The graph panel shows plots for the currently selected measurement. The
plots are selected via the buttons at the top of the graph area.
The various graph types are:
SPL and Phase
Impulse
Filtered IR
Group Delay
RT60
Spectral Decay
Waterfall
Spectrogram
OscilloScope
Options that affect the appearance of the traces can be found in the View Preferences.
Each trace can be turned on or off via the selection buttons to the left
of the trace name in the legend panel. Trace names are in the same colour
as the trace itself, whilst the line style for the trace is shown between
the label and the trace's value at the current cursor position. If a trace
has fractional octave smoothing applied the octave fraction is shown (1/12th
octave in the example below).
This button at the top left corner of the graph area allows the current
graph view to be saved as an image. A dialog pops up to set the desired width of
the image (click default to set the image to be the same width as the graph).
If the "Include Title" box is checked the graph type will be shown at the top
of the graph. If the graph has data that can have smoothing applied to it the
amount of smoothing (if any) will be shown at the top of the graph, alongside
the title (if selected). If the "Include Legend" box is checked the image includes
the graph legend. Text typed into the box that shows "Type any additional text
here" will appear on the graph image near the top of the graph, beneath the title
and/or smoothing settings. The graph will be saved as either JPEG or PNG according
to the type selected.
The Scrollbars button toggles scrollbars for the graph area on/off, hiding
the scrollbars provides more room for the graph. The setting is remembered
for the next startup. If the scrollbars are off, the graph can still be moved
by holding down the right mouse button while in the graph area.
The Freq Axis button toggles the frequency axis between logarithmic and
linear modes. This function is also available via a command in the
Graph menu and the associated shortcut keys.
The Graph Limits button allows desired top, left, bottom and right graph limits
to be defined. A dialog pops up in which the values are entered, they are applied
as they are entered or by clicking the Apply Settings button.
The Graph Controls button brings up a menu of control options for the currently
selected graph type, if there are any.
The horizontal axis zoom buttons appear when the mouse pointer is inside the graph
area, they zoom in or out by a factor of approximately 2 centred around the
cursor position.
The vertical axis zoom buttons appear when the mouse pointer is inside
the graph area, they zoom in and out on the Y axis.
REW provides a variable graphical zoom capability by either pressing and holding the middle mouse button, or pressing the right button then, while the right button is held down, pressing and holding the left button and dragging the pointer.
When variable zoom is active a cross is displayed, split into quadrants allowing horizontal and/or vertical zooming in our out depending on the mouse position. The amount of zoom is governed by how far the mouse pointer is dragged from the start position.
When the Ctrl key is pressed followed by the right mouse button a zoom box can be drawn by dragging the mouse. Measurement cursors are shown on the outside of the box, to zoom to the shaded area click within it. If the shaded area is too small to zoom in to a message will indicate which dimension is too small for zooming and what the limit is to allow zoom.
To undo the last Variable Zoom or Zoom to Area, press Ctrl+Z or select the Undo Zoom entry in the Graph menu. This will restore the graph axes to the settings they had when the right or middle mouse button was last pressed. This Undo feature can be used even if you have not zoomed, just press the right mouse button when the axis settings are to your preference then you can return to these settings (undoing any subsequent movements or control changes) by pressing Ctrl+Z.
The SPL and Phase plot (or Impedance and Phase for an Impedance measurement) shows the frequency (dB or Ohms) and phase (degrees) responses of the measurement. The frequency response is labelled with the measurement name, the phase response uses a brighter shade of the measurement colour and the right hand plot axis. Note that to have valid phase information it is necessary to remove any time delays from the Impulse Response. A time delay causes a phase shift that increases with frequency - for example, a delay of just 1ms results in a phase shift of 36 degrees at 100Hz but 3,600 degrees at 10kHz, because 1ms is 1/10th of the 10ms period of a 100Hz signal but is 10 times the 0.1ms period of a 10kHz signal, and each period is 360 degrees. The time delay of a measurement can be adjusted by changing the zero position of the time axis using the Impulse graph controls, or by using the Estimate IR Delay control described below.
In addition to the measured phase, the plot can show minimum and excess phase plots that result from generating a minimum phase version of the response, described further below. The plot also shows any mic/meter or soundcard calibration data for the measurement. The calibration data can be changed or removed by selecting Change Cal... on the measurement panel.
