音频参数与FFMpeg转码音频

一、音频的本质

音频就是一段连续的波。

在AU里查看一首音乐，就是这样的界面。

• 主界面上分成了两个部分，分别是左右声道；
• 绿色条的高度就是声音的强度，可以理解为音量大小，最大为0dB。

把波形放大查看，

这绿绿的曲线，是不是很像股票，哈哈哈。

二、音频属性

还是使用MediaInfo查看音频的属性，然后对照着说。

2.1 采样率

在真实的物理世界中，声音是连续的。而在电子世界中，数据都是离散的，不管是声音，还是拍照、录像。当采样的频率很大，大到人的眼睛和耳朵都分辨不出来，那么人就认为是连续的。

iPhone的Retina屏幕概念，显示器的抖动算法，视频的原理……，都是这个原理。

采样率最直白的解释就是：一秒钟采样多少个点。

最常见的音频采样率有三种：8kHz系列，44.1kHz系列和48kHz系列。

• 音乐：一般是44.1kHz系列，手机中的有些铃声可能是22.05kHz；
• 电视、电影：一般是48kHz
• 语音通话：一般是8kHz或者16kHz

语音通话的采样率比音乐要低很多，这也就是另一篇博文里我说音乐下采样到通话中时，音质下降很厉害的原因。

在AU里直观查看，

图中一个个的点就是音频的采样点，44.1kHz意思就是：每一秒钟的音频，有44100个点。

2.2 位深

采样率描述的是：采样点在横坐标——时间维度上的密集程度。

位深描述的是：采样点在纵坐标——声压维度上的精度。打个比方：假如最小音量是1，最大音量是100，位深描述的就是精确到小数点后几位。如果不要小数点，音量就只能有100级；如果精确到小数点1位，音量就能有1000级，描述音量就会更加精准、细腻。

