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关于USB-AUDIO使用ALSA编程的一点问题

發(fā)布時(shí)間：2023/12/10 编程问答 35 豆豆

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轉(zhuǎn)載自：http://blog.chinaunix.net/uid-25272011-id-3153434.html

最近在調(diào)試一款原相PAP7501攝像頭中的USB的麥克風(fēng)，USB層走的應(yīng)該是標(biāo)準(zhǔn)的UAC協(xié)議，具體可以見(jiàn)USB的官網(wǎng)：http://www.usb.org/developers/devclass_docs#approved，而音頻部分則可以跑目前Linux標(biāo)準(zhǔn)的ALSA的PCM接口，對(duì)于硬件CODEC來(lái)說(shuō)，與其是完全兼容的。
???? 給出一份參考代碼：
???? 這個(gè)是仿照arecord寫(xiě)的一個(gè)簡(jiǎn)略的測(cè)試代碼，保存為wav格式的。
1、recod.c

This example reads from the default PCM device

and writes to standard output for 5 seconds of data.

/* Use the newer ALSA API */

#define ALSA_PCM_NEW_HW_PARAMS_API

#include <alsa/asoundlib.h>

/**************************************************************/

#define ID_RIFF 0x46464952

#define ID_WAVE 0x45564157

#define ID_FMT 0x20746d66

#define ID_DATA 0x61746164

typedef unsigned long uint32_t;

typedef unsigned short uint16_t;

#define FORMAT_PCM 1

static uint32_t totle_size = 0;

struct wav_header {

????/* RIFF WAVE Chunk */

????uint32_t riff_id;

????uint32_t riff_sz;

????uint32_t riff_fmt;

????/* Format Chunk */

????uint32_t fmt_id;

????uint32_t fmt_sz;

????uint16_t audio_format;

????uint16_t num_channels;

????uint32_t sample_rate;

????uint32_t byte_rate; /* sample_rate * num_channels * bps / 8 */

????uint16_t block_align; /* num_channels * bps / 8 */

????uint16_t bits_per_sample;

????/* Data Chunk */

????uint32_t data_id;

????uint32_t data_sz;

}__attribute__((packed));

static struct wav_header hdr;

/**************************************************************/

int record_file(unsigned rate, unsigned channels, int fd, unsigned count)

{

????long loops;

??? int val;

????int rc;

????int size;

????snd_pcm_t *handle;

????snd_pcm_hw_params_t *params;

????int dir;

????snd_pcm_uframes_t frames;

????char *buffer;????????????????????/* TODO */

????/* Open PCM device for recording (capture). */

????rc = snd_pcm_open(&handle, "plughw:0,0", SND_PCM_STREAM_CAPTURE, 0);

????if (rc < 0) {

????????fprintf(stderr, "unable to open pcm device: %s\n", snd_strerror(rc));

????????exit(1);

????}

????/* Allocate a hardware parameters object. */

????snd_pcm_hw_params_alloca(&params);

????/* Fill it in with default values. */

????snd_pcm_hw_params_any(handle, params);

????/* Set the desired hardware parameters. */

????/* Interleaved mode */

????snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);

????/* Signed 16-bit little-endian format */

????snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE);

????/* Two channels (stereo) */

????snd_pcm_hw_params_set_channels(handle, params, channels);

????/* rate bits/second sampling rate (CD quality) */

????snd_pcm_hw_params_set_rate_near(handle, params, &rate, &dir);

????/* Set period size to 32 frames. */

????frames = 320;???? /* 這邊的大小也不是絕對(duì)的，可以調(diào)整 */

????snd_pcm_hw_params_set_period_size_near(handle, params, &frames, &dir);

????/* Write the parameters to the driver */

????rc = snd_pcm_hw_params(handle, params);

????if (rc < 0) {

????????fprintf(stderr, "unable to set hw parameters: %s\n", snd_strerror(rc));

????????exit(1);

????}

????/* Use a buffer large enough to hold one period */

????snd_pcm_hw_params_get_period_size(params, &frames, &dir);/* 獲取實(shí)際的frames */

????

????size = frames * 2; /* 2 bytes/sample, 1 channels */

????buffer = (char *) malloc(size);

????/* We want to loop for 20 seconds 時(shí)間不一定準(zhǔn)確 */

????snd_pcm_hw_params_get_period_time(params, &val, &dir);

????loops = 20000000 / val;

????

