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) * Utils::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 & 0xf)) / 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.
return $log_gain - $lin_gain;
}
