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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 19:32:06 -0800
- Resent-Date: Mon, 16 Mar 1998 19:24:45 -0800
- Resent-From: telecine at alegria.com
- Resent-Message-ID: <"gRGpE.A.6FC.JzeD1" at sun>
- Resent-Sender: telecine-request at alegria.com
- Resent-To: multiple recipients of <telecine at alegria.com>
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/