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alsa-lib/doc/soundapi.txt
Jaroslav Kysela 5abac67626 Initial revision
1998-08-13 15:42:56 +00:00

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Advanced Linux Sound Architecture - Library API
Jaroslav Kysela <perex@jcu.cz> with assistance from Alan
Robinson
v0.0.3, 25 March 1998
This document describes, in full detail, the Advanced Linux Sound
Architecture library API.
______________________________________________________________________
Table of Contents:
1. Introduction
2. Error Codes
2.1. Error Codes in Detail
2.2. Functions
2.2.1. const char *snd_strerror( int errnum )
3. Control Interface
3.1. Low-Level Layer
3.1.1. int snd_cards( void )
3.1.2. unsigned int snd_cards_mask( void )
3.1.3. int snd_ctl_open( void **handle, int card )
3.1.4. int snd_ctl_close( void *handle )
3.1.5. int snd_ctl_file_descriptor( void *handle )
3.1.6. int snd_ctl_hw_info( void *handle, struct snd_ctl_hw_info
*info )
3.1.7. int snd_ctl_pcm_info( void *handle, int dev, snd_pcm_info_t
*info )
3.1.8. int snd_ctl_pcm_playback_info( void *handle, int dev,
snd_pcm_playback_info_t *info )
3.1.9. int snd_ctl_pcm_record_info( void *handle, int dev,
snd_pcm_record_info_t *info )
3.1.10. int snd_ctl_mixer_info( void *handle, int dev,
snd_mixer_info_t *info )
3.2. Examples
4. Mixer Interface
4.1. Low-Level Layer
4.1.1. int snd_mixer_open( void **handle, int card, int device )
4.1.2. int snd_mixer_close( void *handle )
4.1.3. int snd_mixer_file_descriptor( void *handle )
4.1.4. int snd_mixer_channels( void *handle )
4.1.5. int snd_mixer_info( void *handle, snd_mixer_info_t *info )
4.1.6. int snd_mixer_channel( void *handle, const char *channel_id )
4.1.7. int snd_mixer_exact_mode( void *handle, int enable )
4.1.8. int snd_mixer_channel_info( void *handle, int channel,
snd_mixer_channel_info_t *info )
4.1.9. int snd_mixer_channel_read( void *handle, int channel,
snd_mixer_channel_t *data )
4.1.10. int snd_mixer_channel_write( void *handle, int channel,
snd_mixer_channel_t *data )
4.1.11. int snd_mixer_special_read( void *handle, snd_mixer_special_t
*special )
4.1.12. int snd_mixer_special_write( void *handle, snd_mixer_special_t
*special )
4.1.13. int snd_mixer_read( void *handle, snd_mixer_callbacks_t
*callbacks )
4.2. Examples
5. Digital Audio (PCM) Interface
5.1. Low-Level Layer
5.1.1. int snd_pcm_open( void **handle, int card, int device, int
mode )
5.1.2. int snd_pcm_close( void *handle )
5.1.3. int snd_pcm_file_descriptor( void *handle )
5.1.4. int snd_pcm_block_mode( void *handle, int enable )
5.1.5. int snd_pcm_info( void *handle, snd_pcm_info_t *info )
5.1.6. int snd_pcm_playback_info( void *handle,
snd_pcm_playback_info_t *info )
5.1.7. int snd_pcm_record_info( void *handle, snd_pcm_record_info_t
*info )
5.1.8. int snd_pcm_playback_format( void *handle, snd_pcm_format_t
*format )
5.1.9. int snd_pcm_record_format( void *handle, snd_pcm_format_t
*format )
5.1.10. int snd_pcm_playback_params( void *handle,
snd_pcm_playback_params_t *params )
5.1.11. int snd_pcm_record_params( void *handle,
snd_pcm_record_params_t *params )
5.1.12. int snd_pcm_playback_status( void *handle,
snd_pcm_playback_status_t *status )
5.1.13. int snd_pcm_record_status( void *handle,
snd_pcm_record_status_t *status )
5.1.14. int snd_pcm_drain_playback( void *handle )
5.1.15. int snd_pcm_flush_playback( void *handle )
5.1.16. int snd_pcm_flush_record( void *handle )
5.1.17. int snd_pcm_playback_time( void *handle, int enable )
5.1.18. int snd_pcm_record_time( void *handle, int enable )
5.1.19. ssize_t snd_pcm_write( void *handle, const void *buffer,
size_t size )
5.1.20. ssize_t snd_pcm_read( void *handle, void *buffer, size_t size
)
5.2. Examples
______________________________________________________________________
11.. IInnttrroodduuccttiioonn
The Advanced Linux Sound Architecture comes with a kernel API &
library API. This document describes the library API and how it
interfaces with the kernel API. The kernal API will probably never be
documented in standalone form.
Application programmers should use the library API rather than kernel
API. The Library offers 100% of the functionally of the kernel API,
but add next major improvements in usability, making the application
code simpler and better looking. In addition, some of the some
fixes/compatibility code in, may be placed in the library code instead
of the kernel driver.
For a complete list of all variables and functions in the API you
should look at the following header files:
1. /usr/include/sys/soundlib.h
2. /usr/include/linux/sound.h
3. /usr/include/linux/sounddetect.h
22.. EErrrroorr CCooddeess
All functions return int (or some sort of signed value). If this value
is negative it represents an error code. Codes up to SND_ERROR_BEGIN
(500000) represents standard system errors. Codes equal or greather
than this value represents sound library API errors. All error codes
begin with the prefix _S_N_D___E_R_R_O_R__.
22..11.. EErrrroorr CCooddeess iinn DDeettaaiill
SSNNDD__EERRRROORR__UUNNCCOOMMPPAATTIIBBLLEE__VVEERRSSIIOONN ((550000000000))
This error is caused if the driver uses an incompatible kernel
API for this interface and hence the library doesn't know how
this API can be used.
22..22.. FFuunnccttiioonnss
22..22..11..
ccoonnsstt cchhaarr **ssnndd__ssttrreerrrroorr(( iinntt eerrrrnnuumm ))
This functions converts error code to a string. Its functionality is
the same as the _s_t_r_e_r_r_o_r function from the standard C library, but
this function returns correct strings for sound error codes, too.
33.. CCoonnttrrooll IInntteerrffaaccee
The control interfaces gives application various information about the
currently installed sound driver in the system. The interface should
be used to detect if another sound interface is present for selected
soundcard or, for example, to create a list of devices (MIXER, PCM
etc) from which the user can select.
