03-04-2017, 11:40 AM
Let's think about what we need to test this hypothesis:
For 625, the active line period is 52us, so set your scope to 5us/div
For 405, the active line period is 80.3us, so set your scope to 10us/div
Not perfect, but enough to make the point. It's just one click of the timebase switch.
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.
It's also instructive to play around with a CRT PC monitor if you still have one. I've retained my 17" ADI multiscan unit, which covers a massive range of H and V frequencies, and has a wonderfully sharp Sony CRT. You can display just about anything you like on this - though I doubt it'll go down to 10kHz
- and simply switching between 640x480 and 1280x1024 goes some way towards answering the question. Or keep the resolution the same but alter the frame rate... It also is a useful tool for assessing the quality of your graphics card - back in the 1990s when I bought it, a lot of "generic" graphics cards had significant HF rolloff.
For 625, the active line period is 52us, so set your scope to 5us/div
For 405, the active line period is 80.3us, so set your scope to 10us/div
Not perfect, but enough to make the point. It's just one click of the timebase switch.
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.
It's also instructive to play around with a CRT PC monitor if you still have one. I've retained my 17" ADI multiscan unit, which covers a massive range of H and V frequencies, and has a wonderfully sharp Sony CRT. You can display just about anything you like on this - though I doubt it'll go down to 10kHz
- and simply switching between 640x480 and 1280x1024 goes some way towards answering the question. Or keep the resolution the same but alter the frame rate... It also is a useful tool for assessing the quality of your graphics card - back in the 1990s when I bought it, a lot of "generic" graphics cards had significant HF rolloff.
