撰寫: 2022/1/25
發佈: 2022/2/16
在Raspberry Pi使用USB Audio裝置
為何要在Raspberry Pi上使用USB audio裝置呢?當你發現Raspberry Pi並沒有audio input端口來接麥克風，或甚至Raspberry Pi Zero連audio output端口都沒有的時候，你就會想辦法在Raspberry Pi上加上USB audio裝置來達到audio input/output的目的。
目前在Raspberry Pi上有相當多的USB audio裝置可以使用，只要是因為它們都使用 “C-Media” 晶片。
再加上新版的Raspbian作業系統已經支援Plug-n-Plug，所以只需要將USB audio裝置插入，輸入以下的命令檢查有哪一些USB裝置：
lsusb
可以看到以下回應：
Bus 001 Device 005: ID 8086:0808 Intel Corp.
Bus 001 Device 003: ID 0424:ec00 Standard Microsystems Corp. SMSC9512/9514 Fast Ethernet Adapter
Bus 001 Device 002: ID 0424:9514 Standard Microsystems Corp.
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
其中的Device 005就是我新插入的USB audio裝置
可以再以下列的命令檢查所有Message有關C-Media晶片：
dmesg | grep C-Media
搜尋結果如下：
[ 2.224342] usb 1-1.5: Manufacturer: C-Media Electronics Inc.
[ 2.230660] input: C-Media Electronics Inc. USB PnP Sound Device as /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.5/1-1.5:1.3/0003:8086:0808.0001/input/input0
[ 2.296082] hid-generic 0003:8086:0808.0001: input,hidraw0: USB HID v1.00 Device [C-Media Electronics Inc. USB PnP Sound Device] on usb-3f980000.usb-1.5/input3
[ 20.634107] usb 1-1.5: Manufacturer: C-Media Electronics Inc.
[ 20.670453] input: C-Media Electronics Inc. USB PnP Sound Device as /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.5/1-1.5:1.3/0003:8086:0808.0002/input/input1
[ 20.736492] hid-generic 0003:8086:0808.0002: input,hidraw0: USB HID v1.00 Device [C-Media Electronics Inc. USB PnP Sound Device] on usb-3f980000.usb-1.5/input3
表示確實有C-Media晶片USB audio裝置載入到系統中，所以就可以再以下列命令檢查所有註冊在系統中的所有audio播放裝置：
aplay -l
出現所有註冊在系統中的所有audio裝置如下：
**** List of PLAYBACK Hardware Devices ****
card 0: ALSA [bcm2835 ALSA], device 0: bcm2835 ALSA [bcm2835 ALSA]
Subdevices: 8/8
Subdevice #0: subdevice #0
Subdevice #1: subdevice #1
Subdevice #2: subdevice #2
Subdevice #3: subdevice #3
Subdevice #4: subdevice #4
Subdevice #5: subdevice #5
Subdevice #6: subdevice #6
Subdevice #7: subdevice #7
card 0: ALSA [bcm2835 ALSA], device 1: bcm2835 ALSA [bcm2835 IEC958/HDMI]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 1: Device [USB PnP Sound Device], device 0: USB Audio [USB Audio]
Subdevices: 1/1
Subdevice #0: subdevice #0
其中的card 0就是Raspberry Pi原來內建的音效裝置，card 0第二個很明顯顯示HDMI的音效，所以card 0第一個應該是3.5mm音效輸出。另外card 1就是我們新插入的USB Audio裝置。
設定USB Audio裝置為初始的Audio裝置
我們也可以將USB Audio裝置設定為初始的Audio裝置，在Raspberry Pi 中Audio裝置設定在系統中的alsa.conf檔案，因為是系統檔案，所有權限保護設定，要修改它需要super user權限，所以必須以super user方式修改：
sudo nano /usr/share/alsa/alsa.conf
在文中找到以下的訊息：
defaults.ctl.card 0
defaults.pcm.card 0
修改成為剛才找到的USB Audio裝置 card 1
defaults.ctl.card 1
defaults.pcm.card 1
採用[CTRL-X], [Y], [ENTER]方式，儲存檔案並回到命令欄模式。
或先將alsa.conf檔案拷貝在pi路徑，採用Text Editor編輯，在文中找到以下的訊息：
defaults.ctl.card 0
defaults.pcm.card 0
修改成為剛才找到的USB Audio裝置 card 1
defaults.ctl.card 1