最常见的位深是16位。

2.3 声道

就是有几条音轨，反应在AU里，就是有几个绿条。

音乐一般都是双声道，电影有2.1声道，或者Dolby环绕立体声，包括：5.1声音、7.1声道14.1声道等，其中.1表示低音。

安卓系统在system/media/audio/include/system/audio-base.h里定义了很多的声道：

185enum {
     
186    AUDIO_CHANNEL_REPRESENTATION_POSITION   = 0x0u,
187    AUDIO_CHANNEL_REPRESENTATION_INDEX      = 0x2u,
188    AUDIO_CHANNEL_NONE                      = 0x0u,
189    AUDIO_CHANNEL_INVALID                   = 0xC0000000u,
190
191    AUDIO_CHANNEL_OUT_FRONT_LEFT            = 0x1u,
192    AUDIO_CHANNEL_OUT_FRONT_RIGHT           = 0x2u,
193    AUDIO_CHANNEL_OUT_FRONT_CENTER          = 0x4u,
194    AUDIO_CHANNEL_OUT_LOW_FREQUENCY         = 0x8u,
195    AUDIO_CHANNEL_OUT_BACK_LEFT             = 0x10u,
196    AUDIO_CHANNEL_OUT_BACK_RIGHT            = 0x20u,
197    AUDIO_CHANNEL_OUT_FRONT_LEFT_OF_CENTER  = 0x40u,
198    AUDIO_CHANNEL_OUT_FRONT_RIGHT_OF_CENTER = 0x80u,
199    AUDIO_CHANNEL_OUT_BACK_CENTER           = 0x100u,
200    AUDIO_CHANNEL_OUT_SIDE_LEFT             = 0x200u,
201    AUDIO_CHANNEL_OUT_SIDE_RIGHT            = 0x400u,
202    AUDIO_CHANNEL_OUT_TOP_CENTER            = 0x800u,
203    AUDIO_CHANNEL_OUT_TOP_FRONT_LEFT        = 0x1000u,
204    AUDIO_CHANNEL_OUT_TOP_FRONT_CENTER      = 0x2000u,
205    AUDIO_CHANNEL_OUT_TOP_FRONT_RIGHT       = 0x4000u,
206    AUDIO_CHANNEL_OUT_TOP_BACK_LEFT         = 0x8000u,
207    AUDIO_CHANNEL_OUT_TOP_BACK_CENTER       = 0x10000u,
208    AUDIO_CHANNEL_OUT_TOP_BACK_RIGHT        = 0x20000u,
209    AUDIO_CHANNEL_OUT_TOP_SIDE_LEFT         = 0x40000u,
210    AUDIO_CHANNEL_OUT_TOP_SIDE_RIGHT        = 0x80000u,
211    AUDIO_CHANNEL_OUT_MONO                  = 0x1u,     // OUT_FRONT_LEFT
212    AUDIO_CHANNEL_OUT_STEREO                = 0x3u,     // OUT_FRONT_LEFT | OUT_FRONT_RIGHT
213    AUDIO_CHANNEL_OUT_2POINT1               = 0xBu,     // OUT_FRONT_LEFT | OUT_FRONT_RIGHT | OUT_LOW_FREQUENCY
214    AUDIO_CHANNEL_OUT_2POINT0POINT2         = 0xC0003u, // OUT_FRONT_LEFT | OUT_FRONT_RIGHT | OUT_TOP_SIDE_LEFT | OUT_TOP_SIDE_RIGHT
215    AUDIO_CHANNEL_OUT_2POINT1POINT2         = 0xC000Bu, // OUT_FRONT_LEFT | OUT_FRONT_RIGHT | OUT_TOP_SIDE_LEFT | OUT_TOP_SIDE_RIGHT | OUT_LOW_FREQUENCY
216    AUDIO_CHANNEL_OUT_3POINT0POINT2         = 0xC0007u, // OUT_FRONT_LEFT | OUT_FRONT_CENTER | OUT_FRONT_RIGHT | OUT_TOP_SIDE_LEFT | OUT_TOP_SIDE_RIGHT
217    AUDIO_CHANNEL_OUT_3POINT1POINT2         = 0xC000Fu, // OUT_FRONT_LEFT | OUT_FRONT_CENTER | OUT_FRONT_RIGHT | OUT_TOP_SIDE_LEFT | OUT_TOP_SIDE_RIGHT | OUT_LOW_FREQUENCY
218    AUDIO_CHANNEL_OUT_QUAD                  = 0x33u,    // OUT_FRONT_LEFT | OUT_FRONT_RIGHT | OUT_BACK_LEFT | OUT_BACK_RIGHT
219    AUDIO_CHANNEL_OUT_QUAD_BACK             = 0x33u,    // OUT_QUAD
220    AUDIO_CHANNEL_OUT_QUAD_SIDE             = 0x603u,   // OUT_FRONT_LEFT | OUT_FRONT_RIGHT | OUT_SIDE_LEFT | OUT_SIDE_RIGHT
221    AUDIO_CHANNEL_OUT_SURROUND              = 0x107u,   // OUT_FRONT_LEFT | OUT_FRONT_RIGHT | OUT_FRONT_CENTER | OUT_BACK_CENTER
222    AUDIO_CHANNEL_OUT_PENTA                 = 0x37u,    // OUT_QUAD | OUT_FRONT_CENTER
223    AUDIO_CHANNEL_OUT_5POINT1               = 0x3Fu,    // OUT_FRONT_LEFT | OUT_FRONT_RIGHT | OUT_FRONT_CENTER | OUT_LOW_FREQUENCY | OUT_BACK_LEFT | OUT_BACK_RIGHT
224    AUDIO_CHANNEL_OUT_5POINT1_BACK          = 0x3Fu,    // OUT_5POINT1
225    AUDIO_CHANNEL_OUT_5POINT1_SIDE          = 0x60Fu,   // OUT_FRONT_LEFT | OUT_FRONT_RIGHT | OUT_FRONT_CENTER | OUT_LOW_FREQUENCY | OUT_SIDE_LEFT | OUT_SIDE_RIGHT
