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.
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.
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.