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55 | <H1><A NAME="SECTION0016100000000000000000"> | |
56 | 14.1 <tt class="module">audioop</tt> -- | |
57 | Manipulate raw audio data</A> | |
58 | </H1> | |
59 | ||
60 | <P> | |
61 | <A NAME="module-audioop"></A> | |
62 | ||
63 | <P> | |
64 | The <tt class="module">audioop</tt> module contains some useful operations on sound | |
65 | fragments. It operates on sound fragments consisting of signed | |
66 | integer samples 8, 16 or 32 bits wide, stored in Python strings. This | |
67 | is the same format as used by the <tt class="module"><a href="module-al.html">al</a></tt> and <tt class="module"><a href="module-sunaudiodev.html">sunaudiodev</a></tt> | |
68 | modules. All scalar items are integers, unless specified otherwise. | |
69 | ||
70 | <P> | |
71 | This module provides support for u-LAW and Intel/DVI ADPCM encodings. | |
72 | <a id='l2h-4620' xml:id='l2h-4620'></a> | |
73 | ||
74 | <P> | |
75 | A few of the more complicated operations only take 16-bit samples, | |
76 | otherwise the sample size (in bytes) is always a parameter of the | |
77 | operation. | |
78 | ||
79 | <P> | |
80 | The module defines the following variables and functions: | |
81 | ||
82 | <P> | |
83 | <dl><dt><b><span class="typelabel">exception</span> <tt id='l2h-4594' xml:id='l2h-4594' class="exception">error</tt></b></dt> | |
84 | <dd> | |
85 | This exception is raised on all errors, such as unknown number of bytes | |
86 | per sample, etc. | |
87 | </dd></dl> | |
88 | ||
89 | <P> | |
90 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
91 | <td><nobr><b><tt id='l2h-4595' xml:id='l2h-4595' class="function">add</tt></b>(</nobr></td> | |
92 | <td><var>fragment1, fragment2, width</var>)</td></tr></table></dt> | |
93 | <dd> | |
94 | Return a fragment which is the addition of the two samples passed as | |
95 | parameters. <var>width</var> is the sample width in bytes, either | |
96 | <code>1</code>, <code>2</code> or <code>4</code>. Both fragments should have the same | |
97 | length. | |
98 | </dl> | |
99 | ||
100 | <P> | |
101 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
102 | <td><nobr><b><tt id='l2h-4596' xml:id='l2h-4596' class="function">adpcm2lin</tt></b>(</nobr></td> | |
103 | <td><var>adpcmfragment, width, state</var>)</td></tr></table></dt> | |
104 | <dd> | |
105 | Decode an Intel/DVI ADPCM coded fragment to a linear fragment. See | |
106 | the description of <tt class="function">lin2adpcm()</tt> for details on ADPCM coding. | |
107 | Return a tuple <code>(<var>sample</var>, <var>newstate</var>)</code> where the sample | |
108 | has the width specified in <var>width</var>. | |
109 | </dl> | |
110 | ||
111 | <P> | |
112 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
113 | <td><nobr><b><tt id='l2h-4597' xml:id='l2h-4597' class="function">adpcm32lin</tt></b>(</nobr></td> | |
114 | <td><var>adpcmfragment, width, state</var>)</td></tr></table></dt> | |
115 | <dd> | |
116 | Decode an alternative 3-bit ADPCM code. See <tt class="function">lin2adpcm3()</tt> | |
117 | for details. | |
118 | </dl> | |
119 | ||
120 | <P> | |
121 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
122 | <td><nobr><b><tt id='l2h-4598' xml:id='l2h-4598' class="function">avg</tt></b>(</nobr></td> | |
123 | <td><var>fragment, width</var>)</td></tr></table></dt> | |
124 | <dd> | |
125 | Return the average over all samples in the fragment. | |
126 | </dl> | |
127 | ||
128 | <P> | |
129 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
130 | <td><nobr><b><tt id='l2h-4599' xml:id='l2h-4599' class="function">avgpp</tt></b>(</nobr></td> | |
131 | <td><var>fragment, width</var>)</td></tr></table></dt> | |
132 | <dd> | |
133 | Return the average peak-peak value over all samples in the fragment. | |