If the Generate Minimum Phase control has been used to produce a minimum phase version of the response the minimum and excess phase traces are activated. They show the minimum phase response and the difference between the measured phase and the minimum phase (the "excess"). For more about minimum and excess phase and group delay see Minimum Phase.
The Mic/Meter Cal trace shows the frequency response of the Mic calibration data for this measurement (the calibration file to use for new measurements is specified in the Mic/Meter Preferences). If C Weighted SPL Meter was selected this curve will show the effect of C weighting (outside the range of the calibration data file, if there is one). The trace is not shown if there is no mic/meter calibration data. The trace is drawn relative to the middle of the graph.
The Soundcard Cal trace shows the measured frequency response of the soundcard relative to its level at 1kHz (if a calibration file has been loaded via the Soundcard Preferences). The trace is not shown if cal data has not been loaded. The trace is drawn relative to the middle of the graph. Fractional octave smoothing can be applied or removed via the Graph menu and its shortcut keys. The smoothing is applied to the SPL, phase and Group Delay traces. This is mainly used for full range measurements, as reflections can cause severe comb filtering which makes it difficult to see the underlying trend of the response. Smoothing should rarely be used for low frequency measurements as it obscures the true shape of the response. When smoothing has been applied an indicator appears in the trace legend.
The control panel for the SPL and Phase graph has these controls:
If Show points when zoomed in is selected the individual points that make up the SPL and phase responses are shown on the graph when the zoom level is high enough for them to be distinguished (which may only be over part of the plot)
The phase trace normally wraps at +180/-180 degrees. This is because phase is cyclic over a 360 degree range (+90 is the same phase as -270). The trace can, however, be displayed without wrapping which is what the Unwrap Phase control does. A difficulty with unwrapped phase is knowing where the correct zero phase is, another is being able to view parts of the trace where the unwrapped value has become very large. The unwrapped phase is offset (by a multiple of 360 degrees) so that it is within the range -180..180 degrees at the cursor frequency. The +360 and -360 buttons will also shift the phase trace in 360 degree steps.
Wrap Phase changes the phase trace back to a conventional wrapped view with vertical lines where the trace crosses 180 or -180 degrees.
Generate Minimum Phase will produce a minimum phase version of the measurement using the current IR window settings. The minimum phase trace then shows the lowest phase shift a system with the same frequency response as the measurement could have, while the excess phase trace shows the difference between the measured and minimum phase. Using this control also generates a minimum phase impulse plot and minimum and excess group delay plots, which can be viewed on the respective graphs.
Note that the IR window settings are important as the minimum phase response is derived from the frequency (magnitude) response of the measurement, which in turn is affected by the IR window settings. If the window settings are subsequently changed Generate Minimum Phase should be used again to reflect the new settings. Note also that the shape of the left side window (the window applied before the peak) affects the minimum phase result, a rectangular window will produce a response with lower phase shift than, for example, a Hanning window.
If the system being measured was inherently minimum phase (as most crossovers are, for example) the minimum phase response is the same as removing any time delay from the measurement. Room measurements are typically not minimum phase except in some regions, mainly at low frequencies. For more about minimum and excess phase and group delay see Minimum Phase.
Estimate IR Delay calculates an estimate of the time delay in the measurement by comparing it with a minimum phase version. The delay it calculates can be removed from the impulse response by pressing the Shift IR button on the panel shown after the delay is calculated. Note that shifting the impulse response will clear any spectrogram which had been generated as the plot would no longer be valid.
The Generate Minimum Phase and Estimate IR Delay controls also appear in the Impulse graph control panel.
The trace offset value moves the graph position, but does not alter the data so the legend values do not change. If the Add offset to data button is pressed the current offset value is transferred to the measurement data and the legend readings will update accordingly.
The Impulse graph shows the impulse response for the current measurement. It can also show the left and right windows and the effect of the windows on the data that is used to calculate the frequency response; a minimum phase impulse; the impulse response envelope (ETC) and the step response.
The Y axis used for the impulse response can be selected as % FS or dB FS
(FS = Full Scale) via a control in the top left corner which appears when
the mouse cursor is inside the graph area. The dB Fs scale is equivalent to
a "log squared" view of the impulse.
Dashed vertical black lines show the extents of the impulse response windows, a dashed red line shows the reference position. If the window settings are changed the region outside the new area is shown shaded until the settings are applied. It is best to set the Y axis to dB to adjust the windows as it is then much easier to see where the response has decayed into the noise.
