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Digital Betacam and noise



        Reply to:   RE>Digital Betacam and noisy composite encoding
>>  Why is it that this type of noise is most apparent in blue and
>>  particularily in bright white areas of the image. 16 mm neg seems to
>>  be the largest culprit.Some one in a previous post mentioned rounding
>>  via the renaissance 8:8:8.


>The resulting lack of light reaching the Blue PEC forces us to run the
>Blue PEC at a very high sensitivity level and brings up the noise.  By
>increasing the light that gets to the Blue PEC the Blue signal to noise can be
>drasticallly improved.  This can be done a number of ways;
>1.   Get a brighter CRT.
>2.   Change the cut off wavelength of the Blue Trim Filter in the cell box to
>pass more light to the PEC then correct the errors with a new masking matrix.
>3.   Install a faster gate lens that passes more light and/or Rank's
>"Highlight Kit".  16 mm gate lenses are usually slower than 35mm gate lenses
>and further away from the CRT.  Both of these factors reduce the light
>throughput and thus the signal to noise.  
>(Note, increasing the Blue light throughput can cause the Blue PEC to
>saturate, so the Blue PEC may have to be replaced with the same type used in
>Green).

>URSAs also have the additional problem of an overall background noise in the A
>to D converters.  If the signal out of the Cell Box is not maximized before
>the A-D conversion then the A-D noise becomes very objectionable.


My understanding of the situation was that Highlight doesn't necessarily help,
as Dave mentioned over saturation occurs.  The PEC's that are used were not
actually designed for the purpose we use them for.  Of course logically,
throwing more light around should help the deal, but I am sure most of use have
seen "fold over" occur.  I have personaly seen huge amounts of signal to noise
gain by some of the following methods:  Tuning the head end of the telecine so
that any outside interference is erradicated and then (he shuts his eyes when he
says this!) sorting out the numbers on the A-D convertor (URSA).  I have seen I
think up to a 12dB increase in performance, think of those edges for blue
screen!!

The above is really related to URSA's, but MK 3 can also be helped in a number
of ways.

Some years ago I tried a prototype white phosphur tube in my Jumpscan in
London...  I cannot even begin to describe the results.... FANTASTIC.  There was
only one teeny weeny problem:  The phosphur burned like crazy, in fact so much
that the person who put the tube in on test thought that I was busting his chops
by adjusting the focus!  The after glows had to be adjusted every minute or so
to keep up with the burn!  A great pity as really the picture were stunning for
that minute!!

Ken