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Entries found for search: compress

compression : (1) The process of reducing the dynamic range of an audio signal by reducing the peaks so as to be able to boost the low levels. For every dB of compression applied, the S/N ratio is worsened by 1dB, assuming that the make-up gain is set so that the maximum levels of the compressed and uncompressed signals are the same, as the quieter parts of the original signal, plus any noise contained in these regions, will be raised in level. (2) A dynamic-range problem in loudspeakers caused by nonlinearity under conditions of high input power levels. At very high levels, the acoustic output increases more slowly or ceases to increase altogether as the input power increases, producing nonlinear distortion, i.e., a frequency response curve very different for very high levels. (3) Data compression used on digital audio files is a process ADPCM, MACE, for example. (4) The opposite of rarefaction whereby a quantity of data is reduced in order to occupy less storage space. See ATRAC.

compression driver : A specialized mid- or high-frequency speaker consisting of a small diaphragm and voice coil coupled to a large magnet structure. The unit is mounted to a horn which acoustically matches the impedance of the driver to the impedance of the air and shapes the signal. Expensive due to the precise tolerances required, compression drivers are substantially more efficient than traditional direct-radiating cone speakers.

compression ratio : (1) The ratio of the dB change from input level to output level effected by a compressor, once the threshold has been exceeded. (2) In data compression, the ratio of the number of bytes of uncompressed to compressed data, an indication of the space-saving efficiency of the compression algorithm.

compressor/limiter : A device for reducing the effective dynamic range of an input signal by preventing it from rapidly exceeding or falling below a selected amplitude threshold. The first part of a compander, it is used to make loud parts of a signal softer and soft parts louder. Beyond the threshold, the ratio of the signalís input level to its output level (e.g., 2:1, 4:1) is user-selectable. A compressor is commonly used to keep mic levels within an acceptable range, but because it can slow a signalís rate of decay below the threshold, compressors are also used to add sustain to instruments such as electric guitar and bass. The limiter acts like a compressor, but operates only at the top end of the dynamic range. The limiter has a faster attack time (1Ķs to 1ms) than the compressor alone (1ms to 10ms). A compressor/limiter is inserted between the outputs of a MIDI soundcard, synthesizer, or mixer and the inputs of the mixdown deck. See hard knee compression, soft knee compression.

hard knee compression : A characteristic of certain designs of a compressor wherein nothing happens to an input signal until the signal reaches the threshold limit, but as soon as it does, the full level of gain reduction is applied, as determined by the ratio control setting. A graph of the input gain against the output gain will show a sharp change in slope at the threshold level. Compare with soft knee compression."

high-frequency compression : See HX/HX pro.

split-band compression : Compression where different sections of the audio spectrum are compressed separately. With a wideband compressor, if there is a dominant portion of the spectrum, no matter how small, it affects the whole waveform. With split-band devices, the greatest effect is with high ratios, where the effect is more like localized limiting. Split-band compressors are similar to multiband audio processors in the first stage of audio processing.

soft knee compression : The output of a compressor whose gain reduction is brought in progressively over a range of input signal values, such as over 10db or so, starting a few dBs below the threshold.. When the input signal amplitude comes within the range of the threshold, the compressor starts to apply again reduction, but with a very low ratio setting. As the input level increases, the compression is automatically increased until, at the threshold level, the ratio becomes infinite. Compare with hard knee compression.

velocity compression : Each MIDI Note-On message has a velocity value between 1-127. The velocity corresponds to how hard the key was struck. In velocity scaling, (more accurately called a velocity offset), a group of notes is selected for editing and then their velocities are all cut or boosted in a linear manner: e.g., with a scaling value of -20, three notes originally recorded with velocities of 65, 91, and 37 would be set to play back with velocities of 45, 71, and 17, respectively. In velocity compression (sometimes called velocity scaling), the velocities are multiplied or divided by some factor so that the differences between them get larger or smaller. With a compression value of 75%, for example, the same three notes would be played back with velocities of 49, 68, and 28. This means that the note with the largest starting velocity is reduced the most, while soft notes play back closer to their original velocity, helping to keep them audible. Thus, compression is a better way to smooth out the transients in a passage that were played too loudly, without changing the musical dynamics of the piece.

data compression : See compression(3).

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