33..11.. LLooww--LLeevveell LLaayyeerr
33..11..11..
iinntt ssnndd__ccaarrddss(( vvooiidd ))
Returns the number of soundcards present in the system, if any.
Otherwise it returns a negative value, which maps to an error code.
This function will return 0 if no soundcards are detected.
33..11..22..
uunnssiiggnneedd iinntt ssnndd__ccaarrddss__mmaasskk(( vvooiidd ))
Returns the bitmap of soundcards present in the system, if any.
Otherwise it returns a negative value, which maps to an error code.
This function will return 0 if no soundcards are detected. First
soundcard is represented with bit 0.
33..11..33..
iinntt ssnndd__ccttll__ooppeenn(( vvooiidd ****hhaannddllee,, iinntt ccaarrdd ))
Creates a new handle and opens communication with the kernel sound
control interface for soundcard number _c_a_r_d (0-N). The function also
checks if protocol is compatible, so as to prevent the use of old
programs with a new kernel API. Function returns zero if successful,
otherwise an error code is returned.
33..11..44..
iinntt ssnndd__ccttll__cclloossee(( vvooiidd **hhaannddllee ))
Function frees all resources allocated with control handle and closes
the kernel sound control interface. Function returns zero if
successful, otherwise it returns an error code.
33..11..55..
iinntt ssnndd__ccttll__ffiillee__ddeessccrriippttoorr(( vvooiidd **hhaannddllee ))
Function returns file descriptor for the kernel sound control
interface. This function should be used in very special cases.
Function returns a negative error code if some error was encountered.
33..11..66..
iinntt ssnndd__ccttll__hhww__iinnffoo(( vvooiidd **hhaannddllee,, ssttrruucctt ssnndd__ccttll__hhww__iinnffoo **iinnffoo ))
Fills the info structure with data about the sound hardware referenced
by handle. Function returns zero if successful, otherwise it returns
an error code.
______________________________________________________________________
#define SND_CTL_GCAPS_MIDI 0x0000001 /* driver has MIDI interface */
#define SND_CTL_LCAPS_SYNTH 0x0000001 /* soundcard has synthesizer */
#define SND_CTL_LCAPS_RAWFM 0x0000002 /* soundcard has RAW FM/OPL3 */
struct snd_ctl_hw_info {
unsigned int type; /* type of card - see SND_CARD_TYPE_XXXX */
unsigned int gcaps; /* see SND_CTL_GCAPS_XXXX */
unsigned int lcaps; /* see SND_CTL_LCAPS_XXXX */
unsigned int pcmdevs; /* count of PCM devices (0 to N) */
unsigned int mixerdevs; /* count of MIXER devices (0 to N) */
unsigned int mididevs; /* count of raw MIDI devices (0 to N) */
char id[8]; /* ID of card (user selectable) */
char name[80]; /* name/info text about soundcard */
unsigned char reserved[128]; /* reserved for future use */
};
______________________________________________________________________
33..11..77..
iinntt ssnndd__ccttll__ppccmm__iinnffoo(( vvooiidd **hhaannddllee,, iinntt ddeevv,, ssnndd__ppccmm__iinnffoo__tt **iinnffoo ))
Fills the *info structure with data about the PCM device. Function
returns zero if successful, otherwise it returns an error code.
Details about the snd_pcm_info_t structure are in the DDiiggiittaall AAuuddiioo
((PPCCMM)) IInntteerrffaaccee section. The argument _d_e_v selects the device number
for the soundcard referenced by *handle. Its range is 0 to N where N
is _s_t_r_u_c_t _s_n_d___c_t_l___h_w___i_n_f_o _-_> _p_c_m_d_e_v_s _- _1. This function will work if
the selected PCM device is busy, too. It should be used to collect
information about PCM devices without exclusive lock.
33..11..88..
iinntt ssnndd__ccttll__ppccmm__ppllaayybbaacckk__iinnffoo(( vvooiidd **hhaannddllee,, iinntt ddeevv,, ssnndd__ppccmm__ppllaayy--
bbaacckk__iinnffoo__tt **iinnffoo ))
Fills the *info structure with data about the PCM device and playback
direction. Function returns zero if successful, otherwise it returns
an error code. Details about the snd_pcm_playback_info_t structure
are in the DDiiggiittaall AAuuddiioo ((PPCCMM)) IInntteerrffaaccee section. The argument _d_e_v
selects the device number for the soundcard referenced by *handle. Its
range is 0 to N where N is _s_t_r_u_c_t _s_n_d___c_t_l___h_w___i_n_f_o _-_> _p_c_m_d_e_v_s _- _1.
This function will work if the selected PCM device is busy, too. It
should be used to collect information about PCM devices without
exclusive lock.
33..11..99..
iinntt ssnndd__ccttll__ppccmm__rreeccoorrdd__iinnffoo(( vvooiidd **hhaannddllee,, iinntt ddeevv,,
ssnndd__ppccmm__rreeccoorrdd__iinnffoo__tt **iinnffoo ))
Fills the *info structure with data about the PCM device and record
direction. Function returns zero if successful, otherwise it returns
an error code. Details about the snd_pcm_record_info_t structure are
in the DDiiggiittaall AAuuddiioo ((PPCCMM)) IInntteerrffaaccee section. The argument _d_e_v selects
the device number for the soundcard referenced by *handle. Its range
is 0 to N where N is _s_t_r_u_c_t _s_n_d___c_t_l___h_w___i_n_f_o _-_> _p_c_m_d_e_v_s _- _1. This
function will work if the selected PCM device is busy, too. It should
be used to collect information about PCM devices without exclusive
lock.
33..11..1100..
iinntt ssnndd__ccttll__mmiixxeerr__iinnffoo(( vvooiidd **hhaannddllee,, iinntt ddeevv,, ssnndd__mmiixxeerr__iinnffoo__tt **iinnffoo
))
Fills the *info structure with data about the mixer device. Returns
zero if successful, otherwise it returns an error code. Details about
the snd_mixer_info_t structure are in the MMiixxeerr IInntteerrffaaccee section.
The argument _d_e_v specifies the device number for the appropriate
soundcard. Its range is 0 to N where N found from _s_t_r_u_c_t
_s_n_d___c_t_l___h_w___i_n_f_o _-_> _m_i_x_e_r_d_e_v_s _- _1. It should be used to collect
information about mixer devices.
