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Remote Audio I/O

Overview

Remote Audio I/O uses an audio device attached to another MeasureLab computer on the same LAN as a measurement input or duplex input/output. It is an internal MeasureLab AudioEngine backend, not an operating-system virtual audio driver.

Sample position and measurement integrity take priority over concealment. Missing audio is not interpolated. A missing range is replaced by an equal-length zero range and reported visibly as data loss.

Preparing the remote computer

  1. In Settings, select the physical input/output devices, sample rate, buffer size, and channels used for remote measurement.
  2. Open Remote Audio I/O.
  3. Select Share this computer's audio.
  4. Enable Allow remote playback only when remote output is required.
  5. Select Start sharing. By default, MeasureLab listens on every IPv4 LAN interface and enables automatic discovery.
  6. After sharing starts, open Advanced and check Details. Connect from another computer displays each usable LAN endpoint in address:port form, for example 192.168.1.10:40100.
  7. Change Listen address, UDP port, or Allow automatic discovery under Advanced before sharing when the defaults are unsuitable.

On Windows, allow inbound UDP connections to the selected port in Windows Defender Firewall before sharing (default: 40100). An inbound TCP rule is not required. Because the main computer sends the UDP request first, it normally needs no additional inbound rule. Allow local-network access if macOS prompts for it. When a firewall is enabled on Linux, allow the same inbound UDP port on the provider.

If more than one endpoint is displayed, use the address on the LAN shared with the client. Listening on 0.0.0.0:40100 means that the provider accepts connections on all IPv4 interfaces; 0.0.0.0 is not an address to enter on the client.

Provider-only routing and local modules

While sharing is active, Remote Audio I/O displays Provide Local I/O, and the main window switches to a provider-only state:

  • The status is I/O Provider, with Mode: waiting, Mode: Input only, or Mode: Duplex.
  • I/O Routing is read-only because it reports the route owned by the provider rather than offering a local output choice.
  • While waiting, it displays Remote I/O provider — waiting for client.
  • For an input-only connection, it displays Physical input → Remote client · Physical output muted.
  • For a duplex connection, it displays Physical input → Remote client · Remote client → Physical output.

Selecting Start sharing reserves the AudioEngine exclusively for the provider, including while it is waiting for a client. This keeps one owner in control of the physical audio stream, its device clock, sample order, and remote-output permission. Local measurement modules therefore cannot register their audio processing while sharing is active; allowing both paths to run could mix local output into the remote measurement or compromise its timing. Select Stop sharing before using local measurement modules again.

Connecting from the main computer

  1. Open Connect to another computer in Remote Audio I/O.
  2. Select the remote computer under Available MeasureLab computers, then select Connect selected. No address or port entry is required.
  3. If discovery is unavailable, check Connect from another computer in the provider's Advanced details. On the client, open Advanced, enter the endpoint's address part under Remote host and its port part under UDP port, then select Connect by address. For example, enter 192.168.1.10 and 40100 for 192.168.1.10:40100. A reachable host name may be used instead of the displayed IPv4 address. Manual connections use the same UDP protocol as discovered connections.
  4. The fixed network buffer, duplex request, and Recover lost network packets option are also under Advanced. Start with a 100 ms buffer and packet recovery enabled for a typical LAN. Recovery is used only when both computers support it.
  5. Start an ordinary measurement module after the connection is established.

Discovery sends an IPv4 broadcast query from the main computer to the selected UDP port and lists only providers that reply. Normally, use the default port 40100 on both computers. Discovery does not cross subnets and can be blocked by Wi-Fi client isolation, VPNs, or firewalls that filter broadcasts. Manual connection remains available when the provider address is otherwise reachable.

If the provider has not allowed remote playback, a duplex request is reduced to input-only operation. Disabling Allow remote playback during a connection mutes output immediately. If an input-only session is already connected when the option is enabled, the permission takes effect on the next connection for safety.

Calibration profiles are not transferred over the network. For measurements in physical units, select a profile on the main computer that matches the remote audio interface, gain settings, and measurement chain.

Data-integrity monitoring

The Connection quality area at the top displays the following while connected:

  • Missing frame count
  • Event count combining missing packets, corrupt packets, and local queue overflows
  • Frames recovered by optional retransmission and packets retransmitted by this computer
  • Whether data loss is still increasing or has stopped increasing

Open Advanced in the relevant tab to see the connected device and format, or the listening endpoints and connected client.

Do not treat a measurement interval showing Data loss is increasing as complete data. Increase the buffer or move to a wired LAN, then repeat the measurement.

Transport and safety

  • Protocol v2 carries discovery, connection control, keepalives, and uncompressed float32 PCM audio on one UDP port. It has no TCP listener.
  • The main computer sends DISCOVER_QUERY and CONNECT_REQUEST; the provider returns responses and audio to the observed UDP source address. It does not trust a reply address declared in the payload.
  • Session start, playback state, shutdown, and other reliable control packets use acknowledgements and retries with the same message ID. When enabled and supported by both computers, deadline-limited AUDIO_NACK requests recover recent audio packets without extending the fixed network-buffer deadline.
  • Keepalives prevent an unresponsive client from reserving the provider indefinitely.
  • Audio packets include a session ID, sequence number, absolute sample position, and CRC.
  • The remote audio device clock is the sample-time authority for the session.
  • Playback uses a fixed look-ahead buffer. Its delay is not adjusted while streaming.
  • The transport is not encrypted or authenticated. Enable the provider only on a trusted LAN.
  • Protocol v2 supports IPv4, mono/stereo, and one client per provider.
  • Wired Ethernet is recommended for stable measurements.

Stop active measurement modules before connecting or disconnecting. A failed network session never silently falls back to a local microphone or output.