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why the SE/30 display has only two colours?
- Thread starter mc9625
- Start date
A SE\30 at the time sold for $4000. That is as much as a brand new (low end) car back then!
Say you bumped it up from 1bpp (black & white) to 2bpp (four shades of gray, same as the Powerbook 150.) That doubles the amount of memory required for the framebuffer from ~21KB to ~43KB - considering that the machine originally shipped with 1MB, that's not really a huge amount. Double it again to 85.5KB and you get 16 shades of gray, and still only use ~1/12th the amount of RAM in the minimal system configuration.
Now, granted, the SE/30 actually has separate video RAM and they'd need to put in 128KB simply to use available parts, but that's still really not a lot. In fact, if everymac.com is correct, the SE/30 already has 64KB VRAM - well more than enough for 2bpp grayscale. I suspect that rather than cost of memory, the real reason for the limited display is because they already had a perfectly good setup for 512x342 1bpp video engineered from the inception of the Mac and didn't feel any particular need to up the ante, as that was still quite passable for basic business use in 1989.
Now, granted, the SE/30 actually has separate video RAM and they'd need to put in 128KB simply to use available parts, but that's still really not a lot. In fact, if everymac.com is correct, the SE/30 already has 64KB VRAM - well more than enough for 2bpp grayscale. I suspect that rather than cost of memory, the real reason for the limited display is because they already had a perfectly good setup for 512x342 1bpp video engineered from the inception of the Mac and didn't feel any particular need to up the ante, as that was still quite passable for basic business use in 1989.
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Isn't there an expansion card you can add to the SE/30 get full grayscale? I think it was for an external display, but also have the internal display grayscale?
Yeah, that's the Micron Xceed mentioned earlier in the thread.
Ah, missed that post. Thanks. After after a quick google search... welp... looks like I won't be getting one of those then. :O
The limitation is the analog board - the circuit board that drives the display. Apple used the same circuit (more or less) in all the compact Macs (except the Color Classic and CCII) for 1-bit-per-pixel black and white. The signal going to the CRT is either on or off, no in-between levels.
To display greyscale they would have had to both design a new analog board and add more VRAM to the SE/30 logic board.
To display greyscale they would have had to both design a new analog board and add more VRAM to the SE/30 logic board.
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So the Xceed card somehow bypasses the analog board circuit?
In other words, to achieve grayscale, the Xceed replaces the video board that plugs into the end of the CRT with one that connects directly to the video card (which has the proper circuitry for displaying >1-bit-per-pixel), rather than the analog board (which is hard-wired for 1bpp), correct?
I think there was a relatively plentiful IIsi PDS card that, when combined with a version of the reverse-engineered Xceed board, can allow grayscale on the internal CRT. If this is true, what make/model was it?
c
In other words, to achieve grayscale, the Xceed replaces the video board that plugs into the end of the CRT with one that connects directly to the video card (which has the proper circuitry for displaying >1-bit-per-pixel), rather than the analog board (which is hard-wired for 1bpp), correct?
I think there was a relatively plentiful IIsi PDS card that, when combined with a version of the reverse-engineered Xceed board, can allow grayscale on the internal CRT. If this is true, what make/model was it?
c
Two system limitations and the lack of some components that would be needed for greater color depth.
The screen image is stored in memory. This video memory is called a frame buffer. It is a buffer of one image frame. In the case of one bit color (monochrome), each pixel on the screen has just one corresponding bit in memory. When the CRT's electron beam is pointed at that pixel on the screen, the pixel's corresponding memory bit controls a switching transistor (on or off states, not much in between), and that transistor feeds the electron gun. If the transistor is on, the gun is on ans a white spot appears. Transistor off, electron gun off, just blackness at that pixel.
For greater color depth, more bits of memory are used to represent the state of the pixel. This means that the system needs more video memory, and that, for the same display frame rate, the bits must be shifted out of memory from two to several times the monochrome rate.
Suppose the system is built to support 4 bit color. Now, the frame buffer must be shifted to the display circuitry at four times the previous rate. Each pixel needs four bits of data from memory.
This is where the extra components come in, and you can see this on the Xceed CRT yolk board schematic. Those four bits can't just be sent to a transistor. The four bit pixel data is fed to a digital to analog converter. A four bit number goes in one side and at the other end a voltage appears at one of sixteen different possible levels. Of course a resistor ladder can be used to make a crude D to A converter (DAC), but in any case, this adds a component here.
Then the analog voltage level from the DAC is fed to the transistor that feeds the electron gun. Except a simple switching transistor is no use in this position now. Instead a transistor that operates in an analog mode as an amplifier is used. A higher voltage at the gate causes a corresponding increase in current through the transistor to drive the electron gun. As the electron gun is driven more strongly or weakly, the corresponding pixel can appear at sixteen levels of brightness (sixteen in our example, more or less if more or fewer bits are used) from white, through levels of gray down to black.
So grayscale support requires:
1) More video RAM (larger frame buffer)
2) Circuitry that can shift more bits out of video memory per pixel displayed
3) A digital to analog converter (DAC) that supports however many bits of grayscale wanted (no use have 8 bits per pixel in memory if your DAC only has inputs for 4 bit numbers).
4) A linear amplifying transistor instead of a switching transistor to drive the electron gun.
All of this ignores the upstream requirement of telling the operating system to store more bits per pixel when it draws the screen image in the frame buffer.
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