genlock and video taping

genlock and video taping

Post by Jie Yuan, Pharmacology, U. Cincinna » Sat, 29 Oct 1994 01:43:26



Hi!

Have you used the genlock option to record animations?

We have a 4D35 with Genlock option and we do molecular modeling, which
sometimes generate animation data (or movie).

I have heard that only 1/4 of the display on the screen can be recorded into
NTSC (North American) video tape.  I thought it should be able to to better
than this.  Can you tell me the "truth?"

My main application is to generate composite color signal and record
on VHS (or S-VHS if necessary) tape, while monitoring with a TV.  We may do
this only a few times per year.  If it is too complicated, we probably don't
want to bother.

The other way around is to use a camcorder to shoot the movie off the monitor,
which certainly will lose quality to some degree.  Because of the simplicity,
this has been the only way we record movies so far :(

Thanks for your attention!  If there is enough interest, I'll summarize to the
net.  Let me know if you want it :)

Best,

Jie

PS: Our SGI 4D35GT is under Irix 4.0.5.  No plan to move up to Irix 5.x yet.

 
 
 

genlock and video taping

Post by Paul Spenc » Sun, 30 Oct 1994 08:17:52



Quote:

> I have heard that only 1/4 of the display on the screen can be recorded into
> NTSC (North American) video tape.  I thought it should be able to to better
> than this.

The NTSC standard is 646 x 486 resolution. To record onto video tape,
your Personal Iris changes it's video output from the standard 1280

interlaced).

So, you are putting the entire screen display to video tape; but the
resolution is lower than you are used to looking at.

You cannot record a resolution of 1280 x 1024 onto video tape.
Your only option if you want to run a program at 1280 x 1024 and
record it on NTSC is to run the 1280 x 1024 signal into a scan
convertor, which averages it down in size and outputs an NTSC signal.
This averaging process will, of course, cause you to lose some detail
in your original image.

These scan convertor boxes are available from SGI (the VideoCreator),
Folsom Research (the Otto), and several other sources.

....paul

--
Paul Spencer                 Silicon Graphics Advanced Graphics Division


 
 
 

genlock and video taping

Post by David Ge » Wed, 02 Nov 1994 03:36:27



|> >
|> > I have heard that only 1/4 of the display on the screen can be recorded into
|> > NTSC (North American) video tape.  I thought it should be able to to better
|> > than this.
|>
|> The NTSC standard is 646 x 486 resolution. To record onto video tape,
|> your Personal Iris changes it's video output from the standard 1280

|> interlaced).
|>
|> So, you are putting the entire screen display to video tape; but the
|> resolution is lower than you are used to looking at.
|>
|> You cannot record a resolution of 1280 x 1024 onto video tape.
|> Your only option if you want to run a program at 1280 x 1024 and
|> record it on NTSC is to run the 1280 x 1024 signal into a scan
|> convertor, which averages it down in size and outputs an NTSC signal.
|> This averaging process will, of course, cause you to lose some detail
|> in your original image.
|>
|> These scan convertor boxes are available from SGI (the VideoCreator),
|> Folsom Research (the Otto), and several other sources.

Unfortunately, it is even far worse than this.  Most common video recorders,
such as VHS and SVHS are designed to record Y,I and Q signals (or Y, Cr, and
Cb signals), which are derived from the R,G,and B signals your computer
screen uses.  Because of the limited recording bandwidth of the magnetic tape,
bandwidth is allocated between the components, with Y getting about 4MHz, and
the I and Q (or Cr and Cb) getting about 500KHz each.  This is a complicated topic,
but the net result is that the horizontal Luminance resolution (Y) is about 320
lines, and the Chrominance resolution (Cr, Cb) is about 40 lines!

If I had to guess at what you are seeing from the tape, it is overall low image
resolution, especially the color resolution, and color noise in the image.

What is truly amazing to me is that video movies still look O.K. with this kind
of resolution.

Why isn't it better than this?  Because a VHS VCR needs to sell for around $150.
If you need better quality, it can be had, but you better sell your house to get it.
A D1 tape deck, which stores digital video at full resolution, costs around $100K.

David Gere
SGI