????while (loops > 0) {

????????loops--;

????????rc = snd_pcm_readi(handle, buffer, frames);

????????if (rc == -EPIPE) {

???????? /* EPIPE means overrun */

???????? fprintf(stderr, "overrun occurred\n");

???????? snd_pcm_prepare(handle);

????????} else if (rc < 0) {

???????? fprintf(stderr, "error from read: %s\n", snd_strerror(rc));

????????} else if (rc != (int)frames) {

???????? fprintf(stderr, "short read, read %d frames\n", rc);

????????}

????????rc = write(fd, buffer, size);

????????totle_size += rc;????????????????????????/* totle data size */

????????if (rc != size)

???????? fprintf(stderr, "short write: wrote %d bytes\n", rc);

????}

????

????lseek(fd, 0, SEEK_SET);????? /* 回到文件頭，重新更新音頻文件大小 */

????hdr.riff_sz = totle_size + 36;

????hdr.data_sz = totle_size;

????

????if (write(fd, &hdr, sizeof(hdr)) != sizeof(hdr)) {

????????fprintf(stderr, "arec: cannot write header\n");

????????return -1;

????}

????

????snd_pcm_drain(handle);

????snd_pcm_close(handle);

????free(buffer);

????return 0;

}

/**************************************************************/

int rec_wav(const char *fn)

{

????unsigned rate, channels;

????int fd;

????fd = open(fn, O_WRONLY | O_CREAT | O_TRUNC, 0664);

????if (fd < 0) {

????????fprintf(stderr, "arec: cannot open '%s'\n", fn);

????????return -1;

????}

????hdr.riff_id = ID_RIFF;

????hdr.riff_fmt = ID_WAVE;

????hdr.fmt_id = ID_FMT;

????hdr.audio_format = FORMAT_PCM;

????hdr.fmt_sz = 16;

????hdr.bits_per_sample = 16;

????hdr.num_channels = 1;

????hdr.data_sz = 0;????????????????????????/* TODO before record over */

????hdr.sample_rate = 16000;

????hdr.data_id = ID_DATA;

????

????hdr.byte_rate = hdr.sample_rate * hdr.num_channels * hdr.bits_per_sample / 8;

????hdr.block_align = hdr.num_channels * hdr.bits_per_sample / 8;

????

????if (write(fd, &hdr, sizeof(hdr)) != sizeof(hdr)) {

????????fprintf(stderr, "arec: cannot write header\n");

????????return -1;

????}

????fprintf(stderr,"arec: %d ch, %ld hz, %d bit, %s\n",

????????????hdr.num_channels, hdr.sample_rate, hdr.bits_per_sample,

????????????hdr.audio_format == FORMAT_PCM ? "PCM" : "unknown");

????

????return record_file(hdr.sample_rate, hdr.num_channels, fd, hdr.data_sz);

}

int main(int argc, char **argv)

{

????if (argc != 2) {

????????fprintf(stderr,"usage: arec <file>\n");

????????return -1;

????}

????return rec_wav(argv[1]);

}

對(duì)于上述代碼補(bǔ)充說(shuō)明一點(diǎn)，這個(gè)是設(shè)計(jì)ALSA的一點(diǎn)概念：

樣本長(zhǎng)度(sample)：樣本是記錄音頻數(shù)據(jù)最基本的單位，常見(jiàn)的有8位和16位。

通道數(shù)(channel)：該參數(shù)為1表示單聲道，2則是立體聲。

楨(frame)：楨記錄了一個(gè)聲音單元，其長(zhǎng)度為樣本長(zhǎng)度與通道數(shù)的乘積。

采樣率(rate)：每秒鐘采樣次數(shù)，該次數(shù)是針對(duì)楨而言。

周期(period)：音頻設(shè)備一次處理所需要的楨數(shù)，對(duì)于音頻設(shè)備的數(shù)據(jù)訪問(wèn)以及音頻數(shù)據(jù)的存儲(chǔ)，都是以此為單位。

交錯(cuò)模式(interleaved)：是一種音頻數(shù)據(jù)的記錄方式，在交錯(cuò)模式下，數(shù)據(jù)以連續(xù)楨的形式存放，即首先記錄完楨1的左聲道樣本和右聲道樣本（假設(shè)為立體聲格式），再開(kāi)始楨2的記錄。而在非交錯(cuò)模式下，首先記錄的是一個(gè)周期內(nèi)所有楨的左聲道樣本，再記錄右聲道樣本，數(shù)據(jù)是以連續(xù)通道的方式存儲(chǔ)。不過(guò)多數(shù)情況下，我們只需要使用交錯(cuò)模式就可以了。

