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DV CODEC are clipping values

from Phil Glau (CircusRedk@aol.com)
The current Apple and Promax CODEC limit values to 16 and 235. Values above and below this number are clipped to 16 and 235 respectively. So what you say?

Well, many cameras, including the Canon XL1, can record values above 235. My experience with the Canon XL1 is that it can record up to 245 per color channel. When creating a dissolve or other effect using the footage, the transition between the end of the effect and the next frame of footage "pops." For example we have some footage where the subject is sitting and the sky is blown out behind him. When we dissolve this footage the image detail inside of the dissolve is clipped to 235 and at the end of the disolve, the next frame "pops" back to the full range of the source. (That is all the image detail between 235 and 245 is once again visible).

With regards to the Apple DV Codec clipping signal at 235, the ITU-R 601 Standard provides the lock-step reason for this decision. Nonetheless, it is wrongly implemented by Apple.

This is according to the Quantel Guide "The ADC [analog to digital converter] has only a finite set of numbers (an 8-bit ADC can output 256 unique numbers but no more) with which to describe the signal. The importance of this is such that the ITU-R 601 standard specifies this set-up quite precisely saying that black should correspond to level 16 and white to level 235 - leaving headroom for errors, noise and spikes to avoid overflow or underflow on the ADC."

In traditional terms, a level of 16 would correspond to 7.5 IRE and a level of 235 would correspond to 100 IRE. Apple has implemented these standards rigidly clipping the signal to 235 instead of using the overrun buffer up to 255. Even in the old world of analog, it was always possible to "over-modulate" the signal to a "hotter" level of say 105 to 120 IRE. With DigiBeta and D1, this is easily accomplished. What Apple has done is arbitrarily limit the ability to over modulate and thus use the available "headroom" described by the Quantel Guide. The fact that the Canon XL1 (and I suspect other DV cameras) can record a "hotter" signal speaks to the fact that this number of 235 is an artificial ceiling. Thus when dissolves or effects are rendered with the "hotter" footage, the Apple CODEC clips these values instead of using this extra headroom resulting in the "pop" I mentioned earlier. This headroom translates directly to the color spectrum available to the artists using the NTSC signal as well as contrast range. (Many commercials you see on TV exploit this extra headroom to deliver "hotter", "punchier" colors as well as greater latitude.) Furthermore, for those using DV for eventual output to film, the arbitrary broadcast standards of 7.5 and 100 IRE mean nothing and the full headroom of the signal should be available for use.

Since the 16/235 level corresponds to the technical level perhaps the ideal solution would be to write a CODEC which has selectable options for the 16/235 Standard as well as "full range" or user selectable upper and lower limits (similar to the Broadcast Safe filter in After Effects). Thus we could deliver product that either conforms or exploits the additional headroom available on DV, DigiBeta and other digital formats.

Phil

(posted 9139 days ago)

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