Re: Cinematronics > Atari Adapter???

Zonn wrote:

>On Tue, 15 Oct 2002 02:17:44 -0500, Rodger Boots <rlboots@cedar-rapids.net>
>wrote:
>
>
>
>>>You do need 12 bits for the 10 bit vector resolution, using the hardcoded timers
>>>built onto the Cine CPU board. Anything less than 12 bits for 10 bit vectors
>>>and the intensity difference between the start of the vector and the end,
>>>becomes noticeable. That and the timing generator on the CCPU would have to be
>>>changed to compensate for the slower drawing rates of the smaller DACs. And the
>>>instruction set would need to be changed since the Normalization instruction
>>>currently normalizes for 12 bit DACs.
>>>
>>>
>>>
>>>
>>You might be comparing apples to oranges here. The vectors are drawn by
>>the integrator, NOT the DAC. The only difference dropping a few bits
>>will have is less accuracy setting the start and end points. It doesn't
>>affect timing, brightness, or anything else about the vector except the
>>start and end points, and they will be real close to being right.
>>
>>
>
>No, I've got my fruits straight on this. ;-) You could be thinking of how the
>Atari VG works, and not the Cinematronics approach.
>
>You refer to the R/C generator as an integrator, in the classic sense it isn't.
>The capacitor is not connected across the feedback of a op-amp, and does not
>integrate current, but holds a voltage as in a sample and hold circuit.
>
>The capacitor is connected to ground. It is driven to a starting position using
>a DAC set to the actual starting position voltage (only 10 bits of the DAC are
>used to do this), through an analog switch connected directly to the cap. The
>Z-axis is then turned and the DAC is set to a voltage way beyond the endpoint,
>and through a resistor, (using an analog switch) the capacitor is setup to
>charge to the new DAC voltage, which if allowed to go there, would be way off
>the edge of the screen. This is done for both axis simultaneously. This need to
>send double the needed voltage to cap is why a 12 bit DAC is needed for 10 bit
>vectors (you need to double the voltage in both positive and negative directions
>which is why you need 2 additional bits.)
>
>Timing is very critical at this point, when the capacitor reaches the position
>of the original endpoint (not the one loaded into the DAC, but where we *want*
>the end point to be), the analog switch must be opened, and the Z-axis turned
>off. All timing is dependent upon the value of the resistor, the capacitor, and
>the voltage on the DAC.
>
>If the voltage on the DAC is too low, the CRT trace will not have moved far
>enough at the timeout period, and the vector will be too short, if too high, the
>vector will be too long when the timer times out.
>
>As far as brightness. An R/C circuit (as opposed to a real integrator), does
>not charge at a consistent speed. It starts by charging quickly and slows down
>logarithmically as it nears it's final ending voltage. If the DAC were to be
>set to the actual end point voltage, the CRT trace would start by moving quickly
>and then slow down the closer it got to the ending point. The result is a dim
>vector that gets logarithmically brighter as it approaches the end point, and
>would take much longer to draw. By setting the DAC's endpoint to way beyond the
>original end point, only the starting charge of the R/C circuit is used to
>generate the vector, this part of the R/C charge curve is fairly linear and is
>used to draw a vector of consistent intensity and speed.
>
>When building a Cine->WG replacement, one has no control over the timing, this
>is all done on the CCPU board. Same with the voltage values being sent to the
>DACs. The CCPU is sending out 12 bit DAC information, and is expecting to time
>the charge of a preset R/C constant. Changing any of the parameters: DAC
>voltages, R or C, will result in screwed up timings and vectors of incorrect
>lengths.
>
>-Zonn
>
>

For what it's worth, I am well aware of how Cinematronics draws vectors
(it's Atari I've never studied). The original question was how to
reduce the costs of the circuit because someone thought $11 was too much
to pay for a DAC and that's when I said to get rid of the DAC and build
what is in essence a home made DAC. The tradeoff was accuracy for cost.
 And if there is any system out there that would work with inaccurate
DACs, it's Cinematronics.

Yup, I screwed up calling it an integrator. It isn't. It relies on the
concept that the first fifth of a time constant, or so, of an RC
EXPONENTIAL (NOT logarithmic) response is close enough to being linear
to be useful for drawing a line on a screen.

It really doesn't matter to me, I'm not going to bother to build it
anyway. Any time I've ever come up with an idea of designing something
useful to the vector folks half a dozen other people chime in that
they're doing the same thing. (Remember the replacement for the
Amplifone HV supply? Seemed like everyone had a version of it.)
 Nothing ever comes of any of the projects that way. So screw it all, I
don't operate or collect games any more so why should I care, right?

The only exception was the PC vector card project. Good going, guys, on
that one.

>
>

---------------------------------------------------------------------------
** To UNSUBSCRIBE from vectorlist, send a message with "UNSUBSCRIBE" in the
** message body to vectorlist-request@synthcom.com. Please direct other
** questions, comments, or problems to vectorlist-owner@synthcom.com.
Received on Tue Oct 15 23:11:17 2002