SpanDSP - a series of DSP components for telephony. More...

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Data Structures
struct	plc_state_t
Defines
#define	CORRELATION_SPAN 160
#define	PLC_HISTORY_LEN (CORRELATION_SPAN + PLC_PITCH_MIN)
#define	PLC_PITCH_MAX 40
#define	PLC_PITCH_MIN 120
#define	PLC_PITCH_OVERLAP_MAX (PLC_PITCH_MIN >> 2)
Functions
int	plc_fillin (plc_state_t *s, int16_t amp[], int len)
	Fill-in a block of missing audio samples.
plc_state_t *	plc_init (plc_state_t *s)
	Process a block of received V.29 modem audio samples.
int	plc_rx (plc_state_t *s, int16_t amp[], int len)
	Process a block of received audio samples.

Detailed Description

SpanDSP - a series of DSP components for telephony.

plc.h

Author:: Steve Underwood <steveu@coppice.org>

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

This version may be optionally licenced under the GNU LGPL licence.

A license has been granted to Digium (via disclaimer) for the use of this code.

Definition in file plc.h.

Define Documentation

#define CORRELATION_SPAN 160

The length over which the AMDF function looks for similarity (20 ms)

Definition at line 99 of file plc.h.

Referenced by plc_fillin().

#define PLC_HISTORY_LEN (CORRELATION_SPAN + PLC_PITCH_MIN)

History buffer length. The buffer much also be at leat 1.25 times PLC_PITCH_MIN, but that is much smaller than the buffer needs to be for the pitch assessment.

Definition at line 103 of file plc.h.

Referenced by normalise_history(), plc_fillin(), and save_history().

#define PLC_PITCH_MAX 40

Maximum allowed pitch (200 Hz)

Definition at line 95 of file plc.h.

Referenced by plc_fillin().

#define PLC_PITCH_MIN 120

Minimum allowed pitch (66 Hz)

Definition at line 93 of file plc.h.

Referenced by plc_fillin().

#define PLC_PITCH_OVERLAP_MAX (PLC_PITCH_MIN >> 2)

Maximum pitch OLA window

Definition at line 97 of file plc.h.

Function Documentation

int plc_fillin	(	plc_state_t *	s,
		int16_t	amp[],
		int	len
	)

Fill-in a block of missing audio samples.

Parameters:

s	The packet loss concealer context.
amp	The audio sample buffer.
len	The number of samples to be synthesised.

Returns:: The number of samples synthesized.

Definition at line 175 of file plc.c.

References amdf_pitch(), ATTENUATION_INCREMENT, CORRELATION_SPAN, fsaturate(), plc_state_t::history, len(), plc_state_t::missing_samples, normalise_history(), plc_state_t::pitch, plc_state_t::pitch_offset, plc_state_t::pitchbuf, PLC_HISTORY_LEN, PLC_PITCH_MAX, PLC_PITCH_MIN, and save_history().

Referenced by adjust_frame_for_plc().

{
   int i;
   int pitch_overlap;
   float old_step;
   float new_step;
   float old_weight;
   float new_weight;
   float gain;
   int orig_len;

   orig_len = len;
   if (s->missing_samples == 0) {
      /* As the gap in real speech starts we need to assess the last known pitch,
      and prepare the synthetic data we will use for fill-in */
      normalise_history(s);
      s->pitch = amdf_pitch(PLC_PITCH_MIN, PLC_PITCH_MAX, s->history + PLC_HISTORY_LEN - CORRELATION_SPAN - PLC_PITCH_MIN, CORRELATION_SPAN);
      /* We overlap a 1/4 wavelength */
      pitch_overlap = s->pitch >> 2;
      /* Cook up a single cycle of pitch, using a single of the real signal with 1/4
      cycle OLA'ed to make the ends join up nicely */
      /* The first 3/4 of the cycle is a simple copy */
      for (i = 0;  i < s->pitch - pitch_overlap;  i++)
         s->pitchbuf[i] = s->history[PLC_HISTORY_LEN - s->pitch + i];
      /* The last 1/4 of the cycle is overlapped with the end of the previous cycle */
      new_step = 1.0/pitch_overlap;
      new_weight = new_step;
      for ( ; i < s->pitch; i++) {
         s->pitchbuf[i] = s->history[PLC_HISTORY_LEN - s->pitch + i] * (1.0 - new_weight) + s->history[PLC_HISTORY_LEN - 2 * s->pitch + i]*new_weight;
         new_weight += new_step;
      }
      /* We should now be ready to fill in the gap with repeated, decaying cycles
      of what is in pitchbuf */

