mirror of https://gerrit.osmocom.org/libosmocore
345 lines
11 KiB
C
345 lines
11 KiB
C
/*
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* (C) 2017 by sysmocom - s.f.m.c. GmbH
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* (C) 2017 by Philipp Maier <pmaier@sysmocom.de>
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* All Rights Reserved
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*
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* Significantly reworked in 2023 by Mother
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* Mychaela N. Falconia <falcon@freecalypso.org> - however,
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* Mother Mychaela's contributions are NOT subject to copyright.
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* No rights reserved, all rights relinquished.
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* Portions of this code are based on Themyscira libgsmfrp,
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* a public domain library by the same author.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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*
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* The present ECU implementation for GSM-FR is closely based on the
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* TS 46.011 spec from 3GPP; more specifically, it is based on the
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* Example solution presented in Chapter 6 of that spec, adapted for
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* libosmocodec ECU architecture, and comes as close to fulfilling
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* the spec's officially stated requirements (Chapter 5) as is
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* possible within this Osmocom-imposed architecture. Please note
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* the following areas where the present implementation fails to
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* fulfill the original intent of GSM spec authors:
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*
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* - The "lost SID" criterion, defined in GSM 06.31, is based on the
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* TAF bit from the Radio Subsystem. However, libosmocodec ECU API
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* does not include this flag, thus spec requirements related to
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* lost SID conditions cannot be implemented in a strictly compliant
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* manner. The present implementation improvises its own "lost SID"
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* detector (not strictly spec-compliant) by counting frame_out()
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* calls in between good traffic frame inputs via frame_in().
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*
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* - In the architecture envisioned and assumed in the GSM specs,
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* the ECU function of GSM 06.11 was never intended to be a fully
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* modular component with its own bona fide I/O interfaces - this
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* approach appears to be an Osmocom invention - instead this ECU
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* function was intended to be subsumed in the Rx DTX handler
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* component of GSM 06.31, also incorporating the comfort noise
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* generator of GSM 06.12 - and unlike the narrower-scope ECU,
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* this slightly-larger-scope Rx DTX handler is a modular component
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* with well-defined I/O interfaces. In the case of BFI conditions
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* following a SID, GSM 06.11 spec was written with the assumption
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* that the ECU controls the comfort noise generator via internal
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* signals, as opposed to emitting "corrected" SID frames on a
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* modular interface going to a CN generator located somewhere else.
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* Thus the "correct" behavior for a fully modularized ECU is unclear,
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* and an argument can be made that the very existence of such a
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* fully modularized ECU is incorrect in itself. The present
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* implementation re-emits a "rejuvenated" form of the last saved
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* SID frame during BFI conditions following a SID within the
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* permitted window of 48 frames, then starts emitting muted SIDs
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* with Xmaxc decreasing by 4 on each frame, and finally switches
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* to emitting non-SID silence frames (Table 1 of TS 46.011)
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* once Xmaxc reaches 0.
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*/
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#include <stdbool.h>
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#include <string.h>
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#include <stdint.h>
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#include <errno.h>
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#include <osmocom/core/prbs.h>
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#include <osmocom/codec/codec.h>
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#include <osmocom/codec/ecu.h>
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#include <osmocom/core/linuxlist.h>
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/* See TS 46.011, Chapter 6 Example solution */
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#define GSM611_XMAXC_REDUCE 4
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/* The first 5 bytes of RTP encoding neatly contain the magic nibble
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* and LARc parameters, which also happens to be the part of SID frames
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* that needs to be passed through as-is. */
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#define SID_PREFIX_LEN 5
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enum ecu_principal_state {
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STATE_NO_DATA,
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STATE_SPEECH,
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STATE_SP_MUTING,
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STATE_SID,
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STATE_SID_MUTING,
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};
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struct fr_ecu_state {
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struct osmo_ecu_state ecu_state;
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enum ecu_principal_state pr_state;
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uint8_t speech_frame[GSM_FR_BYTES];
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uint8_t sid_prefix[SID_PREFIX_LEN];
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uint8_t sid_xmaxc;
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uint8_t sid_reemit_count;
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struct osmo_prbs_state prng;
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bool last_input_was_sid;
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};
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/* This function is the frame input to the ECU - all inputs to this
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* function have been received by the Radio Subsystem as good traffic
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* frames in the GSM 06.31 definition.