134 | No filtering is done, so the usefulness of this routine is | |
135 | questionable. | |
136 | </dl> | |
137 | ||
138 | <P> | |
139 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
140 | <td><nobr><b><tt id='l2h-4600' xml:id='l2h-4600' class="function">bias</tt></b>(</nobr></td> | |
141 | <td><var>fragment, width, bias</var>)</td></tr></table></dt> | |
142 | <dd> | |
143 | Return a fragment that is the original fragment with a bias added to | |
144 | each sample. | |
145 | </dl> | |
146 | ||
147 | <P> | |
148 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
149 | <td><nobr><b><tt id='l2h-4601' xml:id='l2h-4601' class="function">cross</tt></b>(</nobr></td> | |
150 | <td><var>fragment, width</var>)</td></tr></table></dt> | |
151 | <dd> | |
152 | Return the number of zero crossings in the fragment passed as an | |
153 | argument. | |
154 | </dl> | |
155 | ||
156 | <P> | |
157 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
158 | <td><nobr><b><tt id='l2h-4602' xml:id='l2h-4602' class="function">findfactor</tt></b>(</nobr></td> | |
159 | <td><var>fragment, reference</var>)</td></tr></table></dt> | |
160 | <dd> | |
161 | Return a factor <var>F</var> such that | |
162 | <code>rms(add(<var>fragment</var>, mul(<var>reference</var>, -<var>F</var>)))</code> is | |
163 | minimal, i.e., return the factor with which you should multiply | |
164 | <var>reference</var> to make it match as well as possible to | |
165 | <var>fragment</var>. The fragments should both contain 2-byte samples. | |
166 | ||
167 | <P> | |
168 | The time taken by this routine is proportional to | |
169 | <code>len(<var>fragment</var>)</code>. | |
170 | </dl> | |
171 | ||
172 | <P> | |
173 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
174 | <td><nobr><b><tt id='l2h-4603' xml:id='l2h-4603' class="function">findfit</tt></b>(</nobr></td> | |
175 | <td><var>fragment, reference</var>)</td></tr></table></dt> | |
176 | <dd> | |
177 | Try to match <var>reference</var> as well as possible to a portion of | |
178 | <var>fragment</var> (which should be the longer fragment). This is | |
179 | (conceptually) done by taking slices out of <var>fragment</var>, using | |
180 | <tt class="function">findfactor()</tt> to compute the best match, and minimizing the | |
181 | result. The fragments should both contain 2-byte samples. Return a | |
182 | tuple <code>(<var>offset</var>, <var>factor</var>)</code> where <var>offset</var> is the | |
183 | (integer) offset into <var>fragment</var> where the optimal match started | |
184 | and <var>factor</var> is the (floating-point) factor as per | |
185 | <tt class="function">findfactor()</tt>. | |
186 | </dl> | |
187 | ||
188 | <P> | |
189 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
190 | <td><nobr><b><tt id='l2h-4604' xml:id='l2h-4604' class="function">findmax</tt></b>(</nobr></td> | |
191 | <td><var>fragment, length</var>)</td></tr></table></dt> | |
192 | <dd> | |
193 | Search <var>fragment</var> for a slice of length <var>length</var> samples (not | |
194 | bytes!) with maximum energy, i.e., return <var>i</var> for which | |
195 | <code>rms(fragment[i*2:(i+length)*2])</code> is maximal. The fragments | |
196 | should both contain 2-byte samples. | |
197 | ||
198 | <P> | |
199 | The routine takes time proportional to <code>len(<var>fragment</var>)</code>. | |
200 | </dl> | |
201 | ||
202 | <P> | |
203 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
204 | <td><nobr><b><tt id='l2h-4605' xml:id='l2h-4605' class="function">getsample</tt></b>(</nobr></td> | |
205 | <td><var>fragment, width, index</var>)</td></tr></table></dt> | |
206 | <dd> | |
207 | Return the value of sample <var>index</var> from the fragment. | |
208 | </dl> | |
209 | ||
210 | <P> | |
211 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
