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iOS录音生成Wav格式问题

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iPhone的音质一般，音量偏小，作为听个响系列，表现也是不置可否，这篇文章总结一下，iOS本地录音生成Wav文件格式遇到的一个问题。

Wav文件格式错误

之前开发一个项目，里面需要通过录音生成wav文件，提交给后台实现声纹注册，当我调用系统的API生成Wav文件上传后，后台报文件格式错误，然而android生成的wav文件却没有报这个错….

问题定位

  1. 是否iOS本地生成的Wav文件格式有问题
    使用itunes文件共享功能，将生成的Wav文件导出，进行本地端（Mac OS）播放正常，在windwos端播放正常。

初略的认定生成的Wav格式没有问题。

  1. 是否wav文件上传功能出现问题
    借用android端生成的wav文件重新发送，提示声纹注册成功。排除本地端文件上传功能有问题

  2. 后台解析Wav文件是不是有什么问题？
    后台说不是他这边报的文件格式错误，他只是做了中间转发，错误是微软后台那边报的（项目是跟微软内部合作）

  3. 是不是对录音的参数有什么要求，采样率，位深….?
    各种找人，各种甩锅之后，找到了内部跟微软对接需求的人（DNS迭代查询即视感），没有详细文档，没有参数限制，让我自己试….

  4. 分析两边Wav文件格式的差异
    对比之后，发现苹果生成wav文件格式不是标准的wav协议头的文件。。。

WAV简介

WAVE(Waveform Audio File Format), 采用RIFF（Resource Interchange File Format）文件格式结构

WAV格式的音频文件通常用来保存PCM格式的原始音频数据，通常被称之为无损音频。

WAV音频文件，粗略来说是WAV数据头+PCM数据组成的，裸数据PCM外面包了一层文件头，WAV实质是一个RIFF文件

WAV数据头

关于 WAV 音频文件的数据头定义如下图所示:

一般的 WAV 文件的数据头为 44 个字节, 其后面跟的是 PCM 数据

分析wav数据头

使用hexdump或者xd来看一下 WAV 文件的数据头

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hexdump -n 44 m.wav      //查看前 44 个字节

这是标准的44字节的wav文件头，来看下iOS录音生成wav文件的wav格式

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NSMutableDictionary* settingDic=@{}.mutableCopy;
[settingDic setObject:@(kAudioFormatLinearPCM) forKey:AVFormatIDKey];
[settingDic setValue:@(44100) forKey:AVSampleRateKey];
[settingDic setValue:@(2) forKey:AVNumberOfChannelsKey];
[settingDic setValue:@(16) forKey:AVLinearPCMBitDepthKey];
[settingDic setValue:@(AVAudioQualityHigh) forKey:AVEncoderAudioQualityKey];
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Jack-Mac-mini:~ Jack$ afinfo /Users/Jack/Desktop/Audio/test.wav
File:           /Users/Jack/Desktop/Audio/test.wav
File type ID:   WAVE
Num Tracks:     1
----
Data format:     2 ch,  44100 Hz, 'lpcm' (0x0000000C) 16-bit little-endian signed integer
                no channel layout.
estimated duration: 17.507846 sec
audio bytes: 3088384
audio packets: 772096
bit rate: 1411200 bits per second
packet size upper bound: 4
maximum packet size: 4
audio data file offset: 4096
optimized
source bit depth: I16
----
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Jack-Mac-mini:~ Jack$ xd -cl  4096  /Users/Jack/Desktop/Audio/test.wav
File name: /Users/Jack/Desktop/Audio/test.wav

00000000: 52 49 46 46 f8 2f 2f 00  57 41 56 45 4a 55 4e 4b     RIFF.//.WAVEJUNK
00000010: 1c 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00     ................
00000020: 00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00     ................
00000030: 66 6d 74 20 10 00 00 00  01 00 02 00 44 ac 00 00     fmt.........D...
00000040: 10 b1 02 00 04 00 10 00  46 4c 4c 52 a8 0f 00 00     ........FLLR....
00000050: 00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00     ................
00000060: 00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00     ................
****
00000fd0: 00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00     ................
00000fe0: 00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00     ................
00000ff0: 00 00 00 00 00 00 00 00  64 61 74 61 00 20 2f 00     ........data../.
Total 4096 Byte

