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Telecine blanking



I would like to offer my thoughts in reply to the points raised by Vin
ce Forcier.
     
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 
market?
     
     Robin Hurley. Channel Four Television. London.
     
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