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Re: Green Light

Subject:     Re: Green Light
Sent:        3/16/98 7:29 PM
To:          TIG, telecine at alegria.com

## My apologies to anyone who struggled through the formatting of the 
previous posting.##

I would like to support a comment made by both Dave Corbitt and 
Christopher Bacon (both on 3/16/98).

Dave indicated that the shape of the CRT spectral response to be... 

>very smooth and continuous, albeit somewhat slim in Blue and far Red. 

We have recently measured several CRTs from two different manufactures  
and found them to be strikingly similar, and both consistent with Dave's 
observations.  I will try to get a copy of the response to Rob for 
posting. 

On the same topic of green light, Christopher wrote 

>Of course there is a lot less light on the other side from a
>CRT than a xenon bulb, but that doesn't prove anything since some types of
>sensors need very little light to operate, while other types need more. 

Mr. Bacon brings up a very critical point.  The photomultiplier tubes 
used to  receive the light generated by the CRT have an extremely wide 
gain range, in excess of four usable decades (10^4).  This gain is 
optimized for each transfer by adjusting the voltage supplied to each 
PMT.  Adjusting the gain of the sensor to match the film and CRT output 
is roughly equivalent to adjusting the light output to match the film and 
the sensors.  In both systems it optimizes the operation of the system. 

As has previously been discussed, the advantage of adjusting the light 
output is that the SNR of the sensors remains constant.  This is indeed 
true, but it should be noted that over a significant portion of the 
usable gain of a PMT, the noise does not increase linearly with gain.  
There is a substantial portion of the PMT operating region where the 
noise is effectively fixed, and is determined more by the design and 
construction of the PMT than by the gain the PMT is operated at.   

Our measurements of PMT noise using a low noise preamps and a modified 
bias network indicate that the SNR from the PMTs typically used today can 
be better than 80dB (computed for a 7.5MHz bandwidth). Even with very 
high gain (-1000V PMT voltage) the PMT noise increased by just 10 to 12, 
still providing approximately  70dB of SNR. Our evaluations of several 
high gain/low noise PMTs indicates that  these devices may have 10 to 15 
dB better performance than the PMT currently used in most Cintel 
telecines.  Unfortunately, these high gain PMTs will not operate  
properly with the PMT bias networks which are currently available today. 

I hope to be able to perform some specific performance tests based on the 
film  types and conditions mentioned by previous TIG contributors. As I 
complete these tests I will attempt to keep the TIG informed. 

I provide this information in an attempt to augment  the discussion which 
has gone  before.  I think it is important to recognize that there is a 
lot of signal processing performed on the output of the sensors, and that 
this processing may contribute as much, or more noise, to the final image 
than the sensor itself. 

Jeff DesCombes 

President, Director of Engineering
Sprocket Digital
Burbank, CA  USA 

Sprocket Digital manufactures professional video equipment which can be 
used with any type of telecine as well as system upgrades for Cintel 
Telecines.  Sprocket  Digital has no marketing or manufacturing 
arrangement with either Cintel or Philips.  Anyone interested in the 
details of the above mentioned results should contact me and I will 
provide the specific test conditions and results.

---
Thanks to Complete Post L.A. for supporting the TIG in 1998..
No product marketing allowed on the main TIG.  Contact rob at alegria.com
957 subscribers in 36 countries on Mon Mar 16 19:23:47 PST 1998 
complete information on the TIG website http://www.alegria.com/tig3/