codec_g726.c

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/*
 * Asterisk -- An open source telephony toolkit.
 *
 * Copyright (C) 1999 - 2006, Digium, Inc.
 *
 * Mark Spencer <[email protected]>
 * Kevin P. Fleming <[email protected]>
 *
 * Based on frompcm.c and topcm.c from the Emiliano MIPL browser/
 * interpreter.  See http://www.bsdtelephony.com.mx
 *
 * See http://www.asterisk.org for more information about
 * the Asterisk project. Please do not directly contact
 * any of the maintainers of this project for assistance;
 * the project provides a web site, mailing lists and IRC
 * channels for your use.
 *
 * This program is free software, distributed under the terms of
 * the GNU General Public License Version 2. See the LICENSE file
 * at the top of the source tree.
 */

/*! \file
 *
 * \brief codec_g726.c - translate between signed linear and ITU G.726-32kbps (both RFC3551 and AAL2 codeword packing)
 *
 * \ingroup codecs
 */

/*** MODULEINFO
   <support_level>core</support_level>
 ***/

#include "asterisk.h"

#include "asterisk/lock.h"
#include "asterisk/linkedlists.h"
#include "asterisk/module.h"
#include "asterisk/config.h"
#include "asterisk/translate.h"
#include "asterisk/utils.h"

#define WANT_ASM
#include "log2comp.h"

/* define NOT_BLI to use a faster but not bit-level identical version */
/* #define NOT_BLI */

#if defined(NOT_BLI)
#  if defined(_MSC_VER)
typedef __int64 sint64;
#  elif defined(__GNUC__)
typedef long long sint64;
#  else
#     error 64-bit integer type is not defined for your compiler/platform
#  endif
#endif

#define BUFFER_SAMPLES   8096 /* size for the translation buffers */
#define BUF_SHIFT 5

/* Sample frame data */
#include "asterisk/slin.h"
#include "ex_g726.h"

/*
 * The following is the definition of the state structure
 * used by the G.726 encoder and decoder to preserve their internal
 * state between successive calls.  The meanings of the majority
 * of the state structure fields are explained in detail in the
 * CCITT Recommendation G.721.  The field names are essentially identical
 * to variable names in the bit level description of the coding algorithm
 * included in this Recommendation.
 */
struct g726_state {
   long yl; /* Locked or steady state step size multiplier. */
   int yu;     /* Unlocked or non-steady state step size multiplier. */
   int dms; /* Short term energy estimate. */
   int dml; /* Long term energy estimate. */
   int ap;     /* Linear weighting coefficient of 'yl' and 'yu'. */
   int a[2];   /* Coefficients of pole portion of prediction filter.
          * stored as fixed-point 1==2^14 */
   int b[6];   /* Coefficients of zero portion of prediction filter.
          * stored as fixed-point 1==2^14 */
   int pk[2];  /* Signs of previous two samples of a partially
          * reconstructed signal. */
   int dq[6];     /* Previous 6 samples of the quantized difference signal
          * stored as fixed point 1==2^12,
          * or in internal floating point format */
   int sr[2];  /* Previous 2 samples of the quantized difference signal
          * stored as fixed point 1==2^12,
          * or in internal floating point format */
   int td;     /* delayed tone detect, new in 1988 version */
};

static int qtab_721[7] = {-124, 80, 178, 246, 300, 349, 400};
/*
 * Maps G.721 code word to reconstructed scale factor normalized log
 * magnitude values.
 */
static int _dqlntab[16] = {-2048, 4, 135, 213, 273, 323, 373, 425,
            425, 373, 323, 273, 213, 135, 4, -2048};

