近期有个新的需求，定期在服务器上播放某段音频。所以研究了一下如何在linux上播放音乐、调整音量。
ALSA
ALSA(Advanced Linux Sound Architecture)为Linux提供了音频和MIDI功能，具有如下特点(笔者手动翻译欢迎勘误)。
- 支持各类音频接口，包括定制声卡和专业多通道音频接口；
- 完全模块化的音频驱动；
- SMP和线程安全设计；
- 用户态函数库(alsa-lib)简化二次开发；
- 支持OSS的大部分程序。
只是简单地播放音频，用不上二次开发，只需要两个指令aplay和alsamixer。
使用手册指令man来查看aplay的手册。
man aplay
如果不清楚
man是什么，可以用指令man man来查看man的手册。MAN(1) Manual pager utils MAN(1) NAME man - an interface to the system reference manuals SYNOPSIS man [man options] [[section] page ...] ... man -k [apropos options] regexp ... man -K [man options] [section] term ... man -f [whatis options] page ... man -l [man options] file ... man -w|-W [man options] page ... DESCRIPTION man is the system's manual pager. Each page argument given to man is normally the name of a program, utility or function. The manual page associated with each of these arguments is then found and displayed. A section, if provided, will direct man to look only in that section of the manual. The default action is to search in all of the available sections following a pre-defined order (see DEFAULTS), and to show only the first page found, even if page exists in several sections. The table below shows the section numbers of the manual followed by the types of pages they contain. 1 Executable programs or shell commands 2 System calls (functions provided by the kernel) 3 Library calls (functions within program libraries) 4 Special files (usually found in /dev) 5 File formats and conventions, e.g. /etc/passwd 6 Games 7 Miscellaneous (including macro packages and conventions), e.g. man(7), groff(7) Manual page man(1) line 1 (press h for help or q to quit)
通过查阅aplay的手册我们得知：
- 该指令是一个音频播放指令；
- 与其对应的还有一个
arecord录音指令； - 该指令可以传入多个文件；
- 未指定文件的会默认使用
stdin和stdout； - 采样率等音频信息可以自动地从文件头读取。
并且该指令还能列出音频设备。
继续查阅alsamixer的手册。
man alsamixer
ALSAMIXER(1) General Commands Manual ALSAMIXER(1) NAME alsamixer - soundcard mixer for ALSA soundcard driver, with ncurses interface SYNOPSIS alsamixer [options] DESCRIPTION alsamixer is an ncurses mixer program for use with the ALSA soundcard drivers. It supports multiple soundcards with multiple devices. OPTIONS -h, --help Help: show available flags. -c, --card <card number or identification> Select the soundcard to use, if you have more than one. Cards are numbered from 0 (the default). -D, --device <device identification> Select the mixer device to control. -V, --view <mode> Select the starting view mode, either playback, capture or all. -g, --no-color Toggle the using of colors. MIXER VIEWS The top-left corner of alsamixer is the are to show some basic information: the card name, the mixer chip name, the current view mode and the currently selected mixer item. When the mixer item is switched off, [Off] is displayed in its name. Volume bars are located below the basic information area. You can scroll left/right when all controls can't be put in a single screen. The name of each control is shown in the bottom be‐ low the volume bars. The currently selected item is drawn in red and/of emphasized. Manual page alsamixer(1) line 1 (press h for help or q to quit)
通过查阅alsamixer的手册我们得知，这是一个混音器，显然可以调整音量，并且是一个有命令行GUI的指令。
该指令有一个问题，不能通过命令直接设置音量，必须经过UI去调整。笔者最终希望通过配置文件的方式来设置默认音量，所以这个工具并不满足要求。
好在ALSA有另一个混音器工具amixer，同样查看手册。
AMIXER(1) General Commands Manual AMIXER(1) NAME amixer - command-line mixer for ALSA soundcard driver SYNOPSIS amixer [-option] [cmd] DESCRIPTION amixer allows command-line control of the mixer for the ALSA soundcard driver. amixer sup‐ ports multiple soundcards. amixer with no arguments will display the current mixer settings for the default soundcard and device. This is a good way to see a list of the simple mixer controls you can use. COMMANDS help Shows syntax. info Shows the information about a mixer device. scontrols Shows a complete list of simple mixer controls. scontents Shows a complete list of simple mixer controls with their contents. set or sset <SCONTROL> <PARAMETER> ... Sets the simple mixer control contents. The parameter can be the volume either as a percentage from 0% to 100% with % suffix, a dB gain with dB suffix (like -12.5dB), or an exact hardware value. The dB gain can be used only for the mixer elements with available dB