226    AUDIO_CHANNEL_OUT_5POINT1POINT2         = 0xC003Fu, // OUT_5POINT1 | OUT_TOP_SIDE_LEFT | OUT_TOP_SIDE_RIGHT
227    AUDIO_CHANNEL_OUT_5POINT1POINT4         = 0x2D03Fu, // OUT_5POINT1 | OUT_TOP_FRONT_LEFT | OUT_TOP_FRONT_RIGHT | OUT_TOP_BACK_LEFT | OUT_TOP_BACK_RIGHT
228    AUDIO_CHANNEL_OUT_6POINT1               = 0x13Fu,   // OUT_FRONT_LEFT | OUT_FRONT_RIGHT | OUT_FRONT_CENTER | OUT_LOW_FREQUENCY | OUT_BACK_LEFT | OUT_BACK_RIGHT | OUT_BACK_CENTER
229    AUDIO_CHANNEL_OUT_7POINT1               = 0x63Fu,   // OUT_FRONT_LEFT | OUT_FRONT_RIGHT | OUT_FRONT_CENTER | OUT_LOW_FREQUENCY | OUT_BACK_LEFT | OUT_BACK_RIGHT | OUT_SIDE_LEFT | OUT_SIDE_RIGHT
230    AUDIO_CHANNEL_OUT_7POINT1POINT2         = 0xC063Fu, // OUT_7POINT1 | OUT_TOP_SIDE_LEFT | OUT_TOP_SIDE_RIGHT
231    AUDIO_CHANNEL_OUT_7POINT1POINT4         = 0x2D63Fu, // OUT_7POINT1 | OUT_TOP_FRONT_LEFT | OUT_TOP_FRONT_RIGHT | OUT_TOP_BACK_LEFT | OUT_TOP_BACK_RIGHT
232
233    AUDIO_CHANNEL_IN_LEFT                   = 0x4u,
234    AUDIO_CHANNEL_IN_RIGHT                  = 0x8u,
235    AUDIO_CHANNEL_IN_FRONT                  = 0x10u,
236    AUDIO_CHANNEL_IN_BACK                   = 0x20u,
237    AUDIO_CHANNEL_IN_LEFT_PROCESSED         = 0x40u,
238    AUDIO_CHANNEL_IN_RIGHT_PROCESSED        = 0x80u,
239    AUDIO_CHANNEL_IN_FRONT_PROCESSED        = 0x100u,
240    AUDIO_CHANNEL_IN_BACK_PROCESSED         = 0x200u,
241    AUDIO_CHANNEL_IN_PRESSURE               = 0x400u,
242    AUDIO_CHANNEL_IN_X_AXIS                 = 0x800u,
243    AUDIO_CHANNEL_IN_Y_AXIS                 = 0x1000u,
244    AUDIO_CHANNEL_IN_Z_AXIS                 = 0x2000u,
245    AUDIO_CHANNEL_IN_BACK_LEFT              = 0x10000u,
246    AUDIO_CHANNEL_IN_BACK_RIGHT             = 0x20000u,
247    AUDIO_CHANNEL_IN_CENTER                 = 0x40000u,
248    AUDIO_CHANNEL_IN_LOW_FREQUENCY          = 0x100000u,
249    AUDIO_CHANNEL_IN_TOP_LEFT               = 0x200000u,
250    AUDIO_CHANNEL_IN_TOP_RIGHT              = 0x400000u,
251    AUDIO_CHANNEL_IN_VOICE_UPLINK           = 0x4000u,
252    AUDIO_CHANNEL_IN_VOICE_DNLINK           = 0x8000u,
253    AUDIO_CHANNEL_IN_MONO                   = 0x10u,     // IN_FRONT
254    AUDIO_CHANNEL_IN_STEREO                 = 0xCu,      // IN_LEFT | IN_RIGHT
255    AUDIO_CHANNEL_IN_FRONT_BACK             = 0x30u,     // IN_FRONT | IN_BACK
256    AUDIO_CHANNEL_IN_6                      = 0xFCu,     // IN_LEFT | IN_RIGHT | IN_FRONT | IN_BACK | IN_LEFT_PROCESSED | IN_RIGHT_PROCESSED
257    AUDIO_CHANNEL_IN_2POINT0POINT2          = 0x60000Cu, // IN_LEFT | IN_RIGHT | IN_TOP_LEFT | IN_TOP_RIGHT
258    AUDIO_CHANNEL_IN_2POINT1POINT2          = 0x70000Cu, // IN_LEFT | IN_RIGHT | IN_TOP_LEFT | IN_TOP_RIGHT | IN_LOW_FREQUENCY
259    AUDIO_CHANNEL_IN_3POINT0POINT2          = 0x64000Cu, // IN_LEFT | IN_CENTER | IN_RIGHT | IN_TOP_LEFT | IN_TOP_RIGHT
260    AUDIO_CHANNEL_IN_3POINT1POINT2          = 0x74000Cu, // IN_LEFT | IN_CENTER | IN_RIGHT | IN_TOP_LEFT | IN_TOP_RIGHT | IN_LOW_FREQUENCY
261    AUDIO_CHANNEL_IN_5POINT1                = 0x17000Cu, // IN_LEFT | IN_CENTER | IN_RIGHT | IN_BACK_LEFT | IN_BACK_RIGHT | IN_LOW_FREQUENCY
262    AUDIO_CHANNEL_IN_VOICE_UPLINK_MONO      = 0x4010u,   // IN_VOICE_UPLINK | IN_MONO
263    AUDIO_CHANNEL_IN_VOICE_DNLINK_MONO      = 0x8010u,   // IN_VOICE_DNLINK | IN_MONO
264    AUDIO_CHANNEL_IN_VOICE_CALL_MONO        = 0xC010u,   // IN_VOICE_UPLINK_MONO | IN_VOICE_DNLINK_MONO

