Frye FONIX 8000 - 1.1 Signal Generation Overview

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2 Maintenance Manual

1.1 Signal Generation Overview

1.1.1 Digital Basis of the Composite Signal

The composite signal is generated by reading 256 16-bit data words in RAM sequential-

ly and doing it over and over again. The program is able to adjust the individual am-

plitude of each signal component in the series so that it provides a correct amplitude

of drive at the test point in the chamber for each test frequency. The test microphone

is placed at the reference point in the chamber. Pushing the LEVEL button starts the

amplitude correction process. A composite analysis of the chamber is done and correc-

tion factors are calculated which alter all of the individual drive components so that

the chamber response is ﬂat and at the desired amplitude at this reference point.

One way to think of the test waveform is to picture one sine wave of 100 Hz being built

using 256 points, or steps, through the sequential reading of a 16-bit digital RAM. If

we wanted to create a signal with two equal value components, we would arithmeti-

cally add to this wave a second sinewave, say of 400 Hz. Each cycle of the 400 Hz wave

would take exactly one fourth as many steps as the 100 Hz wave to produce (i.e., 64

steps). When these two sets of steps are added together we would get a composite wave

with two frequency components.

1.1.2 Component Amplitude Weighting Considerations

The amplitude of each frequency may also be dependent upon its placement in the

frequency spectrum. If a white noise equivalent is used, then every multiple of 100 Hz

will be used and will have an equal amplitude value. If the ANSI spectrum is to be ap-

proximated, the amplitudes of the components in the spectrum will drop at a rate of 6

dB per octave above 900 Hz when viewed on a standard Fast Fourier analyzer.

After the signal is received from the hearing aid and passed through the preampliﬁer,

it is run through a high frequency emphasis ampliﬁer to restore the 6 dB/octave loss so

that we can get a compensated gain response picture.

The RMS amplitude of the drive signal increases in proportion to the square root of the

sum of the squares of all of its components. Thus, if a particular RMS value of drive

signal is needed, both the number and the individual amplitudes of all of the compo-

nents must be taken into account. A multiple frequency signal will then have smaller

component amplitudes to produce the same RMS drive to the device under test as that

of a sine wave signal drive which has only one frequency component.

1.1.3 Phase of the Components

Another interesting problem encountered is that the individual components of the

wave cannot be allowed to line up in phase, or the composite signal will consist of