information. When plus(+) or minus(-) letter is appended after volume value, the volume is incremented or decremented from the current value, respectively. The parameters cap, nocap, mute, unmute, toggle are used to change capture (recording) and muting for the group specified. The optional modifiers can be put as extra parameters to specify the stream direction or channels to apply. The modifiers playback and capture specify the stream, and the modifiers front, rear, center, woofer are used to specify channels to be changed. A simple mixer control must be specified. Only one device can be controlled at a time. get or sget <SCONTROL> Shows the simple mixer control contents. A simple mixer control must be specified. Only one device can be controlled at a time. controls Shows a complete list of card controls. contents Shows a complete list of card controls with their contents. Manual page amixer(1) line 1 (press h for help or q to quit)
注意到set指令可以设置音量，而且还有百分比和分贝两种单位可选，完美符合需求。
检视音频设备
使用指令sudo aplay -l来查看所有音频设备。这里以笔者的树莓派为例。
$ sudo aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: Headphones [bcm2835 Headphones], device 0: bcm2835 Headphones [bcm2835 Headphones]
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 1: vc4hdmi0 [vc4-hdmi-0], device 0: MAI PCM i2s-hifi-0 [MAI PCM i2s-hifi-0]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 2: vc4hdmi1 [vc4-hdmi-1], device 0: MAI PCM i2s-hifi-0 [MAI PCM i2s-hifi-0]
Subdevices: 1/1
Subdevice #0: subdevice #0
一共有三个设备，其中card1和card2都是树莓派的HDMI接口，笔者的音响(其实是耳机)接在3.5mm音频接口上，所以应该是这里的card0，也就是Headphones设备。
使用指令sudo aplay -L查看所有PCM(Pulse-code modulation 脉冲编码调制)。
$ sudo aplay -L
null
Discard all samples (playback) or generate zero samples (capture)
hw:CARD=Headphones,DEV=0
bcm2835 Headphones, bcm2835 Headphones
Direct hardware device without any conversions
plughw:CARD=Headphones,DEV=0
bcm2835 Headphones, bcm2835 Headphones
Hardware device with all software conversions
default:CARD=Headphones
bcm2835 Headphones, bcm2835 Headphones
Default Audio Device
sysdefault:CARD=Headphones
bcm2835 Headphones, bcm2835 Headphones
Default Audio Device
dmix:CARD=Headphones,DEV=0
bcm2835 Headphones, bcm2835 Headphones
Direct sample mixing device
hw:CARD=vc4hdmi0,DEV=0
vc4-hdmi-0, MAI PCM i2s-hifi-0
Direct hardware device without any conversions
plughw:CARD=vc4hdmi0,DEV=0
vc4-hdmi-0, MAI PCM i2s-hifi-0
Hardware device with all software conversions
default:CARD=vc4hdmi0
vc4-hdmi-0, MAI PCM i2s-hifi-0
Default Audio Device
sysdefault:CARD=vc4hdmi0
vc4-hdmi-0, MAI PCM i2s-hifi-0
Default Audio Device
hdmi:CARD=vc4hdmi0,DEV=0
vc4-hdmi-0, MAI PCM i2s-hifi-0
HDMI Audio Output
dmix:CARD=vc4hdmi0,DEV=0
vc4-hdmi-0, MAI PCM i2s-hifi-0
Direct sample mixing device
hw:CARD=vc4hdmi1,DEV=0
vc4-hdmi-1, MAI PCM i2s-hifi-0
Direct hardware device without any conversions
plughw:CARD=vc4hdmi1,DEV=0
vc4-hdmi-1, MAI PCM i2s-hifi-0
Hardware device with all software conversions
default:CARD=vc4hdmi1
vc4-hdmi-1, MAI PCM i2s-hifi-0
Default Audio Device
sysdefault:CARD=vc4hdmi1
vc4-hdmi-1, MAI PCM i2s-hifi-0
Default Audio Device
hdmi:CARD=vc4hdmi1,DEV=0
vc4-hdmi-1, MAI PCM i2s-hifi-0
HDMI Audio Output
dmix:CARD=vc4hdmi1,DEV=0
vc4-hdmi-1, MAI PCM i2s-hifi-0
Direct sample mixing device
注意其中的bcm2835 Headphones，这表明对应的设备是我们的耳机。
播放音频
了解了设备之后可以开始播放音频了。
笔者的工作目录具有如下结构。
$ pwd
/home/pi/Musics
$ tree
.
└── 01 Love Never Felt So Good.wav
0 directories, 1 file
通过设备Headphones播放这首歌。
可以通过Tab自动补全文件名。
$ aplay -D plughw:Headphones 01\ Love\ Never\ Felt\ So\ Good.wav
Playing WAVE '01 Love Never Felt So Good.wav' : Signed 16 bit Little Endian, Rate 44100 Hz, Stereo
注意到Michael悦耳的歌声从耳机中传出，这表明我们成功播放了音乐。
调整音量
使用amixer来调整音量。
$ amixer
Simple mixer control 'Headphone',0
Capabilities: pvolume pvolume-joined pswitch pswitch-joined
Playback channels: Mono
Limits: Playback -10239 - 400
Mono: Playback -664 [90%] [-6.64dB] [on]
记得刚才的Headphones，这里设置音量的指令需要这个参数。比如将音量设置为$90\%$。
$ amixer -c Headphones set Headphone 90%
Simple mixer control 'Headphone',0
Capabilities: pvolume pvolume-joined pswitch pswitch-joined
Playback channels: Mono
Limits: Playback -10239 - 400
Mono: Playback -664 [90%] [-6.64dB] [on]
-c后面跟的是刚才列出的PCM名称，而set后面的是amixer控制的名称，不要搞混，有些机器上可能不一样。
总结
手册是个好东西。