[rec.arts.tv.uk] Converting PAL to NTSC on the fly

lars@salt.acc.com (Lars J Poulsen) (07/13/89)

In article <10350003@otter.hpl.hp.com> (on rec.arts.tv.uk), Lee Carter writes:
>=The problems of converting between PAL and NTSC are numerous and
>=unpleasant; They use entirely different methods to encode the television
>=signal

Certainly the conversion requires buffering through a digital
framestore. I believe that the cost of fast enough RAM used to be the
driving cost factor. You probably need 24 bitplanes and about 600 x 800
resolution. But this is well within what's available in newer computer
display cards, and it seems to me that several of the new display cards
for the Mac-II are beginning to have framegrabbers.

The idea of mixing computer graphics with video is not new. I have been
told that one of the selling points of the Amiga is that it can produce
NTSC video output and can overlay the graphics screen on a live video
image.

Anyway, video/framegrabber cards allow for video in, frame storage and
video output from the frame store. It seems to me that this means we
have the most expensive pieces of a scanconverter in place.

This still leaves some problems:
(1) 525 lines versus 625 lines.
    Not a big deal: Just use 600 lines. 525 line-video resides in the
    upper 525 lines. Or even better: set a programmable starting point
    so you have the choice of a window menu bar on top or a subtitle
    area on the bottom for 525 line video. Chop off the bottom 25 lines
    for 625 line video input.
(2) 50 Hz vs 60 Hz scan rates.
    This requires the memory to be dual-ported. To reduce the effect of
    access collisions, you'd probably need to pre-fetch a line-buffer's
    worth of data on each port.
(3) Interlace versus non-interlace scan.
    Broadcast video is usually interlaced to reduce flicker, computer
    displays are usually non-interlaced. This means a little extra
    logic, but not much. The 50 Hz interlaced video is really skipping
    every other line; i.e. it really is only 25 Hz. Some users might be
    willing to forego interlaced output (i.e. only view the euro movies
    on the computer screen).
(4) Distortion due to 525-600-625 rate differences. Yes, it looks ugly
    in broadcast, but you can live with it on home viewing by fiddling with
    the height and/or horizontal gain.
    Professional systems, of course, skip/duplicate lines at even
    intervals in the picture.

Am I the only person with family in Europe that would like to run
videomovies through the computer ? Will this open up a new vertical
market for Commodore, Apple or NeXT ?

Followups to sci.electronics, only, please.

/ Lars Poulsen <lars@salt.acc.com>   (800) 222-7308  or (805) 963-9431 ext 358
  ACC Customer Service              Affiliation stated for identification only
                My employer probably would not agree if he knew what I said !!