2.4 格式

最常见的音频格式是 MP3。还有一些非常普遍的，比如：Flac、Ape、AAC、Ogg、Opus等。

安卓系统在/system/media/audio/include/system/audio-base.h定义的音频格式如下。

71typedef enum {
     
72    AUDIO_FORMAT_INVALID             = 0xFFFFFFFFu,
73    AUDIO_FORMAT_DEFAULT             = 0,
74    AUDIO_FORMAT_PCM                 = 0x00000000u,
75    AUDIO_FORMAT_MP3                 = 0x01000000u,
76    AUDIO_FORMAT_AMR_NB              = 0x02000000u,
77    AUDIO_FORMAT_AMR_WB              = 0x03000000u,
78    AUDIO_FORMAT_AAC                 = 0x04000000u,
79    AUDIO_FORMAT_HE_AAC_V1           = 0x05000000u,
80    AUDIO_FORMAT_HE_AAC_V2           = 0x06000000u,
81    AUDIO_FORMAT_VORBIS              = 0x07000000u,
82    AUDIO_FORMAT_OPUS                = 0x08000000u,
83    AUDIO_FORMAT_AC3                 = 0x09000000u,
84    AUDIO_FORMAT_E_AC3               = 0x0A000000u,
85    AUDIO_FORMAT_DTS                 = 0x0B000000u,
86    AUDIO_FORMAT_DTS_HD              = 0x0C000000u,
87    AUDIO_FORMAT_IEC61937            = 0x0D000000u,
88    AUDIO_FORMAT_DOLBY_TRUEHD        = 0x0E000000u,
89    AUDIO_FORMAT_EVRC                = 0x10000000u,
90    AUDIO_FORMAT_EVRCB               = 0x11000000u,
91    AUDIO_FORMAT_EVRCWB              = 0x12000000u,
92    AUDIO_FORMAT_EVRCNW              = 0x13000000u,
93    AUDIO_FORMAT_AAC_ADIF            = 0x14000000u,
94    AUDIO_FORMAT_WMA                 = 0x15000000u,
95    AUDIO_FORMAT_WMA_PRO             = 0x16000000u,
96    AUDIO_FORMAT_AMR_WB_PLUS         = 0x17000000u,
97    AUDIO_FORMAT_MP2                 = 0x18000000u,
98    AUDIO_FORMAT_QCELP               = 0x19000000u,
99    AUDIO_FORMAT_DSD                 = 0x1A000000u,
100    AUDIO_FORMAT_FLAC                = 0x1B000000u,
101    AUDIO_FORMAT_ALAC                = 0x1C000000u,
102    AUDIO_FORMAT_APE                 = 0x1D000000u,
103    AUDIO_FORMAT_AAC_ADTS            = 0x1E000000u,
104    AUDIO_FORMAT_SBC                 = 0x1F000000u,
105    AUDIO_FORMAT_APTX                = 0x20000000u,
106    AUDIO_FORMAT_APTX_HD             = 0x21000000u,
107    AUDIO_FORMAT_AC4                 = 0x22000000u,
108    AUDIO_FORMAT_LDAC                = 0x23000000u,
109    AUDIO_FORMAT_MAT                 = 0x24000000u,
110    AUDIO_FORMAT_MAIN_MASK           = 0xFF000000u,
111    AUDIO_FORMAT_SUB_MASK            = 0x00FFFFFFu,
112
113    /* Subformats */
114    AUDIO_FORMAT_PCM_SUB_16_BIT        = 0x1u,
115    AUDIO_FORMAT_PCM_SUB_8_BIT         = 0x2u,
116    AUDIO_FORMAT_PCM_SUB_32_BIT        = 0x3u,
117    AUDIO_FORMAT_PCM_SUB_8_24_BIT      = 0x4u,
118    AUDIO_FORMAT_PCM_SUB_FLOAT         = 0x5u,
119    AUDIO_FORMAT_PCM_SUB_24_BIT_PACKED = 0x6u,
120
121    AUDIO_FORMAT_MP3_SUB_NONE          = 0x0u,
122
123    AUDIO_FORMAT_AMR_SUB_NONE          = 0x0u,
124
125    AUDIO_FORMAT_AAC_SUB_MAIN          = 0x1u,
126    AUDIO_FORMAT_AAC_SUB_LC            = 0x2u,
127    AUDIO_FORMAT_AAC_SUB_SSR           = 0x4u,
128    AUDIO_FORMAT_AAC_SUB_LTP           = 0x8u,
129    AUDIO_FORMAT_AAC_SUB_HE_V1         = 0x10u,
130    AUDIO_FORMAT_AAC_SUB_SCALABLE      = 0x20u,
131    AUDIO_FORMAT_AAC_SUB_ERLC          = 0x40u,
132    AUDIO_FORMAT_AAC_SUB_LD            = 0x80u,
