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Re: Green Light
- To: "TIG" <telecine at alegria.com>
- Subject: Re: Green Light
- From: "Jeffery W. DesCombes" <jdescomb at sprocketdig.com>
- Date: Mon, 16 Mar 98 18:55:35 -0800
- Resent-Date: Mon, 16 Mar 1998 18:48:39 -0800
- Resent-From: telecine at alegria.com
- Resent-Message-ID: <"4uuk_.A.AwB.KReD1" at sun>
- Resent-Sender: telecine-request at alegria.com
- Resent-To: multiple recipients of <telecine at alegria.com>
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
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
receive the light generated by the CRT have an extremely wide gain range,
excess of four usable decades (10^4). This gain is optimized for each
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
light output to match the film and the sensors. In both systems it
the operation of the system.
As has previously been discussed, the advantage of adjusting the light
is that the SNR of the sensors remains constant. This is indeed true,
should be noted that over a significant portion of the usable gain of a
the noise does not increase linearly with gain. There is a substantial
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
PMT is operated at.
Our measurements of PMT noise using a low noise preamps and a modified
network indicate that the SNR from the PMTs typically used today can be
than 80dB (computed for a 7.5MHz bandwidth). Even with very high gain
voltage) the PMT noise increased by just 10 to 12, still providing
70dB of SNR. Our evaluations of several high gain/low noise PMTs
these devices may have 10 to 15 dB better performance than the PMT
in most Cintel telecines. Unfortunately, these high gain PMTs will not
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
types and conditions mentioned by previous TIG contributors. As I
tests I will attempt to keep the TIG informed.
I provide this information in an attempt to augment the discussion which
before. I think it is important to recognize that there is a lot of
processing performed on the output of the sensors, and that this
contribute as much, or more noise, to the final image than the sensor
President, Director of Engineering
Burbank, CA USA
Sprocket Digital manufactures professional video equipment which can be
any type of telecine as well as system upgrades for Cintel Telecines.
Digital has no marketing or manufacturing arrangement with either Cintel
Philips. Anyone interested in the details of the above mentioned results
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 18:47:32 PST 1998
complete information on the TIG website http://www.alegria.com/tig3/