Staging: add echo cancelation module

This is used by mISDN and Zaptel drivers.

From: Steve Underwood <steveu@coppice.org>
From: David Rowe <david@rowetel.com>
Cc: Tzafrir Cohen <tzafrir.cohen@xorcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 632 insertions, 0 deletions

diff --git a/drivers/staging/echo/echo.c b/drivers/staging/echo/echo.c
new file mode 100644
index 00000000000..4a281b14fc5
--- /dev/null
+++ b/drivers/staging/echo/echo.c
@@ -0,0 +1,632 @@
+/*
+ * SpanDSP - a series of DSP components for telephony
+ *
+ * echo.c - A line echo canceller.  This code is being developed
+ *          against and partially complies with G168.
+ *
+ * Written by Steve Underwood <steveu@coppice.org>
+ *         and David Rowe <david_at_rowetel_dot_com>
+ *
+ * Copyright (C) 2001, 2003 Steve Underwood, 2007 David Rowe
+ *
+ * Based on a bit from here, a bit from there, eye of toad, ear of
+ * bat, 15 years of failed attempts by David and a few fried brain
+ * cells.
+ *
+ * All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2, as
+ * published by the Free Software Foundation.
+ *
+ * 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.
+ *
+ * $Id: echo.c,v 1.20 2006/12/01 18:00:48 steveu Exp $
+ */
+
+/*! \file */
+
+/* Implementation Notes
+   David Rowe
+   April 2007
+
+   This code started life as Steve's NLMS algorithm with a tap
+   rotation algorithm to handle divergence during double talk.  I
+   added a Geigel Double Talk Detector (DTD) [2] and performed some
+   G168 tests.  However I had trouble meeting the G168 requirements,
+   especially for double talk - there were always cases where my DTD
+   failed, for example where near end speech was under the 6dB
+   threshold required for declaring double talk.
+
+   So I tried a two path algorithm [1], which has so far given better
+   results.  The original tap rotation/Geigel algorithm is available
+   in SVN http://svn.rowetel.com/software/oslec/tags/before_16bit.
+   It's probably possible to make it work if some one wants to put some
+   serious work into it.
+
+   At present no special treatment is provided for tones, which
+   generally cause NLMS algorithms to diverge.  Initial runs of a
+   subset of the G168 tests for tones (e.g ./echo_test 6) show the
+   current algorithm is passing OK, which is kind of surprising.  The
+   full set of tests needs to be performed to confirm this result.
+
+   One other interesting change is that I have managed to get the NLMS
+   code to work with 16 bit coefficients, rather than the original 32
+   bit coefficents.  This reduces the MIPs and storage required.
+   I evaulated the 16 bit port using g168_tests.sh and listening tests
+   on 4 real-world samples.
+
+   I also attempted the implementation of a block based NLMS update
+   [2] but although this passes g168_tests.sh it didn't converge well
+   on the real-world samples.  I have no idea why, perhaps a scaling
+   problem.  The block based code is also available in SVN
+   http://svn.rowetel.com/software/oslec/tags/before_16bit.  If this
+   code can be debugged, it will lead to further reduction in MIPS, as
+   the block update code maps nicely onto DSP instruction sets (it's a
+   dot product) compared to the current sample-by-sample update.
+
+   Steve also has some nice notes on echo cancellers in echo.h
+
+
+   References:
+
+   [1] Ochiai, Areseki, and Ogihara, "Echo Canceller with Two Echo
+       Path Models", IEEE Transactions on communications, COM-25,
+       No. 6, June
+       1977.
+       http://www.rowetel.com/images/echo/dual_path_paper.pdf
+
+   [2] The classic, very useful paper that tells you how to
+       actually build a real world echo canceller:
+         Messerschmitt, Hedberg, Cole, Haoui, Winship, "Digital Voice
+         Echo Canceller with a TMS320020,
+         http://www.rowetel.com/images/echo/spra129.pdf
+
+   [3] I have written a series of blog posts on this work, here is
+       Part 1: http://www.rowetel.com/blog/?p=18
+
+   [4] The source code http://svn.rowetel.com/software/oslec/
+
+   [5] A nice reference on LMS filters:
+         http://en.wikipedia.org/wiki/Least_mean_squares_filter
+
+   Credits:
+
+   Thanks to Steve Underwood, Jean-Marc Valin, and Ramakrishnan
+   Muthukrishnan for their suggestions and email discussions.  Thanks
+   also to those people who collected echo samples for me such as
+   Mark, Pawel, and Pavel.
