Linux ALSA音频系统：soundcard

8.1声卡和PCM设备的建立过程

　前面分析了codec,platform,machine驱动的组成部分及其注册过程，这三者都是物理设备相关的。

　pcm逻辑设备，我们习惯称之为PCM中间层或pcm native,起着承上启下的作用：往上是与用户态接口的交互，实现音频数据在用户和内核态之间的拷贝；往下是触发codec,platform,machine的操作函数，实现音频数据在dma_buffer<->cpu_dai<->codec之间的传输。

　　声卡驱动中，一般挂载着多个逻辑设备，看看我们计算机的声卡驱动有几个逻辑设备：

$ cat /proc/asound/devices 2: [ 0]   : control3: [ 0- 0]: digital audio playback4: [ 0- 0]: digital audio capture5: [ 0- 3]: digital audio playback6: [ 0- 7]: digital audio playback7: [ 0- 8]: digital audio playback8: [ 0- 0]: hardware dependent9: [ 0- 3]: hardware dependent33:        : timer

Device	Description
digital audio playback	用于回放的 PCM 设备
digital audio capture	用于录制的 PCM 设备
control	用于声卡控制的 CTL 设备，如通路控制、音量调整等
timer	定时器设备
sequencer	音序器设备

嵌入式系统中，我们更关心PCM和CTL这两种设备。

$ ll /dev/snd/
total 0
drwxr-xr-x  2 root root       60 7月  14 22:19 by-path
crw-rw----+ 1 root audio 116,  2 7月  14 22:19 controlC0
crw-rw----+ 1 root audio 116,  8 7月  14 22:19 hwC0D0
crw-rw----+ 1 root audio 116,  9 7月  14 22:19 hwC0D3
crw-rw----+ 1 root audio 116,  4 7月  15 13:43 pcmC0D0c
crw-rw----+ 1 root audio 116,  3 7月  15 15:35 pcmC0D0p
crw-rw----+ 1 root audio 116,  5 7月  14 22:20 pcmC0D3p
crw-rw----+ 1 root audio 116,  6 7月  14 22:20 pcmC0D7p
crw-rw----+ 1 root audio 116,  7 7月  14 22:20 pcmC0D8p
crw-rw----+ 1 root audio 116,  1 7月  14 22:19 seq
crw-rw----+ 1 root audio 116, 33 7月  14 22:19 timer

可以看到这些设备节点的Major=116,Minor则与/proc/asound/devices所列的对应起来，都是字符设备。上层可以通过open/close/read/write/ioctl等系统调用来操作声卡设备，这和其他字符设备类似，但一般情况下我们会使用已封装好的用户接口库如alsa-lib。

8.2声卡结构概述

　　回顾下ASoC是如何注册声卡的，这里仅简单陈述下：

Machine驱动初始化时，.name = "soc-audio"的platform_device与platform_driver匹配成功，触发soc_probe()调用；
继而调用snd_soc_register_card():

为每个音频物理链路找到对应的codec,codec_dai,cpu_dai,platform设备实例，完成dai_link的绑定；
调用snd_card_create()创建声卡；
依次回调cpu_dai,codec,platform的probe()函数，完成物理设备的初始化；

随后调用soc_new_pcm():

设置pcm native中要使用的pcm操作函数，这些函数用于驱动音频物理设备，包括machine,codec_dai,cpu_dai,platform;
调用snd_pcm_new()创建pcm逻辑设备，回放子流和录制子流都在这里创建；
回调platform驱动的pcm_new()，完成音频dma设备初始化和dma buffer内存分配；

最后调用snd_card_register()注册声卡。

下面详细分析声卡和PCM逻辑设备的注册过程。

上面提到声卡驱动上挂着多个逻辑子设备，有pcm音频数据流，control混音器，midi，timer定时器，sequencer音序器等。

                  +-----------+| snd_card  |+-----------+|   |   |+-----------+   |   +------------+|               |                |
+-----------+    +-----------+    +-----------+|  snd_pcm  |    |snd_control|    | snd_timer |    ...+-----------+    +-----------+    +-----------+

