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<section id="pcm-terminology-and-concepts">
<h1>PCM Terminology and Concepts<a class="headerlink" href="#pcm-terminology-and-concepts" title="Permalink to this heading">¶</a></h1>
<p>In order to use PCM devices it is useful to be familiar with some concepts and
terminology.</p>
<dl>
<dt>Sample</dt><dd><p>PCM audio, whether it is input or output, consists of <em>samples</em>.
A single sample represents the amplitude of one channel of sound
at a certain point in time. A lot of individual samples are
necessary to represent actual sound; for CD audio, 44100 samples
are taken every second.</p>
<p>Samples can be of many different sizes, ranging from 8 bit to 64
bit precision. The specific format of each sample can also vary -
they can be big endian byte integers, little endian byte integers, or
floating point numbers.</p>
<p>Musically, the sample size determines the dynamic range. The
dynamic range is the difference between the quietest and the
loudest signal that can be reproduced.</p>
</dd>
<dt>Frame</dt><dd><p>A frame consists of exactly one sample per channel. If there is only one
channel (Mono sound) a frame is simply a single sample. If the sound is
stereo, each frame consists of two samples, etc.</p>
</dd>
<dt>Frame size</dt><dd><p>This is the size in bytes of each frame. This can vary a lot: if each sample
is 8 bits, and we’re handling mono sound, the frame size is one byte.
For six channel audio with 64 bit floating point samples, the frame size
is 48 bytes.</p>
</dd>
<dt>Rate</dt><dd><p>PCM sound consists of a flow of sound frames. The sound rate controls how
often the current frame is replaced. For example, a rate of 8000 Hz
means that a new frame is played or captured 8000 times per second.</p>
</dd>
<dt>Data rate</dt><dd><p>This is the number of bytes which must be consumed or provided per
second at a certain frame size and rate.</p>
<p>8000 Hz mono sound with 8 bit (1 byte) samples has a data rate of
8000 * 1 * 1 = 8 kb/s or 64kbit/s. This is typically used for telephony.</p>
<p>At the other end of the scale, 96000 Hz, 6 channel sound with 64
bit (8 bytes) samples has a data rate of 96000 * 6 * 8 = 4608
kb/s (almost 5 MB sound data per second).</p>
<p>If the data rate requirement is not met, an overrun (on capture) or
underrun (on playback) occurs; the term “xrun” is used to refer to
either event.</p>
</dd>
</dl>
<dl id="term-period">
<dt>Period</dt><dd><p>The CPU processes sample data in chunks of frames, so-called periods
(also called fragments by some systems). The operating system kernel’s
sample buffer must hold at least two periods (at any given time, one
is processed by the sound hardware, and one by the CPU).</p>
<p>The completion of a <em>period</em> triggers a CPU interrupt, which causes
processing and context switching overhead. Therefore, a smaller period
size causes higher CPU resource usage at a given data rate.</p>
<p>A bigger size of the <em>buffer</em> improves the system’s resilience to xruns.
The buffer being split into a bigger number of smaller periods also does
that, as it allows it to be drained / topped up sooner.</p>
<p>On the other hand, a bigger size of the <em>buffer</em> also increases the
playback latency, that is, the time it takes for a frame from being
sent out by the application to being actually audible.</p>
<p>Similarly, a bigger <em>period</em> size increases the capture latency.</p>
<p>The trade-off between latency, xrun resilience, and resource usage
must be made depending on the application.</p>
</dd>
<dt>Period size</dt><dd><p>This is the size of each period in frames. <em>Not bytes, but frames!</em>
In <a class="reference internal" href="libalsaaudio.html#module-alsaaudio" title="alsaaudio (Linux)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">alsaaudio</span></code></a> the period size is set directly, and it is
therefore important to understand the significance of this
number. If the period size is configured to for example 32,
each write should contain exactly 32 frames of sound data, and each
read will return either 32 frames of data or nothing at all.</p>
</dd>
</dl>
<p>Once you understand these concepts, you will be ready to use the PCM API. Read
on.</p>
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