133    AUDIO_FORMAT_AAC_SUB_HE_V2         = 0x100u,
134    AUDIO_FORMAT_AAC_SUB_ELD           = 0x200u,
135    AUDIO_FORMAT_AAC_SUB_XHE           = 0x300u,
136
137    AUDIO_FORMAT_VORBIS_SUB_NONE       = 0x0u,
138
139    AUDIO_FORMAT_E_AC3_SUB_JOC         = 0x1u,
140
141    AUDIO_FORMAT_MAT_SUB_1_0           = 0x1u,
142    AUDIO_FORMAT_MAT_SUB_2_0           = 0x2u,
143    AUDIO_FORMAT_MAT_SUB_2_1           = 0x3u,
144
145    /* Aliases */
146    AUDIO_FORMAT_PCM_16_BIT            = 0x1u,        // (PCM | PCM_SUB_16_BIT)
147    AUDIO_FORMAT_PCM_8_BIT             = 0x2u,        // (PCM | PCM_SUB_8_BIT)
148    AUDIO_FORMAT_PCM_32_BIT            = 0x3u,        // (PCM | PCM_SUB_32_BIT)
149    AUDIO_FORMAT_PCM_8_24_BIT          = 0x4u,        // (PCM | PCM_SUB_8_24_BIT)
150    AUDIO_FORMAT_PCM_FLOAT             = 0x5u,        // (PCM | PCM_SUB_FLOAT)
151    AUDIO_FORMAT_PCM_24_BIT_PACKED     = 0x6u,        // (PCM | PCM_SUB_24_BIT_PACKED)
152    AUDIO_FORMAT_AAC_MAIN              = 0x4000001u,  // (AAC | AAC_SUB_MAIN)
153    AUDIO_FORMAT_AAC_LC                = 0x4000002u,  // (AAC | AAC_SUB_LC)
154    AUDIO_FORMAT_AAC_SSR               = 0x4000004u,  // (AAC | AAC_SUB_SSR)
155    AUDIO_FORMAT_AAC_LTP               = 0x4000008u,  // (AAC | AAC_SUB_LTP)
156    AUDIO_FORMAT_AAC_HE_V1             = 0x4000010u,  // (AAC | AAC_SUB_HE_V1)
157    AUDIO_FORMAT_AAC_SCALABLE          = 0x4000020u,  // (AAC | AAC_SUB_SCALABLE)
158    AUDIO_FORMAT_AAC_ERLC              = 0x4000040u,  // (AAC | AAC_SUB_ERLC)
159    AUDIO_FORMAT_AAC_LD                = 0x4000080u,  // (AAC | AAC_SUB_LD)
160    AUDIO_FORMAT_AAC_HE_V2             = 0x4000100u,  // (AAC | AAC_SUB_HE_V2)
161    AUDIO_FORMAT_AAC_ELD               = 0x4000200u,  // (AAC | AAC_SUB_ELD)
162    AUDIO_FORMAT_AAC_XHE               = 0x4000300u,  // (AAC | AAC_SUB_XHE)
163    AUDIO_FORMAT_AAC_ADTS_MAIN         = 0x1e000001u, // (AAC_ADTS | AAC_SUB_MAIN)
164    AUDIO_FORMAT_AAC_ADTS_LC           = 0x1e000002u, // (AAC_ADTS | AAC_SUB_LC)
165    AUDIO_FORMAT_AAC_ADTS_SSR          = 0x1e000004u, // (AAC_ADTS | AAC_SUB_SSR)
166    AUDIO_FORMAT_AAC_ADTS_LTP          = 0x1e000008u, // (AAC_ADTS | AAC_SUB_LTP)
167    AUDIO_FORMAT_AAC_ADTS_HE_V1        = 0x1e000010u, // (AAC_ADTS | AAC_SUB_HE_V1)
168    AUDIO_FORMAT_AAC_ADTS_SCALABLE     = 0x1e000020u, // (AAC_ADTS | AAC_SUB_SCALABLE)
169    AUDIO_FORMAT_AAC_ADTS_ERLC         = 0x1e000040u, // (AAC_ADTS | AAC_SUB_ERLC)
170    AUDIO_FORMAT_AAC_ADTS_LD           = 0x1e000080u, // (AAC_ADTS | AAC_SUB_LD)
171    AUDIO_FORMAT_AAC_ADTS_HE_V2        = 0x1e000100u, // (AAC_ADTS | AAC_SUB_HE_V2)
172    AUDIO_FORMAT_AAC_ADTS_ELD          = 0x1e000200u, // (AAC_ADTS | AAC_SUB_ELD)
173    AUDIO_FORMAT_AAC_ADTS_XHE          = 0x1e000300u, // (AAC_ADTS | AAC_SUB_XHE)
174    AUDIO_FORMAT_E_AC3_JOC             = 0xA000001u,  // (E_AC3 | E_AC3_SUB_JOC)
175    AUDIO_FORMAT_MAT_1_0               = 0x24000001u, // (MAT | MAT_SUB_1_0)
176    AUDIO_FORMAT_MAT_2_0               = 0x24000002u, // (MAT | MAT_SUB_2_0)
177    AUDIO_FORMAT_MAT_2_1               = 0x24000003u, // (MAT | MAT_SUB_2_1)
178} audio_format_t;

