Re: This can't be true...

["Michael J. Mahon" <mjmahon@aol.com>]

Jorge Chamorro wrote:
> Apple IIe: Video Signal "NTSC-Compatible", Not "NTSC-Compliant"

Right.  There were some very expensive genlock cards made in the
late 1970's that put the Apple video on standard NTSC-standard
sync and timing specs.  The much less expensive Video Overlay Card
will also do this, if you really need it.

> I bought Sharp XA305 video recorders to record the video from my Apple
> IIe systems: however, I am able to record only black and white pictures.
>
> I tried the same setup with an older Sears model VCR and it worked
> perfectly. I tried a new camcorder and got only black and white pictures
> again. Sharp told me that this is because the Apple IIe has a
> substandard NTSC signal.
>
> Can you help?
>
>
>
> The composite video signal coming from the Apple IIe is not stated to be
> an NTSC-compliant signal. The signal coming from the video-out port is
> only considered NTSC-compatible video. "Compatible" means that generally
> the Apple IIe video will work with an NTSC device, although it may not
> work in all situations. Specifically, the Apple IIe video signal should
> work with all NTSC display devices.
>
> When the Apple II was first designed, economy was of great importance.
> The video circuitry was built with as few parts as possible. In the
> design process, it was discovered that certain portions of the NTSC
> broadcast specification could be omitted, yet still display a color
> image on a standard television set. The one element that affects the
> described situation is color burst. The Apple IIe does not contain the
> color burst element of the NTSC video signal. By modulating other
> portions of the video signal, the Apple IIe simulates this missing
> information and allows an image to be produced on a television. To
> provide a consistent color video image for video tape, this color burst
> needs to be present.

This is not so.  In all graphics modes, a color burst is present
(though it may not of "perfect" position or duration).  Almost
every NTSC device is *extraordinarily* sensitive to this color
burst, which is gated and used to phase lock a crystal oscillator
for chroma decoding.

In fact, there was an early change to the Apple II circuit to provide
more isolation when the color burst was switched off (in text mode)
so that TV monitors wouldn't pick up the tiny leak-through burst and
go into color mode!

As Don points out, only a *very* picky video processing system would
fail to lock to the Apple color burst.

> In summary, the Apple IIe composite video signal was designed for
> display proposes, not for videotaping purposes. The Apple IIe composite
> signal is close to the NTSC specification and allows the video signal to
> be displayed on television screens. This, however, does not indicate
> that the signal can be recorded. Since the signal is similar to the full
> NTSC specification, video sometimes records properly on videotape with
> certain VCRs. In other environments, the signal may not record at all.
> The capability to record a monochrome image falls between these
> extremes.
>
> To have a true NTSC signal designed for videotaping, the Video Overlay
> Card is required.

That's a very conservative statement.  Most VHS video recorders will
record the Apple video signal just fine--there were several applications
for titling videotapes that depended on it.

As the generation of spec-perfect NTSC timing and sync has become a
piece of cake (or a tiny sliver of silicon), the sloppy NTSC of the
early days has become rarer, so modern equipment is likely to be more
dependent on precise timing.

-michael

Music synthesis for 8-bit Apple II's!
Home page:  http://members.aol.com/MJMahon/

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