Waveform Monitor boo-boo forewarned

Posted by dcouzin 
Waveform Monitor boo-boo forewarned
October 26, 2013 04:05PM
Graeme Nattress wrote the excellent report "Video Levels in Final Cut Pro" back in 2005. It explains what the FCP Waveform Monitor really shows. The report is quizzical about what the FCP -9 level represents, and it warns that it should not represent code value 1 (in 8-bit). Unfortunately the report gives no example of the Waveform Monitor reading below Black = 0, leaving the question whether the warning was necessary hanging.

The FCP7 Waveform Monitor is pretty flaky about sub-blacks. Sometimes it shows them and sometimes not. I made a grey target with Y' taking the full 10-bit range 4-to-1019. It has exactly one pixel per code value. Miraculously the Waveform Monitor displayed the sub-blacks of this target. Since Waveform Monitor is 8-bit it sees this target as having a 1-to-254 range.

Waveform Monitor gives the curve a sharp bend at Black = 0 which it shouldn't have. The scope has indeed put code value 1 (in 8-bit) at the -9 level (just above the -10 level). Once again, Apple's coders have done the arithmetic wrong. Nattress's warning was necessary.

Dennis Couzin
Berlin, Germany
Re: Waveform Monitor boo-boo forewarned
October 28, 2013 01:54PM
Interesting. Thanks for researching that.
Re: Waveform Monitor boo-boo forewarned
October 29, 2013 02:59AM
This seems like a really important post, but I'm having difficulty grasping it. The waveform monitor sometimes shows sub blacks, sometimes not? Is there a fix or is this just something to be aware of and take into account when color correcting in FCP?
Re: Waveform Monitor boo-boo forewarned
October 29, 2013 04:35PM
My post abbreviated the advice of an experienced NLE programmer:

Don't rely on the FCP scope to confirm the presence of sub-blacks. Their behaviour is flakey, to say the least. Sometimes the scope shows the image being clamped at black, but then you nudge the timeline a frame forward and the sub-blacks are revealed.

Waveform Monitor just did it to me! But it wouldn't do it again, ephemera being what they are. So I emptied caches, restored prefs, and rebooted. This time Waveform Monitor did an interesting dance. The clip is a wedge, so Waveform Monitor shows a ramp. For about a second it showed a ramp clipped at black = 0. Then it changed to the full (bent) ramp.

I suspect FCP's rendering and not-rendering, its strange RT methodology, lies behind Waveform Monitor's misbehavior.

Dennis Couzin
Berlin, Germany
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