Qlab 5 and LTC issues

[José Miguel Mayoral]

Hi.

. I'm having a problem with a timecode cue. I'm playing a video with audio that simultaneously triggers the timecode cue. A grandma receives the timecode, but after 5 seconds it drops out. I'm using Qlab 5 on a Mac Studio and a Focusrite 18i20, channels 1 and 2 for the video's audio and output 3 for the timecode. I stumbled upon the conclusion that the problem was the volume of the audio in the video. If I lower it to -10, the timecode doesn't drop out, but I don't understand why, since I'm supposedly using three independent channels.

Thanks.

[Sam Kusnetz]

I think you may need to share your workspace for us to figure it out.

sk

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Sam Kusnetz (he/him) | Figure 53

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[Paul]

The LTC audio signal should sound uncomfortably loud if you listen to it (not recommended!). Generally close to 0dB. I would adjust the level  of the third output (either in the video cue or the audio patch), so that output 3 is significantly louder than the actual audio (ch1&2) from the video.

[Rich Walsh]

I don’t think that’s entirely accurate, and also doesn’t explain why the effectiveness of LTC on output 3 should be a function of the level on outputs 1 & 2. The grandMA appears to expect an analogue signal between -11dBu and +15dBu: https://help.malighting.com/grandMA3/2.0/HTML/fs_connect_smpte_ltc.html. How that relates to “0dB” (presumably you mean 0dBFS?) depends on the gain structure of the audio interface.

In fact, since LTC is a continuous audio signal, you often find you have to drop its level to avoid crosstalk in analogue multicores. Perhaps that’s the reverse issue here: crosstalk from the audio signal is disrupting the LTC signal post interface? The Focusrite 18i20 has a maximum output level of +16dBu (which would clip the grandMA’s inputs). Adjusting the levels so that the LTC signal is leaving QLab at -16dBFS should generate a healthy 0dBu analogue signal, in the middle of the grandMA’s acceptable range.

It could also be something really stupid like the Focusrite 18i20 not having enough power and therefore dropping signal levels once the load becomes too high… Does something exciting happen in the audio at 5s that would correlate with a dip in LTC level? Is there a parallel box available to split the signal and see what is going on with it – maybe by routing it back into an input to monitor?

Rich

[Dominic Bilkey]

Hello

Is it possible the audio from the video is also routed to output 3 somehow?

Lowering the volume then below the LTC signal would explain how that fixes the problem and, from experience of assisting someone in a similar situation before, is the very simple but obvious problem. 

Thanks

Dom

On 10 Jun 2026, at 09:37, 'Rich Walsh' via QLab <ql...@googlegroups.com> wrote:

I don’t think that’s entirely accurate, and also doesn’t explain why the effectiveness of LTC on output 3 should be a function of the level on outputs 1 & 2. The grandMA appears to expect an analogue signal between -11dBu and +15dBu: https://help.malighting.com/grandMA3/2.0/HTML/fs_connect_smpte_ltc.html. How that relates to “0dB” (presumably you mean 0dBFS?) depends on the gain structure of the audio interface.

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[Rich Walsh]

Oh, that’s a much more obvious thing to check! Didn’t think of that one.

Rich

[José Miguel Mayoral]

I can't check it yet, but I think there might be some leakage through output 3 where I'm sending LTC, and the two signals are mixing.

[Charlie Pilzer]

On Jun 10, 2026, at 1:09 PM, José Miguel Mayoral <mode...@gmail.com> wrote:

<IMG_2578.jpeg><IMG_2577.jpeg>

I can't check it yet, but I think there might be some leakage through output 3 where I'm sending LTC, and the two signals are mixing.

I suggest looking at the mixer/routing inside the Focusrite Scarlett control software. Qlab is likely sending to the proper channels and the software is doing the combining.

Charlie Pilzer

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