After each measurement the left window width is automatically set up. For full range measurements (and down to end frequencies of 1kHz) the width is 125ms, below that it increases to allow for pre-ringing effects of using a limited sweep range. To change the window settings for a measurement click the IR Windows button:
The impulse response is that of the whole system, including the mic/meter and the soundcard. The mic/meter and soundcard calibrations are only applied when calculating the frequency response.
If the Generate Minimum Phase control has been used to produce a minimum phase version of the current measurement's magnitude response a minimum phase impulse trace is activated, showing the impulse response the minimum phase system would have.
The envelope of the impulse, also called the energy-time curve or ETC, is useful to identify reflections and see the overall shape of the impulse response. The plot below shows the envelope, the spikes after the initial peak are due to reflections from room surfaces, the first spike occurs 3.25ms after the initial peak indicating that the sound travelled an additional 1.11m or 3.7 feet to reach the microphone.
The step response shows the output which would result if the input signal jumped to a fixed level and stayed there. It is the integral of the impulse response. If there is an offset in the measurement input chain the step response will show an overall rise or fall as time progresses, rather than tending back to zero.
A property of the log sweep analysis method is that the various harmonic distortion components appear as additional impulses at negative time, with decreasing spacing as the distortion order increases. For example, this plot shows spikes from distortion components up to the 8th harmonic on a laptop soundcard loopback measurement:
Here is a similar measurement for an external USB soundcard, it is a 44.1k card rather than 48k, which limits us to the 6th harmonic in the 1s pre-impulse period - however, only the 2nd, 3rd and 5th harmonic peaks are evident, the 4th harmonic peak is barely visible above the noise floor (which is about 10dB lower than the laptop card). The extended lobes after the impulse are due to the card's much lower -3dB frequency, 1.0Hz versus 22.1Hz (note that the right side of the time axis is 2.0s in this plot compared to 0.5s in the previous plot):
The control panel for the Impulse graph has these controls:
The impulse response may be plotted with or without normalisation to its peak value according to the setting of the Plot Normalised control. When normalised plotting is selected the peak will be at 100% or 0dBFS.
If Show points when zoomed in is selected the individual points that make up the response are shown on the graph when the zoom level is high enough for them to be distinguished.
The response may be plotted inverted according to the setting of the Invert Impulse control. Note that this has no effect when the Y axis is set to dB FS. If the soundcard you are using inverts its inputs that can be corrected using the Invert checkbox in the Soundcard Preferences Input Channel controls.
Generate Minimum Phase will produce a minimum phase version of the measurement using the current IR window settings. The minimum phase impulse then shows the response of a system having the same frequency response as the measurement but with the lowest phase shift such a system could have. This control also activates minimum and excess phase and group delay traces on the SPL & Phase and GD graphs respectively.
Note that the IR window settings are important as the minimum phase response is derived from the frequency (magnitude) response of the measurement, which in turn is affected by the IR window settings. If the window settings are subsequently changed Generate Minimum Phase should be used again to reflect the new settings. Note also that the shape of the left side window (the window applied before the peak) affects the minimum phase result, a rectangular window will produce a response with lower phase shift than, for example, a Hanning window.
If the system being measured was inherently minimum phase (as most crossovers are, for example) the minimum phase response is the same as removing any time delay from the measurement. Room measurements are typically not minimum phase except in some regions, mainly at low frequencies. For more about minimum and excess phase and group delay see Minimum Phase.
Estimate IR Delay calculates an estimate of the time delay in the measurement by comparing it with a minimum phase version. The delay it calculates can be removed from the impulse response by pressing the Shift IR button on the panel shown after the delay is calculated.
The t=0 offset controls can be used to shift the zero time position by either a specified number of samples or a specified time. These controls can be used to manually remove measurement time delays or determine the correct delay to align measurements of different speakers or drive units. Note that shifting the impulse response will clear any spectrogram which had been generated as the plot would no longer be valid. If a loopback was used as a timing reference the System Delay figure (which can be viewed in the measurement Info panel) is shifted by the same amount as the zero time.
The Scale FR Peak control re-scales the impulse response to achieve a desired maximum SPL figure in the corresponding frequency response. This may be useful to rescale an imported impulse response.
ETC Smoothing is used to smooth the envelope (ETC) trace using a moving average filter of the duration specified in the spinner.
