Audio linearity

The linear power supply finds a very strong niehe within applieations where its ineffieieney is not important. These inelude wall-powered, ground-base equipment where foreed air eooling is not a problem and also those applieations in whieh the instrument is so sensitive to eleetrieal noise that it requires an eleetrieally quiet power supply—these produets might inelude audio and video amplifiers, RF reeeivers, and so forth. Linear regulators are also popular as loeal, board-level regulators. Here only a few watts are needed by the board, so the few watts of loss ean be aeeommodated by a simple heatsink. If dielee-trie isolation is desired from an ae input power souree it is provided by an ae transformer or bulk power supply. 

Newman and Lerner (N2) have used an arrangement where the signal picked up by a microphone attached to the flat surface below the orifice plate is amplified and fed to a loud speaker. The amplified bubble signal is then fed to one pair of fixed contacts of a double-pole double-throw switch of which the other pair of fixed contacts is connected to an audio-frequency generator. The movable contacts of the switch are connected to the vertical and ground terminals of an oscilloscope. This arrangement permits the observation of either the bubble signal or the sine wave as a function of the internal linear time-base of the oscilloscope. 

For linear, time-invariant systems a complete characterization is given by the impulse or complex frequency response [Papoulis, 1977], With perceptual interpretation of this characterization one can determine the audio quality of the system under test. If the design goal of the system under test is to be transparent (no audible differences between input and output) then quality evaluation is simple and brakes down to the... 

Until now only time-frequency smearing of the audio signal by the ear, which leads to an excitation representation, has been described. This excitation representation is generally measured in dB SPL (Sound Pressure Level) as a function of time and frequency. For the frequency scale one does, in most cases, not use the linear Hz scale but the non-linear Bark scale. This Bark scale is a pitch scale representing the... 

After having applied the time-frequency smearing operation one gets an excitation pattern representation of the audio signal in (dl 1 exc, seconds, Bark). This representation is then transformed to an internal representation using a non-linear compression function. The form of this compression function can be derived from loudness experiments. 

Many examples exist of audio recordings which are subject to non-linear amplitude distortion. Distortion can be caused by a number of different mechanisms such as deficiencies in the original recording system and degradation of the recording through... 

The techniques introduced in this section perform well on audio data which have been distorted by the appropriate model. However extensive testing is required to determine whether or not the non-linear models proposed are sufficiently flexible to model real distortion mechanisms. 

Lawrence et al., 1983] Lawrence, R., Moore, B., and Glasberg, B. (1983). A comparison of behind-the-ear high-fidelity linear hearing aids and two-channel compression aids in the laboratory and in everyday life. Brit. J. Audio , 17 31-48. 

For systems that are nearly linear or time-variant, the concept of the impulse (complex frequency) response is still applicable. For weakly non-linear systems the characterization can be extended by including measurements of the non-linearity (noise, distortion, clipping point). For time-variant systems the characterization can be extended by including measurements of the time dependency of the impulse response. Some of the additional measurements incorporate knowledge of the human auditory system which lead to system characterizations that have a direct link to the perceived audio quality (e.g. the perceptually weighted signal to noise ratio). 

The big advantage of the perceptual approach is that it is system independent and can be applied to any system, including systems that show a non-linear and time-variant behavior. A disadvantage is that for the characterization of the audio quality of a system one needs a large set of relevant test signals (speech and music signals). 

If the perceptual approach is used for the prediction of subjectively perceived audio quality of the output of a linear, time-invariant system then the system characterization approach and the perceptual approach must lead to the same answer, In the system characterization approach one will first characterize the system and then interpret the results using knowledge of both the auditory system and the input signal for which one wants to determine the quality. In the perceptual approach one will characterize the perceptual quality of the output signals with the input signals as a reference. 

A model which has found application in many areas of time series processing, including audio restoration (see sections 4.3 and 4.7), is the autoregressive (AR) or allpole model (see Box and Jenkins [Box and Jenkins, 1970], Priestley [Priestley, 1981] and also Makhoul [Makhoul, 1975] for an introduction to linear predictive analysis) in which the current value of a signal is represented as a weighted sum of P previous signal values and a white noise term ... 

A linear modelling approach to noise pulse removal is presented in [Godsill, 1993], In this it is assumed that the corrupted waveform x consists of a linear sum of the underlying audio waveform s and resonant noise pulses v ... 

We note that s and v are the responses of the playback system, including mechanical components and amplification/ equalization circuitry, to the recorded audio and noise signals, respectively. The assumption of a linear system allows the overall response x to be written as the linear superposition of individual responses to signal and noise components. 

An audio recording may be subject to various forms of non-linear distortion, some of which are listed below ... 

In the field of computer music, it is customary to use simple piecewise linear functions for functions other than signals at the audio sampling rate, e.g., for amplitude envelopes, FM-index functions, and so on [Roads and Strawn, 1985, Roads, 1989]. Along these lines, good initial results were obtained using the simplified qualitatively chosen table... 

Fuchs, 1995] Fuchs, H. (1995). Improving MPEG audio coding by backward adaptive linear stereo prediction. In Proc. of the 99th. AES-Convention. Preprint 4086. 

Mice were placed into an enclosed chamber with a high-frequency speaker in the center top lid. An audio signal generator was used to produce a continuous sinusoidal tone that was swept linearly in frequency between 8 and 16 kHz once every 10 milliseconds. The average sound pressure level during stimulation was approximately 100 dB at the chamber floor. DBA/2 mice in the vehicle-treated group... 

The field is modulated at some audio- or radio-frequency (usually -100 kHz), and this produces signals at the crystal detector whose output can be amplified at the modulation frequency. In general, sensitivity is higher at high-modulation frequencies because of the linear variation of excess crystal noise with the inverse of frequency. However, the recently developed Schottky diode makes it possible to obtain good sensitivity at relatively low frequency. 

There are several pieces of backup hardware that are currently available. You can back up your information to magnetic tape, Digital Audio Tape (DAT), Digital Linear Tape (DLT), optical disk, removable hard disk, and many other removable media. The key here is that all of these media can be removed from the drive and stored in a safe place. 

D. Of the items listed, the following are valid backup hardware digital audio tape, digital linear tape, and optical disk. Because hard disks are more failure-prone than the other types listed, they make a poor backup medium. 

Quiet, easy hum of a motor. Tap tap tap of feet on a hard floor—an audio discrimination program went primary— two sets of feet, plus the level breathing of two people. Constant pulse of light pressure on the enhanced retinas below closed eyelids indicated linear movement, backed up by inner ear fluid motion estimated at a fast walking pace. Posture was level he was still lying on his bed. 

Most computer audio systems use two or three types of audio data words. 16-bit (per channel) data is quite conunon, and this is the data format used in Compact Disk systems. 8-bit data is common for speech data in PC and telephone systems. There are also methods of quantization that are non linear, meaning that the quantum is not constant over the range of input signal values. In these systems, the quantum is smaller for small ampUtudes, and larger for large amplitudes. This nonlinear quantization will be discussed more in Section 1.5.1. 

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