+*/
+
+#include <linux/kernel.h>       /* We're doing kernel work */
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/slab.h>
+#define malloc(a) kmalloc((a), GFP_KERNEL)
+#define free(a) kfree(a)
+
+#include "bit_operations.h"
+#include "echo.h"
+
+#define MIN_TX_POWER_FOR_ADAPTION   64
+#define MIN_RX_POWER_FOR_ADAPTION   64
+#define DTD_HANGOVER               600     /* 600 samples, or 75ms     */
+#define DC_LOG2BETA                  3     /* log2() of DC filter Beta */
+
+/*-----------------------------------------------------------------------*\
+                               FUNCTIONS
+\*-----------------------------------------------------------------------*/
+
+/* adapting coeffs using the traditional stochastic descent (N)LMS algorithm */
+
+
+#ifdef __BLACKFIN_ASM__
+static void __inline__ lms_adapt_bg(echo_can_state_t *ec, int clean, int shift)
+{
+    int i, j;
+    int offset1;
+    int offset2;
+    int factor;
+    int exp;
+    int16_t *phist;
+    int n;
+
+    if (shift > 0)
+	factor = clean << shift;
+    else
+	factor = clean >> -shift;
+
+    /* Update the FIR taps */
+
+    offset2 = ec->curr_pos;
+    offset1 = ec->taps - offset2;
+    phist = &ec->fir_state_bg.history[offset2];
+
+    /* st: and en: help us locate the assembler in echo.s */
+
+    //asm("st:");
+    n = ec->taps;
+    for (i = 0, j = offset2;  i < n;  i++, j++)
+    {
+       exp = *phist++ * factor;
+       ec->fir_taps16[1][i] += (int16_t) ((exp+(1<<14)) >> 15);
+    }
+    //asm("en:");
+
+    /* Note the asm for the inner loop above generated by Blackfin gcc
+       4.1.1 is pretty good (note even parallel instructions used):
+
+    	R0 = W [P0++] (X);
+	R0 *= R2;
+	R0 = R0 + R3 (NS) ||
+	R1 = W [P1] (X) ||
+	nop;
+	R0 >>>= 15;
+	R0 = R0 + R1;
+	W [P1++] = R0;
+
+	A block based update algorithm would be much faster but the
+	above can't be improved on much.  Every instruction saved in
+	the loop above is 2 MIPs/ch!  The for loop above is where the
+	Blackfin spends most of it's time - about 17 MIPs/ch measured
+	with speedtest.c with 256 taps (32ms).  Write-back and
+	Write-through cache gave about the same performance.
+    */
+}
+
+/*
+   IDEAS for further optimisation of lms_adapt_bg():
+
+   1/ The rounding is quite costly.  Could we keep as 32 bit coeffs
+   then make filter pluck the MS 16-bits of the coeffs when filtering?
+   However this would lower potential optimisation of filter, as I
+   think the dual-MAC architecture requires packed 16 bit coeffs.
+
+   2/ Block based update would be more efficient, as per comments above,
+   could use dual MAC architecture.
+
+   3/ Look for same sample Blackfin LMS code, see if we can get dual-MAC
+   packing.
+
+   4/ Execute the whole e/c in a block of say 20ms rather than sample
+   by sample.  Processing a few samples every ms is inefficient.
