"Video Levels in Final Cut Pro" is a good, very detailed article on how Final Cut Pro deals with video levels. It just happens that at my facility we have been using a non-standard "wrong" way of handling video levels since we started editing. We now have to make a transition to the standard way of representing luminance, which is going to be a bit painful. Essentially we have not been using the appropriate level for black in Final Cut Pro because of how our equipment handles the conversion of analog video to digital video (and vice versa, converting digital to analog.) In analog video, the luminance value of an image is represented on the IRE scale. Black should equal 7.5 IRE while white is 100 IRE. Grays are in between with higher numbers being lighter than lower numbers.
However, North America is the only place in the world where black is 7.5--the rest of the world defines black as 0 IRE. The use of 7.5 IRE is also called "setup". This can create a problem when you are digitizing an analog signal. Does the analog-to-digital converter know which standard of black you are using (i.e. 7.5 IRE or 0 IRE.) In our case, the converter we used to digitize did not know what the appropriate black level was for us. It assumed our black level was 0 IRE when in fact our tapes, like all analog tapes in North America, were calibrated to have black at 7.5 IRE. This scenario is described in the article under the heading "Not Adding or Removing 7.5 IRE Setup." I made a diagram (fig. 1) to represent this.
Final Cut Pro, like digital tape formats (e.g. DVCam, HDV, DigiBeeta), does not have any use for "setup". For example, in the waveform monitor display in FCP, black levels read 0 on the scale. The waveform display is a representation of the luminance of a picture. The graphic at the top of this post shows a waveform display of a gradient from black to white. Technically, this isn't an IRE scale because IRE is an analog-only scale. The waveform display is meant give you an approximation of what the luminance values of your video should look like in the analog world. On all of our old footage, our black levels coming from our Beta tapes read 7.5 on FCP's luminance scale. Really the appropriate level for black in FCP would have been down to 0 on the FCP waveform. Unfortunately when I started at Link TV, we didn't have any way of adjusting our black levels while digitizing--we did not have a processor amplifier that could have adjusted the setup/black levels nor did we have an analog-digital converter that we could tell to "remove the setup". The Sony DVMC-DA2, which we used to digitize our Beta tapes, assumed that our blacks were 0 IRE and couldn't remove the setup from our 7.5 IRE black analog video.
To compensate for black levels on our digitzied video, we essentially redefined what black was in FCP. FCP wanted black down at the bottom of its scale at 0 on its waveform monitor. But all of our analog video that had been digitzed had blacks at 7.5 on the scale. We couldn't just leave a blank space in the timeline for black because this was blacker than the blacks in our analog video. So we had to make our own black clips with a color matte with a lumiance value of 4. (Logically you would think the matte clips would have a luminance value of 7.5 but that was too bright. To know why read farther down in Graeme's article on how FCP adjusts the gamma of still images.)
Well, we are finally moving over most of our editing systems to professional analog-digital converters from AJA and BlackMagic. These converters have software settings that allow you to control whether setup is removed while digitzing an analog signal. So it is possible to capture analog video with 7.5 black levels that gets digitized to FCP's default value of 0 for black. The AJA or BlackMagic hardware will also add the setup to the signal as it goes out the analog outputs back to Beta tapes. So when we lay off, our Beta master tapes will also have the standard 7.5 IRE black for analog videotape. So the FCP settings are right (black at 0), and the black levels on our Beta masters are right for North America analog video (black at 7.5 IRE.)
We haven't made the switch yet. We currently have a hundreds of promos that we have to update every couple of months that we keep on the system. Ideally we will make a clean break in the near future. After that every project that we either start or have to lay off while have blacks at 0 in FCP. That will be a lot of busy work, but we will finally be using the correct standard for our video. To get the old projects using black at 0 in FCP, we will probably use the color corrector to lower the black levels down to 0 in FCP. It would be great to redigitze all of our projects, but I think that is just too much work.
The article goes into more detail by describing the ITU-601 standard as well, which defines luminance values on an 8-bit scale of 0-255 (I skipped that scale in this description to try and make this complex issue simpler.) The article also describes some interesting issues with luminance values when importing graphics (e.g., from Photoshop.) The author Graeme Nattress is a great engineer type. He authored a good plug-in for converting PAL video to NTSC that we use sometimes at the office. Compressor does a better job of doing standards conversion, but his plugin is easier to use and faster.
No comments:
Post a Comment