> ## Documentation Index
> Fetch the complete documentation index at: https://bedrockdynamics.studio/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Record & Replay Data

> Capture and play back telemetry sessions

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.

## 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.

<Steps>
  <Step title="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.
  </Step>

  <Step title="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.
  </Step>

  <Step title="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`.
  </Step>
</Steps>

<Note>
  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.
</Note>

## 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.

### Playback Controls

| Control      | Action                                                   |
| ------------ | -------------------------------------------------------- |
| Play / Pause | Toggle continuous playback                               |
| Step Forward | Advance by a single message timestamp                    |
| Speed        | Cycle through 0.5x, 1x, 2x, and 4x playback rates        |
| Seek         | Click anywhere on the timeline to jump to that timestamp |
| Scroll Zoom  | Scroll on the timeline to zoom into a time range         |

<Tabs>
  <Tab title="Keyboard Shortcuts">
    * **Space** -- Play / Pause
    * **Right Arrow** -- Step forward one frame
    * **Left Arrow** -- Step backward one frame
    * **+** / **-** -- Increase / decrease playback speed
  </Tab>

  <Tab title="Panel Synchronization">
    All panels receive the same playback timestamp. A 3D viewer shows the scene state at that instant, plots draw a vertical cursor at the current time, and log panels scroll to the corresponding entries. This synchronization is automatic and cannot be disabled.
  </Tab>
</Tabs>

<Tip>
  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.
</Tip>

## 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.

<Note>
  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.
</Note>