From: ab@nova.cc.purdue.edu (Allen B)
Newsgroups: comp.sys.amiga.graphics
Subject: Re: OpalVision
Keywords: RGB port  bandwidth  no free lunch  OpalVision  DCTV  NTSC
Date: 23 Jul 92 15:40:39 GMT

In article <1992Jul19.145635.6063@mullet.gu.uwa.edu.au>  
brendan@mullet.gu.uwa.edu.au (Brendan Langoulant) writes:
> OpalVision Motherboard
> * The motherboard is the core of the system. It is from here that you add
>   all the extra bits and pieces. It is a true 24-Bit frame buffer and display
>   device with 16.8 million colours available for every pixel.
> * Operates in all standard Amiga resolutions up to maximum 768 x 580
>   (480 in NTSC)
> * Double buffered 24bit and 15bit animation can be performed in low
>   resolution modes while 8 bit double buffered animation is available in all
>   resolutions.
> * Available in an internal version which occupies the video slot of the
>   Amiga 2000 and 3000 series computers.  The external version version
>   connects to the RGB port of any Amiga, including the Amiga 500 and 600
>   series computers.
> * Equipped with 1.5Mb of display RAM

OK, I've been confused by this before (in the ads for the
older product (Colorburst they called it?)), so
straighten me out.  The device can run a 24 bit buffer at 768
x 480 from just the data available on the RGB port?  I'm not
contesting the video slot version, but the RGB port? 

Really?

Assuming you can get all the bits displayed by the Amiga
(I'm going to talk NTSC throughout, PAL people can scale
everything), which is all there are, what are the limits? 
(I've done this math before- that's how DCTV works,
remember?) 

You can make a screen as wide as you want, but it quits
showing after about 784 (392 lo-res).  You can make a
screen as tall as you want, but, again, it really stops at
482 or so.  These are generous.  I doubt they go beyond the
768 x 480 they advertise. 

At those resolutions, you can display 4 bits per pixel. 
Ideally, you can display 30 frames per second:

784 * 482 * 30 * 4 = 45346560 bits per second

or 5668320 bytes (5.4MB) per second.

So the device can fill it's 1.5MB (I assume you meant
bytes) 3 times a second- if your Amiga can completely and
continuously update the screen at 30 frames per second on
a HIRES LACE screen.  Mine can't (with a 28MHz '040), so I
doubt yours can.  CHIP memory is the bottleneck.


If you used that for a double-buffered display, you could
run an 8 bit screen at about: 

5668320 bytes/s / (768 * 480) bytes/frame = 15 fps

You need to tell the thing to flip pages too, so it'll be a
hair slower.  Can't do 30.  Ever.  No way. 

Bits is bits, as I always say.  You can't get something for
nothing.  To get 8 bits per pixel, you need to lose half the
frame rate or half the pixels.  Where does OpalVision get
its juice from? 

If you want to interpret the data stream as 24 bits per
pixel, you need to drop the number of pixels or the frame
rate by a factor of 6.  (Or a combination, you get the idea.) 


Did someone just say compression?  If you can compress (or
bandwidth limit (as DCTV does)) each frame uniformly,
you can do it.  If you can't (because you're using a
symmetrical compression scheme), you just can't. 
You'll get bitten when your compression fails- and I
assure you it will. 

If you do compress, you're not "really 24 bit" as so many
people like to whine about DCTV.  I'm recording to
bandwidth-limited NTSC videotape anyway, so I'll let you
do that as long as you tell me what your limits are.  


So what's the scoop?  Tell me tell me tell me.  I think the
makers of OpalVision are not telling us the whole truth.  I
want numbers and I want explanations that match with
reality.

ab