forked from auracaster/pyalsaaudio
df89c12581
items. Argument parsing errors are reported with the methodname (minor improvement). Smallish documentation improvements. git-svn-id: svn://svn.code.sf.net/p/pyalsaaudio/code/trunk@23 ec2f30ec-7544-0410-870e-f70ca00c83f0
185 lines
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185 lines
7.1 KiB
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<link rel="first" href="pyalsaaudio.html" title='PyAlsaAudio' />
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<title>4.1 PCM Terminology and Concepts</title>
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<h2><a name="SECTION002410000000000000000">
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4.1 PCM Terminology and Concepts</a>
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</h2>
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<p>
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In order to use PCM devices it is useful to be familiar with some concepts and
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terminology.
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<p>
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<dl>
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<dt><strong>Sample</strong></dt>
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<dd>PCM audio, whether it is input or output, consists at
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the lowest level of a number of single samples. A sample represents
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the sound in a single channel in a brief interval. If more than one
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channel is in use, more than one sample is required for each
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interval to describe the sound. Samples can be of many different
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sizes, ranging from 8 bit to 64 bit presition. The specific format
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of each sample can also vary - they can be big endian byte order,
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little endian byte order, or even floats.
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<p>
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</dd>
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<dt><strong>Frame</strong></dt>
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<dd>A frame consists of exactly one sample per channel. If
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there is only one channel (Mono sound) a frame is simply a single
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sample. If the sound is stereo, each frame consists of two samples,
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etc.
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<p>
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</dd>
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<dt><strong>Frame size</strong></dt>
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<dd>This is the size in bytes of each frame. This can
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vary a lot: if each sample is 8 bits, and we're handling mono sound,
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the frame size is one byte. Similarly in 6 channel audio with 64 bit
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floating point samples, the frame size is 48 bytes
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<p>
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</dd>
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<dt><strong>Rate</strong></dt>
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<dd>PCM sound consists of a flow of sound frames. The sound
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rate controls how often the current frame is replaced. For example,
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a rate of 8000 Hz means that a new frame is played or captured 8000
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times per second.
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<p>
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</dd>
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<dt><strong>Data rate</strong></dt>
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<dd>This is the number of bytes, which must be recorded
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or provided per second at a certain frame size and rate.
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<p>
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8000 Hz mono sound with 8 bit (1 byte) samples has a data rate of
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8000 * 1 * 1 = 8 kb/s
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<p>
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At the other end of the scale, 96000 Hz, 6 channel sound with 64 bit
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(8 bytes) samples has a data rate of 96000 * 6 * 8 = 4608 kb/s
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(almost 5 Mb sound data per second)
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<p>
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</dd>
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<dt><strong>Period</strong></dt>
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<dd>When the hardware processes data this is done in chunks
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of frames. The time interval between each processing (A/D or D/A
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conversion) is known as the period. The size of the period has
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direct implication on the latency of the sound input or output. For
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low-latency the period size should be very small, while low CPU
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resource usage would usually demand larger period sizes. With ALSA,
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the CPU utilization is not impacted much by the period size, since
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the kernel layer buffers multiple periods internally, so each period
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generates an interrupt and a memory copy, but userspace can be
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slower and read or write multiple periods at the same time.
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<p>
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</dd>
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<dt><strong>Period size</strong></dt>
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<dd>This is the size of each period in Hz. <em>Not
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bytes, but Hz!.</em> In <tt class="module">alsaaudio</tt> the period size is set
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directly, and it is therefore important to understand the
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significance of this number. If the period size is configured to for
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example 32, each write should contain exactly 32 frames of sound
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data, and each read will return either 32 frames of data or nothing
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at all.
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<p>
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</dd>
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</dl>
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<p>
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Once you understand these concepts, you will be ready to use the PCM
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API. Read on.
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<p>
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<td align="center" width="100%">PyAlsaAudio</td>
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<td class='online-navigation'><a rel="contents" title="Table of Contents"
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href="contents.html"><img src='contents.png'
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border='0' height='32' alt='Contents' width='32' /></a></td>
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<span class="release-info">Release 0.4.</span>
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