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Old 11-29-2011, 12:11 AM
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Penthode Penthode is offline
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Quote:
Originally Posted by old_tv_nut

It's the chroma flicker translating into luminance changes that is visible. During the development of HDTV, Dr. Schreiber at MIT proposed going to a true constant luminance system, and demonstrated that 30 Hz true chroma-only flicker (or even 15 Hz!) is invisible. This could hypothetically be used to reduce the frame rate of chroma to save bandwidth, but would run into problems when the camera pans or the eye tracks a moving object. It also could be used to more crudely encode the chroma without producing visible artifacts.
The thought of true constant luminance is intriguing. Unfortunately, the benefit of transmission noise suppression in the blacks would be lost as the gama curve happily compensated for it. Not to mention the complication of adding a CRT compensation circuit at the receiver and having to reconcile some sort of backward compatibility if a fully constant luminance system was achieved.

With regards to the hue control critical adjustment, I recall working with "Simple PAL" receivers. The Delay PAL Monitors in the BBC studios had a simple PAL switch. It was great for checking PAL chroma subcarrier phase because an out-of-phase condition would yield "Hannover Bars". Each alternate line would have a noticable hue difference and even a slight phase misadjustment was very annoying. I would certainly prefer to see a noticable NTSC IQ system overall hue error to having to put up with the Hannover Bars. And this was the UK systems which allowed fully symmetrical upper and lower chroma sidebands which should eliminate the quadrature crosstalk problem! (Remember the UK channel was 8MHz wide with the sound carrier at 6MHz instead of 5.5MHz for their continental cousins. This allowed full +/- 1.5 MHz chroma bandwidth).

CPA was chosen as a means to help widen the chroma bandwidth (hence chroma resolution) and the Phase Alternate Line or Field was to help cancel the quadrature crosstalk resulting from the highly non-symmetrical chroma channels. Even with the hue control set to minimize the flickering, the flickering I would expect to be very noticable. In fact I would guess the hue control adjustment to be more critical for CPA than the later IQ system. It is which ever suits you: a slightly wrong overal hue versus a maddening flickery picture. (I, of course, use the term "hue" control loosely: it is the subcarrier phase adjustment I am referring to).

Last edited by Penthode; 11-29-2011 at 12:14 AM.
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