33..22.. EExxaammpplleess
The following example shows how all PCM devices can be detected for
the first soundcard (#0) in the system.
______________________________________________________________________
int card = 0, err;
void *handle;
stuct snd_ctl_hw_info info;
if ( (err = snd_ctl_open( &handle, card )) < 0 ) {
fprintf( stderr, "open failed: %s\n", snd_strerror( err ) );
return;
}
if ( (err = snd_ctl_hw_info( handle, &info )) < 0 ) {
fprintf( stderr, "hw info failed: %s\n", snd_strerror( err ) );
snd_ctl_close( handle );
return;
}
printf( "Installed PCM devices for card #i: %i\n", card + 1, info.pcmdevs );
snd_ctl_close( handle );
______________________________________________________________________
44.. MMiixxeerr IInntteerrffaaccee
The Mixer Interface allows applications to change the volume level of
a soundcard's input/output channels in both the linear range (0-100)
and in decibels. It also supports features like hardware mute, input
sound source, etc.
44..11.. LLooww--LLeevveell LLaayyeerr
Mixer devices aren't opened exclusively. This allows applications to
open a device multiple times with one or more processes.
44..11..11..
iinntt ssnndd__mmiixxeerr__ooppeenn(( vvooiidd ****hhaannddllee,, iinntt ccaarrdd,, iinntt ddeevviiccee ))
Creates new handle and opens a connection to the kernel sound mixer
interface for soundcard number _c_a_r_d (0-N) and mixer device number
_d_e_v_i_c_e. Also checks if protocol is compatible to prevent use of old
programs with new kernel API. Function returns zero if successful,
otherwise it returns an error code.
44..11..22..
iinntt ssnndd__mmiixxeerr__cclloossee(( vvooiidd **hhaannddllee ))
Frees all resources allocated to the mixer handle and closes its
connection to the kernel sound mixer interface. Function returns zero
if successful, otherwise it returns an error code.
44..11..33..
iinntt ssnndd__mmiixxeerr__ffiillee__ddeessccrriippttoorr(( vvooiidd **hhaannddllee ))
Returns the file descriptor for the connection to the kernel sound
mixer interface. This function should be used only in very special
cases. Function returns a negative error code if an error was
encountered.
The file descriptor should be used for the _s_e_l_e_c_t synchronous
multiplexer function for deterimeing read direction. Applications
should call _s_n_d___m_i_x_e_r___r_e_a_d function if some data is waiting to be
read. It is recomended that you do this, since it leaves place for
this function to handle some new kernel API specifications.
44..11..44..
iinntt ssnndd__mmiixxeerr__cchhaannnneellss(( vvooiidd **hhaannddllee ))
Returns the count of mixer channels for appropriate mixer device,
otherwise the return value is negative, and signifies an error code.
Never returns zero.
44..11..55..
iinntt ssnndd__mmiixxeerr__iinnffoo(( vvooiidd **hhaannddllee,, ssnndd__mmiixxeerr__iinnffoo__tt **iinnffoo ))
Fills the *info structure with information about the mixer associated
with *handle. Returns zero if successful, otherwise it returns an
error code.
______________________________________________________________________
#define SND_MIXER_INFO_CAP_EXCL_RECORD 0x00000001
struct snd_mixer_info {
unsigned int type; /* type of soundcard - SND_CARD_TYPE_XXXX */
unsigned int channels; /* count of mixer devices */
unsigned int caps; /* some flags about this device (SND_MIXER_INFO_CAP_XXXX) */
unsigned char id[32]; /* ID of this mixer */
unsigned char name[80]; /* name of this device */
char reserved[ 32 ]; /* reserved for future use */
};
______________________________________________________________________
44..11..66..
iinntt ssnndd__mmiixxeerr__cchhaannnneell(( vvooiidd **hhaannddllee,, ccoonnsstt cchhaarr **cchhaannnneell__iidd ))
Returns the channel number (index) associated with channel_id (channel
name), or returns an error code.
______________________________________________________________________
#define SND_MIXER_ID_MASTER "Master"
#define SND_MIXER_ID_BASS "Bass"
#define SND_MIXER_ID_TREBLE "Treble"
#define SND_MIXER_ID_SYNTHESIZER "Synth"
#define SND_MIXER_ID_SYNTHESIZER1 "Synth 1"
#define SND_MIXER_ID_FM "FM"
#define SND_MIXER_ID_EFFECT "Effect"
#define SND_MIXER_ID_PCM "PCM"
#define SND_MIXER_ID_PCM1 "PCM 1"
#define SND_MIXER_ID_LINE "Line-In"
#define SND_MIXER_ID_MIC "MIC"
#define SND_MIXER_ID_CD "CD"
#define SND_MIXER_ID_GAIN "Record-Gain"
#define SND_MIXER_ID_IGAIN "In-Gain"
#define SND_MIXER_ID_OGAIN "Out-Gain"
#define SND_MIXER_ID_LOOPBACK "Loopback"
#define SND_MIXER_ID_SPEAKER "PC Speaker"
#define SND_MIXER_ID_AUXA "Aux A"
#define SND_MIXER_ID_AUXB "Aux B"
#define SND_MIXER_ID_AUXC "Aux C"
______________________________________________________________________
44..11..77..
iinntt ssnndd__mmiixxeerr__eexxaacctt__mmooddee(( vvooiidd **hhaannddllee,, iinntt eennaabbllee ))
Turns on or off (by default) exact mode. This mode allows to
application set/get volume values in exact range which uses hardware.
In non-exact mode is range always from 0 to 100 and conversion to
hardware range does driver. Function returns zero if successful,
otherwise it returns an error code.
44..11..88..
iinntt ssnndd__mmiixxeerr__cchhaannnneell__iinnffoo(( vvooiidd **hhaannddllee,, iinntt cchhaannnneell,, ssnndd__mmiixxeerr__cchhaann--
nneell__iinnffoo__tt **iinnffoo ))
Fills the *info structure. The argument _c_h_a_n_n_e_l specifies channel (0
to N) for which is the info requested. Function returns zero if
successful, otherwise it returns an error code.