这些与声音相关的逻辑设备都在结构体snd_card管理之下，可以说snd_card是alsa中最顶层的结构。我们在看看alsa声卡驱动的大致结构图。

snd_cards:记录着所注册的声卡实例，每个声卡实例有着各自的逻辑设备，如PCM设备，CTL设备，MIDI设备等，并一一记录到snd_card的device链表上

snd_minors:记录着所有逻辑设备的上下文信息，它是声卡逻辑设备与系统调用api之间的桥梁；每个snd_minor在逻辑设备注册时被填充，在逻辑设备使用时就可以从该结构中的到相应的信息(主要是系统调用函数集file_operations)

8.3声卡的创建

声卡实例通过函数snd_card_new()来创建，其函数原型：

/***  snd_card_new - create and initialize a soundcard structure*  @parent: the parent device object*  @idx: card index (address) [0 ... (SNDRV_CARDS-1)]*  @xid: card identification (ASCII string)*  @module: top level module for locking*  @extra_size: allocate this extra size after the main soundcard structure*  @card_ret: the pointer to store the created card instance**  Creates and initializes a soundcard structure.**  The function allocates snd_card instance via kzalloc with the given*  space for the driver to use freely.  The allocated struct is stored*  in the given card_ret pointer.**  Return: Zero if successful or a negative error code.*/int snd_card_new(struct device *parent, int idx, const char *xid,                                                                                                                struct module *module, int extra_size,struct snd_card **card_ret)

主要参数说明：

parent:　父设备对象
idx:声卡的编号,如为-1,则由系统自动分配
xid:声卡标识符，如为NULL，则以snd_card的shortname或longname代替
card_ret:返回所创建的声卡实例的指针

如下是我ubuntu16.04的声卡信息：

$ cat /proc/asound/cards0 [PCH            ]: HDA-Intel - HDA Intel PCHHDA Intel PCH at 0xf2530000 irq 31

number: 0
id: PCH
shortname: HDA Intel PCH
longname:HDA Intel PCH at 0xf7c30000 irq 47

shortname,longname常用于打印信息，上面的声卡信息是通过如下函数打印出来的：

static void snd_card_info_read(struct snd_info_entry *entry,struct snd_info_buffer *buffer){int idx, count;struct snd_card *card;for (idx = count = 0; idx < SNDRV_CARDS; idx++) {mutex_lock(&snd_card_mutex);if ((card = snd_cards[idx]) != NULL) {count++;snd_iprintf(buffer, "%2i [%-15s]: %s - %s\n",idx,card->id,card->driver,card->shortname);snd_iprintf(buffer, "                      %s\n",card->longname);}mutex_unlock(&snd_card_mutex);}if (!count)snd_iprintf(buffer, "--- no soundcards ---\n");}

8.4逻辑设备的创建

当声卡实例建立后，接着可以创建声卡下面的各个逻辑设备看。每个逻辑设备创建式，都会调用snd_device_new()生成一个snd_device实例，并把该实例挂到声卡snd_card的devices链表上。alsa驱动为各种逻辑设备提供了创建接口，如下：

Device	Interface
PCM	snd_pcm_new()
CONTROL	snd_ctl_create()
MIDI	snd_rawmidi_new()
TIMER	snd_timer_new()
SEQUENCER	snd_seq_device_new()
JACK	snd_jack_new()

这些接口的一般过程如下：

int snd_xxx_new()
{//这些接口提供逻辑设备注册时回调static struct snd_device_ops ops = {.dev_free = snd_xxx_dev_free,.dev_register = snd_xxx_dev_register,.dev_disconnect = snd_xxx_dev_disconnect,};//逻辑设备实例初始化//新建一个设备实例snd_device,挂到snd_card的devices链表上，把该逻辑设备纳入声卡的管理当中，SNDRV_DEV_XXX是逻辑设备的类型snd_device_new(card, SNDRV_DEV_XXX, xxx, &ops);
}

