Installation guide

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The group delay term comes from the concept that groups of frequencies, which form an energy
band, will be delayed by the average group delay of that band, provided the frequency-response
is slowly varying in that band. Group delay is defined as the rate of change of the total phase shift
with respect to frequency.
Impulse-response
The response that results at the output of an audio component (or, in the case of the PDC, also
the output of a filter) when the input of the audio component (or filter) receives an impulse signal.
In theory an impulse should have infinite magnitude and zero duration, but in practice this is not
achievable. The PDC instead uses a swept sine signal as the input signal and uses signal
processing to convert the resulting measured response to an equivalent impulse-response.
Linear phase-response/Linear phase filter
A digital filter in which the phase-response is related to frequency by a simple linear relationship.
In analog systems the bessel filter is used to approximate linear phase in a band of frequencies.
Linear response/Linear system
A term used to define when a systems output is related to input by a simple gain factor. A
perfectly linear system always has a THD of zero.
Non-linear response/Non-linear system
The opposite of a linear response. A term used to define when a systems output is not related to
its input by a simple gain factor.
Off-Axis response
The response of a loudspeaker when measured to the left or right of the on-axis response.
Listening angles to the left (when listener is directly in front of the speaker) are considered
negative while angles to the right are positive.
On-Axis response
The response of a loudspeaker when measured directly in front. The on-axis response is known
as the zero degree listening angle.
Psycho-acoustic phenomena
How the human ear perceives Sound Pressure Level (SPL). The main psycho-acoustic
phenomena are perceived loudness (flat gain with respect to frequency does not sound flat) and
masking effects (some gain variations, even large ones, cannot be perceived).
Signal to Noise ratio (SNR)
A measure of the strength of the audio signal in comparison to that of the background noise. For
audio components, the background noise in question is hum and internally generated circuit
noise. SNR is measure in decibels (dB) and is defined as:
or in terms of SPL it can be written as
SNR = SPL_Signal - SPL_Noise
Total Harmonic Distortion (THD)
THD is a measure of the percentage of undesirable frequencies and harmonics added to the
audio signal (by an audio component) which weren't present in the incoming signal. THD will vary
with frequency but often it is quoted at a single frequency, typically 1kHz. THD is measured in
percent and for a single frequency (tone) is defined as: