1998-08-13 15:42:56 +00:00
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<TITLE>Advanced Linux Sound Architecture - Library API: Mixer Interface</TITLE>
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<A HREF="soundapi-3.html">Previous</A>
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<A HREF="soundapi.html#toc4">Table of Contents</A>
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<HR>
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<H2><A NAME="s4">4. Mixer Interface</A></H2>
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<P>The Mixer Interface allows applications to change the volume level of
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a soundcard's input/output channels in both the linear range (0-100)
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and in decibels. It also supports features like hardware mute, input
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sound source, etc.</P>
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<H2><A NAME="ss4.1">4.1 Low-Level Layer</A></H2>
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<P>Mixer devices aren't opened exclusively. This allows applications to
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open a device multiple times with one or more processes.</P>
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<H3>int snd_mixer_open( void **handle, int card, int device ) </H3>
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<P>Creates new handle and opens a connection to the kernel sound
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mixer interface for soundcard number <I>card</I> (0-N) and mixer
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device number <I>device</I>. Also checks if protocol is
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compatible to prevent use of old programs with new kernel API. Function
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returns zero if successful, otherwise it returns an error code.</P>
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<H3>int snd_mixer_close( void *handle ) </H3>
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<P>Frees all resources allocated to the mixer handle and
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closes its connection to the kernel sound mixer interface. Function
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returns zero if successful, otherwise it returns an error code.</P>
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<H3>int snd_mixer_file_descriptor( void *handle ) </H3>
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<P>Returns the file descriptor for the connection to the kernel sound
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mixer interface. This function should be used only in very
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special cases. Function returns a negative error code if an
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error was encountered. </P>
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<P>The file descriptor should be used for the <I>select</I> synchronous
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multiplexer function for deterimeing read direction. Applications should
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call <I>snd_mixer_read</I> function if some data is waiting to be read.
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It is recomended that you do this, since it leaves place for this function
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to handle some new kernel API specifications.</P>
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<H3>int snd_mixer_channels( void *handle ) </H3>
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<P>Returns the count of mixer channels for appropriate mixer device, otherwise
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the return value is negative, and signifies an error code. Never returns
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zero.</P>
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<H3>int snd_mixer_info( void *handle, snd_mixer_info_t *info ) </H3>
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<P>Fills the *info structure with information about the mixer associated with
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*handle. Returns zero if successful, otherwise it returns an error code.
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<HR>
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<PRE>
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#define SND_MIXER_INFO_CAP_EXCL_RECORD 0x00000001
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struct snd_mixer_info {
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unsigned int type; /* type of soundcard - SND_CARD_TYPE_XXXX */
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unsigned int channels; /* count of mixer devices */
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unsigned int caps; /* some flags about this device (SND_MIXER_INFO_CAP_XXXX) */
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unsigned char id[32]; /* ID of this mixer */
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unsigned char name[80]; /* name of this device */
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char reserved[ 32 ]; /* reserved for future use */
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};
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</PRE>
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<HR>
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</P>
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<H3>int snd_mixer_channel( void *handle, const char *channel_id ) </H3>
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<P>Returns the channel number (index) associated with channel_id (channel name),
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or returns an error code.
