Dale Kelly wrote:
I have heard several reasons given over the years for the origins of 7.5% setup in NTSC.flyback1 wrote:In Europe, the camera setup standard was different. The grayscale setup chart used was different from the one used with NTSC, eleven steps instead of nine,In Europe they did not degrade video dynamic range by introducing the 7.5 percent DC voltage off-set above blanking as an artificial black level (known as Setup), as is required in the U.S. As I vaguely recall, Setup was established to compensate for a serious non linearity in some proprietary imaging device, near black level. This could account for much of what you describe and as I have also observed.
The ones I have heard all relate to problems with very early clamping and sync separation circuitry
of the late 1930s and early 40s.They include anecdotes from poor TV set sync separator performance requiring a little 'breathng room' between the sync and the blackest part of the video signal, to difficulties with back porch clamping of the black level in OTA transmitters.
The one which sounds the most plausable to me is that the earliest AT&T 2.7 mHz bandwidth cable system, L1, used by RCA to send B&W signals from New York to Philadelphia and Washington D.C. had difficulty clamping the black level properly if the blackest portions of the video signal dipped into the sync area under the baseline. So 'set up' of 7.5 IRE was added to keep the
AT&T equipment happy.If anyone knows more definitively about this interesting topic including when setup was added to the NTSC specs. (1941?)
I'd really like to know the rest of the story.
-----Original Message----- From: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx]On Behalf Of flyback1 Sent: Thursday, December 14, 2006 3:10 PM To: opendtv@xxxxxxxxxxxxx Subject: [opendtv] Re: 405 line pictures Mark Schubin wrote:Television Digest reported decades ago (in the black-&-white days) about an American television delegation that went to the UK in its 405-line, 3-MHz-video-bandwidth days and found the pictures superior to those in the U.S. There was much discussion about why. TTFN, MarkI agree there may have been more care taken in BBC TV production, but I also believe the 405 line pictures looked better to the U.S. delegation because they had more visible information and detail in the darker parts of the image. In Europe, the camera setup standard was different. The grayscale setup chart used was different from the one used with NTSC, eleven steps instead of nine, and the range of reflectance was also different as was the gamma. This still may be the case today. The result is [as I am remembering, because I can't find my camera notebook] the 50% crossover point on a European waveform monitor is about 45 IRE whereas with NTSC cameras it is around 55 IRE. This brings the lower levels up by 10 IRE on a European camera, and adds subtle shading and nuance in the dark parts of the picture creating a more natural looking image. Sort of like how very old movies shot with low contrast B&W silver nitrate film look.