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- Subject: Telecine blanking
I would like to offer my thoughts in reply to the points raised by Vin
Firstly, analogue 625 PAL horizontal blanking is nominally 12 or
12.05 (depending upon which spec. you refer to) between the half
amplitude points of a black to peak white signal. The same blanking is
used for widescreen transmissions.
With regard to the centering of pictures, I think that the difference
between front and back porches is irrelevant. Referring to CCIR
REC601, although analogue blanking is wider than digital, the
difference is nominally equal on the front and back porches.
Consequently a picture correctly centred on a monitor with analogue
blanking will be correct on digital. Also, REC601 blanking does not
have profiled edges. These, at present, will always be overidden by
analogue blanking. Even for the future, some form of profiled blanking
will have to be imposed since one should not present a D-A convertor
with (potentially) a step function from black to white or vice versa.
Therefore, I suggest that if the picture on a monitor correctly
blanked is satisfactory, it does not matter if frame edges would be
visible on a system with REC601 blanking only.
When I designed the Channel Four test card, I retained the super 16mm
height for 4x3 scanning because I assumed that material shot on this
format for television would have been composed with 16x9 transmission
in mind. If this is not satisfactory the XY zoom can be used scan any
height once the scan geometry has been set and could answer the
question of loss of image with 4x3 scanning.
For the record, there is an SMPTE recommendation that when Super 16 is
intended for 4x3 transmission, a scanned height of 4.35 mm be used and
this may be ratified by EBU at some stage. A future revision of my
test card might incorporate this.
Finally, if the test film were to be revised, what changes would be
most useful? Also, are there any test films needed that are not on the
Robin Hurley. Channel Four Television. London.
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