The Filtered IR graph allow octave and one-third octave filters to be applied to the measurement. It is primarily aimed at examining decay behaviour in different frequency bands and analysing the results per ISO 3382. In addition to the filtered impulse response itself this graph includes traces of the impulse response envelope (ETC) and the Schroeder integral.
Octave and 1/3 octave filters can be selected from the box in the lower left corner of the graph. The selected filter is applied to the Impulse Response upon selection. The filter remains active until "No Filter" is selected. The measurement name on all graphs is shown with an indication of the applied filter, for example "Auditorium [250Hz 1/3]".
The Schroeder Integral is a curve obtained by backwards integration of the squared impulse response, ideally starting from a point where the response falls into the noise and applying a correction (a starting value for the integral) which assumes the rate at which the Schroeder curve is falling continues for the whole response. REW uses an iterative procedure to estimate the best starting point for the integration, often called "Lundeby's Method" (from the paper by A. Lundeby, T. E. Vigran, H. Bietz, and M. Vorländer, “Uncertainties of Measurements in Room Acoustics,” Acustica, vol. 81, pp. 344–355 (1995)). The slope of this curve is used to measure how fast the impulse response is decaying, deriving a figure for "RT60" which is the time it would take sound to decay by 60dB. The curve shown on the Impulse graph is for the currently applied filter, if any. When calculating decay data for the octave and one-third octave RT60 results the impulse is first filtered to the corresponding bandwidth and centre frequency before the Schroeder Integral for that band is determined and the various RT60 measures calculated.
The control panel for the Filtered IR graph has these controls:
The impulse response may be plotted with or without normalisation to its peak value according to the setting of the Plot Responses Normalised control. When normalised plotting is selected the peak will be at 100% or 0dBFS.
The Time Reversed Filtering control applies the octave band filters backwards in time, this reduces the filter's own contribution to the measured decay. When using 1/3 octave filters at low frequencies the filter decay time can be significant, over 200ms for a 100Hz 1/3 filter, for example. Applying the filter in reverse reduces this decay to less than 50ms, but it does affect the response somewhat, such that Early Decay Time (EDT) figures using Time-Reversed filters may not be valid.
The Show Data Panel control shows a panel on the graph containing the results for the decay values. The RT60 figures include the decay range over which they have been calculated and an "r" value, the regression coefficient, which measures how well the data corresponds to a straight line. A value of -1 would indicate a perfect fit, values lower in magnitude than -0.98 indicate the corresponding decay figure may not be reliable. Unreliable figures are italicised and shown orange. The parameters available are:
The graph can also show the "Regression Line", which is a line obtained by carrying out least squares linear regression on the Schroeder curve over the range applicable to any particular decay parameter. The selector for which regression line is to be shown is next to the Show Regression Line check box.
ETC Smoothing is used to smooth the envelope (ETC) trace using a moving average filter of the duration specified in the spinner.
The group delay for the measurement is calculated from the slope of the phase trace. Note that if smoothing has been applied to the measurement that will also smooth the phase and group delay traces. Smoothing can be applied or removed via the Graph menu and its shortcut keys. Peaks and dips in the frequency response will usually be accompanied by corresponding peaks and dips in the group delay. The group delay will include any delay in the measurement due to time delays in the PC or soundcard, processing delays in the equipment and delays due to the time sound takes to travel from source to microphone. Delays in the PC or soundcard can be eliminated by using the Use Loopback as Timing Reference option in the Analysis Preferences. If the group delay is tending towards a level at the upper end of the measurement that level typically corresponds to the overall measurement delay.
The control panel for the Group Delay graph has these controls:
Generate Minimum Phase will produce a minimum phase version of the measurement using the current IR window settings. This activates minimum and excess group delay traces that show how the measurement's group delay compares with the response of a system having the same frequency response but with the lowest phase shift such a system could have. This control also activates minimum and excess phase and minimum phase impulse traces on the SPL & Phase and Impulse graphs respectively.
Note that the IR window settings are important as the minimum phase response is derived from the frequency (magnitude) response of the measurement, which in turn is affected by the IR window settings. If the window settings are subsequently changed Generate Minimum Phase should be used again to reflect the new settings. Note also that the shape of the left side window (the window applied before the peak) affects the minimum phase result, a rectangular window will produce a response with lower phase shift than, for example, a Hanning window.
