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Pan/chop mode switch - burning question

Posted: Sun Apr 17, 2016 9:33 pm
by Fraser
I have owned a P4428 and one function always puzzled me. The PAN/Chop button.

Whilst I understand the principle of the Pyro-Electric Vidicon tube and the switch providing either a spiral chopper wheel induced change in scene (Chop mode) or a camera induced change (Pan mode), how did EEV ensure that in Pan mode, the chopper wheel opaque area did not stop in front of the tube face effectively blinding it ? Or was it a case of the user having to 'pulse' the switch to jog the chopper wheel out of the optical path ?

My P4428's have now gone to a new and deserving home but this question still niggles me.

Finally, why was this feature not included on the ARGUS 1 and 2 ? Was it over complicating use, or simply ineffective in the originally planned usage ?

Someone tell me the answer and put me out of my misery please :D


Re: Pan/chop mode switch - burning question

Posted: Mon Apr 18, 2016 12:10 am
by forum_admin
If you go back to P4221 and P4228, those were mainly used in pan mode as the invert-add digital processing had not yet been invented. You had a choice of Pan or an epilepsy-inducing 25Hz flickering image where the background tube nonuniformity inverted every field while the subject stayed white-hot.

This then got written into procedures so had to be retained on the P4428. The chopper is actively held fully open on a P4428 in pan, as it enters an analogue feedback arrangement to settle in a position where the sensor is partially obscured. On a bench, excessive stray light can foil this circuit.
An Argus 1 can also be set into pan mode to aid setup, but only internally using a PCB link and the chopper does not always stop correctly.


Re: Pan/chop mode switch - burning question

Posted: Tue Apr 19, 2016 2:55 am
by Fraser
Thank you ! You have solved the 'mystery' for me.

I like the clever idea to settle the chopper wheel in the correct position.