How to Use ASIO4ALL in REW: Input, Output, and Measurement Setup

How REW, ASIO4ALL, and Windows fit together

REW generates a test signal, sends it through the selected output, and captures the returned microphone or line input. ASIO4ALL sits between REW's ASIO request and one or more Windows WDM endpoints. It can make a built-in output, USB microphone, HDMI path, or another WDM device visible to an ASIO-capable application, but it cannot repair a missing hardware driver or add a physical input that the device does not expose.

A clean first measurement has four matching layers: the REW driver choice, the ASIO4ALL endpoint state, the REW input/output channel selection, and the physical cable, microphone, speaker, or processor being measured. A moving REW level meter proves that a signal reached an input; it does not by itself prove that the intended speaker or channel was excited.

Configure ASIO4ALL in REW Soundcard Preferences

The exact labels can vary slightly by REW release, but the decision belongs in the Soundcard Preferences panel. On Windows, select the ASIO driver type and then choose ASIO4ALL v2. REW's ASIO path normally uses one ASIO device for input and output; ASIO4ALL can wrap separate WDM endpoints, but that flexibility also makes device ownership, clocking, and sample-rate conflicts more likely.

Keep the first route deliberately narrow. If you are measuring a speaker with a calibrated microphone, enable the intended playback endpoint and microphone input only. Do not activate every laptop speaker, HDMI output, Bluetooth profile, and microphone array before you know which channel is producing and capturing the test signal.

1. Open Soundcard Preferences

   Open REW's Preferences and select Soundcard. Record the current driver and sample rate before changing anything so you have a known rollback point.

2. Choose the ASIO driver path

   Select ASIO as the Windows driver type, then choose ASIO4ALL v2 from the device list. If a supported interface has a current native ASIO driver, test that driver first.

3. Open the ASIO4ALL panel

   Use the driver configuration button to open ASIO4ALL. Enable the exact WDM output and input required for the measurement, and leave unrelated endpoints disabled during diagnosis.

4. Match the sample rate

   Choose a rate offered by both the device and REW, then make the Windows endpoint and project use the same rate where possible. If a device disappears from a list, try the rate it actually reports instead of forcing a value.

5. Select channels and save the baseline

   Choose the input and output channels in REW, note the microphone calibration file and output calibration if used, and save a short description of the route before testing levels.

Run a first level check and measurement

Before a room sweep, use REW's Check Levels function or another short, controlled signal to confirm the complete path. Listen at a safe level, watch the input meter, and verify that the expected speaker or loopback channel responds. A silent output, a clipped input, or an unexpectedly active channel is a routing problem to fix before taking measurements.

For a normal acoustic measurement, load the correct microphone calibration file, place the microphone consistently, select the intended output channel, and start with a conservative sweep level. If you use a timing reference or loopback input, select and verify that reference separately; it is not automatically created by installing ASIO4ALL.

1. Play the level check

   Run Check Levels for a few seconds and confirm the expected input responds. Stop immediately if the level is unsafe or another output is active.

2. Confirm the channel identity

   Use a low-level signal and verify the physical speaker, processor input, or loopback connection that responds. Do not infer channel identity from a similar device name alone.

3. Make one short sweep

   Keep the same input, output, rate, microphone position, and sweep level for the first measurement. Record those settings with the result.

4. Repeat before expanding

   Repeat the measurement once before enabling a second WDM device or changing synchronization. Repeatability is more useful than a larger endpoint list.

Fix no signal, missing channels, and unstable sweeps

If REW cannot open ASIO4ALL or a device is unavailable, close DAWs, browsers playing audio, meeting tools, media players, and control panels that may own the WDM endpoint. Reopen REW, select the same driver again, and begin with one output and one input. Reinstalling the driver is not the first fix for an endpoint that another process still owns.

If the device appears but the channel is missing, check the selected sample rate and the ASIO4ALL endpoint tree. REW may only list channels that the current ASIO device reports for that format. Return to the device's supported rate, disable unrelated endpoints, and reopen the Soundcard Preferences panel before changing the measurement itself.

If the sweep starts but is noisy or drops out, raise the ASIO4ALL buffer one step, reduce the number of active WDM endpoints, and repeat the same sweep. Bluetooth adds transport delay and can add format or ownership problems, so use a wired path while diagnosing measurement routing.

• No driver or device: close other audio applications, reopen REW, and confirm ASIO4ALL v2 is selected.
• Red or unavailable endpoint: inspect the exact WDM child device, not only its parent name.
• Input missing: match the sample rate and confirm Windows microphone permission and the correct ASIO channel.
• Output silent: verify the REW output channel, physical cable, processor input, and volume independently.
• Clipping: reduce sweep level or input gain and repeat the level check before measuring.
• Crackles or dropouts: raise the buffer, disable extra endpoints, and test the same route again.
• Unexpected channel: disable mixing, virtual devices, and unrelated HDMI or Bluetooth endpoints while identifying the path.

When a native ASIO driver or another REW driver is better

ASIO4ALL is most useful when REW needs a Windows WDM endpoint that a native driver does not expose, such as a built-in audio path, a particular HDMI route, or a device without a suitable ASIO driver. It is not automatically better for a supported USB interface. A current manufacturer ASIO driver usually gives clearer device ownership, channel naming, clocking, and hardware-specific controls.

REW also offers Java and other platform-specific driver paths. A USB measurement microphone or a simple stereo output may be easier to configure through the driver that exposes the exact devices you need. Choose the path that produces a repeatable, calibrated measurement, then document the driver, device, channels, sample rate, level, and microphone calibration used for the result.

REW with ASIO4ALL verification checklist

A useful REW result is not defined by selecting ASIO4ALL alone. It is defined by a known signal path that repeats: the same driver, endpoint, channel, sample rate, level, microphone calibration, and physical measurement position produce comparable results.

Keep the first route small and write down the settings with the measurement. If a later result changes, you can identify whether the change came from the driver, endpoint ownership, sample rate, buffer, calibration, room position, or the device under test.

• A current native ASIO driver was tested first when the interface provides one.
• REW Soundcard Preferences uses ASIO4ALL v2 only for a clear WDM routing or compatibility need.
• Only the intended WDM input and output are enabled during the first test.
• The REW sample rate and the device or Windows endpoint rate are compatible.
• Check Levels passes without clipping, excessive noise, or an unexpected active channel.
• The microphone or line-input calibration file is attached to the intended input.
• A short sweep repeats on the same physical channel before more endpoints are enabled.
• The result records the driver, device, channels, sample rate, buffer, level, and measurement position.