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Substrate supports recording live telemetry streams to MCAP files and replaying them for post-session analysis. Every panel — 3D viewers, plots, logs — synchronizes to the same playback clock, so you can correlate events across data sources at any point in time.

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Recording

To capture a session, click the Record button in the toolbar (the circle icon). While recording is active, the button pulses red and all subscribed topic data streams into an MCAP file.
1

Start recording

Click the Record button in the toolbar, or use the Command Palette and search for “Start Recording”. Recording begins immediately for all active topic subscriptions.
2

Monitor recording

The status bar displays the recording duration and file size as data accumulates. All topics that are currently subscribed in any panel are included in the recording.
3

Stop recording

Click the Record button again to stop. The MCAP file is saved to your workspace directory with a timestamp-based filename, for example session_2026-02-10T14-30-00.mcap.
Recording captures raw message data at full fidelity. No downsampling or lossy compression is applied. File sizes depend on the number of topics and their publish rates — a typical PX4 SITL session with telemetry and pose data produces roughly 5-15 MB per minute.

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Replay

Open any .mcap file from the file explorer or via File > Open. Substrate detects the format automatically and switches to replay mode. When an MCAP file is loaded, a playback bar appears at the bottom of the workspace. All panels synchronize to the playback position.

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Playback Controls

ControlAction
Play / PauseToggle continuous playback
Step ForwardAdvance by a single message timestamp
SpeedCycle through 0.5x, 1x, 2x, and 4x playback rates
SeekClick anywhere on the timeline to jump to that timestamp
Scroll ZoomScroll on the timeline to zoom into a time range
  • Space — Play / Pause
  • Right Arrow — Step forward one frame
  • Left Arrow — Step backward one frame
  • + / - — Increase / decrease playback speed
MCAP is the modern replacement for ROS bag files. It supports random-access seeking, self-describing schemas, and multi-topic storage in a single file. It is the standard recording format used by Foxglove and Rerun.

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ULog Flight Logs

Substrate also supports PX4 ULog files (.ulg). When you open a ULog file, Substrate applies a purpose-built layout with a 3D flight path viewer, GPS map, telemetry plot groups, and a log message panel. All panels synchronize to the same playback clock, just like MCAP replay.
ULog files contain PX4-specific parameter snapshots, logging metadata, and multi-rate message topics. Substrate parses all of these and exposes parameters as browsable constant-value channels.