4.1 PCM Terminology and Concepts

From b0b6de5e6c61fe92452bf057d2f0677a3d1fec9b Mon Sep 17 00:00:00 2001 From: Walter Bender Date: Fri, 20 Feb 2009 16:12:44 +0000 Subject: silbe patch to build alsaaudio on first launch --- (limited to 'arch_src/pyalsaaudio-0.2/doc/node7.html') diff --git a/arch_src/pyalsaaudio-0.2/doc/node7.html b/arch_src/pyalsaaudio-0.2/doc/node7.html new file mode 100644 index 0000000..f58333a --- /dev/null +++ b/arch_src/pyalsaaudio-0.2/doc/node7.html @@ -0,0 +1,169 @@ + + + + + + + + + + + + + +4.1 PCM Terminology and Concepts + + + + + +

+4.1 PCM Terminology and Concepts +

+ +

+In order to use PCM devices it is useful to be familiar with some concepts and +terminology. + +

Sample

PCM audio, whether it is input or output, consists at the lowest level +of a number of single samples. A sample represents the sound in a single channel in +a brief interval. If more than one channel is in use, more than one sample is required +for each interval to describe the sound. Samples can be of many different sizes, ranging +from 8 bit to 64 bit presition. The specific format of each sample can also vary - they +can be big endian byte order, little endian byte order, or even floats. + +

Frame

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. + +

Frame size

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. Similarly in 6 channel audio with +64 bit floating point samples, the frame size is 48 bytes + +

Rate

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. + +

Data rate

This is the number of bytes, which must be recorded or provided per second +at a certain frame size and rate. + +

+8000 Hz mono sound with 8 bit (1 byte) samples has a data rate of 8000 * 1 * 1 = 8 kb/s + +

+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) + +

Period

When the hardware processes data this is done in chunks of frames. The time interval +between each processing (A/D or D/A conversion) is known as the period. The size of the period has +direct implication on the latency of the sound input or output. For low-latency the period size should +be very small, while low CPU resource usage would usually demand larger period sizes. With ALSA, the +CPU utilization is not impacted much by the period size, since the kernel layer buffers multiple +periods internally, so each period generates an interrupt and a memory copy, but userspace can be +slower and read or write multiple periods at the same time. + +

Period size

This is the size of each period in Hz. Not bytes, but Hz!. In alsaaudio +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. + +

+ +

+Once you understand these concepts, you will be ready to actually utilize PCM API. Read on. + +

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