+*/
+
+#else
+static __inline__ void lms_adapt_bg(echo_can_state_t *ec, int clean, int shift)
+{
+    int i;
+
+    int offset1;
+    int offset2;
+    int factor;
+    int exp;
+
+    if (shift > 0)
+	factor = clean << shift;
+    else
+	factor = clean >> -shift;
+
+    /* Update the FIR taps */
+
+    offset2 = ec->curr_pos;
+    offset1 = ec->taps - offset2;
+
+    for (i = ec->taps - 1;  i >= offset1;  i--)
+    {
+       exp = (ec->fir_state_bg.history[i - offset1]*factor);
+       ec->fir_taps16[1][i] += (int16_t) ((exp+(1<<14)) >> 15);
+    }
+    for (  ;  i >= 0;  i--)
+    {
+       exp = (ec->fir_state_bg.history[i + offset2]*factor);
+       ec->fir_taps16[1][i] += (int16_t) ((exp+(1<<14)) >> 15);
+    }
+}
+#endif
+
+/*- End of function --------------------------------------------------------*/
+
+echo_can_state_t *echo_can_create(int len, int adaption_mode)
+{
+    echo_can_state_t *ec;
+    int i;
+    int j;
+
+    ec = kmalloc(sizeof(*ec), GFP_KERNEL);
+    if (ec == NULL)
+        return  NULL;
+    memset(ec, 0, sizeof(*ec));
+
+    ec->taps = len;
+    ec->log2taps = top_bit(len);
+    ec->curr_pos = ec->taps - 1;
+
+    for (i = 0;  i < 2;  i++)
+    {
+        if ((ec->fir_taps16[i] = (int16_t *) malloc((ec->taps)*sizeof(int16_t))) == NULL)
+        {
+            for (j = 0;  j < i;  j++)
+                kfree(ec->fir_taps16[j]);
+            kfree(ec);
+            return  NULL;
+        }
+        memset(ec->fir_taps16[i], 0, (ec->taps)*sizeof(int16_t));
+    }
+
+    fir16_create(&ec->fir_state,
+                 ec->fir_taps16[0],
+                 ec->taps);
+    fir16_create(&ec->fir_state_bg,
+                 ec->fir_taps16[1],
+                 ec->taps);
+
+    for(i=0; i<5; i++) {
+      ec->xvtx[i] = ec->yvtx[i] = ec->xvrx[i] = ec->yvrx[i] = 0;
+    }
+
+    ec->cng_level = 1000;
+    echo_can_adaption_mode(ec, adaption_mode);
+
+    ec->snapshot = (int16_t*)malloc(ec->taps*sizeof(int16_t));
+    memset(ec->snapshot, 0, sizeof(int16_t)*ec->taps);
+
+    ec->cond_met = 0;
+    ec->Pstates = 0;
+    ec->Ltxacc = ec->Lrxacc = ec->Lcleanacc = ec->Lclean_bgacc = 0;
+    ec->Ltx = ec->Lrx = ec->Lclean = ec->Lclean_bg = 0;
+    ec->tx_1 = ec->tx_2 = ec->rx_1 = ec->rx_2 = 0;
+    ec->Lbgn = ec->Lbgn_acc = 0;
+    ec->Lbgn_upper = 200;
+    ec->Lbgn_upper_acc = ec->Lbgn_upper << 13;
+
+    return  ec;
+}
+/*- End of function --------------------------------------------------------*/
+
+void echo_can_free(echo_can_state_t *ec)
+{
+	int i;
+
+	fir16_free(&ec->fir_state);
+	fir16_free(&ec->fir_state_bg);
+	for (i = 0;  i < 2;  i++)
+		kfree(ec->fir_taps16[i]);
+	kfree(ec->snapshot);
+	kfree(ec);
+}
+/*- End of function --------------------------------------------------------*/
+
+void echo_can_adaption_mode(echo_can_state_t *ec, int adaption_mode)
+{
+    ec->adaption_mode = adaption_mode;
+}
+/*- End of function --------------------------------------------------------*/
+
+void echo_can_flush(echo_can_state_t *ec)
+{
+    int i;
+
+    ec->Ltxacc = ec->Lrxacc = ec->Lcleanacc = ec->Lclean_bgacc = 0;
+    ec->Ltx = ec->Lrx = ec->Lclean = ec->Lclean_bg = 0;
+    ec->tx_1 = ec->tx_2 = ec->rx_1 = ec->rx_2 = 0;
+
+    ec->Lbgn = ec->Lbgn_acc = 0;
+    ec->Lbgn_upper = 200;
+    ec->Lbgn_upper_acc = ec->Lbgn_upper << 13;
+
+    ec->nonupdate_dwell = 0;
+
+    fir16_flush(&ec->fir_state);
+    fir16_flush(&ec->fir_state_bg);
+    ec->fir_state.curr_pos = ec->taps - 1;
+    ec->fir_state_bg.curr_pos = ec->taps - 1;
+    for (i = 0;  i < 2;  i++)
+        memset(ec->fir_taps16[i], 0, ec->taps*sizeof(int16_t));
+
+    ec->curr_pos = ec->taps - 1;
+    ec->Pstates = 0;
+}
+/*- End of function --------------------------------------------------------*/
+
+void echo_can_snapshot(echo_can_state_t *ec) {
+    memcpy(ec->snapshot, ec->fir_taps16[0], ec->taps*sizeof(int16_t));
+}
+/*- End of function --------------------------------------------------------*/
+
+/* Dual Path Echo Canceller ------------------------------------------------*/
+
+int16_t echo_can_update(echo_can_state_t *ec, int16_t tx, int16_t rx)
+{
+    int32_t echo_value;
+    int clean_bg;
+    int tmp, tmp1;
+
+    /* Input scaling was found be required to prevent problems when tx
+       starts clipping.  Another possible way to handle this would be the
+       filter coefficent scaling. */
+
+    ec->tx = tx; ec->rx = rx;
+    tx >>=1;
+    rx >>=1;
+
+    /*
+       Filter DC, 3dB point is 160Hz (I think), note 32 bit precision required
+       otherwise values do not track down to 0. Zero at DC, Pole at (1-Beta)
+       only real axis.  Some chip sets (like Si labs) don't need
+       this, but something like a $10 X100P card does.  Any DC really slows
+       down convergence.