______________________________________________________________________
#define SND_MIXER_CINFO_CAP_RECORD 0x00000001
#define SND_MIXER_CINFO_CAP_STEREO 0x00000002
#define SND_MIXER_CINFO_CAP_MUTE 0x00000004
#define SND_MIXER_CINFO_CAP_HWMUTE 0x00000008 /* channel supports hardware mute */
#define SND_MIXER_CINFO_CAP_DIGITAL 0x00000010 /* channel does digital (not analog) mixing */
#define SND_MIXER_CINOF_CAP_INPUT 0x00000020 /* input channel */
struct snd_mixer_channel_info {
unsigned int channel; /* channel # (filled by application) */
unsigned int parent; /* parent channel # or SND_MIXER_PARENT */
unsigned char name[12]; /* name of this device */
unsigned int caps; /* some flags about this device (SND_MIXER_CINFO_XXXX) */
int min; /* min. value when exact mode (or always 0) */
int max; /* max. value when exact mode (or always 100) */
int min_dB; /* minimum decibel value (*100) */
int max_dB; /* maximum decibel value (*100) */
int step_dB; /* step decibel value (*100) */
unsigned char reserved[16];
};
______________________________________________________________________
44..11..99..
iinntt ssnndd__mmiixxeerr__cchhaannnneell__rreeaadd(( vvooiidd **hhaannddllee,, iinntt cchhaannnneell,, ssnndd__mmiixxeerr__cchhaann--
nneell__tt **ddaattaa ))
Fills the *data structure. The argument _c_h_a_n_n_e_l specifies the channel
(0 to N) for which is data requested. Function returns zero if
successful, otherwise it returns an error code.
______________________________________________________________________
#define SND_MIXER_FLG_RECORD 0x00000001 /* channel record source flag */
#define SND_MIXER_FLG_MUTE_LEFT 0x00010000
#define SND_MIXER_FLG_MUTE_RIGHT 0x00020000
#define SND_MIXER_FLG_MUTE 0x00030000
#define SND_MIXER_FLG_DECIBEL 0x40000000
#define SND_MIXER_FLG_FORCE 0x80000000
struct snd_mixer_channel {
unsigned int channel; /* channel # (filled by application) */
unsigned int flags; /* some flags to read/write (SND_MIXER_FLG_XXXX) */
int left; /* min - max when exact mode (or 0 - 100) */
int right; /* min - max when exact mode (or 0 - 100) */
int left_dB; /* dB * 100 */
int right_dB; /* dB * 100 */
unsigned char reserved[16];
};
______________________________________________________________________
SSNNDD__MMIIXXEERR__FFLLGG__RREECCOORRDD
Record source flag.
SSNNDD__MMIIXXEERR__FFLLGG__DDEECCIIBBEELL
If this bit is set, driver set volume from dB variables _l_e_f_t___d_B
and _r_i_g_h_t___d_B.
SSNNDD__MMIIXXEERR__FFLLGG__FFOORRCCEE
Force set - this bit shouldn't be used from user space. Reserved
for kernel.
44..11..1100..
iinntt ssnndd__mmiixxeerr__cchhaannnneell__wwrriittee(( vvooiidd **hhaannddllee,, iinntt cchhaannnneell,,
ssnndd__mmiixxeerr__cchhaannnneell__tt **ddaattaa ))
Writes the *data structure to kernel. The _c_h_a_n_n_e_l argument specifies
the channel (0 to N) for which is data is to be applied. Function
returns zero if successful, otherwise it returns an error code. This
functions is the opposite of _s_n_d___m_i_x_e_r___c_h_a_n_n_e_l___r_e_a_d.
44..11..1111..
iinntt ssnndd__mmiixxeerr__ssppeecciiaall__rreeaadd(( vvooiidd **hhaannddllee,, ssnndd__mmiixxeerr__ssppeecciiaall__tt **ssppeecciiaall
))
Not documented...
44..11..1122..
iinntt ssnndd__mmiixxeerr__ssppeecciiaall__wwrriittee(( vvooiidd **hhaannddllee,, ssnndd__mmiixxeerr__ssppeecciiaall__tt **ssppee--
cciiaall ))
Not documented...
44..11..1133..
iinntt ssnndd__mmiixxeerr__rreeaadd(( vvooiidd **hhaannddllee,, ssnndd__mmiixxeerr__ccaallllbbaacckkss__tt **ccaallllbbaacckkss ))
This function reads and parses data from driver. Parsed actions are
returned back to the application using the _c_a_l_l_b_a_c_k_s structure.
Applications should not parse data from the driver in standard cases.
This function returns immediately after all data is read from driver.
Does not block process.
______________________________________________________________________
typedef struct snd_mixer_callbacks {
void *private_data; /* should be used by application */
void (*channel_was_changed)( void *private_data, int channel );
void *reserved[15]; /* reserved for future use - must be NULL!!! */
} snd_mixer_callbacks_t;
______________________________________________________________________
44..22.. EExxaammpplleess
The following example shows installed mixer channels for soundcard #0
and mixer device #0 in the system, and also sets the master volume (if
present) to 50.
______________________________________________________________________
int card = 0, device = 0, err;
void *handle;
snd_mixer_info_t info;
snd_mixer_channel_t channel;
if ( (err = snd_mixer_open( &handle, card, device )) < 0 ) {
fprintf( stderr, "open failed: %s\n", snd_strerror( err ) );
return;
}
if ( (err = snd_mixer_info( handle, &info )) < 0 ) {
fprintf( stderr, "info failed: %s\n", snd_strerror( err ) );
snd_mixer_close( handle );
return;
}
printf( "Installed MIXER channels for card #i and device %i: %i\n",
card + 1, device, info.channels );
master = snd_mixer_channel( handle, SND_MIXER_ID_MASTER );
if ( master >= 0 ) {
if ( (err = snd_mixer_read( handle, master, &channel )) < 0 ) {
fprintf( stderr, "master read failed: %s\n", snd_strerror( err ) );
snd_mixer_close( handle );
return;
}
channel -> left = channel -> right = 50;
if ( (err = snd_mixer_write( handle, master, &channel )) < 0 ) {
fprintf( stderr, "master write failed: %s\n", snd_strerror( err ) );
snd_mixer_close( handle );
return;
}
}
snd_mixer_close( handle );
______________________________________________________________________
55.. DDiiggiittaall AAuuddiioo ((PPCCMM)) IInntteerrffaaccee
Digital audio is the most commonly used method of representing sound
inside a computer. In this method sound is stored as a sequence of
samples taken from the audio signal using constant time intervals. A
sample represents volume of the signal at the moment when it was
measured. In uncompressed digital audio each sample require one or
more bytes of storage. The number of bytes required depends on number
of channels (mono, stereo) and sample format (8 or 16 bits, mu-Law,
etc.). The length of this interval determines the sampling rate.