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<HR>
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<PRE>
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#define SND_MIXER_ID_MASTER "Master"
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#define SND_MIXER_ID_BASS "Bass"
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#define SND_MIXER_ID_TREBLE "Treble"
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#define SND_MIXER_ID_SYNTHESIZER "Synth"
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#define SND_MIXER_ID_SYNTHESIZER1 "Synth 1"
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#define SND_MIXER_ID_FM "FM"
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#define SND_MIXER_ID_EFFECT "Effect"
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#define SND_MIXER_ID_PCM "PCM"
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#define SND_MIXER_ID_PCM1 "PCM 1"
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#define SND_MIXER_ID_LINE "Line-In"
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#define SND_MIXER_ID_MIC "MIC"
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#define SND_MIXER_ID_CD "CD"
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#define SND_MIXER_ID_GAIN "Record-Gain"
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#define SND_MIXER_ID_IGAIN "In-Gain"
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#define SND_MIXER_ID_OGAIN "Out-Gain"
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#define SND_MIXER_ID_LOOPBACK "Loopback"
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#define SND_MIXER_ID_SPEAKER "PC Speaker"
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#define SND_MIXER_ID_AUXA "Aux A"
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#define SND_MIXER_ID_AUXB "Aux B"
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#define SND_MIXER_ID_AUXC "Aux C"
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</PRE>
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<HR>
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</P>
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<H3>int snd_mixer_exact_mode( void *handle, int enable ) </H3>
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<P>Turns on or off (by default) exact mode. This mode allows to application
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set/get volume values in exact range which uses hardware. In non-exact
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mode is range always from 0 to 100 and conversion to hardware range does
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driver. Function returns zero if successful, otherwise it returns an error
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code.</P>
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<H3>int snd_mixer_channel_info( void *handle, int channel, snd_mixer_channel_info_t *info ) </H3>
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<P>Fills the *info structure. The argument <I>channel</I> specifies channel
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(0 to N) for which is the info requested. Function returns zero if
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successful, otherwise it returns an error code.
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<HR>
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<PRE>
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#define SND_MIXER_CINFO_CAP_RECORD 0x00000001
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#define SND_MIXER_CINFO_CAP_STEREO 0x00000002
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#define SND_MIXER_CINFO_CAP_MUTE 0x00000004
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#define SND_MIXER_CINFO_CAP_HWMUTE 0x00000008 /* channel supports hardware mute */
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#define SND_MIXER_CINFO_CAP_DIGITAL 0x00000010 /* channel does digital (not analog) mixing */
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#define SND_MIXER_CINOF_CAP_INPUT 0x00000020 /* input channel */
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struct snd_mixer_channel_info {
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unsigned int channel; /* channel # (filled by application) */
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unsigned int parent; /* parent channel # or SND_MIXER_PARENT */
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unsigned char name[12]; /* name of this device */
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unsigned int caps; /* some flags about this device (SND_MIXER_CINFO_XXXX) */
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int min; /* min. value when exact mode (or always 0) */
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int max; /* max. value when exact mode (or always 100) */
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int min_dB; /* minimum decibel value (*100) */
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int max_dB; /* maximum decibel value (*100) */
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int step_dB; /* step decibel value (*100) */
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unsigned char reserved[16];
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};
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</PRE>
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<HR>
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</P>
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<H3>int snd_mixer_channel_read( void *handle, int channel, snd_mixer_channel_t *data ) </H3>
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<P>Fills the *data structure. The argument <I>channel</I> specifies
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the channel (0 to N) for which is data requested. Function returns
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zero if successful, otherwise it returns an error code.
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<HR>
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<PRE>
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#define SND_MIXER_FLG_RECORD 0x00000001 /* channel record source flag */
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#define SND_MIXER_FLG_MUTE_LEFT 0x00010000
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#define SND_MIXER_FLG_MUTE_RIGHT 0x00020000
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#define SND_MIXER_FLG_MUTE 0x00030000
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#define SND_MIXER_FLG_DECIBEL 0x40000000
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#define SND_MIXER_FLG_FORCE 0x80000000
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struct snd_mixer_channel {
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unsigned int channel; /* channel # (filled by application) */
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unsigned int flags; /* some flags to read/write (SND_MIXER_FLG_XXXX) */
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int left; /* min - max when exact mode (or 0 - 100) */
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int right; /* min - max when exact mode (or 0 - 100) */
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int left_dB; /* dB * 100 */
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int right_dB; /* dB * 100 */
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unsigned char reserved[16];
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};
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</PRE>
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<HR>
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</P>
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<P>
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<DL>
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<DT><B>SND_MIXER_FLG_RECORD</B><DD><P>Record source flag.</P>
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<DT><B>SND_MIXER_FLG_DECIBEL</B><DD><P>If this bit is set, driver set volume from dB variables <I>left_dB</I>
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and <I>right_dB</I>.</P>
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<DT><B>SND_MIXER_FLG_FORCE</B><DD><P>Force set - this bit shouldn't be used from user space. Reserved for
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kernel.</P>
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</DL>
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</P>
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<H3>int snd_mixer_channel_write( void *handle, int channel, snd_mixer_channel_t *data ) </H3>
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<P>Writes the *data structure to kernel. The <I>channel</I> argument
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specifies the channel (0 to N) for which is data is to be applied.
