4.1 PCM Terminology and Concepts

From 2876869020584497e61a340d4ca3c4c8a8cbde7f Mon Sep 17 00:00:00 2001 From: Walter Bender Date: Sat, 21 Feb 2009 12:43:42 +0000 Subject: undoing pyalsaaudio patch --- (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 deleted file mode 100644 index f58333a..0000000 --- a/arch_src/pyalsaaudio-0.2/doc/node7.html +++ /dev/null @@ -1,169 +0,0 @@ - - - - - - - - - - - - - -4.1 PCM Terminology and Concepts - - - - - -

-4.1 PCM Terminology and Concepts -

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-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. - -

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

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