Commonly used sampling rates are between 8 kHz (telephone quality) and
48 kHz (DAT tapes).
The physical devices used in digital audio are called the ADC (Analog
to Digital Converter) and DAC (Digital to Analog Converter). A device
containing both ADC and DAC is commonly known as a codec. The codec
device used in a Sound Blaster cards is called a DSP which is somewhat
misleading since DSP also stands for Digital Signal Processor (the SB
DSP chip is very limited when compared to "true" DSP chips).
Sampling parameters affect the quality of sound which can be
reproduced from the recorded signal. The most fundamental parameter is
sampling rate which limits the highest frequency than can be stored.
It is well known (Nyquist's Sampling Theorem) that the highest
frequency that can be stored in a sampled signal is at most 1/2 of the
sampling frequency. For example, a 8 kHz sampling rate permits the
recording of a signal in which the highest frequency is less than 4
kHz. Higher frequency signals must be filtered out before feeding them
to DAC.
Sample encoding limits the dynamic range of recorded signal
(difference between the faintest and the loudest signal that can be
recorded). In theory the maximum dynamic range of signal is
number_of_bits * 6 dB . This means that 8 bits sampling resolution
gives dynamic range of 48 dB and 16 bit resolution gives 96 dB.
Quality has price. The number of bytes required to store an audio
sequence depends on sampling rate, number of channels and sampling
resolution. For example just 8000 bytes of memory is required to store
one second of sound using 8 kHz/8 bits/mono but 48 kHz/16bit/stereo
takes 192 kilobytes. A 64 kbps ISDN channel is required to transfer a
8kHz/8bit/mono audio stream in real time, and about 1.5 Mbps is
required for DAT quality (48kHz/16bit/stereo). On the other hand it
is possible to store just 5.46 seconds of sound in a megabyte of
memory when using 48kHz/16bit/stereo sampling. With 8kHz/8bits/mono it
is possible to store 131 seconds of sound using the same amount of
memory. It is possible to reduce memory and communication costs by
compressing the recorded signal but this is out of the scope of this
document.
55..11.. LLooww--LLeevveell LLaayyeerr
Audio devices are opened exclusively for a selected direction. This
doesn't allow open from more than one processes for the same audio
device in the same direction, but does allow one open call to each
playback direction and second open call to record direction
independently. Audio devices return EBUSY error to applications when
other applications have already opened the requested direction.
Low-Level layer supports these formats:
______________________________________________________________________
#define SND_PCM_SFMT_MU_LAW 0
#define SND_PCM_SFMT_A_LAW 1
#define SND_PCM_SFMT_IMA_ADPCM 2
#define SND_PCM_SFMT_U8 3
#define SND_PCM_SFMT_S16_LE 4
#define SND_PCM_SFMT_S16_BE 5
#define SND_PCM_SFMT_S8 6
#define SND_PCM_SFMT_U16_LE 7
#define SND_PCM_SFMT_U16_BE 8
#define SND_PCM_SFMT_MPEG 9
#define SND_PCM_SFMT_GSM 10
#define SND_PCM_FMT_MU_LAW (1 << SND_PCM_SFMT_MU_LAW)
#define SND_PCM_FMT_A_LAW (1 << SND_PCM_SFMT_A_LAW)
#define SND_PCM_FMT_IMA_ADPCM (1 << SND_PCM_SFMT_IMA_ADPCM)
#define SND_PCM_FMT_U8 (1 << SND_PCM_SFMT_U8)
#define SND_PCM_FMT_S16_LE (1 << SND_PCM_SFMT_S16_LE)
#define SND_PCM_FMT_S16_BE (1 << SND_PCM_SFMT_S16_BE)
#define SND_PCM_FMT_S8 (1 << SND_PCM_SFMT_S8)
#define SND_PCM_FMT_U16_LE (1 << SND_PCM_SFMT_U16_LE)
#define SND_PCM_FMT_U16_BE (1 << SND_PCM_SFMT_U16_BE)
#define SND_PCM_FMT_MPEG (1 << SND_PCM_SFMT_MPEG)
#define SND_PCM_FMT_GSM (1 << SND_PCM_SFMT_GSM)
______________________________________________________________________
Constants with prefix SSNNDD__PPCCMM__FFMMTT__ are used in info structures and
constants with prefix SSNNDD__PPCCMM__SSFFMMTT__ are used in format structures.
55..11..11..
iinntt ssnndd__ppccmm__ooppeenn(( vvooiidd ****hhaannddllee,, iinntt ccaarrdd,, iinntt ddeevviiccee,, iinntt mmooddee ))
Creates a new handle and opens a connection to kernel sound audio
interface for soundcard number _c_a_r_d (0-N) and audio device number
_d_e_v_i_c_e. Function also checks if protocol is compatible to prevent use
of old programs with a new kernel API. Function returns zero if
successful,ful otherwise it returns an error code. Error code -EBUSY
is returned when some process ownes the selected direction.
Default format after opening is mono _m_u_-_L_a_w at 8000Hz. This device can
be used directly for playback of standard .au (Sparc) files.
The following modes should be used for the _m_o_d_e argument:
______________________________________________________________________
#define SND_PCM_OPEN_PLAYBACK (O_WRONLY)
#define SND_PCM_OPEN_RECORD (O_RDONLY)
#define SND_PCM_OPEN_DUPLEX (O_RDWR)
______________________________________________________________________
55..11..22..
iinntt ssnndd__ppccmm__cclloossee(( vvooiidd **hhaannddllee ))
Frees all resources allocated with audio handle and closes the
connection to the kernel sound audio interface. Function returns zero
if successful, otherwise it returns an error code.
55..11..33..
iinntt ssnndd__ppccmm__ffiillee__ddeessccrriippttoorr(( vvooiidd **hhaannddllee ))
Returns the file descriptor of the connection to the kernel sound
audio interface. Function returns an error code if an error was
encountered.
The file descriptor should be used for the _s_e_l_e_c_t synchronous
multiplexer function for setting the read direction. Application
should call _s_n_d___p_c_m___r_e_a_d or _s_n_d___p_c_m___w_r_i_t_e functions if some data is
waiting for reading or a write can be performed. Calling this function
is highly recomended, as it leaves a place for the API to things like
data conversions, if needed.
