Working with Analog Video

If you need to output video to an analog format, make sure you understand how digital signal levels in Final Cut Pro are translated to analog levels.

How Analog Video Signals Are Measured

Analog video is measured using IRE units. (IRE originally stood for Institute of Radio Engineers, which has since merged into the modern IEEE organization; the measurement is a video-specific unit of voltage.) One IRE is 7.143 millivolts, but it’s easier to remember that an analog video signal has a range of 1 volt, which spans 140 IRE units. Synchronization pulses, which are necessary for an analog television broadcast, are located between -40 and 0 IRE.

PAL and Japanese NTSC systems have luma (or Y′) values between 0 IRE (black) and 100 IRE (white), although higher values are possible. Signals above 100 IRE are considered illegal for broadcast. North American NTSC systems set black at 7.5 IRE. This extra 7.5 IRE is called setup or pedestal (because it pushes the entire image higher on the waveform monitor).

How Digital Video Signals Are Measured in Final Cut Pro

The Final Cut Pro Waveform Monitor displays Y′CBCR values as percentages instead of bit values because digital video can use 8 or 10 bits per luma sample. Using 8 bits provides up to 256 tones from black to white, although the actual range used in Y′CBCR video is 16 (black) to 235 (white). The remaining values, 236 through 254, provide additional headroom used to record super-white levels such as specular highlights on shiny objects.

Percentage in Final Cut Pro Waveform Monitor
Values for 8-bit Y′CBCR
Values for 10-bit Y′CBCR
-10%
n/a
n/a
0% (black)
16
64
100% (white)
235
940
109% (super-white)
254
1019

Using an External Waveform Monitor and Vectorscope to Calibrate Analog Video Levels

The scopes in Final Cut Pro display the digital values of your video as it exists in its captured state, but the software scopes can’t measure input and output analog signal levels from and to your FireWire device or third-party video interface. If you like, you can set up your Final Cut Pro workstation with a dedicated waveform monitor and vectorscope for measuring and adjusting video input and output more accurately.

This involves using two pieces of external video hardware:

  • Processing amplifier (or “proc amp”): A proc amp provides hardware control of luma, or video gain (brightness), chroma gain (saturation), hue, and setup (black level). By connecting a proc amp between your analog VTR and your video interface, you can precisely control the incoming Y′CBCR video signal.

    Note: Some VTRs have a built-in proc amp.

  • Hardware waveform monitor or vectorscope: Having a dedicated hardware waveform monitor or vectorscope enables you to measure the actual analog Y′CBCR output from your video interface.
    Figure. Diagram of a waveform monitor/vectorscope connected to a video editing system.

Outputting Accurate Analog Black Levels Using DV FireWire

Outputting analog video from Final Cut Pro requires a video interface with a digital-to-analog converter. Many third-party interfaces include analog outputs. Alternatively, you can use a DV deck or camcorder to convert a digital signal via FireWire to an analog signal for recording to tape. This is the same configuration you would use to monitor DV FireWire output from Final Cut Pro on a standard video monitor.

If you output to analog tape using the built-in FireWire interface of your computer with a consumer DV device, keep in mind that not all DV devices use the same analog black level. Digital video always has a digital black value of 0 percent, but some DV devices convert this value to 0 IRE and others convert this value to 7.5 IRE.

Professional video facilities are fairly stringent about keeping black setup levels at precisely 0 IRE for PAL and for NTSC in Japan, and at 7.5 IRE for NTSC in North America. If your DV device outputs at the wrong analog black level, you may need to use a hardware proc amp to adjust your analog video signal accordingly.

If you are outputting back to analog tape using a third-party analog video interface, you should check the documentation that came with the video interface to determine how to configure the video interface for the North American standard for setup (7.5 IRE) or the Japanese standard (0 IRE). Most vendors of analog video interfaces include a software control panel that allows you to select which black level to use. Most vendors label this as “7.5 Setup” versus “0 Setup,” or in some cases “NTSC” versus “NTSC-J.” For more information, see How Analog Video Signals Are Measured.

The following is a typical configuration for calibrating the analog black levels coming from your digital-to-analog converter (in a DV deck or digital-to-analog converter):

  • Using FireWire, Final Cut Pro outputs video with black at the industry-standard digital value of 16 (as specified by the ITU-R BT.601-4 engineering specification for video). This is the correct level for black for digital video.

  • The analog outputs of your VTR are connected to a proc amp that you can use to adjust the analog black level, or setup.

  • The signal from the proc amp is connected to a video monitor and then to a waveform monitor, where you can measure to make sure the proc amp is adjusted for proper black levels. For NTSC signals in North America, you should adjust the proc amp so that the waveform monitor shows a black level of 7.5 IRE. For PAL and NTSC in Japan, the output signal should be 0 IRE.

    Figure. Diagram of a video editing system showing a proc amp.

Using Built-in Video Scopes During Capture

If you are capturing analog video footage using a third-party video interface, you can use the Log and Capture video scopes to calibrate the video interface input settings to the color bars at the beginning of each tape. Final Cut Pro saves individual image-control settings with each clip you log. You can then have Final Cut Pro capture your offline clips using these individually saved image-control settings.

Adjusting Capture Settings with the Log and Capture Video Scopes

The video scopes in the Log and Capture window include the Waveform Monitor and the Vectorscope. If you are capturing to a Y′CBCR video format, the entire range of the scope is displayed (from –10 percent to 109 percent). However, if you are capturing to an RGB codec, the range is limited to 0 percent to 100 percent.

To use the built-in Log and Capture video scopes to adjust your clip settings
  1. Make sure your video deck is connected to the video interface in your computer.

  2. Cue the videotape to the color bars recorded at the beginning of the tape.

  3. Choose File > Log and Capture, click the Clip Settings tab, then click the Video Scopes button.

    The Waveform Monitor and Vectorscope appear below the Log and Capture window. On the left is the Waveform Monitor, which displays the brightness levels of the color bars within the video frame as a graph. Each “step” of the graph corresponds to one of the color bars. The goal is to adjust the brightness and contrast so that the levels of the bars match their ideal targets, shown in purple.

    Figure. Line Waveform Monitor and Vectorscope.

    Note: Don’t worry if the graph appears to be “noisy”; this is normal for analog video played back from tape.

  4. To display the ideal targets for color bars, Control-click in the Waveform Monitor or the Vectorscope and choose Targets from the shortcut menu.

    Figure. Shortcut menu showing the Targets command.
  5. Adjust video brightness and contrast using the Waveform Monitor for reference.

    Figure. Clip Settings tab showing Brightness and Contrast sliders.

    Different third-party video interfaces have different controls for adjusting the video signal being captured.

    • If your video interface uses Brightness and Contrast sliders: Make the following adjustments.
      • Adjust the Brightness slider so that all of the bars in the Waveform Monitor are set near the upper purple targets in the background.

      • Adjust the Contrast slider so that the tops of the bars and the bottoms in the Waveform Monitor are within the boundaries of the top and bottom targets.

      • Continue adjusting the Brightness and Contrast sliders until there is an acceptable range of brightness between the 100 and 0 percent targets.

    • If your interface uses Black Level and White Level sliders: Make the following adjustments.
      • Adjust the Black Level slider so that the third bar from the left is at the 0 percent target toward the bottom of the Waveform Monitor.

      • Adjust the White Level slider so that the second bar from the left is at the 100 percent target at the top of the Waveform Monitor.

  6. Adjust the clip’s hue and saturation using the Vectorscope for reference.

    The Vectorscope is on the right and displays the color values of the different areas of the video frame as a point graph. Each point on the circular graph represents one of the color bars. The goal is to line up these points with their respective targets.

    Figure.  Line Waveform Monitor and Vectorscope.
    • Adjust the Hue slider to rotate the points around the center of the graph.

    • Adjust the Saturation slider to move the points closer to or farther from the center of the graph. If this slider is properly adjusted, each point falls within a purple target box.

Adjusting Capture Settings on Source Tapes Without Color Bars

If there are no color bars on your source tapes, you can adjust your tapes using the flesh tones of people in the shot. As long as a person’s skin tones look accurate, chances are the rest of the colors in that shot will look fine. To help you find the correct flesh tone, a special marker appears in the Vectorscope to the left of the center of the Vectorscope scale. This line indicates the hue of a person’s skin tones.

Regardless of race, the hues of human flesh tones, when recorded to videotape and measured on a vectorscope, fall along a fairly narrow range (although the saturation and brightness vary).

To adjust color on source tapes using flesh tones
  1. Cue your tape to a section that has a well-lit facial closeup.

  2. Check to see if there’s a spike on the Vectorscope near the Flesh Tone line.

  3. Adjust this spike so that it falls approximately along the Flesh Tone line. While doing so, check the image quality of your clip on a broadcast monitor to make sure that your adjustment is accurate.

    The Flesh Tone line is only meant to be an approximation; it’s more important that the shot look correct than that it fit this line exactly.

Figure. Vectorscope showing the Flesh Tone line.