Using Serial Device Control

Nearly all professional VTRs support serial device control. Serial device control sends and receives timecode and transport control data. A 9-pin serial connector is usually used for serial device control. Each pin can carry a separate signal, although typically only four pins are used (two for transmitting data, and two for receiving data).

Figure. An RS-422 connector.

Professional video and audio decks may use one of two serial interfaces.

Timecode Transferred via Serial Device Control

Final Cut Pro can use two kinds of timecode sent over a remote serial connection.

  • LTC: LTC (longitudinal timecode) is recorded as an audio signal on a dedicated timecode track.
  • VITC: VITC (vertical interval timecode) is recorded as part of the video signal, using several video lines that are normally masked by consumer televisions. Lines 16 and 18 of the vertical blanking portion of the video signal are commonly used.

Final Cut Pro can also automatically switch between LTC and VITC if they are both available:

  • LTC+VITC: If you choose this setting, Final Cut Pro looks at both timecodes so that accurate timecode can be read no matter what speed the tape is playing (LTC is used for normal and high-speed playback; VITC is used for slow motion and pause).

For more information about LTC and VITC timecode, see Timecode on Tape.

To take advantage of the Edit to Tape feature (described in Assemble and Insert Editing Using Edit to Tape), your video device must support insert editing. These VTRs can use timecode In and Out points, along with defined video and audio tracks, and perform a frame-accurate edit.

Calibrating Timecode Capture with Serial Device Control

Unless you are capturing a DV video format via FireWire, the timecode and video signals are sent separately from the video deck to the computer. Because the signals are separate, they can possibly arrive at different times, which causes the wrong timecode number to be recorded with captured video frames. You can calibrate your timecode and video capture setup by entering the number of frames by which the timecode signals are offset from the video into the Capture Offset field.

Note: Remember that timecode is captured via the device control connection (the RS-422 connection in most cases), and video is captured via your video capture interface. In the case of DV formats, FireWire is conveniently used for both.

You need to calibrate the video and timecode signal every time you change decks or cables. If you regularly switch decks, it’s a good idea to create different device control presets, with different Capture Offset settings, for each deck you use. Because each deck will always have the same capture offset, a set of precalibrated presets lets you quickly change decks without having to go through the whole recalibration procedure again.

For more information on setting up your hardware for device control, see Connecting Professional Video and Audio Equipment. For more information on setting up device control presets, see About Device Control Presets.

Determining and Entering the Timecode Offset

You can easily determine if there is a difference between the timecode and video signals by capturing test footage that has timecode “burned in” to the video picture information. By comparing the numbers in the timecode track to the timecode numbers in the video picture, you can determine the offset between them.

If there is an offset, you can calibrate your timecode and video capture setup by entering the number of frames by which the timecode signals are offset from the video in the Capture Offset field of the current device control preset.

To determine the timecode offset
  1. Do one of the following:

    • Create a window-burn (or window dub) test tape for timecode offset testing. You can do this by transferring a tape with timecode to another tape of the same format, making sure you connect an output that can display timecode information visually.

    • Connect the monitor out (or super out) output of your VTR to the input of your video interface. A monitor output is a dedicated output for displaying VTR status information such as a timecode counter superimposed (or “burned-in”) over the video information. Some VTRs don’t have a separate monitor output, but instead have an option for turning on the deck status display and timecode information on the main output. Turn on this option.

  2. Capture a clip of video from a tape with LTC (Longitudinal TimeCode) or VITC (Vertical Interval TimeCode) timecode on it.

    For more information, see Capturing Video from Tape.

  3. Compare the timecode value recorded in the captured media file (in the Current Timecode field) with the burned-in timecode on the video frame.

    Figure. Viewer window showing the Current Timecode field and the burned-in timecode.

    If both timecode values match, then there is no offset, so no adjustment is necessary. If the timecode values do not match, there’s an offset and you’ll need to compensate for this to capture your video properly.

To enter a timecode offset
  1. Choose Final Cut Pro > Audio/Video Settings, then click the Device Control Presets tab.

  2. Make sure your current method of device control is checked, then click Edit.

    If you’re using a locked preset, a new copy of the preset is created.

  3. Enter a new name and description for this device control preset indicating which camcorder or deck it is calibrated for.

    Figure. Device Control Preset Editor dialog showing the Capture Offset field.
  4. Enter a number of frames in the Capture Offset field, then click OK.

    Enter a negative number if the clip’s timecode track has higher numbers than the burned-in timecode. For example, if the clip’s Media Start timecode is 01:00:00:04, and the first video frame has a burned-in timecode number of 01:00:00:00, enter –4 in the Capture Offset field.

    After entering the timecode offset, it’s a good idea to capture a few more clips and compare the timecode track numbers with the burned-in timecode to determine if the timecode offset you entered is correct.

Important: Some devices may require fractional offsets to maintain accuracy when batch capturing over a long duration. For example, suppose that capturing a single clip using a 2-frame offset results in an accurately captured clip, but when you batch capture ten clips, some clips are not accurately captured. In this case, try adjusting the offset by a fractional value (for example, 2.5 instead of 2).