      /* We need to OLA the first 1/4 wavelength of the synthetic data, to smooth
      it into the previous real data. To avoid the need to introduce a delay
      in the stream, reverse the last 1/4 wavelength, and OLA with that. */
      gain = 1.0;
      new_step = 1.0 / pitch_overlap;
      old_step = new_step;
      new_weight = new_step;
      old_weight = 1.0 - new_step;
      for (i = 0; i < pitch_overlap; i++) {
         amp[i] = fsaturate(old_weight * s->history[PLC_HISTORY_LEN - 1 - i] + new_weight * s->pitchbuf[i]);
         new_weight += new_step;
         old_weight -= old_step;
         if (old_weight < 0.0)
            old_weight = 0.0;
      }
      s->pitch_offset = i;
   } else {
      gain = 1.0 - s->missing_samples*ATTENUATION_INCREMENT;
      i = 0;
   }
   for ( ; gain > 0.0 && i < len; i++) {
      amp[i] = s->pitchbuf[s->pitch_offset] * gain;
      gain -= ATTENUATION_INCREMENT;
      if (++s->pitch_offset >= s->pitch)
         s->pitch_offset = 0;
   }
   for ( ; i < len; i++)
      amp[i] = 0;
   s->missing_samples += orig_len;
   save_history(s, amp, len);
   return len;
}

plc_state_t* plc_init ( plc_state_t * s )

Process a block of received V.29 modem audio samples.

Parameters:

s	The packet loss concealer context.

Returns:: A pointer to the he packet loss concealer context.

Definition at line 244 of file plc.c.

{
   memset(s, 0, sizeof(*s));
   return s;
}

int plc_rx	(	plc_state_t *	s,
		int16_t	amp[],
		int	len
	)

Process a block of received audio samples.

Parameters:

s	The packet loss concealer context.
amp	The audio sample buffer.
len	The number of samples in the buffer.

Returns:: The number of samples in the buffer.

Definition at line 132 of file plc.c.

References ATTENUATION_INCREMENT, fsaturate(), len(), plc_state_t::missing_samples, plc_state_t::pitch, plc_state_t::pitch_offset, plc_state_t::pitchbuf, and save_history().

Referenced by adjust_frame_for_plc().

{
   int i;
   int pitch_overlap;
   float old_step;
   float new_step;
   float old_weight;
   float new_weight;
   float gain;
   
   if (s->missing_samples) {
      /* Although we have a real signal, we need to smooth it to fit well
      with the synthetic signal we used for the previous block */

      /* The start of the real data is overlapped with the next 1/4 cycle
         of the synthetic data. */
      pitch_overlap = s->pitch >> 2;
      if (pitch_overlap > len)
         pitch_overlap = len;
      gain = 1.0 - s->missing_samples*ATTENUATION_INCREMENT;
      if (gain < 0.0)
         gain = 0.0;
      new_step = 1.0/pitch_overlap;
      old_step = new_step*gain;
      new_weight = new_step;
      old_weight = (1.0 - new_step)*gain;
      for (i = 0; i < pitch_overlap; i++) {
         amp[i] = fsaturate(old_weight * s->pitchbuf[s->pitch_offset] + new_weight * amp[i]);
         if (++s->pitch_offset >= s->pitch)
            s->pitch_offset = 0;
         new_weight += new_step;
         old_weight -= old_step;
         if (old_weight < 0.0)
            old_weight = 0.0;
      }
      s->missing_samples = 0;
   }
   save_history(s, amp, len);
   return len;
}

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Data Structures

Defines

Functions

Detailed Description

Define Documentation

Function Documentation