Metric Halo SpectraFoo - Manual (Page 55)

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4. The Transfer Function
Overview
For live sound, equipment testing, or any application where you need to analyze the characteristics of acous-
tical and electrical audio systems, SpectraFoo provides Music Based Measurement (MBM™). SpectraFoo us-
es a transfer function display to show you the relative power and phase response between the left and right
channels. The transfer function display assumes that the signal connected to the computer’s right input is a
source signal and the signal connected to the left input is a response signal.
SpectraFoo
Source
Audio or
Acoustic System
Source
Response
Right Left
Delay
Figure 4.1: SpectraFoo Signal Arrangement for MBM Transfer Function
When the source signal is the input to some audio processing arrangement and the response signal is the mea-
sured output of the system, you can use music as your “test tone”. SpectraFoo uses the source signal as a point
of reference and the transfer function display shows the differences in amplitude and phase between the source
and response as function of frequency. This allows you to measure the properties of audio processing systems,
including systems that contain acoustic elements. You can determine the amplitude and phase response of an
equalizer in the presence of a musical signal as easily as measuring the sound coloration of an acoustic space.
The transfer function really only makes sense when the source signal is the input to some audio processing
arrangement and the response signal is the measured output of the system. It does not generally make sense
if the the source and response are truly unrelated, as in the case of the two channels of a stereo mix (in this
case meaning a multi–track mix, with panned elements, as opposed to a true stereo program, such as an X–
Y recording).
Even in the case of a stereo mix, the transfer function may provide some information about the spectral balance
of the recording, but, in general, the spectral balance is not constant in time.
Even when the source and response signals are related, they usually will not be time-aligned. For example,
if you are measuring the room response of an auditorium, there will be the speed–of–sound acoustic delay
between the source signal and the response signal received at the measurement microphone. In order to
properly measure the power and phase of the response signal relative to the source signal, the source signal
must be delayed to time-align it with the response signal.