其中snd_device_ops是声卡逻辑设备的注册函数集，dev_register()回调尤其重要，它在声卡注册时被调用，用于建立系统的设备节点，/dev/snd/目录的设备节点都是在这里创建的，通过这些设备节点可系统调用open/release/read/write/ioctl..访问操作该逻辑设备。

snd_ctl_dev_register():

static const struct file_operations snd_ctl_f_ops ={                                                                                                                                   .owner =    THIS_MODULE,.read =     snd_ctl_read,.open =     snd_ctl_open,.release =  snd_ctl_release,.llseek =   no_llseek,.poll =     snd_ctl_poll,.unlocked_ioctl =   snd_ctl_ioctl,.compat_ioctl = snd_ctl_ioctl_compat,.fasync =   snd_ctl_fasync,};/** registration of the control device*/static int snd_ctl_dev_register(struct snd_device *device){struct snd_card *card = device->device_data;return snd_register_device(SNDRV_DEVICE_TYPE_CONTROL, card, -1,&snd_ctl_f_ops, card, &card->ctl_dev);}
/*** snd_register_device - Register the ALSA device file for the card* @type: the device type, SNDRV_DEVICE_TYPE_XXX                                                                                     * @card: the card instance* @dev: the device index* @f_ops: the file operations* @private_data: user pointer for f_ops->open()* @device: the device to register** Registers an ALSA device file for the given card.* The operators have to be set in reg parameter.** Return: Zero if successful, or a negative error code on failure.*/int snd_register_device(int type, struct snd_card *card, int dev,const struct file_operations *f_ops,void *private_data, struct device *device){int minor;int err = 0;struct snd_minor *preg; if (snd_BUG_ON(!device))return -EINVAL; preg = kmalloc(sizeof *preg, GFP_KERNEL);if (preg == NULL)return -ENOMEM;preg->type = type;preg->card = card ? card->number : -1;preg->device = dev;preg->f_ops = f_ops;preg->private_data = private_data;preg->card_ptr = card;mutex_lock(&sound_mutex);minor = snd_find_free_minor(type, card, dev);if (minor < 0) {err = minor;goto error;}preg->dev = device;device->devt = MKDEV(major, minor);err = device_add(device);if (err < 0)goto error;snd_minors[minor] = preg;error:mutex_unlock(&sound_mutex);if (err < 0)kfree(preg);return err;}

