz

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On Fri, 2 Jan 1998 16:45:37 +0000 Jim Main <spam_OUTjimTakeThisOuTspamEWCOMM.DEMON.CO.UK>
writes:

>Most modern tv's have no problem in coping with a vertical interval
>devoid of equalising pulses - in fact many early videogame consoles
>didn't output any (the Sinclair Spectrum for one)

This isn't limited to early videogame consoles.  Many of the Sony
Playstation's NTSC video modes use timing that is a little nonstandard (I
think some of them are not even 525 lines).  Some TVs with digital
horizontal and vertical timing generation have trouble getting a vertical
lock.  A Zenith circuit is particularly troublesome.  The picture will
"bounce" up and down about 20 lines at a rate of 2 or 3 times per second.
Though the Playstation design is a few years old, it still is in very
active production and one of the market leaders.


>So long as the field sync is present and correctly timed, the tv
>should
>lock up to it.

Driving the video level to "sync" for 3 full line periods usually works.
Another example of nonstandard video comes from a VCR.  In normal play
mode the sync pulses are read from the tape and are quite standard
(though the frequency is related to the speed of the motor and thus isn't
perfectly constant).  When the pause or fast scan modes are engaged, the
VCR applies a simple vertical pulse to the video to improve vertical lock
compared to trying to read the vertical sync from the tape.  There is
also the problem of switching the heads, which occurs at the bottom of
the visible part of the picture.  Often the picture appears bent or
"torn" here  because the scan of the next field on the tape (from the
other head) doesn't start exactly in phase.  I assume the designers of
the VCR placed the switching point here, rather than in the vertical
blanking interval, to allow as much time as possible for the TV to
recover.

1998\01\04@171923 by Martin McCormick

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face
       I believe that if the field switch was made during the vertical
blanking interval, one would run the risk of loosing all the digital
information such as VIR, Closed Captionning, Teletext ,etc

Martin McCormick

1998\01\05@013536 by Mike Keitz

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On Sun, 4 Jan 1998 16:17:08 -0600 Martin McCormick
<.....martinKILLspamspam@spam@DC.CIS.OKSTATE.EDU> writes:
>        I believe that if the field switch was made during the
>vertical
>blanking interval, one would run the risk of loosing all the digital
>information such as VIR, Closed Captionning, Teletext ,etc

I think VCRs were invented well before that stuff was in widespread use.
Thinking about it some more, it would be important to have the vertical
sync pulses in phase with the rest of the field, i.e. switch heads before
the vertical sync pulse.  Otherwise the interlace wouldn't work properly
(not that it ever does).

>
>Martin McCormick
>

1998\01\05@175834 by Nigel Goodwin

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In message <19980105.013549.3022.5.mkeitzspamKILLspamjuno.com>, Mike Keitz
<.....mkeitzKILLspamspam.....JUNO.COM> writes
>On Sun, 4 Jan 1998 16:17:08 -0600 Martin McCormick
><EraseMEmartinspam_OUTspamTakeThisOuTDC.CIS.OKSTATE.EDU> writes:
>>        I believe that if the field switch was made during the
>>vertical
>>blanking interval, one would run the risk of loosing all the digital
>>information such as VIR, Closed Captionning, Teletext ,etc
>
>I think VCRs were invented well before that stuff was in widespread use.
>Thinking about it some more, it would be important to have the vertical
>sync pulses in phase with the rest of the field, i.e. switch heads before
>the vertical sync pulse.  Otherwise the interlace wouldn't work properly
>(not that it ever does).


Teletext was in widespread use well before the introduction of domestic
VCR's - unless you count the old Philips 1500 ones (but they were mainly
used by schools). By the start of VHS and Betamax there were many
millions of teletext TV's in use in the UK - it was only the USA who
didn't seem interested in Teletext.
--

Nigel.

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