Why did 405 look so good?

[Mark Hennessy]

But before drawing any firm conclusions from this, double-check that the hold-off period is the same at both settings. On the Telequipment DM63 - using that as an example because I know it well - there are 3 discrete values of hold-off timing capacitance (and no change in resistance) as you go around the timebase switch. As a result, there are noticeable steps changes in brightness between certain adjacent settings because a different capacitor has been switched in - nothing to do with write speed, just duty cycle. Easy to check with a second 'scope connected to the "Gate" output on the front panel.

Just checked mine - the second change in hold-off time happens between 10 and 20us. The two values of hold-off are 37us and 2.8us, which leads to a duty cycle change of 85% to 97% as you turn from 10 to 20us/DIV. The change in brightness is actually more subtle than I'd remembered it. As you speed the timebase up, the duty cycle falls to 44% at the fastest setting (200ns/DIV - it's only a 20MHz 'scope), and that obviously results in a dimmer trace.

As mentioned, the 'scope is free-running in BBL mode. When triggered to a fixed-frequency signal, then the duty cycle of the trace will be a product of the timebase speed and the trigger event frequency. As a TV shows "full" lines rather than just the first part of them, what I've done makes most sense.

So, a bit tangential, but very easy to measure with the Rigol, so why not? Either way, having looked into this, I'd like to think that the write speed angle is not a big issue in the difference between 405 and 625. Not unless there's something special about the phosphors used (but a dual-standard set would be the way to test for that).