从snd_ctl_dev_register函数中可以看到：

分配并初始化一个snd_minor实例，
分配设备节点
保存snd_minor实例到snd_minors数组中

1016 /**1017  * device_add - add device to device hierarchy.                                                                                     1018  * @dev: device.1019  *1020  * This is part 2 of device_register(), though may be called1021  * separately _iff_ device_initialize() has been called separately.1022  *1023  * This adds @dev to the kobject hierarchy via kobject_add(), adds it1024  * to the global and sibling lists for the device, then1025  * adds it to the other relevant subsystems of the driver model.1026  *1027  * Do not call this routine or device_register() more than once for1028  * any device structure.  The driver model core is not designed to work1029  * with devices that get unregistered and then spring back to life.1030  * (Among other things, it's very hard to guarantee that all references1031  * to the previous incarnation of @dev have been dropped.)  Allocate1032  * and register a fresh new struct device instead.1033  *1034  * NOTE: _Never_ directly free @dev after calling this function, even1035  * if it returned an error! Always use put_device() to give up your1036  * reference instead.1037  */1038 int device_add(struct device *dev)1039 {1040     struct device *parent = NULL;1041     struct kobject *kobj;1042     struct class_interface *class_intf;1043     int error = -EINVAL;1044     struct kobject *glue_dir = NULL;1045 1046     dev = get_device(dev);1047     if (!dev)1048         goto done;
1050     if (!dev->p) {1051         error = device_private_init(dev);1052         if (error)1053             goto done;1054     }1055 1056     /*1057      * for statically allocated devices, which should all be converted1058      * some day, we need to initialize the name. We prevent reading back1059      * the name, and force the use of dev_name()1060      */1061     if (dev->init_name) {1062         dev_set_name(dev, "%s", dev->init_name);1063         dev->init_name = NULL;1064     }1065 1066     /* subsystems can specify simple device enumeration */1067     if (!dev_name(dev) && dev->bus && dev->bus->dev_name)1068         dev_set_name(dev, "%s%u", dev->bus->dev_name, dev->id);1069 1070     if (!dev_name(dev)) {1071         error = -EINVAL;1072         goto name_error;1073     }1074 1075     pr_debug("device: '%s': %s\n", dev_name(dev), __func__);1076 1077     parent = get_device(dev->parent);1078     kobj = get_device_parent(dev, parent);1079     if (kobj)1080         dev->kobj.parent = kobj;                                                                                                    1081 1082     /* use parent numa_node */1083     if (parent && (dev_to_node(dev) == NUMA_NO_NODE))1084         set_dev_node(dev, dev_to_node(parent));1085 1086     /* first, register with generic layer. */1087     /* we require the name to be set before, and pass NULL */1088     error = kobject_add(&dev->kobj, dev->kobj.parent, NULL);1089     if (error) {1090         glue_dir = get_glue_dir(dev);1091         goto Error;1092     }1093 1094     /* notify platform of device entry */1095     if (platform_notify)1096         platform_notify(dev);1097 1098     error = device_create_file(dev, &dev_attr_uevent);1099     if (error)1100         goto attrError;1101 1102     error = device_add_class_symlinks(dev);1103     if (error)1104         goto SymlinkError;1105     error = device_add_attrs(dev);1106     if (error)1107         goto AttrsError;1108     error = bus_add_device(dev);1109     if (error)1110         goto BusError;1111     error = dpm_sysfs_add(dev);1112     if (error)goto DPMError;device_pm_add(dev); if (MAJOR(dev->devt)) {error = device_create_file(dev, &dev_attr_dev);if (error)goto DevAttrError;error = device_create_sys_dev_entry(dev);if (error)goto SysEntryError;devtmpfs_create_node(dev);}/* Notify clients of device addition.  This call must come* after dpm_sysfs_add() and before kobject_uevent().*/if (dev->bus)blocking_notifier_call_chain(&dev->bus->p->bus_notifier,BUS_NOTIFY_ADD_DEVICE, dev); kobject_uevent(&dev->kobj, KOBJ_ADD);bus_probe_device(dev);                                                                                                          if (parent)klist_add_tail(&dev->p->knode_parent,&parent->p->klist_children);if (dev->class) {mutex_lock(&dev->class->p->mutex);/* tie the class to the device */klist_add_tail(&dev->knode_class,&dev->class->p->klist_devices);/* notify any interfaces that the device is here */list_for_each_entry(class_intf,&dev->class->p->interfaces, node)if (class_intf->add_dev)class_intf->add_dev(dev, class_intf);mutex_unlock(&dev->class->p->mutex);}done:put_device(dev);return error;SysEntryError:if (MAJOR(dev->devt))device_remove_file(dev, &dev_attr_dev);DevAttrError:device_pm_remove(dev);dpm_sysfs_remove(dev);DPMError:bus_remove_device(dev);BusError:device_remove_attrs(dev);AttrsError:device_remove_class_symlinks(dev);SymlinkError:device_remove_file(dev, &dev_attr_uevent);attrError:kobject_uevent(&dev->kobj, KOBJ_REMOVE);glue_dir = get_glue_dir(dev);kobject_del(&dev->kobj);Error:cleanup_glue_dir(dev, glue_dir);put_device(parent);name_error:kfree(dev->p);dev->p = NULL;goto done;}

