Amplifier compression

Application of the Bernoulli Equation to the simple apparatus which is an air amplifier implements what is known as the Coanda Effect (named after Romanian aerodynamics pioneer Henri Coanda, who, in 1934, was the first to recognize the practical application of the phenomenon in development of aircraft which would have adequate lift at low air speed). Specifically, in an air amplifier, compressed air expands across a specially configured surface. A high velocity jet is produced. The jet clings to the walls of the surface (the Coanda Effect). A zone of low pressure is created. Available air at a higher pressure is pulled into the zone of lower pressure. The level of low pressure is dependent upon the dimensions of the surface configuration (as predicted by the Bernoulli Equation). 

Pump characteristic Simple single- head Single- head smooth pulse Simple dual-head Dual-head, compressibility-corrected, smooth pulse Dual-head, closed loop flow control Triple head low-volume Syringe- type Hydraulic amplifier Simple Amplifier Amplifier with flow control... 

Figure 6.14 Input/output relationship for a typical hearing-aid compression amplifier...

Figure 6.16 Compression amplifier input/output curves derived from a simplified model of hearing loss.

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. 

B gives input AGC, in which the gain and maximum output level are simultaneously adjusted by the volume control but where the input-referred compression threshold is unaffected. A separate trimmer adjustment is normally provided for the compression threshold. Control point A for the volume control gives an example of output AGC, in which the volume control simultaneously adjusts the gain and compression threshold, and a separate trimmer is used to set the maximum output level. Another option is feedforward compression, in which the detector is driven directly by the microphone signal a delay in the amplified signal relative to the control signal can then be used to reduce the attack overshoot in the compression circuit [Verschuure and Dreschler, 1993],... 

The choice of optimum compression parameters to maximize speech intelligibility or speech quality is still open to contention. Rapid attack time constants (less than 5 ms) are accepted in the industry and also by researchers to prevent transients from saturating the output power amplifier. Arguments for fast release times (less than 20 ms), also termed syllabic compression, are based on considerations of the syllabic variations in speech and the desire to amplify soft speech sounds on a nearly instantaneous basis... 

Multi-channel compression systems divide the speech spectrum into several frequency bands, and provide a compression amplifier for each band. The compression may be independent in each of the bands, or the compression control signals and/or gains may be cross-linked. Independent syllabic compression has not been found to offer any consistent advantage over linear amplification [Braida et al., 1979][Lippmann et al., 1981][Walker et al., 1984], One problem in multi-channel compression systems has been the unwanted phase and amplitude interactions that can occur in the filters used for frequency analysis/synthesis [Walker et al., 1984] and which can give unwanted peaks or notches in the system frequency response as the gains change in each channel. 

D. Strickland, G. Mourou, Compression of amplified chirped optical pulses, Optics Communications 56, 219 (1985)... 

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