55..11..44..
iinntt ssnndd__ppccmm__bblloocckk__mmooddee(( vvooiidd **hhaannddllee,, iinntt eennaabbllee ))
Sets up block (default) or nonblock mode for a handle. Block mode
suspends execution of a program when _s_n_d___p_c_m___r_e_a_d or _s_n_d___p_c_m___w_r_i_t_e is
called for the time which is needed for the actual playback or record
over of the entire buffer. In nonblock mode, programs aren't suspended
and the above functions returns immediately with the count of bytes
which were read or written by the driver. When used in this way, don't
try to use the entire buffer after the call, but instead process the
number of bytes returned, and call the function again.
55..11..55..
iinntt ssnndd__ppccmm__iinnffoo(( vvooiidd **hhaannddllee,, ssnndd__ppccmm__iinnffoo__tt **iinnffoo ))
Fills the *info structure with data about the PCM device selected by
*handle. Function returns zero if successful, otherwise it returns an
error code.
______________________________________________________________________
#define SND_PCM_INFO_CODEC 0x00000001
#define SND_PCM_INFO_DSP SND_PCM_INFO_CODEC
#define SND_PCM_INFO_MMAP 0x00000002 /* reserved */
#define SND_PCM_INFO_PLAYBACK 0x00000100
#define SND_PCM_INFO_RECORD 0x00000200
#define SND_PCM_INFO_DUPLEX 0x00000400
#define SND_PCM_INFO_DUPLEX_LIMIT 0x00000800 /* rate for playback & record are same */
struct snd_pcm_info {
unsigned int type; /* soundcard type */
unsigned int flags; /* see SND_PCM_INFO_XXXX */
unsigned char id[32]; /* ID of this PCM device */
unsigned char name[80]; /* name of this device */
unsigned char reserved[64]; /* reserved for future use */
};
______________________________________________________________________
SSNNDD__PPCCMM__IINNFFOO__MMMMAAPP
This flag is reserved and should be never used. It remains for
compatibility with Open Sound System driver.
SSNNDD__PPCCMM__IINNFFOO__DDUUPPLLEEXX__LLIIMMIITT
If this bit is set, rate must be same for playback and record
direction.
55..11..66..
iinntt ssnndd__ppccmm__ppllaayybbaacckk__iinnffoo(( vvooiidd **hhaannddllee,, ssnndd__ppccmm__ppllaayybbaacckk__iinnffoo__tt **iinnffoo
))
Fills the *info structure with data about PCM playback. Function
returns zero if successful, otherwise it returns an error code.
______________________________________________________________________
#define SND_PCM_PINFO_BATCH 0x00000001
#define SND_PCM_PINFO_8BITONLY 0x00000002
#define SND_PCM_PINFO_16BITONLY 0x00000004
struct snd_pcm_playback_info {
unsigned int flags; /* see SND_PCM_PINFO_XXXX */
unsigned int formats; /* supported formats */
unsigned int min_rate; /* min rate (in Hz) */
unsigned int max_rate; /* max rate (in Hz) */
unsigned int min_channels; /* min channels (probably always 1) */
unsigned int max_channels; /* max channels */
unsigned int buffer_size; /* playback buffer size */
unsigned int min_fragment_size; /* min fragment size in bytes */
unsigned int max_fragment_size; /* max fragment size in bytes */
unsigned int fragment_align; /* align fragment value */
unsigned char reserved[64]; /* reserved for future use */
};
______________________________________________________________________
SSNNDD__PPCCMM__PPIINNFFOO__BBAATTCCHH
Driver implements double buffering with this device. This means
that the chip used for data processing has its own memory, and
output should be more delayed than if a traditional codec chip
is used.
SSNNDD__PPCCMM__PPIINNFFOO__88BBIITTOONNLLYY
If this bit is set, the driver uses 8-bit format for 16-bit
samples and does software conversion. This bit is set on broken
SoundBlaster 16/AWE soundcards which can't do full 16-bit
duplex. If this bit is set application or highter digital audio
layer should do the conversion from 16-bit samples to 8-bit
samples rather than making the driver to do it in the kernel.
SSNNDD__PPCCMM__PPIINNFFOO__1166BBIITTOONNLLYY
If this bit is set, driver uses 16-bit format for 8-bit samples
and does software conversion. This bit is set on broken
SoundBlaster 16/AWE soundcards which can't do full 8-bit duplex.
If this bit is set the application or highter digital audio
layer should do conversion from 8-bit samples to 16-bit samples
rather than making the driver to do it in the kernel.
55..11..77..
iinntt ssnndd__ppccmm__rreeccoorrdd__iinnffoo(( vvooiidd **hhaannddllee,, ssnndd__ppccmm__rreeccoorrdd__iinnffoo__tt **iinnffoo ))
Fills the *info structure. Returns zero if successful, otherwise it
returns an error code.
______________________________________________________________________
#define SND_PCM_RINFO_BATCH 0x00000001
#define SND_PCM_RINFO_8BITONLY 0x00000002
#define SND_PCM_RINFO_16BITONLY 0x00000004
struct snd_pcm_record_info {
unsigned int flags; /* see to SND_PCM_RINFO_XXXX */
unsigned int formats; /* supported formats */
unsigned int min_rate; /* min rate (in Hz) */
unsigned int max_rate; /* max rate (in Hz) */
unsigned int min_channels; /* min channels (probably always 1) */
unsigned int max_channels; /* max channels */
unsigned int buffer_size; /* record buffer size */
unsigned int min_fragment_size; /* min fragment size in bytes */
unsigned int max_fragment_size; /* max fragment size in bytes */
unsigned int fragment_align; /* align fragment value */
unsigned char reserved[64]; /* reserved for future... */
};
______________________________________________________________________
SSNNDD__PPCCMM__PPIINNFFOO__BBAATTCCHH
Driver implements buffering for this device. This means that the
chip used for data processing has its own memory and output
should be more delayed than if a traditional codec chip is used.
SSNNDD__PPCCMM__PPIINNFFOO__88BBIITTOONNLLYY
If this bit is set, the device uses 8-bit format for 16-bit
samples and does software conversion. This bit is set on broken
SoundBlaster 16/AWE soundcards which can't do full 16-bit
duplex. If this bit is set the application or highter digital
audio layer should do conversion from 16-bit samples to 8-bit
samples rather than making the driver to do it in the kernel.
