Michael J. Mahon wrote:
(snip)
The problem is that when the D/A's are (re-)loaded, the voltage outputs
will swing rapidly and exponentially damped toward the new values. This
results in a trajectory that is not usually a straight line, nor is it
of constant velocity, so the "vector" is curved and of non-uniform
intensity. In fact, if the values of both D/A's cannot be latched
simultaneously, there will be a "hook" at the start of each vector.
Either linear ramp generators must be controlled by the D/A's or they
must be low-pass filtered and fed redundant points along the desired
vector to get a straight line (and always to get a curved line).
The Tektronix terminals, such as the 401x series, somehow did it.
Well, they only had to do it once, they used a storage CRT to
keep the image. I don't remember noticing that they lines were
ever curved, but sometimes they were brighter on one end if the
velocity wasn't uniform. Though it should saturate the CRT so it
doesn't have to be as uniform as when doing continuous vector refresh.