avthumb参数acodec格式及解释
http://developer.qiniu.com/code/v6/api/dora-api/av/avthumb.html
参数 /acodec/<AudioCodec>：音频编码方案，支持方案：libmp3lame ， libfaac ， libvorbis ，libfdk_aac 等。
| audioEncoders | description |
|---|---|
| aac | AAC (Advanced Audio Coding) |
| libfaac | |
| ac3 | ATSC A/52A (AC-3) |
| ac3_fixed | ATSC A/52A (AC-3) (codec ac3) |
| adpcm_adx | SEGA CRI ADX ADPCM |
| g722 | G.722 ADPCM (codec adpcm_g722) |
| g726 | G.726 ADPCM (codec adpcm_g726) |
| adpcm_ima_qt | ADPCM IMA QuickTime |
| adpcm_ima_wav | ADPCM IMA WAV |
| adpcm_ms | ADPCM Microsoft |
| adpcm_swf | ADPCM Shockwave Flash |
| adpcm_yamaha | ADPCM Yamaha |
| alac | ALAC (Apple Lossless Audio Codec) |
| libopencore_amrnb | OpenCORE AMR-NB (Adaptive Multi-Rate Narrow-Band) (codec amr_nb) |
| comfortnoise | RFC 3389 comfort noise generator |
| dca | DCA (DTS Coherent Acoustics) (codec dts) |
| eac3 | ATSC A/52 E-AC-3 |
| flac | FLAC (Free Lossless Audio Codec) |
| g723_1 | G.723.1 |
| libilbc | iLBC (Internet Low Bitrate Codec) (codec ilbc) |
| mp2 | MPEG audio layer 2 High Frequency |
| libmp3lame | libmp3lame MP3 (MPEG audio layer 3) (codec mp3) |
| nellymoser | Nellymoser Asao |
| pcm_alaw | PCM A-law / G.711 A-law |
| pcm_f32be | PCM 32-bit floating point big-endian |
| pcm_f32le | PCM 32-bit floating point little-endian |
| pcm_f64be | PCM 64-bit floating point big-endian |
| pcm_f64le | PCM 64-bit floating point little-endian |
| pcm_mulaw | PCM mu-law / G.711 mu-law |
| pcm_s16be | PCM signed 16-bit big-endian |
| pcm_s16be_planar | PCM signed 16-bit big-endian planar |
| pcm_s16le | PCM signed 16-bit little-endian |
| pcm_s16le_planar | PCM signed 16-bit little-endian planar |
| pcm_s24be | PCM signed 24-bit big-endian |
| pcm_s24daud | PCM D-Cinema audio signed 24-bit |
| pcm_s24le | PCM signed 24-bit little-endian |
| pcm_s24le_planar | PCM signed 24-bit little-endian planar |
| pcm_s32be | PCM signed 32-bit big-endian |
| pcm_s32le | PCM signed 32-bit little-endian |
| pcm_s32le_planar | PCM signed 32-bit little-endian planar |
| pcm_s8 | CM signed 8-bit |
| pcm_s8_planar | PCM signed 8-bit planar |
| pcm_u16be | PCM unsigned 16-bit big-endian |
| pcm_u16le | PCM unsigned 16-bit little-endian |
| pcm_u24be | PCM unsigned 24-bit big-endian |
| pcm_u24le | PCM unsigned 24-bit little-endian |
| pcm_u32be | PCM unsigned 32-bit big-endian |
| pcm_u32le | PCM unsigned 32-bit little-endian |
| pcm_u8 | PCM unsigned 8-bit |
| real_144 | RealAudio 1.0 (14.4K) (codec ra_144) |
| roq_dpcm | id RoQ DPCM |
| s302m | SMPTE 302M |
| sonic | Sonic |
| sonicls | Sonic lossless |
| tta | TTA (True Audio) |
| vorbis | Vorbis |
| libvorbis | ibvorbis (codec vorbis) |
| wavpack | 无损音频压缩格式 |
| wmav1 | Windows Media Audio 1 |
| wmav2 | Windows Media Audio 2 |
| copy | 编码同原始音频 |
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