上面过程是声卡注册时才被回调的。

8.5声卡的注册

当声卡下的所有逻辑设备都已经准备就绪后，就可以调用snd_card_register()注册声卡了：

创建声卡的sysfs设备；
调用snd_device_register_all()注册所有挂在该声卡下的逻辑设备；
建立proc信息文件和sysfs属性文件。

   724 /**725  *  snd_card_register - register the soundcard726  *  @card: soundcard structure727  *728  *  This function registers all the devices assigned to the soundcard.729  *  Until calling this, the ALSA control interface is blocked from the730  *  external accesses.  Thus, you should call this function at the end731  *  of the initialization of the card.732  *733  *  Return: Zero otherwise a negative error code if the registration failed.734  */735 int snd_card_register(struct snd_card *card)736 {737     int err;738 739     if (snd_BUG_ON(!card))740         return -EINVAL;741 　　　//创建sysfs设备，声卡的class将会出现在/sys/class/sound下面742     if (!card->registered) {743         err = device_add(&card->card_dev);744         if (err < 0)745             return err;746         card->registered = true;747     }748 //遍历挂在该声卡的所有逻辑设备，回调各snd_device的ops->dev_register()完成各逻辑设备的注册749     if ((err = snd_device_register_all(card)) < 0)750         return err;751     mutex_lock(&snd_card_mutex);752     if (snd_cards[card->number]) {753         /* already registered */754         mutex_unlock(&snd_card_mutex);755         return snd_info_card_register(card); /* register pending info */756     }757     if (*card->id) {758         /* make a unique id name from the given string */759         char tmpid[sizeof(card->id)];760         memcpy(tmpid, card->id, sizeof(card->id));761         snd_card_set_id_no_lock(card, tmpid, tmpid);762     } else {763         /* create an id from either shortname or longname */764         const char *src;765         src = *card->shortname ? card->shortname : card->longname;766         snd_card_set_id_no_lock(card, src,767                     retrieve_id_from_card_name(src));                                                                                                                            768     }769     snd_cards[card->number] = card;//把该声卡实例保存到snd_cards数组中770     mutex_unlock(&snd_card_mutex);//声卡相关信息，见:/proc/asound/card0771     init_info_for_card(card);772 #if IS_ENABLED(CONFIG_SND_MIXER_OSS)773     if (snd_mixer_oss_notify_callback)774         snd_mixer_oss_notify_callback(card, SND_MIXER_OSS_NOTIFY_REGISTER);775 #endif776     return 0;777 }

至此完成了声卡及声卡下的所有逻辑设备的注册，用户态可以通过系统调用来访问这些设备。

8.6PCM设备的创建

snd_pcm_set_ops:设置PCM设备的操作接口，设置完成后，在PCM设备层即可访问操作底层音频物理设备。

snd_pcm_new:

创建一个PCM设备实例snd_pcm;
创建playback stream和capture　stream，旗下的substream也同时建立；
调用snd_device_new()把pcm设备挂到声卡的devices链表上。

