Chasing external LTC

Chase mode turns LTCast into a timecode follower. Another device is the master clock; LTCast reads its LTC signal and follows it. The big timecode display shows the external time, the playhead and Next Cue countdowns move with it, the setlist song that matches that time is loaded for you, and the cues and triggers of that song fire as the external clock reaches them. Playing the song's audio in step with the master is optional.

How LTCast finds the song

Chase matches the incoming timecode to a setlist song only through LTC that is inside the song's own audio file and was found by LTCast's LTC scan. When you add songs, LTCast scans them in the background (see LTC scan badges) and remembers which stretches of timecode each file contains. While chasing, it looks the incoming time up in those stretches:

• If it falls inside a scanned stretch, that song becomes the active song and the playhead goes to the matching position. If two songs contain the same timecode, the one higher in the setlist wins.
• If it falls outside every scanned stretch, nothing changes: the active song stays and the playhead and Next Cue panel hold still, although the big timecode display keeps showing the external time.

Songs that contain no LTC cannot be chased yet. To chase a song, its file must carry the LTC — a music-plus-LTC file, or a song you merged from separate files with Separate music + LTC import. For a quick test, export a file with LTC WAV Export, add it to the setlist as a song and wait for its scan to finish.

Very long songs and AIFF files are an exception to the background scan. A song too long for the normal decoder (about 1 h 33 min at 48 kHz), larger than about 500 MB, or in AIFF format is loaded by streaming and the background scan cannot read it. For such a song LTCast takes the LTC from the file when you load it, so chase finds the song from that first load on and the result is saved with the show — but a long song that has never been loaded is not found by chase. See Loading audio files.

Enabling chase mode

1. In Settings → Devices, choose the LTC Input Device — the audio input that carries the external LTC. It can be a physical audio interface input, a virtual device (VB-CABLE Output, BlackHole) or a system input. Names appear only after the operating system has given microphone permission; click Grant microphone access if prompted. Next to it, choose the channel: Auto-detect (parallel) listens on channels 1–4 and locks onto the first one that carries LTC, or pick CH 1 to CH 4.
2. Turn chase on with any of these: Settings → Timecode In → Chase → Enable chase mode; the CHASE pill in the status bar; or Ctrl+Shift+C (Cmd+Shift+C on macOS).

The first time you enable chase while audio output is off, a message explains that the audio is muted by default and links to the setting. If no input device is chosen yet, the Chase tab shows an amber link that jumps to Devices.

Chase is free: the Chase tab and its settings work without Pro. Sending the timecode on — TC Bridge, and the MTC, Art-Net and OSC outputs — needs Pro.

Status indicators

Chase has four states:

• Idle — chase is on but no LTC has been heard yet.
• Chasing — valid LTC is arriving and LTCast is following it. Gaps shorter than half a second are ignored.
• Freewheeling — the LTC stopped and Freewheel is on: the chase clock keeps counting on the computer's own clock.
• Lost — the LTC stopped and Freewheel is off.

You see the state in several places. The CHASE pill dot is cyan while chasing, amber while freewheeling, red when lost and grey when idle. The Chase tab in Settings spells the state out. While chasing, the timecode display carries an EXT badge, and when the signal is lost a red LTC SIGNAL LOST banner covers it. The setlist header shows a CHASE LOCKED badge.

The setlist while chasing

Once chase has heard LTC (Chasing, Freewheeling or Lost), the external source decides which song is current, so the setlist is locked: a "Chase locked — Disable Chase to switch songs manually" overlay covers it, and clicking a song only shows a message to that effect. The keyboard shortcuts that preselect or start songs (↑, ↓, Enter) and the arrow-key scrub are ignored too. Changing the song or the show by hand is locked as well. Dropping an audio file or a setlist row onto the window, File → Load Song and a MIDI remote Go to song are refused with "Chase is running — stop Chase to change the song manually". Opening, importing or starting another show — the File menu, Ctrl+O, the saved-show picker, New show, or a .ltcast file opened from the operating system — is refused with "Chase is running — stop Chase to open or start another show". Turn chase off to pick songs by hand. In the Next Cue panel and the Calling Script, clicking a cue to jump is also disabled while chase owns the playhead.

Cues while chasing

The triggers (MIDI and OSC) of the active song fire when the external time passes their trigger timecode, with the same one-frame precision as normal playback. If the master jumps or scrubs, LTCast treats it as a seek: triggers before the new position are marked as already fired and the ones after it are armed again. When chase enters a song part-way through, the earlier triggers are not fired in a burst. Triggers need incoming frames, so they do not fire while the chase clock is freewheeling.

Audio output while chasing

By default LTCast does not play the song's audio while chasing — you get the display, the playhead and the cues only. The status bar shows 🔇 MIDI only next to the CHASE pill; click it, or tick Output audio while chasing in Settings → Timecode In → Chase, and it changes to 🔊 AUDIO. With audio on:

• LTCast plays the matched song from the master's current position and re-aligns if the master jumps by more than half a second.
• When chase moves to another song, the song loads and its audio starts at the master's current position — not at the start of the song's LTC, and not where the master was when the song change was detected. A long song can take a few seconds to load, and the audio still lands where the master is by then.
• If the LTC is lost and Freewheel is off, LTCast pauses the audio so it does not run on unsynchronised.
• A deliberate Pause is respected: chase will not restart audio you paused by hand until you press Play.

This is useful when the master carries no music and LTCast supplies both sync and playback. The outputs that come from that local playback — the file's LTC, MTC, Art-Net and OSC — behave as in normal playback, including the Pro rule for outputs.