+
+       Note: removes some low frequency from the signal, this reduces
+       the speech quality when listening to samples through headphones
+       but may not be obvious through a telephone handset.
+
+       Note that the 3dB frequency in radians is approx Beta, e.g. for
+       Beta = 2^(-3) = 0.125, 3dB freq is 0.125 rads = 159Hz.
+    */
+
+    if (ec->adaption_mode & ECHO_CAN_USE_RX_HPF) {
+      tmp = rx << 15;
+#if 1
+        /* Make sure the gain of the HPF is 1.0. This can still saturate a little under
+           impulse conditions, and it might roll to 32768 and need clipping on sustained peak
+           level signals. However, the scale of such clipping is small, and the error due to
+           any saturation should not markedly affect the downstream processing. */
+        tmp -= (tmp >> 4);
+#endif
+      ec->rx_1 += -(ec->rx_1>>DC_LOG2BETA) + tmp - ec->rx_2;
+
+      /* hard limit filter to prevent clipping.  Note that at this stage
+	 rx should be limited to +/- 16383 due to right shift above */
+      tmp1 = ec->rx_1 >> 15;
+      if (tmp1 > 16383) tmp1 = 16383;
+      if (tmp1 < -16383) tmp1 = -16383;
+      rx = tmp1;
+      ec->rx_2 = tmp;
+    }
+
+    /* Block average of power in the filter states.  Used for
+       adaption power calculation. */
+
+    {
+	int new, old;
+
+	/* efficient "out with the old and in with the new" algorithm so
+	   we don't have to recalculate over the whole block of
+	   samples. */
+	new = (int)tx * (int)tx;
+	old = (int)ec->fir_state.history[ec->fir_state.curr_pos] *
+              (int)ec->fir_state.history[ec->fir_state.curr_pos];
+	ec->Pstates += ((new - old) + (1<<ec->log2taps)) >> ec->log2taps;
+	if (ec->Pstates < 0) ec->Pstates = 0;
+    }
+
+    /* Calculate short term average levels using simple single pole IIRs */
+
+    ec->Ltxacc += abs(tx) - ec->Ltx;
+    ec->Ltx = (ec->Ltxacc + (1<<4)) >> 5;
+    ec->Lrxacc += abs(rx) - ec->Lrx;
+    ec->Lrx = (ec->Lrxacc + (1<<4)) >> 5;
+
+    /* Foreground filter ---------------------------------------------------*/
+
+    ec->fir_state.coeffs = ec->fir_taps16[0];
+    echo_value = fir16(&ec->fir_state, tx);
+    ec->clean = rx - echo_value;
+    ec->Lcleanacc += abs(ec->clean) - ec->Lclean;
+    ec->Lclean = (ec->Lcleanacc + (1<<4)) >> 5;
+
+    /* Background filter ---------------------------------------------------*/
+
+    echo_value = fir16(&ec->fir_state_bg, tx);
+    clean_bg = rx - echo_value;
+    ec->Lclean_bgacc += abs(clean_bg) - ec->Lclean_bg;
+    ec->Lclean_bg = (ec->Lclean_bgacc + (1<<4)) >> 5;
+
+    /* Background Filter adaption -----------------------------------------*/
+
+    /* Almost always adap bg filter, just simple DT and energy
+       detection to minimise adaption in cases of strong double talk.