/**800  * snd_pcm_new - create a new PCM instance801  * @card: the card instance802  * @id: the id string803  * @device: the device index (zero based)804  * @playback_count: the number of substreams for playback805  * @capture_count: the number of substreams for capture806  * @rpcm: the pointer to store the new pcm instance807  *808  * Creates a new PCM instance.809  *810  * The pcm operators have to be set afterwards to the new instance811  * via snd_pcm_set_ops().812  *813  * Return: Zero if successful, or a negative error code on failure.814  */815 int snd_pcm_new(struct snd_card *card, const char *id, int device,816         int playback_count, int capture_count, struct snd_pcm **rpcm)817 {818     return _snd_pcm_new(card, id, device, playback_count, capture_count,819             false, rpcm);820 }
755 static int _snd_pcm_new(struct snd_card *card, const char *id, int device,756         int playback_count, int capture_count, bool internal,757         struct snd_pcm **rpcm)758 {759     struct snd_pcm *pcm;760     int err;761     static struct snd_device_ops ops = {762         .dev_free = snd_pcm_dev_free,763         .dev_register = snd_pcm_dev_register,764         .dev_disconnect = snd_pcm_dev_disconnect,765     };766 767     if (snd_BUG_ON(!card))768         return -ENXIO;769     if (rpcm)770         *rpcm = NULL;771     pcm = kzalloc(sizeof(*pcm), GFP_KERNEL);772     if (!pcm)773         return -ENOMEM;774     pcm->card = card;775     pcm->device = device;776     pcm->internal = internal;777     mutex_init(&pcm->open_mutex);778     init_waitqueue_head(&pcm->open_wait);779     INIT_LIST_HEAD(&pcm->list);780     if (id)781         strlcpy(pcm->id, id, sizeof(pcm->id));782     if ((err = snd_pcm_new_stream(pcm, SNDRV_PCM_STREAM_PLAYBACK, playback_count)) < 0) {783         snd_pcm_free(pcm);784         return err;785     }786     if ((err = snd_pcm_new_stream(pcm, SNDRV_PCM_STREAM_CAPTURE, capture_count)) < 0) {787         snd_pcm_free(pcm);788         return err;789     }790     if ((err = snd_device_new(card, SNDRV_DEV_PCM, pcm, &ops)) < 0) {791         snd_pcm_free(pcm);792         return err;793     }794     if (rpcm)795         *rpcm = pcm;796     return 0;                                                                                                                                       797 }

在看看pcm设备的系统调用：

const struct file_operations snd_pcm_f_ops[2] = {3681     {3682         .owner =        THIS_MODULE,3683         .write =        snd_pcm_write,3684         .write_iter =       snd_pcm_writev,3685         .open =         snd_pcm_playback_open,3686         .release =      snd_pcm_release,3687         .llseek =       no_llseek,3688         .poll =         snd_pcm_playback_poll,3689         .unlocked_ioctl =   snd_pcm_playback_ioctl,3690         .compat_ioctl =     snd_pcm_ioctl_compat,3691         .mmap =         snd_pcm_mmap,3692         .fasync =       snd_pcm_fasync,3693         .get_unmapped_area =    snd_pcm_get_unmapped_area,3694     },3695     {3696         .owner =        THIS_MODULE,3697         .read =         snd_pcm_read,3698         .read_iter =        snd_pcm_readv,3699         .open =         snd_pcm_capture_open,3700         .release =      snd_pcm_release,3701         .llseek =       no_llseek,3702         .poll =         snd_pcm_capture_poll,3703         .unlocked_ioctl =   snd_pcm_capture_ioctl,3704         .compat_ioctl =     snd_pcm_ioctl_compat,3705         .mmap =         snd_pcm_mmap,3706         .fasync =       snd_pcm_fasync,3707         .get_unmapped_area =    snd_pcm_get_unmapped_area,3708     }3709 };

snd_pcm_f_ops作为snd_register_device_for_dev()的参数传入，并被记录在snd_minors[minor]中的字段f_ops中。snd_pcm_f_ops[0]是回放使用的系统调用接口，snd_pcm_f_ops[1]是录制使用的系统调用接口。

９．Frame && Period

音频数据中的几个重要概念：

Sample:样本长度，音频数据最基本的单位，常见的有8bit和16bit;
channel:声道数，分为单声道mono和立体声stereo;
Frame:帧，构成一个完整的声音单元，所谓的声音单元是☞一个采样样本， Frame = Sample * channel;
Rate:又称sample rate,采样率，即毎秒的采样次数，针对帧而言；
Period Size:周期，每次硬件中断处理音频数据的帧数，对于音频设备的数据读写，以此为单位；
Buffer Size:数据缓冲区大小，这里这指runtime的buffer size,而不是结构图snd_pcm_hardware中定义的buffer_bytes_max;一般来说buffer_size = period_size * period_count,period_count相当于处理完一个buffer数据所需的硬件中断次数。

