Installation guide

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measurement - for example, if the tweeter was high-pass limited to 2000 Hz during the speaker
measurement then the crossover frequency is set by the user to 1500 Hz.
The optimum crossover frequency for a combination of drivers is best determined by
experimentation and listening - it cannot be reliably determined from inspection of the frequency-
response alone of the drivers. Fortunately the PDC is ideal for this sort of experimentation, as it
allows for the quick creation of filters with a range of crossovers. These different filters can be
loaded into the different profiles of one configuration, allowing for quick A-B comparisons of the
effect of changing the crossover frequency.
The Boost Limit and Cut Limit specify the maximum boost and cut that will be applied in
generating the correction filter. These maxima and minima can be set individually for each
Boost/Cut Band. The Number of Boost/Cut Bands defaults to the number of amplifiers per stereo
channel (ie single-amp: one band, bi-amp: two bands, etc) but can be set to up to 10 for any
template regardless of configuration. In the Limits tab the boost and cut limits can be adjusted.
This adjustment is provided because in principle the boost should be high enough to provide a flat
frequency-response across the audible spectrum, but in practice you may find through
experimentation that, for example, too much boost causes distortion, or that significant boosting
of low frequencies requires more amplifier power than is available. However, by default the scale
is automatically adjusted so that the lower limit of the measured response is 0dB. This generally
results in most of the frequency range of the speaker(s) being cut rather than boosted.
As an example of the use of the effective frequency range and the boost and cut limits, in the
figure above the effective frequency range of the speaker correction is 30 Hz to 20 kHz, the boost
limit is 6 dB below 2.665 kHz and 4 dB above 2.665 kHz. The purple (wiggly) line is the Full
Range response of the measured speaker, and the red line (flat for most of the effective
frequency range) is the Desired Response.
The Full Range response is the measured frequency-response of the speaker before it has been
corrected, but taking account of the truncation that was done in the anechoic window (see the
paragraph on windowing in the section on Correcting a Loudspeaker) and any crossovers set.
The Desired Response is an approximation of the frequency-response that will be achieved,
given the measured response and the chosen calibration parameters.
A brief description of the filter design process
The filter correction algorithm corrects in both the time, frequency and phase domains subject to
the parameters that the user enters into the user interface. The user specifies the desired
corrected frequency-response using the limit filters (as described above), but only a few
parameters are required by the user to specify how the phase and time response will be
corrected.
A certain amount of phase correction can be achieved without introducing any additional delay
into the system, but additional phase correction occurs at the expense of introducing delay in the
filter. The user can choose how much of this delay is acceptable by specifying the Maximum
Delay parameter in milliseconds (thousands of a second).
Any design process is not without error, but the uniqueness of the DEQX algorithm is that you
can control the amount of error in the correction filter by specifying particular error tolerances.
This can be achieved in both magnitude 'ripple' (Magnitude Tolerance), and group delay 'ripple'
(Group Delay Tolerance). The routine will design the filters with as much phase and time
correction as possible without violating these requirements.
The final design parameter specifies how the routine is optimised. This can be specified for
Minimum Delay or Minimum Error. A Minimum Delay selection designs a filter that reduces the
delay in the filter to a minimum, without exceeding the error bounds. In this case, it is likely that
the filter will have errors as large as your specified maximum. A Minimum Error