212 | <td><nobr><b><tt id='l2h-4606' xml:id='l2h-4606' class="function">lin2lin</tt></b>(</nobr></td> | |
213 | <td><var>fragment, width, newwidth</var>)</td></tr></table></dt> | |
214 | <dd> | |
215 | Convert samples between 1-, 2- and 4-byte formats. | |
216 | </dl> | |
217 | ||
218 | <P> | |
219 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
220 | <td><nobr><b><tt id='l2h-4607' xml:id='l2h-4607' class="function">lin2adpcm</tt></b>(</nobr></td> | |
221 | <td><var>fragment, width, state</var>)</td></tr></table></dt> | |
222 | <dd> | |
223 | Convert samples to 4 bit Intel/DVI ADPCM encoding. ADPCM coding is an | |
224 | adaptive coding scheme, whereby each 4 bit number is the difference | |
225 | between one sample and the next, divided by a (varying) step. The | |
226 | Intel/DVI ADPCM algorithm has been selected for use by the IMA, so it | |
227 | may well become a standard. | |
228 | ||
229 | <P> | |
230 | <var>state</var> is a tuple containing the state of the coder. The coder | |
231 | returns a tuple <code>(<var>adpcmfrag</var>, <var>newstate</var>)</code>, and the | |
232 | <var>newstate</var> should be passed to the next call of | |
233 | <tt class="function">lin2adpcm()</tt>. In the initial call, <code>None</code> can be | |
234 | passed as the state. <var>adpcmfrag</var> is the ADPCM coded fragment | |
235 | packed 2 4-bit values per byte. | |
236 | </dl> | |
237 | ||
238 | <P> | |
239 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
240 | <td><nobr><b><tt id='l2h-4608' xml:id='l2h-4608' class="function">lin2adpcm3</tt></b>(</nobr></td> | |
241 | <td><var>fragment, width, state</var>)</td></tr></table></dt> | |
242 | <dd> | |
243 | This is an alternative ADPCM coder that uses only 3 bits per sample. | |
244 | It is not compatible with the Intel/DVI ADPCM coder and its output is | |
245 | not packed (due to laziness on the side of the author). Its use is | |
246 | discouraged. | |
247 | </dl> | |
248 | ||
249 | <P> | |
250 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
251 | <td><nobr><b><tt id='l2h-4609' xml:id='l2h-4609' class="function">lin2ulaw</tt></b>(</nobr></td> | |
252 | <td><var>fragment, width</var>)</td></tr></table></dt> | |
253 | <dd> | |
254 | Convert samples in the audio fragment to u-LAW encoding and return | |
255 | this as a Python string. u-LAW is an audio encoding format whereby | |
256 | you get a dynamic range of about 14 bits using only 8 bit samples. It | |
257 | is used by the Sun audio hardware, among others. | |
258 | </dl> | |
259 | ||
260 | <P> | |
261 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
262 | <td><nobr><b><tt id='l2h-4610' xml:id='l2h-4610' class="function">minmax</tt></b>(</nobr></td> | |
263 | <td><var>fragment, width</var>)</td></tr></table></dt> | |
264 | <dd> | |
265 | Return a tuple consisting of the minimum and maximum values of all | |
266 | samples in the sound fragment. | |
267 | </dl> | |
268 | ||
269 | <P> | |
270 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
271 | <td><nobr><b><tt id='l2h-4611' xml:id='l2h-4611' class="function">max</tt></b>(</nobr></td> | |
272 | <td><var>fragment, width</var>)</td></tr></table></dt> | |
273 | <dd> | |
274 | Return the maximum of the <em>absolute value</em> of all samples in a | |
275 | fragment. | |
276 | </dl> | |
277 | ||
278 | <P> | |
279 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
280 | <td><nobr><b><tt id='l2h-4612' xml:id='l2h-4612' class="function">maxpp</tt></b>(</nobr></td> | |
281 | <td><var>fragment, width</var>)</td></tr></table></dt> | |
282 | <dd> | |
283 | Return the maximum peak-peak value in the sound fragment. | |
284 | </dl> | |
285 | ||
286 | <P> | |
287 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