/* Maps G.721 code word to log of scale factor multiplier. */
static int _witab[16] = {-12, 18, 41, 64, 112, 198, 355, 1122,
            1122, 355, 198, 112, 64, 41, 18, -12};
/*
 * Maps G.721 code words to a set of values whose long and short
 * term averages are computed and then compared to give an indication
 * how stationary (steady state) the signal is.
 */
static int _fitab[16] = {0, 0, 0, 0x200, 0x200, 0x200, 0x600, 0xE00,
            0xE00, 0x600, 0x200, 0x200, 0x200, 0, 0, 0};


/*
 * g72x_init_state()
 *
 * This routine initializes and/or resets the g726_state structure
 * pointed to by 'state_ptr'.
 * All the initial state values are specified in the CCITT G.721 document.
 */
static void g726_init_state(struct g726_state *state_ptr)
{
   int      cnta;

   state_ptr->yl = 34816;
   state_ptr->yu = 544;
   state_ptr->dms = 0;
   state_ptr->dml = 0;
   state_ptr->ap = 0;
   for (cnta = 0; cnta < 2; cnta++) {
      state_ptr->a[cnta] = 0;
      state_ptr->pk[cnta] = 0;
#ifdef NOT_BLI
      state_ptr->sr[cnta] = 1;
#else
      state_ptr->sr[cnta] = 32;
#endif
   }
   for (cnta = 0; cnta < 6; cnta++) {
      state_ptr->b[cnta] = 0;
#ifdef NOT_BLI
      state_ptr->dq[cnta] = 1;
#else
      state_ptr->dq[cnta] = 32;
#endif
   }
   state_ptr->td = 0;
}

/*
 * quan()
 *
 * quantizes the input val against the table of integers.
 * It returns i if table[i - 1] <= val < table[i].
 *
 * Using linear search for simple coding.
 */
static int quan(int val, int *table, int size)
{
   int      i;

   for (i = 0; i < size && val >= *table; ++i, ++table)
      ;
   return i;
}

#ifdef NOT_BLI /* faster non-identical version */

/*
 * predictor_zero()
 *
 * computes the estimated signal from 6-zero predictor.
 *
 */
static int predictor_zero(struct g726_state *state_ptr)
{  /* divide by 2 is necessary here to handle negative numbers correctly */
   int i;
   sint64 sezi;
   for (sezi = 0, i = 0; i < 6; i++)         /* ACCUM */
      sezi += (sint64)state_ptr->b[i] * state_ptr->dq[i];
   return (int)(sezi >> 13) / 2 /* 2^14 */;
}

/*
 * predictor_pole()
 *
 * computes the estimated signal from 2-pole predictor.
 *
 */
static int predictor_pole(struct g726_state *state_ptr)
{  /* divide by 2 is necessary here to handle negative numbers correctly */
   return (int)(((sint64)state_ptr->a[1] * state_ptr->sr[1] +
                 (sint64)state_ptr->a[0] * state_ptr->sr[0]) >> 13) / 2 /* 2^14 */;
}

#else /* NOT_BLI - identical version */
/*
 * fmult()
 *
 * returns the integer product of the fixed-point number "an" (1==2^12) and
 * "floating point" representation (4-bit exponent, 6-bit mantessa) "srn".
 */
static int fmult(int an, int srn)
{
   int      anmag, anexp, anmant;
   int      wanexp, wanmant;
   int      retval;

   anmag = (an > 0) ? an : ((-an) & 0x1FFF);
   anexp = ilog2(anmag) - 5;
   anmant = (anmag == 0) ? 32 :
       (anexp >= 0) ? anmag >> anexp : anmag << -anexp;
   wanexp = anexp + ((srn >> 6) & 0xF) - 13;

   wanmant = (anmant * (srn & 077) + 0x30) >> 4;
   retval = (wanexp >= 0) ? ((wanmant << wanexp) & 0x7FFF) :
       (wanmant >> -wanexp);

   return (((an ^ srn) < 0) ? -retval : retval);
}

static int predictor_zero(struct g726_state *state_ptr)
{
   int      i;
   int      sezi;
   for (sezi = 0, i = 0; i < 6; i++)         /* ACCUM */
      sezi += fmult(state_ptr->b[i] >> 2, state_ptr->dq[i]);
   return sezi;
}

static int predictor_pole(struct g726_state *state_ptr)
{
   return (fmult(state_ptr->a[1] >> 2, state_ptr->sr[1]) +
         fmult(state_ptr->a[0] >> 2, state_ptr->sr[0]));
}

