Timecode Cues

Timecode cues comprise QLab's mechanism for generating outgoing timecode.

Unlike most timecode-enabled applications which have a single, fixed timeline from which timecode can be generated directly, or which can be driven by incoming timecode, QLab allows for multiple independent timelines running concurrently. By encapsulating timecode output into a cue, QLab can generate multiple independent timecode streams to drive other devices.

When a Timecode cue is selected, two tabs will appear in the Inspector:

  • Basics
  • Settings

Basics

Please refer to the section on the inspector in the Getting Started section of this documentation.

Settings

Type. Timecode cues can generate either MIDI Timecode (MTC) or Linear Timecode (LTC). Depending on which type you select here, the inspector will show different output options.

LTC (Linear, or Longitudinal, Timecode) is meant to be carried on a line-level audio connection. When LTC is selected, the Destination controls allow you to select an audio device and a channel number to output to. The channel number defaults to 0, which is not valid, so a channel number valid for the selected device must be specified.

Warning: Use caution when setting up the routing of an LTC signal, from QLab or any source. LTC contains strong high-frequency components which, if accidentally routed out to speakers, can cause damage to hearing and to friendships.

MTC (MIDI Timecode) carries most of the same information as LTC, but on a MIDI connection rather than audio. This can be a physical MIDI port, or an IAC bus. No channel number is required, as MTC messages are not associated with a MIDI channel.

Sending MTC over a network is discouraged, as the inconsistent latency of a network connection can often render the timecode signal inaccurate, or even unreadable, on the receiving end. Inexpensive MIDI interfaces often create the same problem, even those that work well for normal musical MIDI. Always use a reliable, high-quality MIDI interface when working with MTC.

Framerate. QLab supports both video speed and film speed options at all common framerates. Be sure to match the framerate on the receiving end precisely.

Broadly speaking, "film speed" refers to the timecode formats with integer framerates (24, 25, 30 drop, 30 non-drop), while "video speed" refers to their non-integer counterparts (23.976, 24.975, 29.97 drop, and 29.97 non-drop).

Each video speed framerate is an identical timecode format to its film speed counterpart, but pulled down by 0.1%. For example, a frame of 29.97 non-drop timecode consists of the same data as the same frame of 30 non-drop, but at a 0.1% slower rate. Neither LTC nor MTC differentiates between video speed and film speed in how the bits are encoded, so timecode at the wrong speed will initially appear correct on the receiving end. However, the timecode will drift noticeably over time from what is expected unless the speeds match.

Timecode cues are clocked differently depending on the type selected. LTC follows the clock of the audio device to which it outputs, and is guaranteed not to drift from that clock. MTC, on the other hand, follows the computer's internal clock. Under normal use, drift between high-quality devices is usually minimal, but if drift-free synchronization with another machine is required over long stretches of time, the best option is to output LTC to an audio device that can resolve to the same master clock (via word clock, etc.) as the other machine.

Start time. This control allows you to specify the first frame of timecode that is transmitted when the cue is triggered. Bear in mind that both LTC and MTC can take up to a few frames to transmit enough information to read. If an event needs to be triggered on a specific frame, it is best to start timecode output a few frames ahead, as preroll into that event. This is why the default start time for a Timecode cue is 1:00:00:00 rather than 0:00:00:00. Because there is no room for preroll before hour 0, the best practice is to treat hour 1 as the beginning of the timeline, with space for preroll beforehand.

Broken Cues

Timecode cues can become broken for the following reasons:

No MIDI destination.

Choose a MIDI destination in the Settings tab of the inspector. You may also need to visit the MIDI section of Settings and connect a MIDI device to the desired patch. This is only relevant to Timecode cues set to the MTC type.

Invalid LTC audio channel.

Choose an audio device and channel in the Settings tab of the inspector. You may also need to visit the Audio section of Settings and connect a MIDI device to the desired patch. This is only relevant to Timecode cues set to the LTC type.

A Pro license is required to reactivate this saved cue.

You'll need to install a Pro Audio, Pro Video, or Pro Bundle license in order to use this cue.