SSNNDD__PPCCMM__PPIINNFFOO__1166BBIITTOONNLLYY
If this bit is set, the device uses a 16-bit format for 8-bit
samples and does software conversion. This bit is set on broken
SoundBlaster 16/AWE soundcards which can't do full 8-bit duplex.
If this bit is set the application or highter digital audio
layer should do the conversion from 8-bit samples to 16-bit
samples rather than making the driver to do it in the kernel.
55..11..88..
iinntt ssnndd__ppccmm__ppllaayybbaacckk__ffoorrmmaatt(( vvooiidd **hhaannddllee,, ssnndd__ppccmm__ffoorrmmaatt__tt **ffoorrmmaatt ))
Sets up format, rate (in Hz) and number of channels for playback, in
the desired direction. Function returns zero if successful, otherwise
it returns an error code.
______________________________________________________________________
struct snd_pcm_format {
unsigned int format; /* SND_PCM_SFMT_XXXX */
unsigned int rate; /* rate in Hz */
unsigned int channels; /* channels (voices) */
unsigned char reserved[16];
};
______________________________________________________________________
55..11..99..
iinntt ssnndd__ppccmm__rreeccoorrdd__ffoorrmmaatt(( vvooiidd **hhaannddllee,, ssnndd__ppccmm__ffoorrmmaatt__tt **ffoorrmmaatt ))
Sets up format, rate (in Hz) and number of channels for used for
recording in the specified direction. Function returns zero if
successful, otherwise it returns an error code.
______________________________________________________________________
struct snd_pcm_format {
unsigned int format; /* SND_PCM_SFMT_XXXX */
unsigned int rate; /* rate in Hz */
unsigned int channels; /* channels (voices) */
unsigned char reserved[16];
};
______________________________________________________________________
55..11..1100..
iinntt ssnndd__ppccmm__ppllaayybbaacckk__ppaarraammss(( vvooiidd **hhaannddllee,, ssnndd__ppccmm__ppllaayybbaacckk__ppaarraammss__tt
**ppaarraammss ))
Sets various parameters for playback direction. Function returns zero
if successful, otherwise it returns an error code.
______________________________________________________________________
struct snd_pcm_playback_params {
int fragment_size;
int fragments_max;
int fragments_room;
unsigned char reserved[16]; /* must be filled with zero */
};
______________________________________________________________________
ffrraaggmmeenntt__ssiizzee
Requested size of fragment. This value should be aligned for
current format (for example to 4 if stereo 16-bit samples are
used) or with the _f_r_a_g_m_e_n_t___a_l_i_g_n variable from
_s_n_d___p_c_m___p_l_a_y_b_a_c_k___i_n_f_o___t structure. Its range can be from
_m_i_n___f_r_a_g_m_e_n_t___s_i_z_e to _m_a_x___f_r_a_g_m_e_n_t___s_i_z_e.
ffrraaggmmeennttss__mmaaxx
Maximum number of fragments in queue for wakeup. This number
doesn't counts partly used fragment. If current count of filled
playback fragments is greater than this value driver block
application or return immediately back if nonblock mode is
active.
ffrraaggmmeennttss__rroooomm
Minumum number of fragments writeable for wakeup. This value
should be in most cases 1 which means return back to application
if at least one fragment is free for playback. This value
includes partly used fragments, too.
55..11..1111..
iinntt ssnndd__ppccmm__rreeccoorrdd__ppaarraammss(( vvooiidd **hhaannddllee,, ssnndd__ppccmm__rreeccoorrdd__ppaarraammss__tt
**ppaarraammss ))
Function sets various parameters for the recording direction. Function
returns zero if successful, otherwise it returns an error code.
______________________________________________________________________
struct snd_pcm_record_params {
int fragment_size;
int fragments_min;
unsigned char reserved[16];
};
______________________________________________________________________
ffrraaggmmeenntt__ssiizzee
Requested size of fragment. This value should be aligned for
current format (for example to 4 if stereo 16-bit samples are
used) or set to the _f_r_a_g_m_e_n_t___a_l_i_g_n variable from
_s_n_d___p_c_m___p_l_a_y_b_a_c_k___i_n_f_o___t structure. Its range can be from
_m_i_n___f_r_a_g_m_e_n_t___s_i_z_e to _m_a_x___f_r_a_g_m_e_n_t___s_i_z_e.
ffrraaggmmeennttss__mmiinn
Minimum filled fragments for wakeup. Driver blocks the
application (if block mode is selected) until it isn't filled
with number of fragments specified with this value.
55..11..1122..
iinntt ssnndd__ppccmm__ppllaayybbaacckk__ssttaattuuss(( vvooiidd **hhaannddllee,, ssnndd__ppccmm__ppllaayybbaacckk__ssttaattuuss__tt
**ssttaattuuss ))
Fills the *status structure. Function returns zero if successful,
otherwise it returns an error code.
______________________________________________________________________
struct snd_pcm_playback_status {
unsigned int rate;
int fragments;
int fragment_size;
int count;
int queue;
int underrun;
struct timeval time;
struct timeval stime;
unsigned char reserved[16];
};
______________________________________________________________________
rraattee
Real playback rate. This value reflects hardware limitations.
ffrraaggmmeennttss
Currently allocated fragments by the driver for playback
direction.
ffrraaggmmeenntt__ssiizzee
Current fragment size used by driver for the playback direction.
ccoouunntt
Count of bytes writeable without blocking.
qquueeuuee
Count of bytes in queue. Note: _(_f_r_a_g_m_e_n_t_s _* _f_r_a_g_m_e_n_t___s_i_z_e_) _-
_q_u_e_u_e should not be equal to _c_o_u_n_t.
uunnddeerrrruunn
This value tells the application the number of underruns since
the ast call of _s_n_d___p_c_m___p_l_a_y_b_a_c_k___s_t_a_t_u_s.
ttiimmee
Delay till played of the first sample from next write. This
value should be used for time synchronization. Returned value is
in the same format as returned from the standard C function
_g_e_t_t_i_m_e_o_f_d_a_y_( _&_t_i_m_e_, _N_U_L_L _). This variable contains right value
only if playback time mode is enabled (look to
_s_n_d___p_c_m___p_l_a_y_b_a_c_k___t_i_m_e function).
ssttiimmee
Time when playback was started. This variable contains right
value only if playback time mode is enabled (look to
_s_n_d___p_c_m___p_l_a_y_b_a_c_k___t_i_m_e function).