#endif /* NOT_BLI */

/*
 * step_size()
 *
 * computes the quantization step size of the adaptive quantizer.
 *
 */
static int step_size(struct g726_state *state_ptr)
{
   int y, dif, al;

   if (state_ptr->ap >= 256) {
      return state_ptr->yu;
   }

   y = state_ptr->yl >> 6;
   dif = state_ptr->yu - y;
   al = state_ptr->ap >> 2;

   if (dif > 0) {
      y += (dif * al) >> 6;
   } else if (dif < 0) {
      y += (dif * al + 0x3F) >> 6;
   }
   return y;
}

/*
 * quantize()
 *
 * Given a raw sample, 'd', of the difference signal and a
 * quantization step size scale factor, 'y', this routine returns the
 * ADPCM codeword to which that sample gets quantized.  The step
 * size scale factor division operation is done in the log base 2 domain
 * as a subtraction.
 */
static int quantize(
   int      d, /* Raw difference signal sample */
   int      y, /* Step size multiplier */
   int      *table,  /* quantization table */
   int      size) /* table size of integers */
{
   int      dqm;  /* Magnitude of 'd' */
   int      exp;  /* Integer part of base 2 log of 'd' */
   int      mant; /* Fractional part of base 2 log */
   int      dl;      /* Log of magnitude of 'd' */
   int      dln;  /* Step size scale factor normalized log */
   int      i;

   /*
    * LOG
    *
    * Compute base 2 log of 'd', and store in 'dl'.
    */
   dqm = abs(d);
   exp = ilog2(dqm);
   if (exp < 0) {
      exp = 0;
   }
   mant = ((dqm << 7) >> exp) & 0x7F;  /* Fractional portion. */
   dl = (exp << 7) | mant;

   /*
    * SUBTB
    *
    * "Divide" by step size multiplier.
    */
   dln = dl - (y >> 2);

   /*
    * QUAN
    *
    * Obtain codword i for 'd'.
    */
   i = quan(dln, table, size);
   if (d < 0) {         /* take 1's complement of i */
      return ((size << 1) + 1 - i);
   } else if (i == 0) {    /* take 1's complement of 0 */
      return ((size << 1) + 1); /* new in 1988 */
   } else {
      return i;
   }
}

/*
 * reconstruct()
 *
 * Returns reconstructed difference signal 'dq' obtained from
 * codeword 'i' and quantization step size scale factor 'y'.
 * Multiplication is performed in log base 2 domain as addition.
 */
static int reconstruct(
   int      sign, /* 0 for non-negative value */
   int      dqln, /* G.72x codeword */
   int      y) /* Step size multiplier */
{
   int      dql;  /* Log of 'dq' magnitude */
   int      dex;  /* Integer part of log */
   int      dqt;
   int      dq;   /* Reconstructed difference signal sample */

   dql = dqln + (y >> 2);  /* ADDA */

   if (dql < 0) {
#ifdef NOT_BLI
      return (sign) ? -1 : 1;
#else
      return (sign) ? -0x8000 : 0;
#endif
   } else {    /* ANTILOG */
      dex = (dql >> 7) & 15;
      dqt = 128 + (dql & 127);
#ifdef NOT_BLI
      dq = ((dqt << 19) >> (14 - dex));
      return (sign) ? -dq : dq;
#else
      dq = (dqt << 7) >> (14 - dex);
      return (sign) ? (dq - 0x8000) : dq;
#endif
   }
}

