用 lame 把 wav 转换成 mp3

前言

项目中用到了录制音频转换成 mp3 后上传，网上查的都是 OC 写的，这里用的 swift 4.2，记录一下。

录制

recorder = try AVAudioRecorder(url: filePathURL, settings: [
                AVFormatIDKey: Int(kAudioFormatLinearPCM),
                AVSampleRateKey: 44100,
                AVNumberOfChannelsKey: 1,
                AVLinearPCMBitDepthKey: 16,
                AVEncoderAudioQualityKey:AVAudioQuality.high.rawValue
            ])

编译 lame 静态库

https://github.com/wuqiong/mp3lame-for-iOS

转换

var read = 0
var write = 0
            
let pcm = fopen(wavPath, "rb")
fseek(pcm, 4*1024, SEEK_CUR)
let mp3 = fopen(mp3Path, "wb+")
            
let pcmSize = 8192
let mp3Size = 8192
            
let pcmBuffer = UnsafeMutablePointer<Int16>.allocate(capacity: Int(pcmSize*2))
let mp3Buffer = UnsafeMutablePointer<UInt8>.allocate(capacity: Int(mp3Size))

            
let lame = lame_init()
lame_set_num_channels(lame, 1)
//                lame_set_mode(lame, MONO)
lame_set_in_samplerate(lame, 22050)
//                lame_set_brate(lame, rate)
lame_set_VBR(lame, vbr_default)
lame_init_params(lame)
            
repeat {
		read = fread(pcmBuffer, 2*MemoryLayout<Int16>.size, pcmSize, pcm)
		if read == 0 {
				write = Int(lame_encode_flush(lame, mp3Buffer, Int32(mp3Size)))
		} else {
				write = Int(lame_encode_buffer_interleaved(lame, pcmBuffer, Int32(read), mp3Buffer, Int32(mp3Size)))
		}
                
		print("write\(write)")
		fwrite(mp3Buffer, write, 1, mp3)
                
} while read != 0
//                lame_mp3_tags_fid(lame, mp3)
lame_close(lame)
fclose(mp3)
fclose(pcm)

遗留问题

录制时候采样率为 44100，转换时如果设置 44100 的采样率，发现会播放两倍速，减半成 22050 以后正常，这个等我学习点理论知识以后再来补充。