Quote:
Originally Posted by Gorf
Look at the calculation Thomas provided - there you will find a division by four, which is a shift by two - or on transistor level just connecting the right wires.
(Again: this is if you would do it digitally, which would probably not be the way it would have been done …)
|
I know what is proposed by Thomas, even provided results for maximum values and 2 bits removed from 4 bits mean half of the colors lost...
How difficult is to understand that something coded in software with plenty of bits, perhaps even floats will not work such simple in limited HW.
You have 4 bits per Y, 4 bits per U, 4 bits per V - you shifting 2 bits and you have only 2 bits left - you don't have 8 bit to loose 2 and keep 6.
Increasing bitdepth to improve accuracy will be anyway costly and inefficient as you can dedicate this HW for 5 bit RGB so instead 2^12 color space you will have 2^15 space but still you need 3 more pins for this so you no longer use CERDIP 48 but you need to use CERDIP 52 and your fab machines may not have such possibility so you need to put this in PLCC but not sure if in 1985 they could use PLCC as Agnus is 48 pin CERDIP.
Quote:
Originally Posted by Gorf
Probably as they just could not wait to change their already working YUV based design to RGB when Commodore took over …  |
Please allow me to address your concerns by quoting Jay Miner (excuse for poor OCR from Byte 1985/11 - not my work) - 36 years ago Jay says:
Quote:
DIGITAL RGB, ANALOG
RGB, AND NTSC
BYTE: When you put out RGB |red-green-
blue| data. how does it come out?
Miner: It comes out as 4 bits. 4 bits.
and 4 bits.
BYTE: And that's how RGB monitors nor-
mally take their information?
Miner: Off chip it goes into a ladder.
There are three groups of 4 bits com-
ing right out of the 3 2 color registers.
and then there's a four-resistor ladder
on each one of those that converts it
into three analog values. That's what
goes to the monitors.
BYTE: Then the values of that analog
data-which you've changed from the digital
data-determine how strong each of the RGB
guns is when it's firing at a particular
point?
Miner: Yes. on the so-called analog
RGB. There are two kinds of RGB:
digital and analog. This is important
to stress because IBM talks about 16
colors. but what IBM really means is
two shades of eight colors. and those
two shades are always the same color.
There's no way to change them. That's
what's called digital RGB. or RGBi. It's
got red on and off. it's got green on
and off. it's got blue on and off. and
it's got an intensity level that deter-
mines brightness or darkness for each
one of those. It's a four-wire control.
but it's completely digital. We put that
out too. in order to be compatible. but
we also put out the analog RGB. which
has 4 bits. 4 bits. and 4 bits. into lad-
ders. so you get 16 values of red. 16
values of green. and 16 values of blue.
It's equivalent to 2^12 total colors and
luminances~
BYTE: So on the analog output. you could
have any number of bits that you wanted?
You could put out I 0 bits on each line?
178
BYTE • NOVEMBER 1985
AMIGA'S CUSlDM CHIPS
Miner: Yes. if you had big enough
registers. In fact. that's probably one
of the things we'll be expanding in the
future chip set.
BYTE: Why did you choose 4 bits in the first
place?
Miner: Originally. this wasn't going to
be RGB; it was going to follow the
NTSC !National Tulevision System
Committee! standard. NTSC works on
intensity. hue. and saturation. Color
and luminance. YIO is what they call
it. The Y is the intensity. and the I and
the 0 define a vector that determines
the saturation. Having 4 bits of each
was about the best we could tackle in
terms of having on-chip ladders that
would take the 4 bits for each one of
these and convert them into an actual
phase angle.
BYTE: So that ladder is on Denise?
Miner: No. it was when we had the
YIO; when we were emphasizing
NTSC. we had a ladder on board.
Then we deemphasized it. We found
a Motorola chip that did a good job
of converting RGB into NTSC. We
needed the extra room on the Denise
chip for extra resolution on the color
registers. so we dropped the YIO
NTSC completely. But we've still stuck
with the 4. 4. 4 bits. Also. you've got
a real pin limitation on a chip like this.
We tried to keep the chip simple and
low-cost to manufacture. and on-chip
ladders take up a lot of area. They're
notoriously inaccurate. and you can
buy I percent resistors external for a
penny apiece.
BYTE: Is there still NTSC output from the
Amiga?
Miner: In the box there is. yes. But not
in the chips.
|