/*
 * update()
 *
 * updates the state variables for each output code
 */
static void update(
   int      code_size,  /* distinguish 723_40 with others */
   int      y,    /* quantizer step size */
   int      wi,      /* scale factor multiplier */
   int      fi,      /* for long/short term energies */
   int      dq,      /* quantized prediction difference */
   int      sr,      /* reconstructed signal */
   int      dqsez,      /* difference from 2-pole predictor */
   struct g726_state *state_ptr) /* coder state pointer */
{
   int      cnt;
   int      mag;     /* Adaptive predictor, FLOAT A */
#ifndef NOT_BLI
   int      exp;
#endif
   int      a2p=0;      /* LIMC */
   int      a1ul;    /* UPA1 */
   int      pks1;    /* UPA2 */
   int      fa1;
   int      tr;         /* tone/transition detector */
   int      ylint, thr2, dqthr;
   int      ylfrac, thr1;
   int      pk0;

   pk0 = (dqsez < 0) ? 1 : 0; /* needed in updating predictor poles */

#ifdef NOT_BLI
   mag = abs(dq / 0x1000); /* prediction difference magnitude */
#else
   mag = dq & 0x7FFF;      /* prediction difference magnitude */
#endif
   /* TRANS */
   ylint = state_ptr->yl >> 15;  /* exponent part of yl */
   ylfrac = (state_ptr->yl >> 10) & 0x1F; /* fractional part of yl */
   thr1 = (32 + ylfrac) << ylint;      /* threshold */
   thr2 = (ylint > 9) ? 31 << 10 : thr1;  /* limit thr2 to 31 << 10 */
   dqthr = (thr2 + (thr2 >> 1)) >> 1;  /* dqthr = 0.75 * thr2 */
   if (state_ptr->td == 0) {     /* signal supposed voice */
      tr = 0;
   } else if (mag <= dqthr) {    /* supposed data, but small mag */
      tr = 0;        /* treated as voice */
   } else {          /* signal is data (modem) */
      tr = 1;
   }
   /*
    * Quantizer scale factor adaptation.
    */

   /* FUNCTW & FILTD & DELAY */
   /* update non-steady state step size multiplier */
   state_ptr->yu = y + ((wi - y) >> 5);

   /* LIMB */
   if (state_ptr->yu < 544) { /* 544 <= yu <= 5120 */
      state_ptr->yu = 544;
   } else if (state_ptr->yu > 5120) {
      state_ptr->yu = 5120;
   }

   /* FILTE & DELAY */
   /* update steady state step size multiplier */
   state_ptr->yl += state_ptr->yu + ((-state_ptr->yl) >> 6);

   /*
    * Adaptive predictor coefficients.
    */
   if (tr == 1) {       /* reset a's and b's for modem signal */
      state_ptr->a[0] = 0;
      state_ptr->a[1] = 0;
      state_ptr->b[0] = 0;
      state_ptr->b[1] = 0;
      state_ptr->b[2] = 0;
      state_ptr->b[3] = 0;
      state_ptr->b[4] = 0;
      state_ptr->b[5] = 0;
   } else {       /* update a's and b's */
      pks1 = pk0 ^ state_ptr->pk[0];      /* UPA2 */

      /* update predictor pole a[1] */
      a2p = state_ptr->a[1] - (state_ptr->a[1] >> 7);
      if (dqsez != 0) {
         fa1 = (pks1) ? state_ptr->a[0] : -state_ptr->a[0];
         if (fa1 < -8191) {   /* a2p = function of fa1 */
            a2p -= 0x100;
         } else if (fa1 > 8191) {
            a2p += 0xFF;
         } else {
            a2p += fa1 >> 5;
         }

         if (pk0 ^ state_ptr->pk[1]) {
            /* LIMC */
            if (a2p <= -12160) {
               a2p = -12288;
            } else if (a2p >= 12416) {
               a2p = 12288;
            } else {
               a2p -= 0x80;
            }
         } else if (a2p <= -12416) {
            a2p = -12288;
         } else if (a2p >= 12160) {
            a2p = 12288;
         } else {
            a2p += 0x80;
         }
      }

