A Film Restoration Diary

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    Assessing the Scans Part 3 — Video Levels

    24 Jun 2015

    This post discusses video levels in the four kine scans. For the purposes of this discussion, we will talk about luminance values only since the Tarax Show scans are black and white and the colour information has been removed from the scans.

    First, let’s look at a screenshot of a “video scope” and the corresponding frame, as displayed in Final Cut Pro.

    A frame from one of the kine scans together with a video scope display that plots the luminance values in each scan line.

    This kind of display has been around since the very early days of television — engineers used them to adjust camera video levels to ensure they fell within the required broadcast-safe range. In those days it would have been a CRT screen displaying analog voltages representing video levels.

    In modern digital video software like Final Cut Pro, the “video scopes” are a software simulation of the old analog scopes. The vertical scale measures the numerical luminance values of the pixels in the video image. The horizontal scale represents a video scan line, or in this case a row of pixels. Every scan line or pixel row that makes up the video image has its luminance values “plotted” on the graph.

    When adjusting the tonal range of any still image or video, two crucial concepts are “black point” and “white point”. These are the darkest and lightest levels (or pixel values) in the image.

    Levels in Digital Video

    With digital images and video, levels are often expressed using an 8-bit/256-step scale. The darkest expressible value would be 0 and the lightest would be 255.

    For broadcast and DVD digital video, the nominal black point is mapped to 16 and the white point to 235, which permits level excursions outside this range without clipping: the range 0–16 for shadow detail and the range 235–255 for highlight recovery.

    The earlier graphic with overlays showing the video levels reserved for shadow detail and highlight recovery (red) and the range within which the pixel values should occur (yellow).

    The Tarax Show scans were made on scanners which capture data using 16-bits per pixel (allowing theoretically up to 65536 luminance levels for each pixel). These scans were then saved the Apple ProRes 422 HQ codec which provides 10-bits per pixel of video resolution, corresponding to 1024 possible luminance values in the range 0–1023. This is four times the 256 levels provided by 8-bit DVD video. This will allow us leeway to grade the scans for optimal reproduction on DVD without throwing away any tonal resolution.

    Observed Levels in the Kine Scans

    So, what do we see when we review these scans on the video scopes in Final Cut Pro?

    Although we are looking at 10-bit video, Final Cut Pro displays video levels as if we were looking at 8-bit/256 level video. The observed luminance values fall slightly outside the broadcast-safe levels 16–235, but are still well within the overall 0–255 range, which means that no digital “clipping” of whites or blacks has occurred — that’s a good thing.

    In a subsequent step, we will adjust the levels back to the to 16–235 broadcast/DVD safe range.

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