Threshold signal level

Figure 6.43 Failure to detect smaller flaws at the same threshold signal level.

The steady-state input/output relationship for a hearing-aid compressor is shown in Fig. 6.14. For input signal levels below the compression threshold, the system is linear. Above the compression threshold, the gain is reduced so that the output increases by 1/CR dB for each dB increase in the input where CR is the compression ratio. 

The most common form of compression in hearing aids is a single-channel system used to reduce amplifier overload. For this application, a rapid attack time is desired so as to give a rapid response to a sudden intense sound, a high compression ratio is desired to limit the maximum signal level, and a high compression threshold is desired so as not to limit sounds that could otherwise be amplified without distortion. 

Mass spectra were obtained on a AEI MS9 (Manchester, U.K.) instrument equipped with a fast atom bombardment source (Figures 10 and 11). The medium was either glycerol or Cleland and the sample was introdiKxd by means of direct insertion. Instrument settings were 92-963 total scans in run, 4 sampling rate, 256 signal level threshold, 30 minimum peak width, 5 scan rate (sec/dec), 10.0. 

Finally, it is possible to have a combination of PWM and PPM where one has variable width pulses and variable periods. In all cases, the output PWM stream is a series of pulses in which the dc voltage level is proportional to the ratio of the amount of on time with respect to the amount of off time for the pulses in the stream. Myopulse modulation, as implemented by Childress (1973) in his myoelectric controller, is an example of this form of pulse modulation (Fig. 32.25). This is an elegant proportional controller that can be implemented with a minimum of components. Because percentage myoelectric signal on time (EMG signal level above an arbitrary threshold) is monoton-ically related to muscle output force level, a proportional signal can be achieved very simply. This... 

Stimulated Brillouin scattering (SBS) occurs at the lowest threshold power levels of the various non-linear effects and can be observed at powers as low as several miUiwatts, depending on the transmitter source and fiber characteristics. SBS occurs when signal power reaches a level that results in the generation of acoustic waves in the optical waveguide medium, through a process caUed electrostriction. The actual process... 

The measured response distribution reflects the variance in the NDE measurement process and is typical of that obtained for any measurement process. The response from the surrounding material constitutes the baseline level for use in discrimination of responses from internal anomalies. The baseline response may be termed noise, and both the discrimination capability and anomaly sizing capability of the NDE procedure are dependent on the relative amplitudes and the rate of change of the anomaly response with increasing anomaly size (slope). The considerable flaw-to-flaw variance and the variance in signal response to flaws of equal size cause increased spread in the probability density distribution of the signal response. If a threshold decision (amplitude) level is applied to the responses, clear flaw discrimination (detection) can be achieved, as shown in Fig. 6.42. If the same threshold decision level (acceptance criterion) is applied to a set of flaws of a smaller size (as shown in Fig. 6.43), clear discrimination cannot be accomplished. 

Channel AEBIL signal conditioning modules (variable amplifier, RMS measurement, floating threshold level, analog to digital conversion signal) ... 

---