      /* TRIGB & DELAY */
      state_ptr->a[1] = a2p;

      /* UPA1 */
      /* update predictor pole a[0] */
      state_ptr->a[0] -= state_ptr->a[0] >> 8;
      if (dqsez != 0) {
         if (pks1 == 0)
            state_ptr->a[0] += 192;
         else
            state_ptr->a[0] -= 192;
      }
      /* LIMD */
      a1ul = 15360 - a2p;
      if (state_ptr->a[0] < -a1ul) {
         state_ptr->a[0] = -a1ul;
      } else if (state_ptr->a[0] > a1ul) {
         state_ptr->a[0] = a1ul;
      }

      /* UPB : update predictor zeros b[6] */
      for (cnt = 0; cnt < 6; cnt++) {
         if (code_size == 5) {      /* for 40Kbps G.723 */
            state_ptr->b[cnt] -= state_ptr->b[cnt] >> 9;
         } else {       /* for G.721 and 24Kbps G.723 */
            state_ptr->b[cnt] -= state_ptr->b[cnt] >> 8;
         }
         if (mag) {  /* XOR */
            if ((dq ^ state_ptr->dq[cnt]) >= 0) {
               state_ptr->b[cnt] += 128;
            } else {
               state_ptr->b[cnt] -= 128;
            }
         }
      }
   }

   for (cnt = 5; cnt > 0; cnt--)
      state_ptr->dq[cnt] = state_ptr->dq[cnt-1];
#ifdef NOT_BLI
   state_ptr->dq[0] = dq;
#else
   /* FLOAT A : convert dq[0] to 4-bit exp, 6-bit mantissa f.p. */
   if (mag == 0) {
      state_ptr->dq[0] = (dq >= 0) ? 0x20 : 0x20 - 0x400;
   } else {
      exp = ilog2(mag) + 1;
      state_ptr->dq[0] = (dq >= 0) ?
          (exp << 6) + ((mag << 6) >> exp) :
          (exp << 6) + ((mag << 6) >> exp) - 0x400;
   }
#endif

   state_ptr->sr[1] = state_ptr->sr[0];
#ifdef NOT_BLI
   state_ptr->sr[0] = sr;
#else
   /* FLOAT B : convert sr to 4-bit exp., 6-bit mantissa f.p. */
   if (sr == 0) {
      state_ptr->sr[0] = 0x20;
   } else if (sr > 0) {
      exp = ilog2(sr) + 1;
      state_ptr->sr[0] = (exp << 6) + ((sr << 6) >> exp);
   } else if (sr > -0x8000) {
      mag = -sr;
      exp = ilog2(mag) + 1;
      state_ptr->sr[0] =  (exp << 6) + ((mag << 6) >> exp) - 0x400;
   } else
      state_ptr->sr[0] = 0x20 - 0x400;
#endif

   /* DELAY A */
   state_ptr->pk[1] = state_ptr->pk[0];
   state_ptr->pk[0] = pk0;

   /* TONE */
   if (tr == 1) {    /* this sample has been treated as data */
      state_ptr->td = 0;   /* next one will be treated as voice */
   } else if (a2p < -11776) { /* small sample-to-sample correlation */
      state_ptr->td = 1;   /* signal may be data */
   } else {          /* signal is voice */
      state_ptr->td = 0;
   }

