1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
|
/* ------------------------------------------------------------------
* Copyright (C) 1998-2009 PacketVideo
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
* express or implied.
* See the License for the specific language governing permissions
* and limitations under the License.
* -------------------------------------------------------------------
*/
/*
Pathname: ./src/find_adts_syncword.c
------------------------------------------------------------------------------
REVISION HISTORY
Description: Fixed error in logic that determines whether there are enough
bits available to conduct a search for the syncword. The plus sign in
the following condition should be a minus.
if (pInputStream->usedBits <
(pInputStream->availableBits + syncword_length)
The length of the syncword should subtract from the number of available
bits, not add.
Description: Fixed condition when the end of file was found, unsigned
comparison produced a undesired search. Fixed by casting comparison
if ((Int)pInputStream->usedBits <
((Int)pInputStream->availableBits - syncword_length) )
Description:
------------------------------------------------------------------------------
INPUT AND OUTPUT DEFINITIONS
Inputs:
pSyncword = Pointer to variable containing the syncword that the
function should be scanning for in the buffer. [ UInt32 * ]
pInputStream = Pointer to a BITS structure, used by the function getbits
to retrieve data from the bitstream. [ BITS * ]
syncword_length = The length of the syncword. [ Int ]
syncword_mask = A mask to be applied to the bitstream before comparison
with the value pointed to by pSyncword. [ UInt32 ]
Local Stores/Buffers/Pointers Needed:
None
Global Stores/Buffers/Pointers Needed:
None
Outputs:
None
Pointers and Buffers Modified:
None
Local Stores Modified:
None
Global Stores Modified:
None
------------------------------------------------------------------------------
FUNCTION DESCRIPTION
This module scans the bitstream for a syncword of any length between 1 and 32.
If certain bits in the syncword are to be ignored, that bit position should
be set to 0 in both parameters *(pSyncword) and syncword_mask. This allows
for a syncword to be constructed out of non-contiguous bits.
Upon finding the syncword's position in the bitstream, a value denoting the
syncword's degree of deviance from being byte-aligned (byte_align_offset)
is set in the structure pointed to by pInputStream.
This is a value between 0 and 7.
If no syncword is found, the function returns status == ERROR.
------------------------------------------------------------------------------
REQUIREMENTS
"Don't care" bits must be set to '0' in both *(pSyncword) and syncword_mask.
This function should not be called if there are less than
(8 + syncword_length) bits in the buffer.
------------------------------------------------------------------------------
REFERENCES
(1) ISO/IEC 13818-7:1997(E)
Part 7
Subpart 6.2 (Audio_Data_Transport_Stream frame, ADTS)
(2) ISO/IEC 11172-3:1993(E)
Part 3
Subpart 2.4.3 The audio decoding process
------------------------------------------------------------------------------
PSEUDO-CODE
IF (pInputStream->usedBits <
(pInputStream->availableBits + syncword_length) )
max_search_length = (pInputStream->availableBits - pInputStream->usedBits);
max_search_length = max_search_length - syncword_length;
search_length = 0;
adts_header =
CALL getbits(syncword_length, pInputStream);
MODIFYING pInputStream->usedBits
RETURNING bits from bitstream of length (syncword_length)
test_for_syncword = adts_header AND syncword_mask;
test_for_syncword = test_for_syncword XOR syncword;
WHILE ( (test_for_syncword != 0) && (search_length > 0) )
search_length = search_length - 1;
adts_header = adts_header << 1;
adts_header = adts_header OR ...
CALL getbits(syncword_length, pInputStream);
MODIFYING pInputStream->usedBits
RETURNING 1 bit from the bitstream
test_for_syncword = adts_header AND syncword_mask;
test_for_syncword = test_for_syncword XOR syncword;
ENDWHILE
IF (search_length == 0)
status = ERROR;
ENDIF
*(pSyncword) = adts_header;
pInputStream->byteAlignOffset =
(pInputStream->usedBits - syncwordlength) AND 0x7;
ELSE
status = ERROR;
ENDIF
return (status);
------------------------------------------------------------------------------
RESOURCES USED
When the code is written for a specific target processor the
the resources used should be documented below.