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*/
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static void fr_ecu_input(struct fr_ecu_state *fr, const uint8_t *frame)
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{
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enum osmo_gsm631_sid_class sidc;
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sidc = osmo_fr_sid_classify(frame);
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switch (sidc) {
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case OSMO_GSM631_SID_CLASS_SPEECH:
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memcpy(fr->speech_frame, frame, GSM_FR_BYTES);
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fr->pr_state = STATE_SPEECH;
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fr->last_input_was_sid = false;
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return;
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case OSMO_GSM631_SID_CLASS_INVALID:
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/* GSM 06.31 section 6.1.2 says: "an invalid SID frame
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* shall be substituted by the last valid SID frame
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* and the procedure for valid SID frames be applied."
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* However, libosmocodec ECU architecture prevents us
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* from doing what the spec says: the frame_in() method
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* gets a const frame that can't be modified, and
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* frame_out() will never get called when BFI=0, even
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* when the "good traffic frame" (in the BFI=0 sense)
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* is an invalid SID by the bit-counting rule.
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* Thus there is no place where we can re-emit a cached
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* copy of the last valid SID upon receiving an invalid SID.
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*
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* In the standard GSM architecture this problem never
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* arises because the ECU is not a separate component
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* but is coupled with the CN generator, thus the output
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* from the Rx DTX handler block will be a CN frame,
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* for both valid-SID and invalid-SID inputs to the block.
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* But what can we do within the constraints of libosmocodec
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* ECU framework? We treat the invalid SID almost like a
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* BFI, doing almost nothing in the frame_in() method,
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* but we reset sid_reemit_count because by the rules of
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* GSM 06.31 an invalid SID is still an accepted SID frame
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* for the purpose of "lost SID" logic. */
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fr->sid_reemit_count = 0;
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fr->last_input_was_sid = true;
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return;
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case OSMO_GSM631_SID_CLASS_VALID:
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/* save LARc part */
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memcpy(fr->sid_prefix, frame, SID_PREFIX_LEN);
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/* save Xmaxc from the last subframe */
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fr->sid_xmaxc = ((frame[27] & 0x1F) << 1) | (frame[28] >> 7);
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fr->pr_state = STATE_SID;
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fr->sid_reemit_count = 0;
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fr->last_input_was_sid = true;
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return;
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default:
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/* There are only 3 possible SID classifications per GSM 06.31
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* section 6.1.1, thus any other return value is a grave error
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* in the code. */
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OSMO_ASSERT(0);
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}
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}
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/* Reduce all 4 Xmaxc fields in the frame. When all 4 Xmaxc fields
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* reach 0, the function will return true for "mute".
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*/
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static bool reduce_xmaxc(uint8_t *frame)
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{
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bool mute_flag = true;
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uint8_t sub, xmaxc;
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for (sub = 0; sub < 4; sub++) {
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xmaxc = ((frame[sub*7+6] & 0x1F) << 1) | (frame[sub*7+7] >> 7);
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if (xmaxc > GSM611_XMAXC_REDUCE) {
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xmaxc -= GSM611_XMAXC_REDUCE;
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mute_flag = false;
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} else
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xmaxc = 0;
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frame[sub*7+6] &= 0xE0;
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frame[sub*7+6] |= xmaxc >> 1;
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frame[sub*7+7] &= 0x7F;
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frame[sub*7+7] |= (xmaxc & 1) << 7;
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}
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return mute_flag;
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}
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/* TS 46.011 chapter 6, paragraph 4, last sentence: "The grid position
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* parameters are chosen randomly between 0 and 3 during this time."
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* (The "during this time" qualifier refers to the speech muting state.)
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* This sentence in the spec must have been overlooked by previous ECU
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* implementors, as this aspect of the muting logic was missing.
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*/
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static void random_grid_pos(struct fr_ecu_state *fr, uint8_t *frame)
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{
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uint8_t sub;
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for (sub = 0; sub < 4; sub++) {
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frame[sub*7+6] &= 0x9F;
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frame[sub*7+6] |= osmo_prbs_get_ubit(&fr->prng) << 6;
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frame[sub*7+6] |= osmo_prbs_get_ubit(&fr->prng) << 5;
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}
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}
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/* Like reduce_xmaxc() above, but for comfort noise rather than speech. */
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static bool reduce_xmaxc_sid(struct fr_ecu_state *fr)
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{
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if (fr->sid_xmaxc > GSM611_XMAXC_REDUCE) {
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fr->sid_xmaxc -= GSM611_XMAXC_REDUCE;
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return false;
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}
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fr->sid_xmaxc = 0;
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return true;
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}
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/* This function implements the part which is peculiar to the present
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* "standalone" packaging of GSM-FR ECU, without a directly coupled
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* comfort noise generator - it re-emits synthetic SID frames during
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* DTX pauses, initially unchanged from the saved SID and later muted.