   /*
    * Adaptation speed control.
    */
   state_ptr->dms += (fi - state_ptr->dms) >> 5;      /* FILTA */
   state_ptr->dml += (((fi << 2) - state_ptr->dml) >> 7);   /* FILTB */

   if (tr == 1) {
      state_ptr->ap = 256;
   } else if (y < 1536) {              /* SUBTC */
      state_ptr->ap += (0x200 - state_ptr->ap) >> 4;
   } else if (state_ptr->td == 1) {
      state_ptr->ap += (0x200 - state_ptr->ap) >> 4;
   } else if (abs((state_ptr->dms << 2) - state_ptr->dml) >=
       (state_ptr->dml >> 3)) {
      state_ptr->ap += (0x200 - state_ptr->ap) >> 4;
   } else {
      state_ptr->ap += (-state_ptr->ap) >> 4;
   }
}

/*
 * g726_decode()
 *
 * Description:
 *
 * Decodes a 4-bit code of G.726-32 encoded data of i and
 * returns the resulting linear PCM, A-law or u-law value.
 * return -1 for unknown out_coding value.
 */
static int g726_decode(int i, struct g726_state *state_ptr)
{
   int      sezi, sez, se; /* ACCUM */
   int      y;       /* MIX */
   int      sr;         /* ADDB */
   int      dq;
   int      dqsez;

   i &= 0x0f;        /* mask to get proper bits */
#ifdef NOT_BLI
   sezi = predictor_zero(state_ptr);
   sez = sezi;
   se = sezi + predictor_pole(state_ptr); /* estimated signal */
#else
   sezi = predictor_zero(state_ptr);
   sez = sezi >> 1;
   se = (sezi + predictor_pole(state_ptr)) >> 1;   /* estimated signal */
#endif

   y = step_size(state_ptr);  /* dynamic quantizer step size */

   dq = reconstruct(i & 8, _dqlntab[i], y); /* quantized diff. */

#ifdef NOT_BLI
   sr = se + dq;           /* reconst. signal */
   dqsez = dq + sez;       /* pole prediction diff. */
#else
   sr = (dq < 0) ? se - (dq & 0x3FFF) : se + dq;   /* reconst. signal */
   dqsez = sr - se + sez;     /* pole prediction diff. */
#endif

   update(4, y, _witab[i] << 5, _fitab[i], dq, sr, dqsez, state_ptr);

#ifdef NOT_BLI
   return (sr >> 10);   /* sr was 26-bit dynamic range */
#else
   return (sr << 2); /* sr was 14-bit dynamic range */
#endif
}

/*
 * g726_encode()
 *
 * Encodes the input vale of linear PCM, A-law or u-law data sl and returns
 * the resulting code. -1 is returned for unknown input coding value.
 */
static int g726_encode(int sl, struct g726_state *state_ptr)
{
   int      sezi, se, sez;    /* ACCUM */
   int      d;       /* SUBTA */
   int      sr;         /* ADDB */
   int      y;       /* MIX */
   int      dqsez;         /* ADDC */
   int      dq, i;

#ifdef NOT_BLI
   sl <<= 10;        /* 26-bit dynamic range */

   sezi = predictor_zero(state_ptr);
   sez = sezi;
   se = sezi + predictor_pole(state_ptr); /* estimated signal */
#else
   sl >>= 2;         /* 14-bit dynamic range */

   sezi = predictor_zero(state_ptr);
   sez = sezi >> 1;
   se = (sezi + predictor_pole(state_ptr)) >> 1;   /* estimated signal */
#endif

   d = sl - se;            /* estimation difference */

   /* quantize the prediction difference */
   y = step_size(state_ptr);     /* quantizer step size */
#ifdef NOT_BLI
   d /= 0x1000;
#endif
   i = quantize(d, y, qtab_721, 7); /* i = G726 code */

   dq = reconstruct(i & 8, _dqlntab[i], y);  /* quantized est diff */

#ifdef NOT_BLI
   sr = se + dq;           /* reconst. signal */
   dqsez = dq + sez;       /* pole prediction diff. */
#else
   sr = (dq < 0) ? se - (dq & 0x3FFF) : se + dq;   /* reconst. signal */
   dqsez = sr - se + sez;        /* pole prediction diff. */
#endif

   update(4, y, _witab[i] << 5, _fitab[i], dq, sr, dqsez, state_ptr);

   return i;
}

/*
 * Private workspace for translating signed linear signals to G726.
 * Don't bother to define two distinct structs.
 */

struct g726_coder_pvt {
   /* buffer any odd byte in input - 0x80 + (value & 0xf) if present */
   unsigned char next_flag;
   struct g726_state g726;
};

