SMPTE LTC

What is LTC?

Linear Time Code (LTC) is SMPTE timecode encoded as audio, using bi-phase mark encoding. It sounds like a high-pitched buzz. If you've heard the timecode stripe on a tape, or seen a waveform that looks like a wall of square waves, that's LTC.

Because it's audio, it goes anywhere audio goes: XLR, 1/4" jacks, audio interfaces, mixing consoles, or onto a recorded track. That's why it still works at live events where there's no MIDI or network connection to be had.

Why use LTC?

LTC gets to places the other timecode formats can't:

• Audio cable carries it for hundreds of meters. MIDI gives up at about 15 m.
• The venue's installed audio snake or patchbay can route it, so there's nothing new to pull.
• You can record it next to the audio tracks and keep the sync for post-production.
• Anything with an audio input can receive it, including analog gear with no digital protocol support at all.

Virtual audio routing

To route LTC between applications on the same computer, use a virtual audio device. On macOS the usual choice is BlackHole (open source, the 2-channel build is enough). On Windows it is VB-CABLE from VB-Audio (free), which shows up as "CABLE Input" in the output list and "CABLE Output" in the input list.

• Install the virtual audio device, and reboot if the installer asks for it
• In TimecodeLink, select it as the LTC audio output
• In your show control software, select the same device as the audio input and enable LTC chase

To send LTC out of a physical connector instead, pick your audio interface's output in TimecodeLink's LTC settings.

TimecodeLink LTC output

TimecodeLink generates LTC from the current deck's timecode position. It follows the DJ's pitch changes: speed the track up and the LTC runs faster, so the visual cues stay on the music.