defaults.pcm.card 1
儲存檔案離開並回到命令欄模式，以super user拷貝到/usr/share/alsa/路徑下。
設定USB Audio裝置聲音大小
在Raspberry Pi 中可以alsamixer來設定Audio裝置聲音大小，在系統終端機Terminal命令欄輸入下列命令：
alsamixer
終端機Terminal會出現如下的畫面可以設定 “Speaker”, “Mic” and “Auto Gain Control”：
在此可以用左右鍵在不同的 “Speaker”, “Mic”裝置通道間切換，用上下鍵可以改變Gain大小數值，在此特別要注意Mic裝置下方default顯示|MM|表示該裝置初始狀態是設為Mute靜音，因為我們要使用麥克風收音，所以可以按”M” 鍵改變切換為啟動模式。
另外Rasbain在版本可以直接在”偏好設定”中設定音效裝置，如下圖所示：
進入畫面因為初始設定為Raspberry Pi內建的BCM2835所以沒有可選的，可在音效卡欄位選取USB PnP Sound Device如下：
選取後就會出現與alsamixer類似同樣的設定畫面如下，照剛才方式設定就可以了。
不過此Audio Device設定功能在最新版Bullseye及Buster中被移除了，所以還是要以alsamixer來設定Audio裝置聲音大小，在如圖所示：
不過選擇音源可以直接在右上方喇叭圖示按右鍵就會出現選單可以選取。
測試喇叭Speak聲音輸出
將耳機或喇叭插入USB Audio裝置聲音輸出端，你就可以用Raspberry Pi 上的speaker-test公用程式來測試喇叭輸出：
speaker-test
正常喇叭會出現雜訊的聲音及以下的訊息：
speaker-test 1.0.28
Playback device is default
Stream parameters are 48000Hz, S16_LE, 1 channels
Using 16 octaves of pink noise
Rate set to 48000Hz (requested 48000Hz)
Buffer size range from 2048 to 16384
Period size range from 1024 to 1024
Using max buffer size 16384
Periods = 4
was set period_size = 1024
was set buffer_size = 16384
0 - Front Left
Time per period = 2.665282
0 - Front Left
可以發現目前只有左聲道輸出，而且只有pink noise雜訊的聲音一直重複。離開測試可以按CTRL-Z來結束。
至於如何用聲音方式來測雙聲道，可以輸入speaker-test -h來查看如何使用如下：
speaker-test 1.0.28
Usage: speaker-test [OPTION]...
-h,--help help
-D,--device playback device
-r,--rate stream rate in Hz
-c,--channels count of channels in stream
-f,--frequency sine wave frequency in Hz
-F,--format sample format
-b,--buffer ring buffer size in us
-p,--period period size in us
-P,--nperiods number of periods
-t,--test pink=use pink noise, sine=use sine wave, wav=WAV file
-l,--nloops specify number of loops to test, 0 = infinite
-s,--speaker single speaker test. Values 1=Left, 2=right, etc
-w,--wavfile Use the given WAV file as a test sound
-W,--wavdir Specify the directory containing WAV files
-m,--chmap Specify the channel map to override
-X,--force-frequency force frequencies outside the 30-8000hz range
Recognized sample formats are: S8 S16_LE S16_BE FLOAT_LE S32_LE S32_BE
可輸入以下命令來進行雙聲道及1kHz弦波測試如下：
speaker-test -c2 -t sine -f 1000
就會有左右聲道切換tone的聲音，訊息如下：
speaker-test 1.0.28
Playback device is default
Stream parameters are 48000Hz, S16_LE, 2 channels
Sine wave rate is 1000.0000Hz
Rate set to 48000Hz (requested 48000Hz)
Buffer size range from 2048 to 16384
Period size range from 1024 to 1024
Using max buffer size 16384
Periods = 4
was set period_size = 1024
was set buffer_size = 16384
0 - Front Left
1 - Front Right
Time per period = 5.657501
0 - Front Left
1 - Front Right
若要測試音效，可以下載簡單的WAV音效檔如下：
wget https://www.kozco.com/tech/piano2.wav
採用aplay進行播放：
aplay piano2.wav
Playing WAVE 'piano2.wav' : Signed 16 bit Little Endian, Rate 48000 Hz, Stereo
以下是補充aplay命令使用說明：
Usage: aplay [OPTION]... [FILE]...
