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The Impulse Control Window
The Frequency response function subtracts the response of a signal from that of a reference on a frame by frame basis. A second
algorithm then finds the effective time displacement, allowing the software to directly unwrap phase (we do not use minimum
phase calculations). The frequency domain information is then reversed back to the time domain revealing the impulse response,
including reflections, but in this case with an accurate time position within the new time window. A 'start' and 'stop' reference
can then be easily added to the impulse allowing the user to reject room reflections.
Note:
Time 'start' and 'stop' ticks also effect the reponse in the main control window. However, these time ticks can only be defined
when the Impulse Window is active. If the Impulse window is not active, the entire frame is assumed.
Waterfall 'Forward Time' FFT
The 'Forward Time' waterfall FFT plot shows FFT response slices as a function of time as the time window grows from the right
hand side. This view shows how the FFT bin energy grows as time progresses until a steady state condition is achieved. Tick marks
'1' and '2' can be moved to change the starting and stopping time positions. The following example is for a Heil AMT tweeter,
intentionally placed ~1M from the corner of the room to produce a strong reflection.
Click Image to Enlarge
Cumulative Spectral Decay (CSD)
Reversing the window growth and showing FFT slices as a function of time as the time window shrinks from the left hand side
results in the CSD display. In this case the FFT bin energy shows how the spectrum changes as energy is removed. As before Tick
marks '1' and '2' adjust the starting and stopping time positions. The following example is for a Heil AMT tweeter, intentionally
placed ~1M from the corner of the room to produce a strong reflection.
Click Image to Enlarge
Frequency Domain Smoothing and Sharpening
The number of FFT bins that are combined to make an entry in the display data is adjusted here. That is, a per octave resolution
is defined here.
Controlling the Transition Region
The lead in and lead out transition region can be 'smoothed' using a modified raised cosine window function. However, since
the impulse response has been located in time using a frequency domain technique, windowing the impulse in a traditional
sense is not required. Never the less, this adjustment may help in some cases.
Sizing and Rotating the Display
Use the arrow, page up, page down, insert, delete, home and end keys to size and rotate the display. Additional controls are
provided to view data in 'slices', 'sheets' and wire frames.
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