221 lines
4.9 KiB
HTML
221 lines
4.9 KiB
HTML
<HTML
|
|
><HEAD
|
|
><TITLE
|
|
>Sound Card Technology</TITLE
|
|
><META
|
|
NAME="GENERATOR"
|
|
CONTENT="Modular DocBook HTML Stylesheet Version 1.63
|
|
"><LINK
|
|
REL="HOME"
|
|
TITLE="The Linux Sound HOWTO"
|
|
HREF="index.html"><LINK
|
|
REL="PREVIOUS"
|
|
TITLE="Introduction"
|
|
HREF="x24.html"><LINK
|
|
REL="NEXT"
|
|
TITLE="Supported Hardware"
|
|
HREF="x96.html"></HEAD
|
|
><BODY
|
|
CLASS="SECT1"
|
|
BGCOLOR="#FFFFFF"
|
|
TEXT="#000000"
|
|
LINK="#0000FF"
|
|
VLINK="#840084"
|
|
ALINK="#0000FF"
|
|
><DIV
|
|
CLASS="NAVHEADER"
|
|
><TABLE
|
|
WIDTH="100%"
|
|
BORDER="0"
|
|
CELLPADDING="0"
|
|
CELLSPACING="0"
|
|
><TR
|
|
><TH
|
|
COLSPAN="3"
|
|
ALIGN="center"
|
|
>The Linux Sound HOWTO</TH
|
|
></TR
|
|
><TR
|
|
><TD
|
|
WIDTH="10%"
|
|
ALIGN="left"
|
|
VALIGN="bottom"
|
|
><A
|
|
HREF="x24.html"
|
|
>Prev</A
|
|
></TD
|
|
><TD
|
|
WIDTH="80%"
|
|
ALIGN="center"
|
|
VALIGN="bottom"
|
|
></TD
|
|
><TD
|
|
WIDTH="10%"
|
|
ALIGN="right"
|
|
VALIGN="bottom"
|
|
><A
|
|
HREF="x96.html"
|
|
>Next</A
|
|
></TD
|
|
></TR
|
|
></TABLE
|
|
><HR
|
|
ALIGN="LEFT"
|
|
WIDTH="100%"></DIV
|
|
><DIV
|
|
CLASS="SECT1"
|
|
><H1
|
|
CLASS="SECT1"
|
|
><A
|
|
NAME="AEN71"
|
|
>2. Sound Card Technology</A
|
|
></H1
|
|
><P
|
|
>This section gives a <EM
|
|
>very</EM
|
|
> cursory overview of
|
|
computer audio technology, in order to help you understand
|
|
the concepts used later in the document. You should consult a book on
|
|
digital audio or digital signal processing in order to learn more.</P
|
|
><P
|
|
>Sound is an <EM
|
|
>analog</EM
|
|
> property; it can take on any value over a
|
|
continuous range. Computers are <EM
|
|
>digital</EM
|
|
>; they like to work
|
|
with discrete values. Sound cards use a device known as an <EM
|
|
>Analog
|
|
to Digital Converter</EM
|
|
> (A/D or ADC) to convert voltages
|
|
corresponding to analog sound waves into digital or numeric values
|
|
which can be stored in memory. Similarly, a <EM
|
|
>Digital to Analog
|
|
Converter</EM
|
|
> (D/A or DAC) converts numeric values back to an analog
|
|
voltage which can in turn drive a loudspeaker, producing sound.</P
|
|
><P
|
|
>The process of analog to digital conversion, known as
|
|
sampling, introduces some error. Two factors are key in determining
|
|
how well the sampled signal represents the original. <EM
|
|
>Sampling
|
|
rate</EM
|
|
> is the number of samples made per unit of time (usually
|
|
expresses as samples per second or Hertz). A low sampling
|
|
rate will provide a less accurate representation of the analog
|
|
signal. Sample size is the range of values used to represent each
|
|
sample, usually expressed in bits. The larger the sample size, the
|
|
more accurate the digitized signal will be.</P
|
|
><P
|
|
>Sound cards commonly use 8 or 16 bit samples at sampling rates from
|
|
about 4000 to 44,000 samples per second. The samples may also be
|
|
contain one channel (mono) or two (stereo).</P
|
|
><P
|
|
><EM
|
|
>FM Synthesis</EM
|
|
> is an older technique for producing
|
|
sound. It is based on combining different waveforms (e.g. sine,
|
|
triangle, square). FM synthesis is simpler to implement in
|
|
hardware that D/A conversion, but is more difficult to program and
|
|
less flexible. Many sound cards provide FM synthesis for backward
|
|
compatibility with older cards and software. Several independent sound
|
|
generators or <EM
|
|
>voices</EM
|
|
> are usually provided.</P
|
|
><P
|
|
><EM
|
|
>Wavetable Synthesis</EM
|
|
> combines the flexibility of
|
|
D/A conversion with the multiple channel capability of FM
|
|
synthesis. With this scheme digitized voices can be downloaded into
|
|
dedicated memory, and then played, combined, and modified with little
|
|
CPU overhead. State of the art sound cards all support wavetable
|
|
synthesis.</P
|
|
><P
|
|
>Most sound cards provide the capability of <EM
|
|
>mixing</EM
|
|
>, combining
|
|
signals from different input sources and controlling gain levels.</P
|
|
><P
|
|
><EM
|
|
>MIDI</EM
|
|
> stands for Musical Instrument Digital
|
|
Interface, and is a standard hardware and software protocol for
|
|
allowing musical instruments to communicate with each other. The
|
|
events sent over a MIDI bus can also be stored as MIDI files for later
|
|
editing and playback. Many sound cards provide a MIDI interface. Those
|
|
that do not can still play MIDI files using the on-board capabilities
|
|
of the sound card.</P
|
|
><P
|
|
><EM
|
|
>MOD files</EM
|
|
> are a common format for computer
|
|
generated songs. As well as information about the musical notes to be
|
|
played, the files contain digitized samples for the instruments (or
|
|
voices). MOD files originated on the Amiga computer, but can be played
|
|
on other systems, including Linux, with suitable software.</P
|
|
><P
|
|
><EM
|
|
>MP3 files</EM
|
|
> are a popular format for distributing
|
|
computer music and speech. MP3 uses a sophisticated encoding scheme
|
|
(MPEG layer 3) to compress audio by roughly a factor of 10 with little
|
|
reduction in quality as compared to CD audio.</P
|
|
></DIV
|
|
><DIV
|
|
CLASS="NAVFOOTER"
|
|
><HR
|
|
ALIGN="LEFT"
|
|
WIDTH="100%"><TABLE
|
|
WIDTH="100%"
|
|
BORDER="0"
|
|
CELLPADDING="0"
|
|
CELLSPACING="0"
|
|
><TR
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="left"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="x24.html"
|
|
>Prev</A
|
|
></TD
|
|
><TD
|
|
WIDTH="34%"
|
|
ALIGN="center"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="index.html"
|
|
>Home</A
|
|
></TD
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="right"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="x96.html"
|
|
>Next</A
|
|
></TD
|
|
></TR
|
|
><TR
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="left"
|
|
VALIGN="top"
|
|
>Introduction</TD
|
|
><TD
|
|
WIDTH="34%"
|
|
ALIGN="center"
|
|
VALIGN="top"
|
|
> </TD
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="right"
|
|
VALIGN="top"
|
|
>Supported Hardware</TD
|
|
></TR
|
|
></TABLE
|
|
></DIV
|
|
></BODY
|
|
></HTML
|
|
> |