OSC feedback (TC acknowledgements)
OSC and Art-Net are one-way: LTCast sends and never learns whether anything received it. TC Feedback closes that loop for OSC. A downstream rig — a media server, a patch you wrote yourself — sends back the timecode it has just received, and LTCast shows it next to its own so you can see at a glance whether the receiver is alive and how far behind or ahead it is.
What the receiver has to send
LTCast listens for a single message:
- Address
/ltcast/tc_ack - Four integers, in this order: hours, minutes, seconds, frames
- As a plain OSC message — bundles are ignored, as is anything with another address or argument pattern
Send it to the computer running LTCast, on the port you set below. Nothing else is needed on the LTCast side: feedback does not change what LTCast sends.
Turning it on
The setting is at the bottom of Settings → Outputs, under TC Feedback (Inbound). It is off by default, and for safety it is switched off again every time LTCast starts, so tick it again each session.
- Port — the UDP port LTCast listens on (default
9001, allowed range 1024–65535). - Bind — Local only (127.0.0.1), the default, accepts feedback only from software on the same computer. Choose LAN (0.0.0.0) if the receiver is another machine; LTCast then listens on every network interface, and on Windows the firewall may ask you to allow it.
Set the port and bind first, then tick the checkbox — those two fields are hidden while the listener is running. The status reads Listening.
Reading the device list
While listening, LTCast lists every sender it hears from, one row each:
- the sender (its IP address and port);
- the timecode it last reported;
- the drift in frames — the reported timecode minus the timecode LTCast is showing right now. A positive number means the receiver is ahead. It is green within 1 frame, amber up to 4 frames, and red beyond that.
Until something arrives the list says "No devices reporting yet". LTCast tracks at most 32 senders and ignores a sender that floods it with more than 200 packets per second.
Drift is a comparison of two clocks seen a moment apart, so a difference of a frame or so is normal, especially over Wi-Fi. Use it to spot a receiver that has stopped or fallen several frames behind, not to measure latency to the frame.
Pre-Show Check
With OSC output on, Pre-Show Check has an OSC Reachability row. It is green only when the target IP address is valid and an acknowledgement has arrived within the last 60 seconds; otherwise it stays amber. Without feedback turned on it cannot be green, so treat amber there as "not confirmed" rather than "broken".