The Left Side and Right Side window selectors offer a choice of window types to be applied to the impulse response data before and after the peak. These are the defaults applied to new measurements, window types for existing measurement can be altered via the IR Windows toolbar button. By default REW will set the widths of the windows automatically to show the whole room response, to override this uncheck the Set IR Window Widths Automatically box and set the default widths you wish to be applied to new measurements.
The Spectral Decay Left and Spectral Decay Right window selections are applied to the impulse response data when generating the Spectral Decay and Waterfall plots.
The Spectrogram Left and Spectrogram Right window selections are applied to the impulse response data when generating the Spectrogram plots.
The Use Loopback as Timing Reference selection controls whether REW uses a loopback on the soundcard as a timing reference for the channel being captured, to eliminate propagation delays within the computer and soundcard. The reference channel signal must be looped back from output to input for this option to work. If this is not checked REW will set the IR zero time according to the setting of Set t=0 at IR Peak.
If using a loopback as a reference REW can calculate the delay through the system being measured and show it in the measurement Info panel as "System Delay" in milliseconds, with the equivalent distance in feet and metres shown in brackets. 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 Set t=0 at IR Peak selection controls whether REW sets the zero time for the impulse response to correspond to the location of the peak or to the first point in the response the energy is more than 1% of the peak energy (level is more than 10% of the peak level).
The Sub-sample Timing Adjustment selection controls whether REW adjusts the impulse response timing to resolution below a single sample when setting t=0 at the IR peak or using the other channel as timing reference. Sub-sample timing adjustment requires a resampling of the impulse response to perform the adjustment, which slightly raises the noise floor of the measurement - however the increase is far below the noise floor of a typical acoustic measurement and sub-sample adjustment provides more accurate phase information at high frequencies.
The Decimate IR selection controls whether REW reduces the sample rate of the impulse response to correspond to the range of frequencies in the measurement. Selecting this option greatly reduces the impulse response size for low frequency measurements and speeds up processing of the data.
The Allow 96 PPO Log Spacing selection controls whether REW is permitted to convert frequency responses from linearly spaced data to logarithmically spaced data at 96 points per octave. The FFT that calculates the frequency responses produces data that is linearly spaced in frequency, i.e. there is a constant frequency step from each value to the next. For the high frequency parts of responses this means there are a very large number of points, using a lot of memory but not contributing anything useful to the displayed data. When this option is selected (it is on by default) REW will automatically convert frequency responses to more efficient logarithmic spacing with 96 data points in each octave of the response if this will reduce memory usage (which is usually the case for sweeps that end above 300Hz or so). As part of the conversion process REW first applies a 1/24th octave smoothing filter to the data to remove any high frequency combing from the response. The conversion takes place on any new measurement or when an IR Window is applied. Whether a measurement is log spaced or linearly spaced can be seen by bringing up the measurement info window by clicking the info button in the toolbar.
Note that conversion to 96 PPO is inhibited if the impulse peak is far from the impulse zero time, where "far" means the peak is offset from zero by a time that corresponds to more than 90 degrees of phase shift between samples at the measurement end frequency. This is to prevent aliased phase data at high frequencies which would lead to incorrect group delay figures.
The Show Response Below Window Limit selection controls whether REW displays the frequency and phase responses at frequencies lower than those valid for the current impulse response window width. For example, if the window width were 10ms frequencies below 100Hz would not be valid and are normally not displayed. There are circumstances in which it may be helpful to see that data, which this option allows, but the responses are drawn dashed to indicate they lie below the window cutoff.
The default smoothing to apply to new measurements can be selected from the drop-down box next to the log spacing check box.