Appendix F 213
Digital Speech Technical Details
The digital speech signal on the FP35 analyzer works by turning the compos-
ite signal on for a set period of time, taking a measurement, turning the signal
off for a random period of time, and turning the signal back on again. This
sequence is repeated until the user ends the test. Usually, either the ANSI S3.42
or the ICRA speech weighting is used to take the measurements.
The signal “on” time takes a minimum of 50 milliseconds. This includes the
time to turn the signal on, take the measurement, and turn the signal off.
Some aids may require an additional delay before the measurement is taken
to allow their circuits to settle. This additional delay time can be changed in
the SHORT SETTLE setting in the Advanced Menu of Real-Ear or Coupler. The
SHORT SETTLE is automatically adjusted according to the aid type selected.
The settle time is added to the 50 ms minimum measurement time.
The default settings are:
•STANDARD(linear):0ms
•AGC:50ms
•ADAPTIVE:100ms
The signal “off” time is random between 100 and 300 milliseconds. It is not
controllable by the user.
Digital Speech is usually used with the ANSI or ICRA speech spectra (see
Section 4.4.4 for a discussion on filters). There is a sizable difference between
these two filters. The ICRA rolls off the high frequencies of the composite signal
much more rapidly than the ANSI weighting. See Figure F-1 for a graphical rep-
resentation of the differences between the two weightings.
Since the speech weightings of the signals are different, the responses of hear-
ing aids to ANSI and ICRA will also be different. Figure F-2 shows an example
of a digital hearing aid being tested with the ANSI and the ICRA digital speech
signals.
