Soundcard Preferences

The Soundcard Preferences panel is used to configure the audio input and output used for measurement, calibrate the audio interface and establish the correct levels for making measurements.

The various controls on the panel are as follows:

Drivers: On Windows platforms there is a choice of Java or ASIO drivers for the audio interface. The Java drivers support 44.1, 48, 88.2, 96 and 192 kHz sample rates and 16-bit data. On macOS and Linux 32-bit or 24-bit data is used if the interface offers it. Java drivers permit the input and output to be on different devices and allow volume control from REW.

The ASIO drivers support up to 1536 kHz and a variety of formats depending on the driver. ASIO drivers support one ASIO device which must be used for both input and output and REW has no control over levels. Pseudo-ASIO drivers such as ASIO4All create an ASIO wrapper around the WDM drivers for devices, allowing input and output through different devices.
Sample Rate: With Java drivers the sample rate may be set to 44.1, 48, 88.2, 96 or 192 kHz, the default is 48kHz. To prevent resampling in the OS make sure the audio interface is configured to operate at the sample rate selected in REW. With ASIO drivers the choice of sample rates offered will reflect those the interface supports, with a maximum sample rate of 1536 kHz. Note that the lists of input and output devices only include those devices that report they support the selected sample rate, if your device does not appear in the lists try changing the sample rate.
Stereo only: Java drivers can be restricted to requesting stereo connections to the audio interface by selecting Stereo only.
Inputs and Outputs (Java drivers): The input and output device lists show the physical devices that Java has found that report they support the selected sample rate, along with some OS virtual devices. The lists of inputs and outputs are specific to the selected input device and output device. The Default Device settings tell REW to request the defaults that have been set in your OS (in the Sounds and Audio Devices control under Windows or the Audio and Midi Setup utility under macOS). When the default devices have been selected REW leaves all control of the audio inputs and outputs and their associated volume controls to you, use the controls provided by your interface's mixer or OS controls to set levels and select inputs and outputs as required. Note that when using a USB mic with a cal file that contains a sensitivity figure REW needs to read the input volume setting to correctly show SPL, to allow that the input device and input for the mic must be selected (they must not be left as "Default Device").
Output Channel (Java drivers): REW can place its test signals on either or both of the output channels. If there are more than 2 available output channels the first and second can be driven at the same time - which channels those are can be configured using the Output Channel Mapping dialog (see below). The output channel selection can also be made directly on the measurement panel or signal generator.
Timing Reference Output Channel (Java drivers): When using a timing reference the output channel will be the channel selected here. The selection can also be made directly on the measurement panel.
Output Mapping (Java drivers): On some platforms Java supports multichannel output - currently Windows does not offer this unless using the Wasapi Exclusive (EXCL) entries. If the platform supports multichannel and the current output has more than 2 channels an output channel mapping button is shown:

The button brings up a dialog to select up to 17 channels for use while measuring:

Each of the channels can be assigned to any of the available hardware outputs. They can be labelled with surround channel names, or the hardware channel number, or the output number.
Input Channel (Java drivers): REW only uses one interface channel to capture the output of your SPL meter or mic preamp, the Input Channel control tells REW which channel you have connected to. The default is the Right channel. If Use loopback as timing reference has been selected in the Analysis Preferences the other channel will be used a reference to eliminate time delays within the computer and interface, this requires a loopback connection on the reference channel.
Input options: If the interface (or something else in the input chain) inverts its input select the Invert checkbox to restore correct polarity. If the input has a DC offset check the High Pass box to have REW apply a 2 Hz high pass filter.
Loopback Input Channel (Java drivers): When the input is stereo the loopback defaults to the channel not being used for measurement. If multiple input channels are available the input to use for the loopback can be configured here.
Virtual balanced input: If this option is selected a Balanced input selection is offered. The balanced input will be subtracted from the measurement input and the result scaled by 0.5. This simulates the behaviour of a balanced input, and is appropriate if the balanced input is driven by an inverted signal (such as by selecting the Invert second output option on the signal generator).
Volume Controls (Java drivers): The Output and Input volume controls are only enabled if you have selected specific input and output devices, have checked the boxes to allow REW to Control output volume and Control input volume and REW has been able to obtain controls for the selected devices from the OS. Under those conditions REW will set the volume controls to the levels last used for measurement and select the chosen input. These controls are not enabled on macOS, macOS only permits values corresponding to specific dB increments which vary according to the device and its volume range. Use Audio MIDI Setup to control volume settings on macOS.
Sweep Level: The Sweep Level control sets the RMS level at which REW will generate its measurement sweep, relative to digital full scale. The highest level possible is -3 dBFS, unless the View preference Full scale sine rms is 0 dBFS has been selected, in which case the maximum is 0 dBFS. Using the maximum value places the peaks of the signal at digital full scale. A typical setting is -12 dBFS (the default). This selection can also be made directly on the measurement panel.
Output Buffer, Input Buffer (Java drivers): The Output Buffer and Input Buffer controls set the size of the buffers used when accessing the interface. The default settings are 32k (meaning the buffer sizes are 32,768 pairs of audio samples). If you experience occasional glitches or interruptions in the signal generator output try increasing the replay buffer size, but note that there are other possible causes of this, such as interference from wireless cards. Similarly if the captured audio signals (as shown in the Scope graph panel) have occasional dropouts try increasing the record buffer. Using larger buffers will increase latency (delays when starting and stopping replay and recording) but should otherwise not be detrimental. If you are not experiencing any problems with audio input or output you may wish to reduce the buffer sizes to minimise latency.
Treat 32-bit data as 24-bit: 32-bit integer sample formats often carry 24-bit data. Treating them as 32-bit data generates low level harmonic artefacts (at around -160 dBFS), to prevent that they should be treated as 24-bit samples. If a device is genuinely 32-bit turning this option off will provide the full resolution of the device, but note that REW uses 32-bit float to distribute audio data internally which means the effective integer-equivalent dynamic range is 25 bits.
Calibration Panel: The controls in the Calibration panel are used to calibrate the interface.