/*! \brief init a new instance of g726_coder_pvt. */
static int lintog726_new(struct ast_trans_pvt *pvt)
{
   struct g726_coder_pvt *tmp = pvt->pvt;

   g726_init_state(&tmp->g726);

   return 0;
}

/*! \brief decode packed 4-bit G726 values (AAL2 packing) and store in buffer. */
static int g726aal2tolin_framein (struct ast_trans_pvt *pvt, struct ast_frame *f)
{
   struct g726_coder_pvt *tmp = pvt->pvt;
   unsigned char *src = f->data.ptr;
   int16_t *dst = pvt->outbuf.i16 + pvt->samples;
   unsigned int i;

   for (i = 0; i < f->datalen; i++) {
      *dst++ = g726_decode((src[i] >> 4) & 0xf, &tmp->g726);
      *dst++ = g726_decode(src[i] & 0x0f, &tmp->g726);
   }

   pvt->samples += f->samples;
   pvt->datalen += 2 * f->samples; /* 2 bytes/sample */

   return 0;
}

/*! \brief compress and store data (4-bit G726 samples, AAL2 packing) in outbuf */
static int lintog726aal2_framein(struct ast_trans_pvt *pvt, struct ast_frame *f)
{
   struct g726_coder_pvt *tmp = pvt->pvt;
   int16_t *src = f->data.ptr;
   unsigned int i;

   for (i = 0; i < f->samples; i++) {
      unsigned char d = g726_encode(src[i], &tmp->g726); /* this sample */

      if (tmp->next_flag & 0x80) {  /* merge with leftover sample */
         pvt->outbuf.c[pvt->datalen++] = ((tmp->next_flag & 0xf)<< 4) | d;
         pvt->samples += 2;   /* 2 samples per byte */
         tmp->next_flag = 0;
      } else {
         tmp->next_flag = 0x80 | d;
      }
   }

   return 0;
}

/*! \brief decode packed 4-bit G726 values (RFC3551 packing) and store in buffer. */
static int g726tolin_framein (struct ast_trans_pvt *pvt, struct ast_frame *f)
{
   struct g726_coder_pvt *tmp = pvt->pvt;
   unsigned char *src = f->data.ptr;
   int16_t *dst = pvt->outbuf.i16 + pvt->samples;
   unsigned int i;

   for (i = 0; i < f->datalen; i++) {
      *dst++ = g726_decode(src[i] & 0x0f, &tmp->g726);
      *dst++ = g726_decode((src[i] >> 4) & 0xf, &tmp->g726);
   }

   pvt->samples += f->samples;
   pvt->datalen += 2 * f->samples; /* 2 bytes/sample */

   return 0;
}

/*! \brief compress and store data (4-bit G726 samples, RFC3551 packing) in outbuf */
static int lintog726_framein(struct ast_trans_pvt *pvt, struct ast_frame *f)
{
   struct g726_coder_pvt *tmp = pvt->pvt;
   int16_t *src = f->data.ptr;
   unsigned int i;

   for (i = 0; i < f->samples; i++) {
      unsigned char d = g726_encode(src[i], &tmp->g726); /* this sample */

      if (tmp->next_flag & 0x80) {  /* merge with leftover sample */
         pvt->outbuf.c[pvt->datalen++] = (d << 4) | (tmp->next_flag & 0xf);
         pvt->samples += 2;   /* 2 samples per byte */
         tmp->next_flag = 0;
      } else {
         tmp->next_flag = 0x80 | d;
      }
   }