+       However this is not critical for the dual path algorithm.
+    */
+    ec->factor = 0;
+    ec->shift = 0;
+    if ((ec->nonupdate_dwell == 0)) {
+	int   P, logP, shift;
+
+	/* Determine:
+
+	   f = Beta * clean_bg_rx/P ------ (1)
+
+	   where P is the total power in the filter states.
+
+	   The Boffins have shown that if we obey (1) we converge
+	   quickly and avoid instability.
+
+	   The correct factor f must be in Q30, as this is the fixed
+	   point format required by the lms_adapt_bg() function,
+	   therefore the scaled version of (1) is:
+
+	   (2^30) * f  = (2^30) * Beta * clean_bg_rx/P
+	       factor  = (2^30) * Beta * clean_bg_rx/P         ----- (2)
+
+	   We have chosen Beta = 0.25 by experiment, so:
+
+	       factor  = (2^30) * (2^-2) * clean_bg_rx/P
+
+                                       (30 - 2 - log2(P))
+	       factor  = clean_bg_rx 2                         ----- (3)
+
+	   To avoid a divide we approximate log2(P) as top_bit(P),
+	   which returns the position of the highest non-zero bit in
+	   P.  This approximation introduces an error as large as a
+	   factor of 2, but the algorithm seems to handle it OK.
+
+	   Come to think of it a divide may not be a big deal on a
+	   modern DSP, so its probably worth checking out the cycles
+	   for a divide versus a top_bit() implementation.
+	*/
+
+	P = MIN_TX_POWER_FOR_ADAPTION + ec->Pstates;
+	logP = top_bit(P) + ec->log2taps;
+	shift = 30 - 2 - logP;
+	ec->shift = shift;
+
+	lms_adapt_bg(ec, clean_bg, shift);
+    }
+
+    /* very simple DTD to make sure we dont try and adapt with strong
+       near end speech */
+
+    ec->adapt = 0;
+    if ((ec->Lrx > MIN_RX_POWER_FOR_ADAPTION) && (ec->Lrx > ec->Ltx))
+	ec->nonupdate_dwell = DTD_HANGOVER;
+    if (ec->nonupdate_dwell)
+	ec->nonupdate_dwell--;
+
+    /* Transfer logic ------------------------------------------------------*/
+
+    /* These conditions are from the dual path paper [1], I messed with
+       them a bit to improve performance. */
+
+    if ((ec->adaption_mode & ECHO_CAN_USE_ADAPTION) &&
+	(ec->nonupdate_dwell == 0) &&
+	(8*ec->Lclean_bg < 7*ec->Lclean) /* (ec->Lclean_bg < 0.875*ec->Lclean) */ &&
+	(8*ec->Lclean_bg < ec->Ltx)      /* (ec->Lclean_bg < 0.125*ec->Ltx)    */ )
+    {
+	if (ec->cond_met == 6) {
+	    /* BG filter has had better results for 6 consecutive samples */
+	    ec->adapt = 1;
+	    memcpy(ec->fir_taps16[0], ec->fir_taps16[1], ec->taps*sizeof(int16_t));
+	}
+	else
+	    ec->cond_met++;
+    }
+    else
+	ec->cond_met = 0;
+
+    /* Non-Linear Processing ---------------------------------------------------*/
+
+    ec->clean_nlp = ec->clean;
+    if (ec->adaption_mode & ECHO_CAN_USE_NLP)
+    {
+        /* Non-linear processor - a fancy way to say "zap small signals, to avoid
+           residual echo due to (uLaw/ALaw) non-linearity in the channel.". */
+
+      if ((16*ec->Lclean < ec->Ltx))
+      {
+	/* Our e/c has improved echo by at least 24 dB (each factor of 2 is 6dB,
+	   so 2*2*2*2=16 is the same as 6+6+6+6=24dB) */
+        if (ec->adaption_mode & ECHO_CAN_USE_CNG)
+	{
+	    ec->cng_level = ec->Lbgn;
+
+	    /* Very elementary comfort noise generation.  Just random
+	       numbers rolled off very vaguely Hoth-like.  DR: This
+	       noise doesn't sound quite right to me - I suspect there
+	       are some overlfow issues in the filtering as it's too
+	       "crackly".  TODO: debug this, maybe just play noise at
+	       high level or look at spectrum.
