getid3_ac3::heavyCompression

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Syntax Syntax

getid3_ac3::heavyCompression( int $compre )

Parameters Parameters

$compre

(int) (Required)

Return Return

(float|int)

Source Source

File: wp-includes/ID3/module.audio.ac3.php

	 *
	 * @return float|int
	 */
	public static function heavyCompression($compre) {
		// The first four bits indicate gain changes in 6.02dB increments which can be
		// implemented with an arithmetic shift operation. The following four bits
		// indicate linear gain changes, and require a 5-bit multiply.
		// We will represent the two 4-bit fields of compr as follows:
		//   X0 X1 X2 X3 . Y4 Y5 Y6 Y7
		// The meaning of the X values is most simply described by considering X to represent a 4-bit
		// signed integer with values from -8 to +7. The gain indicated by X is then (X + 1) * 6.02 dB. The
		// following table shows this in detail.

		// Meaning of 4 msb of compr
		//  7    +48.16 dB
		//  6    +42.14 dB
		//  5    +36.12 dB
		//  4    +30.10 dB
		//  3    +24.08 dB
		//  2    +18.06 dB
		//  1    +12.04 dB
		//  0     +6.02 dB
		// -1         0 dB
		// -2     -6.02 dB
		// -3    -12.04 dB
		// -4    -18.06 dB
		// -5    -24.08 dB
		// -6    -30.10 dB
		// -7    -36.12 dB
		// -8    -42.14 dB

		$fourbit = str_pad(decbin(($compre & 0xF0) >> 4), 4, '0', STR_PAD_LEFT);
		if ($fourbit[0] == '1') {
			$log_gain = -8 + bindec(substr($fourbit, 1));
		} else {
			$log_gain = bindec(substr($fourbit, 1));
		}
		$log_gain = ($log_gain + 1) * getid3_lib::RGADamplitude2dB(2);

		// The value of Y is a linear representation of a gain change of up to -6 dB. Y is considered to
		// be an unsigned fractional integer, with a leading value of 1, or: 0.1 Y4 Y5 Y6 Y7 (base 2). Y can
		// represent values between 0.111112 (or 31/32) and 0.100002 (or 1/2). Thus, Y can represent gain
		// changes from -0.28 dB to -6.02 dB.

		$lin_gain = (16 + ($compre & 0x0F)) / 32;

		// The combination of X and Y values allows compr to indicate gain changes from
		//  48.16 - 0.28 = +47.89 dB, to
		// -42.14 - 6.02 = -48.16 dB.

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