   return 0;
}

static struct ast_translator g726tolin = {
   .name = "g726tolin",
   .src_codec = {
      .name = "g726",
      .type = AST_MEDIA_TYPE_AUDIO,
      .sample_rate = 8000,
   },
   .dst_codec = {
      .name = "slin",
      .type = AST_MEDIA_TYPE_AUDIO,
      .sample_rate = 8000,
   },
   .format = "slin",
   .newpvt = lintog726_new,   /* same for both directions */
   .framein = g726tolin_framein,
   .sample = g726_sample,
   .desc_size = sizeof(struct g726_coder_pvt),
   .buffer_samples = BUFFER_SAMPLES,
   .buf_size = BUFFER_SAMPLES * 2,
};

static struct ast_translator lintog726 = {
   .name = "lintog726",
   .src_codec = {
      .name = "slin",
      .type = AST_MEDIA_TYPE_AUDIO,
      .sample_rate = 8000,
   },
   .dst_codec = {
      .name = "g726",
      .type = AST_MEDIA_TYPE_AUDIO,
      .sample_rate = 8000,
   },
   .format = "g726",
   .newpvt = lintog726_new,   /* same for both directions */
   .framein = lintog726_framein,
   .sample = slin8_sample,
   .desc_size = sizeof(struct g726_coder_pvt),
   .buffer_samples = BUFFER_SAMPLES,
   .buf_size = BUFFER_SAMPLES/2,
};

static struct ast_translator g726aal2tolin = {
   .name = "g726aal2tolin",
   .src_codec = {
      .name = "g726aal2",
      .type = AST_MEDIA_TYPE_AUDIO,
      .sample_rate = 8000,
   },
   .dst_codec = {
      .name = "slin",
      .type = AST_MEDIA_TYPE_AUDIO,
      .sample_rate = 8000,
   },
   .format = "slin",
   .newpvt = lintog726_new,   /* same for both directions */
   .framein = g726aal2tolin_framein,
   .sample = g726_sample,
   .desc_size = sizeof(struct g726_coder_pvt),
   .buffer_samples = BUFFER_SAMPLES,
   .buf_size = BUFFER_SAMPLES * 2,
};

static struct ast_translator lintog726aal2 = {
   .name = "lintog726aal2",
   .src_codec = {
      .name = "slin",
      .type = AST_MEDIA_TYPE_AUDIO,
      .sample_rate = 8000,
   },
   .dst_codec = {
      .name = "g726aal2",
      .type = AST_MEDIA_TYPE_AUDIO,
      .sample_rate = 8000,
   },
   .format = "g726aal2",
   .newpvt = lintog726_new,   /* same for both directions */
   .framein = lintog726aal2_framein,
   .sample = slin8_sample,
   .desc_size = sizeof(struct g726_coder_pvt),
   .buffer_samples = BUFFER_SAMPLES,
   .buf_size = BUFFER_SAMPLES / 2,
};

static int unload_module(void)
{
   int res = 0;

   res |= ast_unregister_translator(&g726tolin);
   res |= ast_unregister_translator(&lintog726);

   res |= ast_unregister_translator(&g726aal2tolin);
   res |= ast_unregister_translator(&lintog726aal2);

   return res;
}

static int load_module(void)
{
   int res = 0;

   res |= ast_register_translator(&g726tolin);
   res |= ast_register_translator(&lintog726);

   res |= ast_register_translator(&g726aal2tolin);
   res |= ast_register_translator(&lintog726aal2);

   if (res) {
      unload_module();
      return AST_MODULE_LOAD_DECLINE;
   }

   return AST_MODULE_LOAD_SUCCESS;
}

AST_MODULE_INFO(ASTERISK_GPL_KEY, AST_MODFLAG_DEFAULT, "ITU G.726-32kbps G726 Transcoder",
   .support_level = AST_MODULE_SUPPORT_CORE,
   .load = load_module,
   .unload = unload_module,
);