The browse button is used to select a calibration file, a plain text file which by default has the extension .cal, though other extensions are also accepted. The file format is detailed below. The delete button clears the calibration data structures, all subsequent measurements will not have any interface calibration corrections applied to them and REW will not load any previously specified interface calibration file on the next startup. Calibrate... starts a process of measuring the interface response via an external loopback connection. Make cal file... is used to save a measurement as a calibration file - this should only be used with the results of a loopback measurement, and then only after checking that the measurement is valid. The measurement data is saved as a text file, with the SPL values offset to give 0dB at 1kHz. The file is automatically loaded on startup and applied to subsequent measurements.
Levels Panel: The controls in the Levels panel are used to set the output and input levels for measurement. Levels can be set using either a subwoofer or one of the main speakers, this is selected in the drop-down box in the panel. The Check Levels... button starts a process of establishing and verifying the levels. The Generate Debug File... button generates a text file with information about all the audio devices and controls that Java has been able to identify. If there are problems configuring the interface for use with REW provide a copy of this file along with a description of the problem. Use pink periodic noise for level checks throughout REW controls whether REW uses random pink noise or periodic pink noise when checking levels. Periodic noise provides much more stable readings at low frequencies but sounds quite different to random noise.

Example Input and Output Settings

Here are some example settings, firstly using Java drivers, a PC's built-in audio interface and a USB mic. REW has been set to control the levels and the Right channel is being used for input.

Soundcard IO Settings

Here are some settings using ASIO drivers, in this case ASIO4All (which provides an ASIO wrapper around Windows audio drivers). Note that it is not necessary to select a timing reference output if a timing reference is not being used. The ASIO Control Panel button launches the ASIO control panel for the interface.

Soundcard IO Settings for ASIO

FlexASIO

If the FlexASIO ASIO driver is being used the ASIO Control Panel button will show a dialog that provides a graphical interface to generate and update the FlexASIO configuration file, FlexASIO.toml.

FleaxASIO control panel

REW's handling of the configuration file only supports the backend and bufferSizeSamples options. The [input] and [output] sections will only contain the selected input and output device names, they will not contain any other options that FlexASIO supports. The configuration file is updated when the Close panel and update config file button is pressed, the FlexASIO driver is then reloaded to update the available inputs and outputs. If the dialog is closed without using the Close panel and update config file button any changes will be discarded.

Soundcard Calibration File Format

The calibration file is a plain text file which by default has the extension .cal, though other extensions are also accepted. It should contain the actual gain (and optionally phase) response of the interface at the frequencies given, these will then be subtracted from subsequent measurements. The values in the calibration file can be separated by spaces, tabs or commas.

Each line of calibration data must have a frequency value and a gain value, a phase value is optional
Frequency is in Hz, gain in dB, phase in degrees
The cal points can be at arbitrary frequency spacing, but each line must have a higher frequency than the one before and there must be at least 2 freq, gain data pairs
Only lines which begin with a number are loaded, others are ignored
In comma-delimited files there must be at least one space after the comma
Spaces before values are ignored
The sample rate at which the data was generated can be indicated by having a line which starts "Sample Rate:" (without the quotation marks) followed by the sample rate in Hz. REW checks for this when loading a file and will warn if the rate does not match the current interface setting - calibration data generated at a different sample rate will not provide accurate correction.

Here is an example section of a valid file format:

* Soundcard Calibration data saved by Room EQ Wizard V5.00
* Source: EDIROL UA-1A, Digital Audio Interface, Right channel, volume: no control
* Format:  256k Log Swept Sine, 1 sweep
* Dated: 21-Nov-2010 21:47:56
* Sample Rate: 44100
*
2.019 -1.424 53.471
2.219 -1.238 49.420
2.419 -1.062 45.118
2.619 -0.929 41.888
2.819 -0.823 39.056
3.019 -0.740 36.590
3.219 -0.668 34.409
3.419 -0.607 32.468
3.619 -0.557 30.736
3.819 -0.513 29.177
4.019 -0.475 27.777
4.219 -0.443 26.495

After a calibration file has been loaded it will be applied to all subsequent measurements. Loading the calibration file does NOT affect any data already measured and does not affect any measurement data that is imported. The graph display is updated to show the calibration curve, offset to lie at the current Target level.

Linear interpolation is used between calibration points. Outside the range of the calibration data the behaviour depends on whether C weighting compensation has been selected. If C weighting compensation is selected, C weighting curve figures will be used for frequencies above or below the range of frequencies in the calibration data. If not, the calibration values for the lowest frequency in the file will also be applied for all lower frequencies and the calibration values for the highest frequency in the file will be applied for all higher frequencies.

The calibration file name and path are remembered for the next startup, the file will be loaded automatically when REW is started. A message confirming loading of the file is given.

To stop calibration data being applied, use the Clear Cal... button. Useful tip: To apply or remove a soundcard calibration file after a measurement has been taken, simply load or clear the cal data as required and press the Apply Windows button in the IR Windows panel to recalculate the frequency response.