下面是一章直观的表示buffer/period/frame/sample之间的关系：

这个buffer中有4个period，每当DMA搬运完一个period的数据就会出生一次中断，因此搬运这个buffer中的数据将产生4次中断。

ALSA为什么这样做?因为数据缓冲区可能很大，一次传输可能会导致不可接收的延迟；为了解决这个问题，alsa把缓冲区拆分成多个周期，以周期为单元传输数据。

9.1frames&periods

alsa官网对periods的解释：https://www.alsa-project.org/main/index.php/FramesPeriods

Frame:帧，构成一个完整的声音单元，它的大小等于sample_bits * channels;
Period:周期大小，即每次dma运输处理音频数据的帧数，如果周期大小设定的较大，则单次处理的数据较多，这意味着单位时间内硬件中断的次数较少，CPU也就有更多时间处理其他任务，功耗也更低，但这样带来一个显著的弊端－数据处理的时延会增大。
period bytes,对于dma处理来说，它直接关心的是数据大小，而非period_size(一个周期的帧数)，有个转换关系：period_bytes =period_size * sample_bits * channels / 8

由于i2s总线采样率是稳定的，我们可以计算i2s传输一个周期的数据所需的时间：transfer_time = 1 * period_size /sample_rate,in second.

例如period_size = 1024, sample_rate = 48khz,那么一个周期数据的传输时间是:１*1024/48000 = 21.3(ms)

Linux ALSA音频系统：soundcard相关推荐

Linux ALSA 音频系统：物理链路篇
原址 1. Overview 硬件平台及软件版本: Kernel - 3.4.5 SoC - Samsung exynos CODEC - WM8994 Machine - goni_wm8994 U ...
（二）Linux ALSA 音频系统：逻辑设备篇
逻辑设备篇转自:https://me.csdn.net/zyuanyun Linux ALSA 音频系统:逻辑设备篇 Linux ALSA 音频系统:逻辑设备篇原创 zyuanyun 最后发布于2 ...
（一）Linux ALSA 音频系统：物理链路篇
物理链路篇转自:https://me.csdn.net/zyuanyun Linux ALSA 音频系统:物理链路篇 Linux ALSA 音频系统:物理链路篇原创 zyuanyun 最后发布于2 ...
Linux ALSA音频系统:platform,machine,codec
版权声明:本文为博主原创文章,遵循 CC 4.0 BY-SA 版权协议,转载请附上原文出处链接和本声明. 本文链接:https://blog.csdn.net/weixin_41965270/arti ...
linux音频时钟bclk,linux alsa音频中采样率fs、比特率BCLK 、主时钟MCLK关系
转:https://blog.csdn.net/lugandong/article/details/72468831 一. 拿512fs说话: 看图知道采样的位深是32bit(位),左右声道各占了8* ...
Linux ALSA音频框架分析五：HDA Driver分析
Linux ALSA音频框架分析五:HDA Driver分析一概述 HDA(High Definition Audio)是intel设计的用来取代AC97的音频标准,硬件架构上由hda dodec ...
Linux ALSA音频子系统二
From 本文以高通平台为例,介绍一下android下的音频结构.android使用的是tinyALSA作为音频系统,使用方法和基本框架与linux中常用的ALSA音频子系统是一致的. ALSA音频框 ...
Android ALSA音频系统架构分析（1）----从Loopback了解Audio
/*********************************** * Author:刘江明 * Environment:MTK Android 6.0 * Date:2017年05月25日 * ...
Linux ALSA音频驱动一：音频系统概述
音频系统概述音频系统通过数据+控制总线与CODEC连接,控制通路用I2C,数据通常为I2S,框图如图1所示. I2C:寄存器读写,用于配置CODEC控制通路. I2S:音频数据传输,通常与platf ...

Linux ALSA音频系统：soundcard

9.1frames&periods

Linux ALSA音频系统：soundcard相关推荐

最新文章

热门文章