288 | <td><nobr><b><tt id='l2h-4613' xml:id='l2h-4613' class="function">mul</tt></b>(</nobr></td> | |
289 | <td><var>fragment, width, factor</var>)</td></tr></table></dt> | |
290 | <dd> | |
291 | Return a fragment that has all samples in the original fragment | |
292 | multiplied by the floating-point value <var>factor</var>. Overflow is | |
293 | silently ignored. | |
294 | </dl> | |
295 | ||
296 | <P> | |
297 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
298 | <td><nobr><b><tt id='l2h-4614' xml:id='l2h-4614' class="function">ratecv</tt></b>(</nobr></td> | |
299 | <td><var>fragment, width, nchannels, inrate, outrate, | |
300 | state</var><big>[</big><var>, weightA</var><big>[</big><var>, weightB</var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt> | |
301 | <dd> | |
302 | Convert the frame rate of the input fragment. | |
303 | ||
304 | <P> | |
305 | <var>state</var> is a tuple containing the state of the converter. The | |
306 | converter returns a tuple <code>(<var>newfragment</var>, <var>newstate</var>)</code>, | |
307 | and <var>newstate</var> should be passed to the next call of | |
308 | <tt class="function">ratecv()</tt>. The initial call should pass <code>None</code> | |
309 | as the state. | |
310 | ||
311 | <P> | |
312 | The <var>weightA</var> and <var>weightB</var> arguments are parameters for a | |
313 | simple digital filter and default to <code>1</code> and <code>0</code> respectively. | |
314 | </dl> | |
315 | ||
316 | <P> | |
317 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
318 | <td><nobr><b><tt id='l2h-4615' xml:id='l2h-4615' class="function">reverse</tt></b>(</nobr></td> | |
319 | <td><var>fragment, width</var>)</td></tr></table></dt> | |
320 | <dd> | |
321 | Reverse the samples in a fragment and returns the modified fragment. | |
322 | </dl> | |
323 | ||
324 | <P> | |
325 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
326 | <td><nobr><b><tt id='l2h-4616' xml:id='l2h-4616' class="function">rms</tt></b>(</nobr></td> | |
327 | <td><var>fragment, width</var>)</td></tr></table></dt> | |
328 | <dd> | |
329 | Return the root-mean-square of the fragment, i.e. | |
330 | <BR><P></P> | |
331 | <DIV ALIGN="CENTER" CLASS="mathdisplay"> | |
332 | <!-- MATH | |
333 | \begin{displaymath} | |
334 | \catcode`_=8 | |
335 | \sqrt{\frac{\sum{{S_{i}}^{2}}}{n}} | |
336 | \end{displaymath} | |
337 | --> | |
338 | ||
339 | <IMG | |
340 | WIDTH="61" HEIGHT="55" BORDER="0" | |
341 | SRC="img3.png" | |
342 | ALT="\begin{displaymath} | |
343 | \catcode\lq _=8 | |
344 | \sqrt{\frac{\sum{{S_{i}}^{2}}}{n}} | |
345 | \end{displaymath}"> | |
346 | </DIV> | |
347 | <BR CLEAR="ALL"> | |
348 | <P></P> | |
349 | This is a measure of the power in an audio signal. | |
350 | </dl> | |
351 | ||
352 | <P> | |
353 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
354 | <td><nobr><b><tt id='l2h-4617' xml:id='l2h-4617' class="function">tomono</tt></b>(</nobr></td> | |
355 | <td><var>fragment, width, lfactor, rfactor</var>)</td></tr></table></dt> | |
356 | <dd> | |
357 | Convert a stereo fragment to a mono fragment. The left channel is | |
358 | multiplied by <var>lfactor</var> and the right channel by <var>rfactor</var> | |
359 | before adding the two channels to give a mono signal. | |
360 | </dl> | |
361 | ||
362 | <P> | |
363 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
364 | <td><nobr><b><tt id='l2h-4618' xml:id='l2h-4618' class="function">tostereo</tt></b>(</nobr></td> | |
365 | <td><var>fragment, width, lfactor, rfactor</var>)</td></tr></table></dt> | |
366 | <dd> | |
367 | Generate a stereo fragment from a mono fragment. Each pair of samples | |
368 | in the stereo fragment are computed from the mono sample, whereby left | |
369 | channel samples are multiplied by <var>lfactor</var> and right channel | |
