Frye FONIX 8000 - 1.2 Signal Detection Overview

Chapter 1: 3
a series of spikes with very little in between, or an impulse drive. The disadvantage
with this type of drive is that it makes very heavy demands on the dynamic range of
the hearing aid under test. Hearing aids are not known for their large dynamic ranges,
so this type of waveform must be avoided. To produce a moderate crest factor noise
equivalent, it is necessary to skew the phases of all of the components so that a given
RMS amplitude of signal has a low peak value. A computer program was used to deter-
mine the optimum set of phase values.
1.1.4 Special Distortion Measurements
Harmonic distortion tests can be run by generating a signal consisting of only one sine
component at a time, and using the Fast Fourier analysis program to determine the
spectral distribution of harmonics.
Intermodulation tests are automatically run using the composite waveform. If present,
this type of distortion is immediately apparent when viewing the gain or power re-
sponse of the hearing aid.
1.2 Signal Detection Overview
1.2.1 Fast Fourier Technique
The assumed technique for making response measurements is the Fast Fourier Trans-
form. This technique is ideally suited to a system that is used to analyze a digitally
generated waveform, since the sampling process can be locked to the signal generation
process, thus eliminating one of the main problems with the FFT, the windowing of
the measured waveform. A rectangular window is thus possible and used. That is, all
sampled components are used as measured.
1.2.1.1 Windowing
The FFT requires that the signal being sampled is exactly repetitive in order to
achieve a clean spectral display. If there is an abrupt discontinuity between the
first and last samples taken, the transform can only assume that large high frequen-
cy components are present in order to generate the discontinuity. Use of the inter-
nally generated signal helps to eliminate this problem; if a random noise is present
in the signal being measured, it will appear in the response display.
When the sound drive is switched to “OFF,” the 8000 is turned into a general pur-
pose spectrum analyzer and can be used for analysis of signals. When this is done,
mathematical windowing is used as is done with standard FFT analyzers. This
windowing is done by multiplying the first and last samples taken by a value close
to zero. The next two samples in from the ends are multiplied by a larger quantity.
In the center of the group of samples, the values are multiplied by a compensating
factor greater than unity. The function used is called a Hanning window.