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Function returns zero if successful, otherwise it returns an error code.
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This functions is the opposite of <I>snd_mixer_channel_read</I>.</P>
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<H3>int snd_mixer_special_read( void *handle, snd_mixer_special_t *special ) </H3>
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<P>Not documented...</P>
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<H3>int snd_mixer_special_write( void *handle, snd_mixer_special_t *special ) </H3>
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<P>Not documented...</P>
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<H3>int snd_mixer_read( void *handle, snd_mixer_callbacks_t *callbacks ) </H3>
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<P>This function reads and parses data from driver. Parsed actions are returned
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back to the application using the <I>callbacks</I> structure. Applications
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should not parse data from the driver in standard cases. This function
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returns immediately after all data is read from driver. Does not
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block process.
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<HR>
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<PRE>
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typedef struct snd_mixer_callbacks {
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void *private_data; /* should be used by application */
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void (*channel_was_changed)( void *private_data, int channel );
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void *reserved[15]; /* reserved for future use - must be NULL!!! */
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} snd_mixer_callbacks_t;
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</PRE>
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<HR>
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</P>
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<H2><A NAME="ss4.2">4.2 Examples</A></H2>
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<P>The following example shows installed mixer channels for soundcard #0 and
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mixer device #0 in the system, and also sets the master volume (if present)
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to 50.</P>
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<P>
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<BLOCKQUOTE><CODE>
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<HR>
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<PRE>
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int card = 0, device = 0, err;
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void *handle;
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snd_mixer_info_t info;
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snd_mixer_channel_t channel;
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1998-08-30 21:08:44 +00:00
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if ( (err = snd_mixer_open( &handle, card, device )) < 0 ) {
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fprintf( stderr, "open failed: %s\n", snd_strerror( err ) );
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return;
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}
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1998-08-30 21:08:44 +00:00
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if ( (err = snd_mixer_info( handle, &info )) < 0 ) {
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fprintf( stderr, "info failed: %s\n", snd_strerror( err ) );
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snd_mixer_close( handle );
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return;
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}
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printf( "Installed MIXER channels for card #i and device %i: %i\n",
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card + 1, device, info.channels );
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master = snd_mixer_channel( handle, SND_MIXER_ID_MASTER );
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if ( master >= 0 ) {
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1998-08-30 21:08:44 +00:00
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if ( (err = snd_mixer_read( handle, master, &channel )) < 0 ) {
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1998-08-13 15:42:56 +00:00
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fprintf( stderr, "master read failed: %s\n", snd_strerror( err ) );
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snd_mixer_close( handle );
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return;
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}
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channel -> left = channel -> right = 50;
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if ( (err = snd_mixer_write( handle, master, &channel )) < 0 ) {
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fprintf( stderr, "master write failed: %s\n", snd_strerror( err ) );
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snd_mixer_close( handle );
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return;
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}
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}
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snd_mixer_close( handle );
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</PRE>
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<HR>
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</CODE></BLOCKQUOTE>
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</P>
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<HR>
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<A HREF="soundapi-3.html">Previous</A>
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<A HREF="soundapi-5.html">Next</A>
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<A HREF="soundapi.html#toc4">Table of Contents</A>
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