对于一个未压缩的双声道、16bit位深、44.1kHz采样率的音频，它的码率是：2 * 16 * 44100/8，这个码率对于早期的网络来说，压力还是比较大的。

而经过MP3、AAC这样的压缩格式后，码率能显著降低到100 ~ 320kbps左右，而且音质损失在可控程度。

音频的压缩主要是基于频率掩蔽效应和时域掩蔽效应，以及人的听力频率范围20 ~ 20kHz。

2.5 Format Profile

与视频压缩的Profile类似，不同的Profile适用不同的场景，AAC格式下最常见的Profile就是LC，Low Complex，低复杂度规格。

2.6 有损

MP3、AAC、Ogg都属于有损压缩格式，意思是：压缩后，再解压回来，已经恢复不到原本的样子了。

2.7 帧率

音频的帧率比较有特点，音频的帧率与音频格式和采样率有关。

• 一帧的采样点数量：
  • AAC： 1024；
  • MP3： 1052；
  • 语音：20 ms
• 帧率：采样率/一帧的采样点数量。

也就是说，如果采样率不同，帧率也是不同的。

截图中的帧率 43.066 = 44100/1024。

三、使用FFMpeg对音频进行转码

使用ffmpeg对音频文件进行转格式。

ffmpeg -i input.mp3 -c:a aac output.aac

使用特定的编码器进行编码。

ffmpeg -i input.mp3 -c:a libfdk_aac output.aac

转码时指定Profile、码率等其他参数。

ffmpeg -i input.mp3 -c:a libfdk_aac -profile:a lc -b:a 160k output.aac