具體可以參照：http://blog.chinaunix.net/uid-25272011-id-3151136.html

一開(kāi)始我犯過(guò)一個(gè)錯(cuò)誤就是rc = snd_pcm_readi(handle, buffer, frames)，這個(gè)函數(shù)的參數(shù)3的單位應(yīng)該是幀大小，而一個(gè)幀的大小是根據(jù)你的量化位數(shù)和聲道數(shù)決定的，對(duì)于本代碼，是16bit單聲道，自然一個(gè)幀大小是2字節(jié)，起初我將申請(qǐng)的buffer大小傳給了這個(gè)參數(shù)，結(jié)果必然導(dǎo)致“卡頓”或者“快進(jìn)”，“卡頓”是因?yàn)槲以陧?xiàng)目中是實(shí)時(shí)傳輸，會(huì)導(dǎo)致阻塞，畢竟數(shù)據(jù)量大了一倍，“快進(jìn)”則是因?yàn)榫彌_區(qū)的大小是實(shí)際讀取數(shù)據(jù)的一半，有一半的數(shù)據(jù)在buffer中被自己給覆蓋掉了，所以要慎重啊。

2、Makefile

exe = record

src = record.c

CC = arm-linux-gcc

INC = -I/nfs/usr/local/arm-alsa/include

LDFLAGS = -lpthread -L/nfs/usr/local/arm-alsa/lib -lasound

$(exe) : $(src) FORCE

????$(CC) -o $@ $(src) $(LDFLAGS) $(INC)

FORCE:

clean:

????rm -f ./*.o $(exe)

????? 此處的alsa-lib庫(kù)就是之前介紹的安裝的庫(kù)的路徑，編譯可以引用該路徑的庫(kù)，而運(yùn)行之后庫(kù)的路徑可不受限制，按照你定義的環(huán)境變量找到即可。
???? 對(duì)于內(nèi)核的配置則在
? ? ?

Device Drivers --->

<*> Sound card support --->

<*> Advanced Linux Sound Architecture --->

[*] USB sound devices --->

<*> USB Audio/MIDI driver

???? 對(duì)于這款USB麥克風(fēng)，我正常的去錄音的時(shí)候，上層的直觀感覺(jué)就是卡頓，這個(gè)與上面提到的是有區(qū)別的，因?yàn)橥瑯拥拇a在arm上是好的，所以就懷疑是底層讀慢了，（我們的應(yīng)用背景是開(kāi)發(fā)板實(shí)時(shí)錄音，通過(guò)USB-WIFI發(fā)到上位機(jī)同步播放）很明顯的是讀取音頻數(shù)據(jù)慢了。而同樣的代碼跑硬件的CODEC是很好的，不卡頓，所以很有可能問(wèn)題出在USB上。我們剛好有USB的協(xié)議分析儀，我們USB是跑的全速模式，其描述符為

Interface Descriptor:

??????bLength 9

??????bDescriptorType 4

??????bInterfaceNumber 3

??????bAlternateSetting 0

??????bNumEndpoints 0

??????bInterfaceClass 1

??????bInterfaceSubClass 2

??????bInterfaceProtocol 0

??????iInterface 0

????Interface Descriptor:

??????bLength 9

??????bDescriptorType 4

??????bInterfaceNumber 3

??????bAlternateSetting 1

??????bNumEndpoints 1

??????bInterfaceClass 1

??????bInterfaceSubClass 2

??????bInterfaceProtocol 0

??????iInterface 0

??????AudioStreaming Interface Descriptor:

????????bLength 7

????????bDescriptorType 36

????????bDescriptorSubtype 1 (AS_GENERAL)

????????bTerminalLink 3

????????bDelay 1 frames

????????wFormatTag 1 PCM

??????AudioStreaming Interface Descriptor:

????????bLength 11

????????bDescriptorType 36

????????bDescriptorSubtype 2 (FORMAT_TYPE)

????????bFormatType 1 (FORMAT_TYPE_I)

????????bNrChannels 1

????????bSubframeSize 2

????????bBitResolution 16

????????bSamFreqType 1 Discrete

????????tSamFreq[ 0] 16000

??????Endpoint Descriptor:

????????bLength 9

????????bDescriptorType 5

????????bEndpointAddress 0x83 EP 3 IN

????????bmAttributes 5

??????????Transfer Type Isochronous

??????????Synch Type Asynchronous

??????????Usage Type Data

????????wMaxPacketSize 0x0020 1x 32 bytes

????????bInterval 4

????????bRefresh 0

????????bSynchAddress 0

????????AudioControl Endpoint Descriptor:

??????????bLength 7

??????????bDescriptorType 37

??????????bDescriptorSubtype 1 (EP_GENERAL)

??????????bmAttributes 0x01

????????????Sampling Frequency

??????????bLockDelayUnits 0 Undefined

??????????wLockDelay 0 Undefined

/************************************************************************/

????Interface Descriptor:

??????bLength 9

??????bDescriptorType 4

??????bInterfaceNumber 3

??????bAlternateSetting 2

??????bNumEndpoints 1

??????bInterfaceClass 1

??????bInterfaceSubClass 2

??????bInterfaceProtocol 0

??????iInterface 0

??????AudioStreaming Interface Descriptor:

????????bLength 7

????????bDescriptorType 36

????????bDescriptorSubtype 1 (AS_GENERAL)

????????bTerminalLink 3

????????bDelay 1 frames

????????wFormatTag 1 PCM

??????AudioStreaming Interface Descriptor:

????????bLength 11

????????bDescriptorType 36

????????bDescriptorSubtype 2 (FORMAT_TYPE)

????????bFormatType 1 (FORMAT_TYPE_I)

????????bNrChannels 1

????????bSubframeSize 2

????????bBitResolution 16

????????bSamFreqType 1 Discrete

????????tSamFreq[ 0] 48000

??????Endpoint Descriptor:

????????bLength 9

????????bDescriptorType 5

????????bEndpointAddress 0x83 EP 3 IN

????????bmAttributes 5

??????????Transfer Type Isochronous

??????????Synch Type Asynchronous

??????????Usage Type Data

????????wMaxPacketSize 0x0060 1x 96 bytes

????????bInterval 4

????????bRefresh 0

????????bSynchAddress 0

????????AudioControl Endpoint Descriptor:

??????????bLength 7

??????????bDescriptorType 37

??????????bDescriptorSubtype 1 (EP_GENERAL)

??????????bmAttributes 0x01

????????????Sampling Frequency

??????????bLockDelayUnits 0 Undefined

??????????wLockDelay 0 Undefined

???
? ? 看到描述符，順便插一句，對(duì)照端點(diǎn)大小計(jì)算一下，藍(lán)色字體，這款USB-AUDIO只支持16K和48K的16bit單聲道錄音，拿16K為例，1s數(shù)據(jù)量應(yīng)該是16K*16/8=32KB，對(duì)應(yīng)于端點(diǎn)的大小32B*1000=32KB，也就是說(shuō)全速模式下應(yīng)該是每幀（1ms）請(qǐng)求一次才對(duì)，而對(duì)于圖中的紅色字體，說(shuō)明的意思是全速模式下的ISO傳輸請(qǐng)求間隔參數(shù)是4，對(duì)應(yīng)我們的USB的控制器，意思即為每8幀才發(fā)起一次ISO請(qǐng)求，抓包驗(yàn)證確實(shí)如此，這一點(diǎn)確實(shí)比較詭異，問(wèn)題可能就出在這里：

????? 但是對(duì)于幾乎同樣的驅(qū)動(dòng)版本，我們611的是2.6.32.9而arm的是2.6.32.2，其sound目錄下的usbaudio.c基本是相同的，描述符又是相同的，所以上層獲取描述符進(jìn)行的配置應(yīng)該也是相同的，唯一的區(qū)別就是USB的控制器，我們611的是musb，而arm的是OHCI，我們musb對(duì)這個(gè)bInterval的配置是

因此驅(qū)動(dòng)固然是8幀請(qǐng)求一次。我在OHCI的控制器中未找到類(lèi)似的寄存器，猜想可能是OHCI默認(rèn)的就是ISO傳輸每一幀會(huì)保證一次，其抓包圖如下：

所以只好強(qiáng)制去修改musb的驅(qū)動(dòng)配置，/drivers/usb/musb/musb_host.c

2013 /* precompute rxtype/txtype/type0 register */

2014 type_reg = (qh->type << 4) | qh->epnum;

2015 switch (urb->dev->speed) {

2016 case USB_SPEED_LOW:

2017 type_reg |= 0xc0;

2018 break;

2019 case USB_SPEED_FULL:

2020 type_reg |= 0x80;

2021 break;

2022 default:

2023 type_reg |= 0x40;

2024 }

2025 qh->type_reg = type_reg;

2026

2027 /* Precompute RXINTERVAL/TXINTERVAL register */

2028 switch (qh->type) {

2029 case USB_ENDPOINT_XFER_INT:

2030 /*

2031 * Full/low speeds use the linear encoding,

2032 * high speed uses the logarithmic encoding.

2033 */

2034 if (urb->dev->speed <= USB_SPEED_FULL) {

2035 interval = max_t(u8, epd->bInterval, 1);

2036 break;

2037 }

2038 /* FALLTHROUGH */

2039 case USB_ENDPOINT_XFER_ISOC:

2040 /* ISO always uses logarithmic encoding */

2041 //interval = min_t(u8, epd->bInterval, 16);

2042 interval = min_t(u8, epd->bInterval, 1); //JGF

2043 break;

2044 default:

這樣USB就是每幀請(qǐng)求一次，同樣的代碼，效果和2440的也一樣了。

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