-h, --help help
--version print current version
-l, --list-devices list all soundcards and digital audio devices
-L, --list-pcms list device names
-D, --device=NAME select PCM by name
-q, --quiet quiet mode
-t, --file-type TYPE file type (voc, wav, raw or au)
-c, --channels=# channels
-f, --format=FORMAT sample format (case insensitive)
-r, --rate=# sample rate
-d, --duration=# interrupt after # seconds
-M, --mmap mmap stream
-N, --nonblock nonblocking mode
-F, --period-time=# distance between interrupts is # microseconds
-B, --buffer-time=# buffer duration is # microseconds
--period-size=# distance between interrupts is # frames
--buffer-size=# buffer duration is # frames
-A, --avail-min=# min available space for wakeup is # microseconds
-R, --start-delay=# delay for automatic PCM start is # microseconds
(relative to buffer size if <= 0)
-T, --stop-delay=# delay for automatic PCM stop is # microseconds from xrun
-v, --verbose show PCM structure and setup (accumulative)
-V, --vumeter=TYPE enable VU meter (TYPE: mono or stereo)
-I, --separate-channels one file for each channel
-i, --interactive allow interactive operation from stdin
-m, --chmap=ch1,ch2,.. Give the channel map to override or follow
--disable-resample disable automatic rate resample
--disable-channels disable automatic channel conversions
--disable-format disable automatic format conversions
--disable-softvol disable software volume control (softvol)
--test-position test ring buffer position
--test-coef=# test coefficient for ring buffer position (default 8)
expression for validation is: coef * (buffer_size / 2)
--test-nowait do not wait for ring buffer - eats whole CPU
--max-file-time=# start another output file when the old file has recorded
for this many seconds
--process-id-file write the process ID here
--use-strftime apply the strftime facility to the output file name
--dump-hw-params dump hw_params of the device
--fatal-errors treat all errors as fatal
Recognized sample formats are: S8 U8 S16_LE S16_BE U16_LE U16_BE S24_LE S24_BE U24_LE U24_BE S32_LE S32_BE U32_LE U32_BE FLOAT_LE FLOAT_BE FLOAT64_LE FLOAT64_BE IEC958_SUBFRAME_LE IEC958_SUBFRAME_BE MU_LAW A_LAW IMA_ADPCM MPEG GSM SPECIAL S24_3LE S24_3BE U24_3LE U24_3BE S20_3LE S20_3BE U20_3LE U20_3BE S18_3LE S18_3BE U18_3LE U18_3BE G723_24 G723_24_1B G723_40 G723_40_1B DSD_U8 DSD_U16_LE
Some of these may not be available on selected hardware
The available format shortcuts are:
-f cd (16 bit little endian, 44100, stereo)
-f cdr (16 bit big endian, 44100, stereo)
-f dat (16 bit little endian, 48000, stereo)
測試麥克風Microphone聲音輸入
麥克風在Raspberry pi系統中可以arecord -l命令檢查所有註冊在系統中的所有audio錄音裝置：
arecord -l
**** List of CAPTURE Hardware Devices ****
card 1: Device [USB PnP Sound Device], device 0: USB Audio [USB Audio]
Subdevices: 1/1
Subdevice #0: subdevice #0
在此只能看到card 1一個裝置，這就是USB audio裝置帶出來的，因為Raspberry pi初始並沒有錄音裝置。
要測試錄音可以下列命令錄一段5secs的錄音檔：
arecord --format=S16_LE --duration=5 --rate=16000 --file-type=wav test.wav
Recording WAVE 'test.wav' : Signed 16 bit Little Endian, Rate 16000 Hz, Mono
記得用Ctrl-Z中斷，不然就會不斷5秒產生一個檔!
