🚀 go-pugleaf

In article <93018.074631HARRAPR@QUCDN.QueensU.CA> HARRAPR@QUCDN.QueensU.CA  
writes:
> Hi:
> I was using a friends NeXTDimension board a while ago and attempted
> to output video from the screen to a vcr. The results were not too
> impressive - color areas came through okay, but window borders
> appeared to vibrate wildly (left to right) and some artifacts, also
> vibrating, appeared throughout. Obviously this is a synch or
> interference problem.
> 
> Is it necessary to use a fantastic vcr (frame-accurate) to get
> reasonable output from the ND, or is it just not possible? Any
> other suggestions on using the ND for video?
> 

One of the unfortunate facts of life with the existing television standards  
(NTSC and PAL) is that they are interlaced.  A complete picture, or frame, is  
made up of two fields - one containing all the odd lines, the other with all  
the even lines.  Drawing of the fields alternates at a 60Hz rate (50 Hz for  
PAL.)

This means that any single pixel wide horizontal line will flicker since it is  
drawn in only one field.  The NeXT user interface uses lots of single pixel  
wide horizontal lines, this looks great on the NeXT monitor but flickers  
annoyingly on the NTSC output.

The NTSC standard will also result in some other artifacts, such as "chroma -  
crawl" on edges where there is a color change.  

I don't know what you mean by "window borders appeared to vibrate wildly (left  
to right)."  I have used many different VCRs (everything from a $199 consumer  
model to an $8000 professional model) to record the ND video output, and with  
the exception of the problems I described above, the video quality has been  
excellent.

On a high resolution screen such as the NeXTdimension's, it is very easy to  
create an image that looks just great, but will look terrible when encoded to  
NTSC.  Reasons for this include the interlace, color encoding, and limited  
frequency response of the NTSC and PAL standards.  Some very basic rules of  
thumb for creating images that will encode well:

	1) Don't use single pixel wide horizontal lines.
	2) Avoid the use of fully saturated colors.
	3) For practical purposes, the luma (Black/White) bandwidth of NTSC
	   is about 4 Mhz.  Simply stated, alternating black and white
	   single pixel wide vertical lines will end up as a grey mush.
	4) The chroma (color) bandwidth of NTSC is 3Mhz at most.  Small color
	   features (less than a few pixels horizontally) will tend to 
	   disappear or cause unpleasant artifacts.

Let me emphasize that these rules of thumb relect limitations of the NTSC  
television standard, not the ND hardware.

The NTSC encoder on the ND is pretty good, but as a result of deliberate cost  
and board space tradeoffs it is not "broadcast" quality.  (Go check the cost of  
a broadcast quality encoder.)  If you want the best possible NTSC image, get a  
professional RGB -> NTSC encoder and drive it from the RGB video output.  

Ross Werner
ND Hardware Architect