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*/
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static void reemit_sid(struct fr_ecu_state *fr, uint8_t *frame)
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{
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uint8_t *p, sub;
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memcpy(frame, fr->sid_prefix, SID_PREFIX_LEN);
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p = frame + SID_PREFIX_LEN;
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for (sub = 0; sub < 4; sub++) {
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*p++ = 0;
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*p++ = fr->sid_xmaxc >> 1;
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*p++ = (fr->sid_xmaxc & 1) << 7;
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*p++ = 0;
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*p++ = 0;
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*p++ = 0;
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*p++ = 0;
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}
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}
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/* This function is responsible for generating the ECU's output
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* in the event that the Radio Subsystem does not have a good
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* traffic frame - conditions corresponding to BFI=1 in the specs.
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*/
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static void fr_ecu_output(struct fr_ecu_state *fr, uint8_t *frame)
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{
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bool mute;
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switch (fr->pr_state) {
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case STATE_NO_DATA:
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memcpy(frame, osmo_gsm611_silence_frame, GSM_FR_BYTES);
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return;
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case STATE_SPEECH:
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/* TS 46.011 chapter 6: "The first lost speech frame is
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* replaced at the speech decoder input by the previous
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* good speech frame." */
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memcpy(frame, fr->speech_frame, GSM_FR_BYTES);
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fr->pr_state = STATE_SP_MUTING;
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return;
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case STATE_SP_MUTING:
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mute = reduce_xmaxc(fr->speech_frame);
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memcpy(frame, fr->speech_frame, GSM_FR_BYTES);
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random_grid_pos(fr, frame);
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if (mute)
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fr->pr_state = STATE_NO_DATA;
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return;
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case STATE_SID:
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fr->sid_reemit_count++;
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if (fr->sid_reemit_count >= 48) {
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fr->pr_state = STATE_SID_MUTING;
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reduce_xmaxc_sid(fr);
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}
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reemit_sid(fr, frame);
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return;
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case STATE_SID_MUTING:
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if (reduce_xmaxc_sid(fr)) {
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fr->pr_state = STATE_NO_DATA;
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memcpy(frame, osmo_gsm611_silence_frame, GSM_FR_BYTES);
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} else
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reemit_sid(fr, frame);
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return;
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default:
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/* a severe bug in the state machine! */
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OSMO_ASSERT(0);
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}
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}
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/***********************************************************************
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* Integration with ECU core
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***********************************************************************/
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static struct osmo_ecu_state *ecu_fr_init(void *ctx, enum osmo_ecu_codec codec)
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{
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struct fr_ecu_state *fr;
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fr = talloc_zero(ctx, struct fr_ecu_state);
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fr->ecu_state.codec = codec;
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fr->pr_state = STATE_NO_DATA;
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osmo_prbs_state_init(&fr->prng, &osmo_prbs15);
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return (struct osmo_ecu_state *) fr;
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}
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static inline struct fr_ecu_state *_osmo_ecu_state_get_fr(struct osmo_ecu_state *st)
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{
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return (struct fr_ecu_state *)container_of(st, struct fr_ecu_state, ecu_state);
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}
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static int ecu_fr_frame_in(struct osmo_ecu_state *st, bool bfi, const uint8_t *frame,
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unsigned int frame_bytes)
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{
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struct fr_ecu_state *fr = _osmo_ecu_state_get_fr(st);
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if (bfi)
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return 0;
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if (frame_bytes != GSM_FR_BYTES)
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return 0;
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if ((frame[0] & 0xF0) != 0xD0)
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return 0;
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fr_ecu_input(fr, frame);
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return 0;
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}
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static int ecu_fr_frame_out(struct osmo_ecu_state *st, uint8_t *frame_out)
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{
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struct fr_ecu_state *fr = _osmo_ecu_state_get_fr(st);
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fr_ecu_output(fr, frame_out);
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return GSM_FR_BYTES;
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}
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static bool ecu_fr_is_dtx_pause(struct osmo_ecu_state *st)
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{
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struct fr_ecu_state *fr = _osmo_ecu_state_get_fr(st);
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return fr->last_input_was_sid;
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}
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static const struct osmo_ecu_ops osmo_ecu_ops_fr = {
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.init = ecu_fr_init,
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.frame_in = ecu_fr_frame_in,
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.frame_out = ecu_fr_frame_out,
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.is_dtx_pause = ecu_fr_is_dtx_pause,
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};
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static __attribute__((constructor)) void on_dso_load_ecu_fr(void)
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{
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osmo_ecu_register(&osmo_ecu_ops_fr, OSMO_ECU_CODEC_FR);
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}
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