Raspbian早期版本(如Pixel 3)可能設定音效卡不支援低取樣率16000，這時可以採用建議的44100高取樣率，這測試的命令如下：
arecord --format=S16_LE --duration=5 --rate=44100 --file-type=wav test.wav
播放此檔就可用前面的aplay命令如下：
aplay --format=S16_LE --rate=16000 test.wav
或是：
aplay --format=S16_LE --rate=44100 test.wav
以下是補充arecord命令使用說明：
Usage: arecord [OPTION]... [FILE]...
-h, --help help
--version print current version
-l, --list-devices list all soundcards and digital audio devices
-L, --list-pcms list device names
-D, --device=NAME select PCM by name
-q, --quiet quiet mode
-t, --file-type TYPE file type (voc, wav, raw or au)
-c, --channels=# channels
-f, --format=FORMAT sample format (case insensitive)
-r, --rate=# sample rate
-d, --duration=# interrupt after # seconds
-M, --mmap mmap stream
-N, --nonblock nonblocking mode
-F, --period-time=# distance between interrupts is # microseconds
-B, --buffer-time=# buffer duration is # microseconds
--period-size=# distance between interrupts is # frames
--buffer-size=# buffer duration is # frames
-A, --avail-min=# min available space for wakeup is # microseconds
-R, --start-delay=# delay for automatic PCM start is # microseconds
(relative to buffer size if <= 0)
-T, --stop-delay=# delay for automatic PCM stop is # microseconds from xrun
-v, --verbose show PCM structure and setup (accumulative)
-V, --vumeter=TYPE enable VU meter (TYPE: mono or stereo)
-I, --separate-channels one file for each channel
-i, --interactive allow interactive operation from stdin
-m, --chmap=ch1,ch2,.. Give the channel map to override or follow
--disable-resample disable automatic rate resample
--disable-channels disable automatic channel conversions
--disable-format disable automatic format conversions
--disable-softvol disable software volume control (softvol)
--test-position test ring buffer position
--test-coef=# test coefficient for ring buffer position (default 8)
expression for validation is: coef * (buffer_size / 2)
--test-nowait do not wait for ring buffer - eats whole CPU
--max-file-time=# start another output file when the old file has recorded
for this many seconds
--process-id-file write the process ID here
--use-strftime apply the strftime facility to the output file name
--dump-hw-params dump hw_params of the device
--fatal-errors treat all errors as fatal
Recognized sample formats are: S8 U8 S16_LE S16_BE U16_LE U16_BE S24_LE S24_BE U24_LE U24_BE S32_LE S32_BE U32_LE U32_BE FLOAT_LE FLOAT_BE FLOAT64_LE FLOAT64_BE IEC958_SUBFRAME_LE IEC958_SUBFRAME_BE MU_LAW A_LAW IMA_ADPCM MPEG GSM SPECIAL S24_3LE S24_3BE U24_3LE U24_3BE S20_3LE S20_3BE U20_3LE U20_3BE S18_3LE S18_3BE U18_3LE U18_3BE G723_24 G723_24_1B G723_40 G723_40_1B DSD_U8 DSD_U16_LE
Some of these may not be available on selected hardware
The available format shortcuts are:
-f cd (16 bit little endian, 44100, stereo)
-f cdr (16 bit big endian, 44100, stereo)
-f dat (16 bit little endian, 48000, stereo)
另外可以進行一個回授測試
arecord --format=S16_LE --rate=16000 | aplay --format=S16_LE --rate=16000
或是
arecord --format=S16_LE --rate=44100 | aplay --format=S16_LE --rate=44100
後記
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