FIR Designer M : Channels – Export
The FIR filter windowing and truncation can be set on this view. The channel channel FIR filter and summary file can also be saved here.
The preceding five FIR filter design views combine to create the channel FIR filter. However the “ideal” FIR filter potentially has a very long temporal length. To use the FIR filter in a practical DSP or processor, the “ideal” filter has to be truncated or shortened to fit within the maximum available FIR filter length or tap count.
Simple truncation can lead to audible discontinuities at the FIR filter start and/or end, and/or differences in the filter response, compared to the “ideal.” Windowing involves fading down the FIR filter coefficients, at the ends, to reduce discontinuities and bring the truncated filter closer in magnitude and phase to the “ideal” filter.
FIR Tab Controls
|Filter delay (samples)||The filter delay represents the time=0 sample index, within the FIR filter. This also represents the effective delay through the FIR filter. All prototype filters are created with reference to this delay or sample index. For example a linear phase filter prototype will have coefficients symmetrically either side of this sample index. Maximum phase filter prototypes have coefficients before this index and minimum phase filter prototypes have coefficients after this index.|
|Filter length (samples)||This represents the number of coefficients or taps in the FIR filter. Different DSP’s have different maximum FIR filter lengths, and so the length can be adjusted for different target DSP’s and processors.|
|Window function||Here different window functions can be chosen. The chosen window function is shown as a light red line on the adjacent plot.|
|Alpha||If the “Kaiser-Bessel” window function is chosen, this setting controls how quickly the window function fades up and down.|
|DC Compensation||The truncation and windowing may cause a slight shift in the DC level of the FIR filter. If the FIR filter includes a high-pass filter prototype, DC Compensation ensures the filter has no DC component. If the FIR filter doesn’t include any high-pass filter prototypes, DC Compensation ensures the filter has exactly 0 dB at DC. This will cause unintended behaviour if the FIR filter has intentional gain at DC, for example from a low shelf filter. In this case, uncheck DC Compensation.|
|Output gain (dB)||Here the gain of the FIR filter can be manually adjusted.|
|Format||Select the FIR filter output format for import into the speaker processor. See also FIR Capable Products.|
|Invert filter polarity when saving||As the text suggests, when checked, the FIR filter will be inverted when the filter is saved.|
|Save FIR||This button brings up a file save dialog.|
|Save FIR + Summary||This button brings up a file save dialog. In addition to the FIR filter file, a summary text file (with the same filename) is saved.|
Any warning messages are displayed here. Such as when the FIR filter coefficients are are too high in level for WAV file export and will be clipped.
FIR Filter Frequency Response
This plot shows the channel FIR filter, before (“ideal”) and after windowing.
|Show FIR filtered loudspeaker||When checked, the plot shows the loudspeaker magnitude and phase after filtering by both the ideal FIR filter and the windowed FIR filter. The plot also includes any IIR filtering.|
This plot shows the difference in magnitude and phase, between the “ideal” FIR filter and the windowed FIR filter. The absolute maximum magnitude and phase difference (with the adjustable frequency range) is also shown.
This plot can be helpful when adjusting the FIR filter length, delay and window to minimise the error or difference, introduced by truncation and windowing.