If the system being measured was inherently minimum phase (as most crossovers are, for example) the minimum phase response is the same as removing any time delay from the measurement. Room measurements are typically not minimum phase except in some regions, mainly at low frequencies. For more about minimum and excess phase and group delay see Minimum Phase.
If Show points when zoomed in is selected the individual points that make up the measured and minimum phase responses are shown on the graph when the zoom level is high enough for them to be distinguished (which may only be over part of the plot)
The RT60 Reverberation Time curves at each octave or one-third octave filter centre frequency are displayed on this graph, with separate traces for the Early Decay time (EDT) and the 60dB decay times T20, T30 and REW's Topt. See below for descriptions of each of these measures.
The control panel for the RT60 graph has these controls:
The Time Reversed Filtering control applies the octave band filters backwards in time, this reduces the filter's own contribution to the measured decay. When using 1/3 octave filters at low frequencies the filter decay time can be significant, over 200ms for a 100Hz 1/3 filter, for example. Applying the filter in reverse reduces this decay to less than 50ms, but it does affect the response somewhat, such that Early Decay Time (EDT) figures using Time-Reversed filters may not be valid.
If the Show Correlation Factor box is checked the graph legend names shows the quality of the line fit for the various decay measures. The "r" value shown after each decay measure is the regression coefficient, which measures how well the data corresponds to a straight line. A value of -1 would indicate a perfect fit, values lower in magnitude than -0.98 indicate the corresponding decay figure may not be reliable. Unreliable values are italicised. The decay measures available are:
The RT60 plot can show horizontal bars centred on each filter frequency and spanning the filter's bandwidth, or lines joining the filter centre frequencies, according to the Use Bars on RT60 Plot control setting.
The reverberation times for the current measurement can be written to a text file using the File -> Export -> RT60 data as text menu entry.
This graph shows spectral decay traces over the region from 10Hz to the end of the measurement sweep. The plot used logarithmically spaced data at 96 points per octave with 1/48th octave smoothing applied. The Spectral Decay plots are generated by shifting the impulse response window to the right by the slice interval to generate each succeeding slice. Two windows are used, a left side window to taper the data prior to the start of the region being analysed and a right side window that spans the selected window width. The default window type for the left side is Hann, for the right side it is Tukey 0.25, other types may be selected via the Spectral Decay entries in the Analysis Preferences. The initial reference point for the windows (end of left window/start of right window) is the peak of the impulse response.
To produce the Decay plot click the Generate button in the bottom left corner of the graph area.
The traces for each slice can be drawn as conventional lines or as filled
areas, selected by the Fill slices check box. The alternative views
are shown below.
The time separation of the slices is controlled by the Slice Interval setting, the width of the impulse response section that is used to generate the slice is set by the Window control. The corresponding frequency resolution is shown at the bottom of the controls panel.
The control settings are remembered for the next time REW runs. The Apply Default Settings button restores the controls to their default values.
This graph shows a waterfall plot over the region from 10Hz to the end of the measurement sweep. The plot uses logarithmically spaced data at 96 points per octave with 1/48th octave smoothing applied. The waterfall plot is generated in the same way as the Spectral Decay plot, shifting the impulse response window to the right by a proportion of the time range to generate each succeeding slice. The window types may be selected via the Spectral Decay entries in the Analysis Preferences. The initial reference point for the windows (end of left window/start of right window) is the peak of the impulse response.
To produce the waterfall plot click the Generate button in the bottom left corner of the graph area.
The labels at the sides of the plot show the time axis values
The Slice slider selects which slice is at the front of the plot - as the slider value is reduced the plot moves forward one slice at a time. The trace value shows the SPL figure for the frontmost slice, the corresponding time for that slice is shown at the top right of the graph.
The x, y and z sliders alter the perspective of the plot, moving it left/right, up/down and forwards/backwards respectively. The check boxes next to the sliders allow the perspective to be disabled in that axis. Disabling the x axis can make it easier to see the frequencies of peaks or dips. Disabling the z axis turns off all the perspective effects which makes the plot like a filled spectral decay. Here is the same plot as above but with the x-axis perspective effect turned off.
The waterfall allows another measurement's plot to be overlaid on the current measurement. The overlay is generated slice-by-slice, plotting a slice of the current measurement's waterfall, then a slice of the overlay, then the next slice of the current measurement and so on. N.B. before a measurement is available to overlay it is necessary to generate the waterfall data for it.
