relnotes_55.txt

lpc 2400 bps语音编解码程序

  LPC version 55 has the following differences from version 54:

 * Preemphasis is restored to the original single-zero filter, 1-.9375**z-1.
   This removes the extra 6 dB boost added around version 47.  The routine
   has been generalized to have the preemphasis coefficient and filter
   state passed as parameters.

 * The 100 Hz high-pass filter hp100 had a missing digit in one of the
   denominator coefficients, which has been corrected.  The two single-zero
   stages have been merged so that the filter now consists of just two
   second order sections.

 * The low pass filter has been increased from 19 to 31 points.  This
   sharpens the transition region and increases the stopband rejection.
   Voicing coefficient calculation now compensates for the filter delay.

 * The inverse filter now checks for zero with a small threshold, to
   avoid precision problems on floating point underflow.

 * Hex bitstream I/O is more robust.

 * The command line parser has improved error checking and better handling
   of options without values.

Future work:

 * The all-zero filter used in the two-stage synthesis procedure is replaced
   by an adaptive postfilter ( A(z**-a) / (1-A(z**-b)) ), where a and b
   are bandwidth widening constants.