Using SMPTE for synchronization with audio recordings

The Qualisys system can be synchronized with audio recordings by using the SMPTE timestamp via the Oqus or Camera Sync Unit. QTM has been tested with the MOTU 828mk3, but in theory any equipment with an SMPTE signal can be used. Check the following information to synchronize QTM with audio recordings. For information about the settings for external timebase see chapters External timebase and Timestamp.

  1. It is important to enter the nominal frequency and the SMPTE frequency on the Synchronization page in the Project options dialog. Otherwise, the data cannot be compared correctly with the audio recordings.

  2. For Oqus systems, make sure to connect the Oqus sync unit to the master camera, the camera with an M in the display, see chapter Using Oqus sync unit for synchronization.

  3. If you want a sync output frequency from the camera system above 5000 Hz, you must use the master camera.

  4. Synchronizing with Word clock or SMPTE
    In theory the Word clock should be better to use as external timebase than SMPTE. However, the SMPTE signal from the MOTU unit is stable enough for the camera synchronization and it has the advantage that the marker capture will start without a variable offset to the SMPTE time code signal.

    1. If you use SMPTE as the sync source then the start of the measurements will be aligned with the SMPTE frames, see image below.

      If you use SMPTE as external timebase it is important that you do not use a too large divisor so that the resulting frequency becomes lower than 2 Hz. This will result in the camera not being able to synchronize to the signal.

    2. When using Word clock it is important that you also use the SMPTE clock for reference of the start of the measurement. However, alignment of the camera frames relative to the SMPTE frames will not be exact and there will be a variable delay of up to one camera frame. For example at 200 Hz the maximum start offset is 5 ms, see image below. This is because the camera can start at any time within an SMPTE frame and then the camera frame will not be aligned with the SMPTE signal.

  5. It is recommended to use a multiple of the SMPTE signal as camera frequency. This makes the data easier to compare to other systems. The data is synchronized even if it is not a multiple, but then you will have to take into consideration that the number of camera frames per SMPTE frame will vary.

  6. The camera frequency with external timebase cannot be increased beyond the maximum frequency at full FOV when using reduced image size. This is because the preview mode uses the full image size of the camera.

Audio recording with the MOTU 828mk3

To be able to synchronize the QTM capture with the audio data it is important that the SMPTE time code is associated with the audio recording. This can be done in the following three ways with the MOTU 828mk3.

  1. When using the MOTU device on a Mac you can use a special setting called Generate from sequencer in the MOTU SMPTE console. With this option the SMPTE signal is generated based on the time position in the MOTU Digital Audio Workstation (DAW) software used for recording audio. However, this requires that you start the recording in the MOTU DAW software before you can start preview in QTM and that the recording must keep running while the cameras are running. Then it is important to select the BWF format for the files in the DAW software so that the start of the SMPTE time can be read from the WAV file header. For detailed information about the BWF standard, see https://tech.ebu.ch/docs/tech/tech3285.pdf.

  2. On Windows you must connect another SMPTE source to the MOTU unit and use a program that supports ASIO synchronization (e.g. Cubase), if you want to see the same SMPTE time in the program as in QTM and save audio files in BWF format with the correct timestamp.

  3. The third alternative works on any multitrack audio recorder and that is to record the SMPTE signal on a parallel audio track. In this case you must decode the SMPTE signal to synchronize the audio files. For Matlab, a solution is available for pairing and cropping audio files to mocap files using a SMPTE decoder. Contact Qualisys support if you are interested.