生成的WAV 文件的数据头为 4096 个字节

转换思路

AVAudioRecorder录音采样生成PCM文件格式，PCM转成标准44字节的Wav文件格式

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/**
 PCM 转 Wav
 
 @param originPath PCM Path
 @return Wav Path
 */
-(NSString*)transformPCM2Wav:(NSString*)originPath
{
    NSString* outPath=[[originPath stringByDeletingPathExtension] stringByAppendingString:@".wav"];
    NSData* data=[NSData dataWithContentsOfFile:originPath];
    BOOL isSuccess=[[self writeWavHead:data] writeToFile:outPath atomically:YES];
    if (isSuccess) {
        return outPath;
    }
    else
    {
        return nil;
    }
}
// 为pcm文件写入wav头
-(NSData*)writeWavHead:(NSData *)audioData {
    long sampleRate = SampleRateKey;
    long numOfChannelsKey = NumberOfChannels;
    Byte waveHead[44];
    waveHead[0] = 'R';
    waveHead[1] = 'I';
    waveHead[2] = 'F';
    waveHead[3] = 'F';
    
    long totalDatalength = [audioData length] + 44;
    waveHead[4] = (Byte)(totalDatalength & 0xff);
    waveHead[5] = (Byte)((totalDatalength >> 8) & 0xff);
    waveHead[6] = (Byte)((totalDatalength >> 16) & 0xff);
    waveHead[7] = (Byte)((totalDatalength >> 24) & 0xff);
    
    waveHead[8] = 'W';
    waveHead[9] = 'A';
    waveHead[10] = 'V';
    waveHead[11] = 'E';
    
    waveHead[12] = 'f';
    waveHead[13] = 'm';
    waveHead[14] = 't';
    waveHead[15] = ' ';
    
    waveHead[16] = 16;  //size of 'fmt '
    waveHead[17] = 0;
    waveHead[18] = 0;
    waveHead[19] = 0;
    
    waveHead[20] = 1;   //format
    waveHead[21] = 0;
    
    waveHead[22] = numOfChannelsKey;   //chanel
    waveHead[23] = 0;
    
    waveHead[24] = (Byte)(sampleRate & 0xff);
    waveHead[25] = (Byte)((sampleRate >> 8) & 0xff);
    waveHead[26] = (Byte)((sampleRate >> 16) & 0xff);
    waveHead[27] = (Byte)((sampleRate >> 24) & 0xff);
    
    long byteRate = sampleRate*numOfChannelsKey*LinearPCMBitDepth/8;//每秒字节数 = 采样频率*采样块大小
    waveHead[28] = (Byte)(byteRate & 0xff);
    waveHead[29] = (Byte)((byteRate >> 8) & 0xff);
    waveHead[30] = (Byte)((byteRate >> 16) & 0xff);
    waveHead[31] = (Byte)((byteRate >> 24) & 0xff);
    
    waveHead[32] =numOfChannelsKey*LinearPCMBitDepth/8;//采样块大小=声道数量*采样点大小/8
    waveHead[33] = 0;
    
    waveHead[34] = LinearPCMBitDepth;//采样点大小
    waveHead[35] = 0;
    
    waveHead[36] = 'd';
    waveHead[37] = 'a';
    waveHead[38] = 't';
    waveHead[39] = 'a';
    
    long totalAudiolength = [audioData length];
    
    waveHead[40] = (Byte)(totalAudiolength & 0xff);
    waveHead[41] = (Byte)((totalAudiolength >> 8) & 0xff);
    waveHead[42] = (Byte)((totalAudiolength >> 16) & 0xff);
    waveHead[43] = (Byte)((totalAudiolength >> 24) & 0xff);
    
    NSMutableData *pcmData = [[NSMutableData alloc]initWithBytes:&waveHead length:sizeof(waveHead)];
    [pcmData appendData:audioData];
    return pcmData;
}

参考

  1. 简单分析 WAV 文件