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音频相关术语
PCM(Pulse Code Modulation)
脉冲编码调制，对连续变化的模拟信号进行抽样、量化和编码，在驱动中一般音频流设备都称为pcm设备I2S
I2S是对PCM格式的数据进行规范化，可以说是PCM的子集，I2S只有左右两通道数据TDM(Time Division Multiplexing)
时分复用，可以用单根线传送多通道数据midi(Musical Instrument Digital Interface)
MIDI是编曲界最广泛的音乐标准格式，可称为“计算机能理解的乐谱”。它用音符的数字控制信号来记录音乐。一首完整的MIDI音乐只有几十KB大，而能包含数十条音乐轨道。几乎所有的现代音乐都是用MIDI加上音色库来制作合成的。MIDI 传输的不是声音信号， 而是音符、控制参数等指令, 它指示MIDI 设备要做什么，怎么做， 如演奏哪个音符、多大音量等。timer(音序器)
音序器，又称声音序列发生器，可将所有MIDI通道中的演奏信息同时自动播放演奏
驱动架构图
OSS(Open Sound System)，linux以前的驱动架构，现在已废弃不用
linux目前的音频框架为ALSA(Advanced Linux Sound Architecture)，框架如下图所示，分三层，其中ASoc适用于目前的嵌入式设备，在ALSA-core基础上又分出一个小架构
其中alsa-core层的主要结构如下，一个声卡用snd_card来表示，在一个声卡上可能集成了好几个子设备，每个子设备负责不用的功能，分为pcm/control/midi/timer等
声卡结构体
一个声卡下是由多个设备的，总声卡用结构体snd_card来表示，各个子设备用结构体snd_device来抽象，当然对于具体的子设备，还会有相应的结构体来表示，这个会在子设备的文章中详细介绍
snd_card
(include/sound/core.h)
struct snd_card {
int number; /* number of soundcard (index to
snd_cards) */
char id[16]; /* id string of this card */
char driver[16]; /* driver name */
char shortname[32]; /* short name of this soundcard */
char longname[80]; /* name of this soundcard */
char mixername[80]; /* mixer name */
char components[128]; /* card components delimited with
space */
struct module *module; /* top-level module */
void *private_data; /* private data for soundcard */
void (*private_free) (struct snd_card *card); /* callback for freeing of
private data */
struct list_head devices; /* devices */---------snd_device加入此链表
unsigned int last_numid; /* last used numeric ID */
struct rw_semaphore controls_rwsem; /* controls list lock */
rwlock_t ctl_files_rwlock; /* ctl_files list lock */
int controls_count; /* count of all controls */
int user_ctl_count; /* count of all user controls */
struct list_head controls; /* all controls for this card */---控制设备snd_device加入此链表
struct list_head ctl_files; /* active control files */
struct mutex user_ctl_lock; /* protects user controls against
concurrent access */
struct snd_info_entry *proc_root; /* root for soundcard specific files */
struct snd_info_entry *proc_id; /* the card id */
struct proc_dir_entry *proc_root_link; /* number link to real id */
struct list_head files_list; /* all files associated to this card */
struct snd_shutdown_f_ops *s_f_ops; /* file operations in the shutdown
state */
spinlock_t files_lock; /* lock the files for this card */
int shutdown; /* this card is going down */
int free_on_last_close; /* free in context of file_release */
wait_queue_head_t shutdown_sleep;
atomic_t refcount; /* refcount for disconnection */
struct device *dev; /* device assigned to this card */
struct device *card_dev; /* cardX object for sysfs */
#ifdef CONFIG_PM
unsigned int power_state; /* power state */
struct mutex power_lock; /* power lock */
wait_queue_head_t power_sleep;
#endif
#if defined(CONFIG_SND_MIXER_OSS) || defined(CONFIG_SND_MIXER_OSS_MODULE)
struct snd_mixer_oss *mixer_oss;
int mixer_oss_change_count;
#endif
};snd_device
(include/sound/core.h)
struct snd_device {
struct list_head list; /* list of registered devices */---加入声卡的链表节点
struct snd_card *card; /* card which holds this device */-对应的声卡
snd_device_state_t state; /* state of the device */
snd_device_type_t type; /* device type */
void *device_data; /* device structure */------------对于具体子设备的私有数据
struct snd_device_ops *ops; /* operations */------------------对子设备注册的操作
};声卡注册
snd_card_register
(sound/core/init.c)
int snd_card_register(struct snd_card *card)
{
int err;
if (snd_BUG_ON(!card))
return -EINVAL;
if (!card->card_dev) {
card->card_dev = device_create(sound_class, card->dev,
MKDEV(0, 0), card,
"card%i", card->number);---注册声卡
if (IS_ERR(card->card_dev))
card->card_dev = NULL;
}
if ((err = snd_device_register_all(card)) < 0)-----注册声卡下的所有子设备
return err;
mutex_lock(&snd_card_mutex);
if (snd_cards[card->number]) {
/* already registered */
mutex_unlock(&snd_card_mutex);
return 0;
}
if (*card->id) {
/* make a unique id name from the given string */
char tmpid[sizeof(card->id)];
memcpy(tmpid, card->id, sizeof(card->id));
snd_card_set_id_no_lock(card, tmpid, tmpid);
} else {
/* create an id from either shortname or longname */
const char *src;
src = *card->shortname ? card->shortname : card->longname;
snd_card_set_id_no_lock(card, src,
retrieve_id_from_card_name(src));
}
snd_cards[card->number] = card;
mutex_unlock(&snd_card_mutex);
init_info_for_card(card);
#if defined(CONFIG_SND_MIXER_OSS) || defined(CONFIG_SND_MIXER_OSS_MODULE)
if (snd_mixer_oss_notify_callback)
snd_mixer_oss_notify_callback(card, SND_MIXER_OSS_NOTIFY_REGISTER);
#endif
if (card->card_dev) {
err = device_create_file(card->card_dev, &card_id_attrs);
if (err < 0)
return err;
err = device_create_file(card->card_dev, &card_number_attrs);
if (err < 0)
return err;
}
return 0;
}
snd_device_register_all
int snd_device_register_all(struct snd_card *card)
{
struct snd_device *dev;
int err;
if (snd_BUG_ON(!card))
return -ENXIO;
list_for_each_entry(dev, &card->devices, list) {----遍历声卡的设备链表
if (dev->state == SNDRV_DEV_BUILD && dev->ops->dev_register) {
if ((err = dev->ops->dev_register(dev)) < 0)
return err;-----------------调用每个设备注册函数，进行注册
dev->state = SNDRV_DEV_REGISTERED;
}
}
return 0;
}change log
| date | content | linux version |
|---|---|---|
| 2017/11/20 | origin | linux 3.10 |