From: rwerner@monolith.next.com (Ross Werner)
Newsgroups: comp.sys.next.hardware
Subject: Re: NeXT Dimension board
Date: 15 Dec 92 19:21:54 GMT
Reply-To: Ross_Werner@next.com


In comp.sys.next.hardware article <1992Dec8.151752.16753@MITL.COM> you wrote:
> Does anyone have any specifications on the NeXT Dimension board? I am looking
> for anything and everything I can find about the hardware.
> 
> For instance, the NTSC standard allows differing scan line lengths and in
> fact, different cameras have different resolution. They are, however,
> less than 640 pixels across. So how is the lower resolutions mapped to the
> 640x480 window?
> 
Actually, the NTSC standard (RS-170a) is very precise at 63.556 uSec per line.  
How many pixels there are on one line (in other words, the camera or monitor 
resolution) is variable.   The quantized image (spatially and temporally) from 
a camera CCD array is converted to an analog signal according to the NTSC 
specification, which further complicates things by the fact that the chroma 
(color) information is at 1/2 the bandwidth of the luma (black/white) 
information, and is then phase modulated onto a sub-carrier frequency.  The 
analog NTSC signal gets converted to digital and decoded - hardly an ideal 
situation, and of course there will be sampling errors.  But NTSC is what we 
are stuck with, at least until HDTV becomes real.

The 640 x 480 resolution commonly used in digital video systems derives from 
the fact that the NTSC standard specifies a 4:3 aspect ratio, with 525 lines 
per field.  40 of these lines are in the vertical blanking interval, leaving 
485 technically visible.  Since all television monitors overscan, all the sides 
get clipped by the edges of the screen.  This makes it so you don't see the top 
and bottom few lines of each field, and the left and right few pixels of each 
line. (Computer monitors do not overscan, that's why you see that black border 
between the physical edge of the display and the image displayed.)  So 640 x 
480 was chosen since it contains the entire area typically displayed, gives a 
4:3 aspect ratio, and is integer multiples of 32, which makes both HW and SW 
types happy.

> Also, it appears that the actual video window is somewhat less than 640,
> somewhere in the neighborhood of 625.
> 
The video window produced by the hardware is 640 pixels wide.  It is a 
"feature" of the Philips video chip set used on the ND board that some of the 
pixels at the beginning and end of each video line are black.  (This has been 
discussed on the net before. )  The NXLiveVideoView  will give you a 640 x 480 
window, the NeXTtv demo app clips this, mimicking the overscan of a television 
monitor.

> Apparently, we received no documentation on the hardware -- only software.

What hardware details do you need?  NeXT does not publish detailed HW 
documentation, no workstation manufacturer does.  If you have specific 
questions, email and I can help you.

For details on video, I recommend these references:

 - Television Engineering Handbook, K. Benson ed., McGraw-Hill.  About $100, 
   worth it if you are seriously into video.  Everything you ever wanted to 
   know about TV, cameras, monitors, VCRs, color perception, signal standards, 
   transmitters, antennas, cable and satellite systems, etc.  More data than 
   you could absorb in a lifetime.

 - Desktop Video Data Handbook, 1992, Philips Semiconductors.  Data sheets on 
   all the Philips video chips, also a good discussion of color decoding and 
   the CCIR 601 spec. Call (800) 227-1817, ext. 900 to find out how to contact 
   your local rep.

 - Television Measurements - NTSC Systems, Tektronix Television Division.  How 
   to use Tekronix equipment for NTSC signal and transmission measurements.  
   Also contains the RS-170a spec.  Call  (503) 627 - 1843 for more 
   information.

Ross Werner
NeXTdimension Hardware Architect
Manager, Visual Media Design
Newsgroups: comp.sys.next.hardware