55..11..1133..
iinntt ssnndd__ppccmm__rreeccoorrdd__ssttaattuuss(( vvooiidd **hhaannddllee,, ssnndd__ppccmm__rreeccoorrdd__ssttaattuuss__tt **ssttaa--
ttuuss ))
Fills the *status structure. Function returns zero if successful,
otherwise it returns an error code.
______________________________________________________________________
struct snd_pcm_record_status {
unsigned int rate;
int fragments;
int fragment_size;
int count;
int free;
int overrun;
struct timeval time;
unsigned char reserved[16];
};
______________________________________________________________________
rraattee
Real record rate. This value reflects hardware limitations.
ffrraaggmmeennttss
Currently allocated fragments by driver for the record
direction.
ffrraaggmmeenntt__ssiizzee
Current fragment size used by driver for the record direction.
ccoouunntt
Count of bytes readable without blocking.
ffrreeee
Count of bytes in buffer still free. Note: _(_f_r_a_g_m_e_n_t_s _*
_f_r_a_g_m_e_n_t___s_i_z_e_) _- _f_r_e_e should not be equal to _c_o_u_n_t.
oovveerrrruunn
This value tells application the count of overruns since the
last call to _s_n_d___p_c_m___r_e_c_o_r_d___s_t_a_t_u_s.
ttiimmee
Lag since the next sample read was recorded. This value should
be used for time synchronization. Returned value is in the same
format as returned by the from standard C function _g_e_t_t_i_m_e_o_f_d_a_y_(
_&_t_i_m_e_, _N_U_L_L _). This variable contains right value only if record
time mode is enabled (look to _s_n_d___p_c_m___r_e_c_o_r_d___t_i_m_e function).
ssttiimmee
Time when record was started. This variable contains right value
only if record time mode is enabled (look to _s_n_d___p_c_m___r_e_c_o_r_d___t_i_m_e
function).
55..11..1144..
iinntt ssnndd__ppccmm__ddrraaiinn__ppllaayybbaacckk(( vvooiidd **hhaannddllee ))
This function drain playback buffers immediately. Function returns
zero if successful, otherwise it returns an error code.
55..11..1155..
iinntt ssnndd__ppccmm__fflluusshh__ppllaayybbaacckk(( vvooiidd **hhaannddllee ))
This function flushes the playback buffers. It blocks the program
while the all the waiting samples in kernel playback buffers are
processed. Function returns zero if successful, otherwise it returns
an error code.
55..11..1166..
iinntt ssnndd__ppccmm__fflluusshh__rreeccoorrdd(( vvooiidd **hhaannddllee ))
This function flushes (destroyes) record buffers. Function returns
zero if successful, otherwise it returns an error code.
55..11..1177..
iinntt ssnndd__ppccmm__ppllaayybbaacckk__ttiimmee(( vvooiidd **hhaannddllee,, iinntt eennaabbllee ))
This function enables or disables time mode for playback direction.
Time mode allows to application better time synchronization. Function
returns zero if successful, otherwise it returns an error code.
55..11..1188..
iinntt ssnndd__ppccmm__rreeccoorrdd__ttiimmee(( vvooiidd **hhaannddllee,, iinntt eennaabbllee ))
This function enables or disables time mode for record direction. Time
mode allows to application better time synchronization. Function
returns zero if successful, otherwise it returns an error code.
55..11..1199..
ssssiizzee__tt ssnndd__ppccmm__wwrriittee(( vvooiidd **hhaannddllee,, ccoonnsstt vvooiidd **bbuuffffeerr,, ssiizzee__tt ssiizzee ))
Writes samples to the device which must be in the proper format
specified by the _s_n_d___p_c_m___p_l_a_y_b_a_c_k___f_o_r_m_a_t function. Function returns
zero or positive value if playback was successful (value represents
count of bytes which was successfuly written to device) or an error
value if error occured. Function should suspend process if block mode
is active.
55..11..2200..
ssssiizzee__tt ssnndd__ppccmm__rreeaadd(( vvooiidd **hhaannddllee,, vvooiidd **bbuuffffeerr,, ssiizzee__tt ssiizzee ))
Function reads samples from driver. Samples are in format specified by
_s_n_d___p_c_m___r_e_c_o_r_d___f_o_r_m_a_t function. Function returns zero or positive
value if record was success (value represents count of bytes which was
successfuly read from device) or negative error value if error
occured. Function should suspend process if block mode is active.
55..22.. EExxaammpplleess
The following example shows how to play the first 512kB from the
/tmp/test.au file with soundcard #0 and PCM device #0:
______________________________________________________________________
int card = 0, device = 0, err, fd, count, size, idx;
void *handle;
snd_pcm_format_t format;
char *buffer;
buffer = (char *)malloc( 512 * 1024 );
if ( !buffer ) return;
if ( (err = snd_pcm_open( &handle, card, device, SND_PCM_OPEN_PLAYBACK )) < 0 ) {
fprintf( stderr, "open failed: %s\n", snd_strerror( err ) );
return;
}
format.format = SND_PCM_SFMT_MU_LAW;
format.rate = 8000;
format.voices = 1;
if ( (err = snd_pcm_playback_format( handle, &format )) < 0 ) {
fprintf( stderr, "format setup failed: %s\n", snd_strerror( err ) );
snd_pcm_close( handle );
return;
}
fd = open( "/tmp/test.au" );
if ( fd < 0 ) {
perror( "open file" );
snd_pcm_close( handle );
return;
}
idx = 0;
count = read( fd, buffer, 512 * 1024 );
if ( count <= 0 ) {
perror( "read from file" );
snd_pcm_close( handle );
return;
}
close( fd );
if ( !memcmp( buffer, ".snd", 4 ) ) {
idx = (buffer[4]<<24)|(buffer[5]<<16)|(buffer[6]<<8)|(buffer[7]);
if ( idx > 128 ) idx = 128;
if ( idx > count ) idx = count;
}
size = snd_pcm_write( handle, &buffer[ idx ], count - idx );
printf( "Bytes written %i from %i...\n", size, count - idx );
snd_pcm_close( handle );
free( buffer );
______________________________________________________________________
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