STACK USAGE: [stack count for this module] + [variable to represent
stack usage for each subroutine called]
where: [stack usage variable] = stack usage for [subroutine
name] (see [filename].ext)
DATA MEMORY USED: x words
PROGRAM MEMORY USED: x words
CLOCK CYCLES: [cycle count equation for this module] + [variable
used to represent cycle count for each subroutine
called]
where: [cycle count variable] = cycle count for [subroutine
name] (see [filename].ext)
------------------------------------------------------------------------------
*/
/*----------------------------------------------------------------------------
; INCLUDES
----------------------------------------------------------------------------*/
#include "pv_audio_type_defs.h"
#include "s_bits.h"
#include "ibstream.h"
#include "find_adts_syncword.h"
/*----------------------------------------------------------------------------
; MACROS
; Define module specific macros here
----------------------------------------------------------------------------*/
/*----------------------------------------------------------------------------
; DEFINES
; Include all pre-processor statements here. Include conditional
; compile variables also.
----------------------------------------------------------------------------*/
#define FIND_ADTS_ERROR -1
/*----------------------------------------------------------------------------
; LOCAL FUNCTION DEFINITIONS
; Function Prototype declaration
----------------------------------------------------------------------------*/
/*----------------------------------------------------------------------------
; LOCAL STORE/BUFFER/POINTER DEFINITIONS
; Variable declaration - defined here and used outside this module
----------------------------------------------------------------------------*/
/*----------------------------------------------------------------------------
; EXTERNAL FUNCTION REFERENCES
; Declare functions defined elsewhere and referenced in this module
----------------------------------------------------------------------------*/
/*----------------------------------------------------------------------------
; EXTERNAL GLOBAL STORE/BUFFER/POINTER REFERENCES
; Declare variables used in this module but defined elsewhere
----------------------------------------------------------------------------*/
/*----------------------------------------------------------------------------
; FUNCTION CODE
----------------------------------------------------------------------------*/
Int find_adts_syncword(
UInt32 *pSyncword,
BITS *pInputStream,
Int syncword_length,
UInt32 syncword_mask)
{
Int status = SUCCESS;
UInt search_length;
UInt32 adts_header = 0;
UInt32 test_for_syncword;
UInt32 syncword = *(pSyncword);
/*
* Determine the maximum number of bits available to this function for
* the syncword search.
*/
if ((Int)pInputStream->usedBits <
((Int)pInputStream->availableBits - syncword_length))
{
search_length = (pInputStream->availableBits - pInputStream->usedBits);
search_length -= syncword_length;
adts_header = getbits(syncword_length, pInputStream);
/*
* Mask the result in adts_header with the syncword_mask, so only the
* bits relevant to syncword detection are compared to *(pSyncword).
*/
test_for_syncword = adts_header & syncword_mask;
test_for_syncword ^= syncword;
/*
* Scan bit-by-bit through the bitstream, until the function either
* runs out of bits, or finds the syncword.
*/
while ((test_for_syncword != 0) && (search_length > 0))
{
search_length--;
adts_header <<= 1;
adts_header |= getbits(1, pInputStream);
test_for_syncword = adts_header & syncword_mask;
test_for_syncword ^= syncword;
}
if (search_length == 0)
{
status = FIND_ADTS_ERROR;
}
/*
* Return the syncword's position in the bitstream. Correct placement
* of the syncword will result in byte_align_offset == 0.
* If the syncword is found not to be byte-aligned, then return
* the degree of disalignment, so further decoding can
* be shifted as necessary.
*
*/
pInputStream->byteAlignOffset =
(pInputStream->usedBits - syncword_length) & 0x7;
} /* END if (pInputStream->usedBits < ...) */
else
{
status = FIND_ADTS_ERROR;
}
*(pSyncword) = adts_header;
return (status);
} /* find_adts_syncword() */
|