Soundcard Settings

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

Soundcard Settings

The various controls on the panel are as follows:
Sample Rate
The sample rate may be set to 48kHz or 44.1kHz, the default is 48kHz. 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. On a Mac it may be necessary to select 44.1kHz to access any audio input or output.
Inputs and Outputs
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 OS X). When the default devices have been selected REW leaves all control of the the audio inputs and outputs and their associated volume controls to you, use the controls provided by your soundcard's mixer or OS controls to set levels and select inputs and outputs as required.
Input Channel
REW only uses one soundcard 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 Left Channel as Calibration Reference has been selected the right channel will be automatically selected for measurement and the channel selection will be disabled.
Volume Controls
The Wave, 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 mixer/volume and Control input mixer/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, mute all output sources except for Wave and select the chosen input.
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 -3dB FS (which has the peak of the sweep at 0dB FS), a typical setting is -12dB FS (the default).
Replay Buffer, Record Buffer
The Replay Buffer and Record Buffer controls set the size of the buffers used when accessing the soundcard. 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 and problems with audio input or output you may wish to reduce the buffer sizes to minimise latency.
Calibration Panel
The controls in the Calibration panel are used to calibrate the soundcard.

The Use Left Channel as Calibration Reference selection controls whether REW uses the soundcard left channel input as a calibration reference for the right channel signal. The left channel signal must be looped back from output to input for this option to work. If this is not checked REW can use a soundcard calibration file (if one has been specified). If it is checked each measurement of the right channel is corrected to remove the frequency response variations found in the left channel. To use the left channel as a reference it is important to check that the left and right channels have near identical responses, if this is the case a loopback measurement of the right channel will give a flat response. To see the response of the right channel reference input independently of the main measurement select the Calculate Ref Response option in the Analysis Settings, but note that this increases the calculation time for each measurement.

If using the left as a reference REW can calculate the delay through the system being measured and show it in the Details section of the measurements panel as "System Delay" in milliseconds. If the mouse pointer is hovered over the delay value a tooltip appears showing the equivalent distance in feet and metres. Note that delay values are not accurate for subwoofer measurements due to the limited bandwidth of the subwoofer response, the delay estimate is based on the location of the peak of the impulse response and subwoofers have a broad peak and a delayed response.

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. Clear Cal clears the calibration data structures, all subsequent measurements will not have any soundcard calibration corrections applied to them and REW will not load any previously specified soundcard calibration file on the next startup. Measure... starts a process of measuring the soundcard response via an external loopback connection. Make Cal... 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 soundcard for use with REW provide a copy of this file along with a description of the problem.

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 soundcard 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.

Here is an example section of a valid file format:

Soundcard Calibration data saved by Room EQ Wizard V3.26
Source: SoundMAX Digital Audio, Line In, Right channel, volume: 0.075
Format: 48000Hz sampling, Log Swept Sine, 176ms pre-impulse, 1,000ms post-impulse 
Dated: 28-Nov-2005 17:19:51
Sample Rate: 48000

0 -9.38
1 -7.69
2 -6.34
3 -5.22
4 -4.26
5 -3.48
6 -2.80
7 -2.20
8 -1.71
9 -1.28
10 -0.87
11 -0.55
12 -0.25
        

After a calibration file has been loaded it wil 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.

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