+	    */
+
+	    ec->cng_rndnum = 1664525U*ec->cng_rndnum + 1013904223U;
+	    ec->cng_filter = ((ec->cng_rndnum & 0xFFFF) - 32768 + 5*ec->cng_filter) >> 3;
+	    ec->clean_nlp = (ec->cng_filter*ec->cng_level*8) >> 14;
+
+        }
+        else if (ec->adaption_mode & ECHO_CAN_USE_CLIP)
+	{
+	    /* This sounds much better than CNG */
+	    if (ec->clean_nlp > ec->Lbgn)
+	      ec->clean_nlp = ec->Lbgn;
+	    if (ec->clean_nlp < -ec->Lbgn)
+	      ec->clean_nlp = -ec->Lbgn;
+	}
+	else
+        {
+	  /* just mute the residual, doesn't sound very good, used mainly
+	     in G168 tests */
+          ec->clean_nlp = 0;
+        }
+      }
+      else {
+	  /* Background noise estimator.  I tried a few algorithms
+	     here without much luck.  This very simple one seems to
+	     work best, we just average the level using a slow (1 sec
+	     time const) filter if the current level is less than a
+	     (experimentally derived) constant.  This means we dont
+	     include high level signals like near end speech.  When
+	     combined with CNG or especially CLIP seems to work OK.
+	  */
+	  if (ec->Lclean < 40) {
+	      ec->Lbgn_acc += abs(ec->clean) - ec->Lbgn;
+	      ec->Lbgn = (ec->Lbgn_acc + (1<<11)) >> 12;
+	  }
+       }
+    }
+
+    /* Roll around the taps buffer */
+    if (ec->curr_pos <= 0)
+        ec->curr_pos = ec->taps;
+    ec->curr_pos--;
+
+    if (ec->adaption_mode & ECHO_CAN_DISABLE)
+      ec->clean_nlp = rx;
+
+    /* Output scaled back up again to match input scaling */
+
+    return (int16_t) ec->clean_nlp << 1;
+}
+
+/*- End of function --------------------------------------------------------*/
+
+/* This function is seperated from the echo canceller is it is usually called
+   as part of the tx process.  See rx HP (DC blocking) filter above, it's
+   the same design.
+
+   Some soft phones send speech signals with a lot of low frequency
+   energy, e.g. down to 20Hz.  This can make the hybrid non-linear
+   which causes the echo canceller to fall over.  This filter can help
+   by removing any low frequency before it gets to the tx port of the
+   hybrid.
+
+   It can also help by removing and DC in the tx signal.  DC is bad
+   for LMS algorithms.
+
+   This is one of the classic DC removal filters, adjusted to provide sufficient
+   bass rolloff to meet the above requirement to protect hybrids from things that
+   upset them. The difference between successive samples produces a lousy HPF, and
+   then a suitably placed pole flattens things out. The final result is a nicely
+   rolled off bass end. The filtering is implemented with extended fractional
+   precision, which noise shapes things, giving very clean DC removal.
+*/
+
+int16_t echo_can_hpf_tx(echo_can_state_t *ec, int16_t tx) {
+    int tmp, tmp1;
+
+    if (ec->adaption_mode & ECHO_CAN_USE_TX_HPF) {
+        tmp = tx << 15;
+#if 1
+        /* Make sure the gain of the HPF is 1.0. The first can still saturate a little under
+           impulse conditions, and it might roll to 32768 and need clipping on sustained peak
+           level signals. However, the scale of such clipping is small, and the error due to
+           any saturation should not markedly affect the downstream processing. */
+        tmp -= (tmp >> 4);
+#endif
+        ec->tx_1 += -(ec->tx_1>>DC_LOG2BETA) + tmp - ec->tx_2;
+        tmp1 = ec->tx_1 >> 15;
+	if (tmp1 > 32767) tmp1 = 32767;
+	if (tmp1 < -32767) tmp1 = -32767;
+	tx = tmp1;
+        ec->tx_2 = tmp;
+    }
+
+    return tx;
+}