370 | samples by <var>rfactor</var>. | |
371 | </dl> | |
372 | ||
373 | <P> | |
374 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
375 | <td><nobr><b><tt id='l2h-4619' xml:id='l2h-4619' class="function">ulaw2lin</tt></b>(</nobr></td> | |
376 | <td><var>fragment, width</var>)</td></tr></table></dt> | |
377 | <dd> | |
378 | Convert sound fragments in u-LAW encoding to linearly encoded sound | |
379 | fragments. u-LAW encoding always uses 8 bits samples, so <var>width</var> | |
380 | refers only to the sample width of the output fragment here. | |
381 | </dl> | |
382 | ||
383 | <P> | |
384 | Note that operations such as <tt class="function">mul()</tt> or <tt class="function">max()</tt> make | |
385 | no distinction between mono and stereo fragments, i.e. all samples | |
386 | are treated equal. If this is a problem the stereo fragment should be | |
387 | split into two mono fragments first and recombined later. Here is an | |
388 | example of how to do that: | |
389 | ||
390 | <P> | |
391 | <div class="verbatim"><pre> | |
392 | def mul_stereo(sample, width, lfactor, rfactor): | |
393 | lsample = audioop.tomono(sample, width, 1, 0) | |
394 | rsample = audioop.tomono(sample, width, 0, 1) | |
395 | lsample = audioop.mul(sample, width, lfactor) | |
396 | rsample = audioop.mul(sample, width, rfactor) | |
397 | lsample = audioop.tostereo(lsample, width, 1, 0) | |
398 | rsample = audioop.tostereo(rsample, width, 0, 1) | |
399 | return audioop.add(lsample, rsample, width) | |
400 | </pre></div> | |
401 | ||
402 | <P> | |
403 | If you use the ADPCM coder to build network packets and you want your | |
404 | protocol to be stateless (i.e. to be able to tolerate packet loss) | |
405 | you should not only transmit the data but also the state. Note that | |
406 | you should send the <var>initial</var> state (the one you passed to | |
407 | <tt class="function">lin2adpcm()</tt>) along to the decoder, not the final state (as | |
408 | returned by the coder). If you want to use <tt class="function">struct.struct()</tt> | |
409 | to store the state in binary you can code the first element (the | |
410 | predicted value) in 16 bits and the second (the delta index) in 8. | |
411 | ||
412 | <P> | |
413 | The ADPCM coders have never been tried against other ADPCM coders, | |
414 | only against themselves. It could well be that I misinterpreted the | |
415 | standards in which case they will not be interoperable with the | |
416 | respective standards. | |
417 | ||
418 | <P> | |
419 | The <tt class="function">find*()</tt> routines might look a bit funny at first sight. | |
420 | They are primarily meant to do echo cancellation. A reasonably | |
421 | fast way to do this is to pick the most energetic piece of the output | |
422 | sample, locate that in the input sample and subtract the whole output | |
423 | sample from the input sample: | |
424 | ||
425 | <P> | |
426 | <div class="verbatim"><pre> | |
427 | def echocancel(outputdata, inputdata): | |
428 | pos = audioop.findmax(outputdata, 800) # one tenth second | |
429 | out_test = outputdata[pos*2:] | |
430 | in_test = inputdata[pos*2:] | |
431 | ipos, factor = audioop.findfit(in_test, out_test) | |
432 | # Optional (for better cancellation): | |
433 | # factor = audioop.findfactor(in_test[ipos*2:ipos*2+len(out_test)], | |
434 | # out_test) | |
435 | prefill = '\0'*(pos+ipos)*2 | |
436 | postfill = '\0'*(len(inputdata)-len(prefill)-len(outputdata)) | |
437 | outputdata = prefill + audioop.mul(outputdata,2,-factor) + postfill | |
438 | return audioop.add(inputdata, outputdata, 2) | |
439 | </pre></div> | |
440 | ||
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475 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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