The overlay is selected using the Overlay selector. Measurements which do not have waterfall data are shown in grey in the selection list. To generate the data for a measurement select it as the current measurement and use the Generate button.
Transparency can be applied to the main plot, the overlay, or both. When transparency is set to 0% both plots are solid. In the image above the main plot is drawn at 75% transparency, allowing the overlay to show through. The transparency mode can be switched between main/overlay/both to ease comparison between the plots.
The Time Range control determines how far the impulse response window is moved from its start position to generate the waterfall, the width of the impulse response section that is used to generate the waterfall is set by the Window control. The corresponding frequency resolution is shown to the right of the window setting. Best results are obtained when the window is smaller than the time span.
The control settings are remembered for the next time REW runs. The Apply Default Settings button restores the controls to their default values.
This graph shows a spectrogram plot over the region from 10Hz to the end of the measurement sweep. The spectrogram is like a waterfall viewed from above, with the level indicated by colour. The scale showing how colour relates to level is displayed to the left of the plot. The vertical axis of the plot shows time, increasing towards the top of the plot. The time starts, by default, the width of the selected window before zero, so that the onset of the response can be seen. The areas where the response is decaying more slowly show up as streaks rising up towards the top of the graph.
The spectrogram plot is generated in the same way as the Spectral Decay plot, shifting the impulse response window to the right by a proportion of the time range to generate each succeeding slice. The window types may be selected via the Spectral Decay entries in the Analysis Preferences. The plot uses logarithmically spaced data at 96 points per octave with 1/48th octave smoothing applied.
To produce the spectrogram plot click the Generate in the bottom left corner of the graph area. The legend panel shows the plot value at the intersection of the vertical and horizontal cursor lines.
An ideal Spectrogram decays very rapidly off the bottom of the scale range. Here is an example of a plot produced from a soundcard loopback measurement.
Match Top of Scale to Peak adjusts the Scale Top value so that it corresponds to the highest level found in the data.
Match Time Scale to Window and Range adjusts the time axis range so that it starts at the Window width before zero (e.g. -300ms for a 300ms Window setting) and ends at the Time Range (e.g. 1000ms for a 1000ms Time Range) so that the plot shows all the generated data.
3D Enhancement gives the plot a more three-dimensional appearance.
Draw Contours adds contour lines at the dB interval set in the adjacent spinner.
The Colour Scheme for the plot can be changed, the plots above use the "Rainbow" scheme, here is a plot using the "Flame" colour scheme.
This plot uses the "Copper" colour scheme with 3D enhancement active.
Scale Gamma adjusts the way colours are distributed along the scale, gamma values below one emphasis variations at the top of the scale, values above one emphasise variations at the bottom of the scale. A gamma value of 0.5 was used for the copper colour scheme image above.
The Scale Top, Scale Bottom and Scale Range controls adjust how the plot colours correspond to the values in the Spectrogram data. Any values higher than the Scale Top are drawn in the colour at the top of the scale, any values lower than the Scale Bottom are drawn in the colour at the bottom. If the Scale Top setting is changed the Scale Bottom will be adjusted to keep the same Scale Range. If the Scale Bottom is changed the Scale range will be adjusted to keep the same Scale Top. If the Scale Range is changed the Scale Bottom will be adjusted, keeping the same Scale Top.
The Time Range control determines how much spectrogram data will be generated after the time = zero point. The width of the window that is moved along the impulse response to generate the spectrogram is set by the Window control. The corresponding frequency resolution is shown below the window setting.
The control settings are remembered for the next time REW runs. The Apply Default Settings button restores the controls to their default values.
This graph shows the generated sweep test signal and the raw captured system
response as acquired via the soundcard, which may be useful for troubleshooting.
This is not a live display, it updates with new content after a sweep has completed.
Only the signals for the last measurement are shown. The Y axis is the percentage
of digital full scale. The generated sweep is shown normalised so that its peak
value is 100%. If the captured trace reaches +100 or -100% it is clipping
and the sweep level or AV processor volume should be reduced.
A check box is provided to invert the captured trace for easier comparison with the test signal if the soundcard input is inverting. As a more permanent solution for this select the Invert checkbox in the soundcard Input Channel settings. If Show points when zoomed in is selected the individual time samples will be shown if the horizontal zoom level is high enough to distinguish them.