WAV format file analysis

Article directory

• WAV format file analysis
• • Introduction to WAV format
  • WAV format composition
  • • RIFF Chunk
    • Format Chunk
    • Data Chunk
    • • 8 bit mono
      • 8 bit dual channel
      • 16 bit mono
      • 16 bit dual channel
  • big and small endian
  • actual file analysis
  • • RIFF Chunk
    • Format Chunk
    • Data Chunk
    • Are there other optional blocks?
  • references

WAV format file analysis

Introduction to WAV format

WAV is one of the most common sound file formats. It is a standard digital audio file specially developed by Microsoft for Windows. The file can record various monophonic or stereo sound information and ensure that the sound is not distorted. It conforms to the Resource Interchange File Format (RIFF) specification and is used to save the audio information resources of the Windows platform, and is widely supported by the Windows platform and its applications. The Wave format supports MSADPCM, CCITT A-law, CCITT μ-law and other compression algorithms, and supports a variety of audio bits, sampling frequencies and channels. It is the most popular sound file format on PCs; however, its file size is large and is mostly used for storage. Short sound clips.

Source: Baidu Encyclopedia

WAV format composition

WAV files follow RIFF ruleschunk , and their content is stored in the smallest unit. A WAV file generally consists of three blocks: RIFF chunk, Format chunkand Data chunk. At the same time, there may also be some optional blocks in the file, such as: Fact chunk, PlayList chunketc. In the process of analysis, we focus on the analysis of the first three blocks: RIFF chunk, Format chunkand Data chunk.

The structure of each block is given in detail below:

RIFF Chunk

name

offset address

number of bytes

endian

content

ID

0x00

4

big endian

RIFF (0x52494646)

Size

0x04

4

little endian

fileSize - 8

Type

0x08

4

big endian

WAVE(0x57415645)

• thought toRIFF identify
• SizeRefers to the size of the entire file minus IDthe Sizelength of the sum. So filesize 8 filesize - 8 filesize8
• TypeTo Waveindicate that there are two sub-blocks behind: FormatandData

Format Chunk

name

offset address

number of bytes

endian

content

ID

0x00

4

big endian

fmt (0x666D7420)

Size

0x04

4

little endian

16/18

AudioFormat

0x08

2

little endian

audio format

NumChannels

0x0A

2

little endian

number of channels

SampleRate

0x0C

4

little endian

Sampling Rate

ByteRate

0x10

4

little endian

data bytes per second

BlockAlign

0x14

2

little endian

data block alignment

BitsPerSample

0x16

2

little endian

Sampling bits

• thought tofmt identify
• SizeWAVIndicates that the header does not contain additional information when the length of the block data (excluding the length of ID and Size) is 16 .
• AudioFormatIndicates Datathe format of the audio data stored in the block, PCMthe value of audio data is 1
• NumChannelsIndicates the number of channels of audio data, 1: mono, 2: dual
• SampleRateIndicates the sample rate of the audio data
• ByteRateSample Rate Num Channels B its Per Sample / 8 SampleRate * NumChannels * BitsPerSample / 8 SampleRateNumChannelsBitsPerSample/8
• BlockAlignNumber of bytes required per sample Num Channels B its Per Sample / 8 NumChannels * BitsPerSample / 8 NumChannelsBitsPerSample/8
• BitsPerSampleThe number of bits stored in each sample, the values ​​are 8, 16, 32

Data Chunk

name

offset address

number of bytes

endian

content

ID

0x00

4

big endian

data(0x64617461)

Size

0x04

4

little endian

It depends on the actual situation

Data

0x08

Depends on file size

little endian

audio data

• Datato identify
• SizeIndicates the length of the audio B yte R ate S econds ByteRate * Seconds ByteRateSeconds
• Datarepresent data

For Data Chunk, the number of channels and the sampling rate are different, resulting in different data layout: (1Byte size per column)

8 bit mono

Sample 1

Sample 2

data 1

data 2

8 bit dual channel

Sample 1

Sample 2

Channel 1 Data 1

Channel 2 Data 1

Channel 1 Data 2

Channel 2 Data 2

16 bit mono

Sample 1

Sample 2

data 1 low byte

Data 1 high byte

data 2 low byte

Data 2 high byte

16 bit dual channel

Sample 1

Channel 1 Data 1 Low Byte

Channel 1 Data 1 High Byte

Channel 2 Data 1 Low Byte

Channel 2 Data 1 High Byte

Sample 2

Channel 1 Data 2 Low Byte

Channel 1 Data 2 High Byte

Channel 2 Data 2 Low Byte

Channel 2 Data 2 High Byte

The following explains the big and small endian problems that often occur in the above content

big and small endian

Wave files store data in little-endian order

• Big-endian mode means that the low bits of data are stored in the high address of the memory , and the high bits of the data are stored in the low address of the memory, such as PNG file format;
• Little-endian mode means that the low-order bits of the data are stored in the low address of the memory , and the high-order bits of the data are stored in the high address of the memory.

Let's analyze a specific .wav file:

actual file analysis

RIFF Chunk

name

The actual data

illustrate

ID

consistent with the above

Size

The entire file size is 45340 bytes

Type

file type is WAVE

Format Chunk

name

The actual data

illustrate

ID

consistent with the description above

Size

The size is 16, and the header contains no additional information

AudioFormat

for PCM audio data

NumChannels

mono audio

SampleRate

The sample rate is 22050

ByteRate

44100 bytes of data per second

BlockAlign

The number of bytes required per sample is 2

BitsPerSample

Each sample stores 16bit

Data Chunk

name

The actual data

illustrate

ID

consistent with the description above

Size

The data length is 45304 bytes

Data

It's a lot if you don't let go...

actually stored data

Are there other optional blocks?

To verify that the file has optional blocks, the following code is added:

#include <iostream>
#include <fstream>
#include <cstdio>
#include <vector>
#include <map>
#include <set>
#define uchar unsigned char

using namespace std;

const string path = "test.wav";
vector<string> ans;

struct RiffHeader
{
    string id = "";
    string type = "";
    unsigned int length = 0;
    uchar len[4];
    uchar Type[4];
    void GetHead(ifstream & in) {
        uchar* buffer = new uchar[4];
        in.read((char *)buffer, 4);
        for (int i = 0;i < 4;i ++) id += (int)buffer[i];
        in.read((char *)len, 4);
        length = (len[1] + (len[0] << 8)) + ((len[3] + (len[2] << 8)) << 8);
        in.read((char *)Type, 4);
        for (auto i : Type) type += (int)i;
        return ; 
    }
};

struct FormatHeader
{
    string id = "";
    uchar data[20];
    void GetHead(ifstream & in) {
        uchar* buffer = new uchar[4];
        in.read((char*)buffer, 4);
        for (int i = 0;i < 4;i ++) id += (int)buffer[i];
        in.read((char*)data, 20);
        return ;
    }
};

struct DataHeader
{
    string id = "";
    uchar* data;
    unsigned int length = 0;
    void GetHead(ifstream & in) {
        uchar* buffer = new uchar[4];
        in.read((char*)buffer, 4);
        for (int i = 0;i < 4;i ++) id += (int)buffer[i];
        uchar* len = new uchar[4];
        in.read((char*)len, 4);
        length = (len[1] + (len[0] << 8)) + ((len[3] + (len[2] << 8)) << 8);
        data = new uchar[length];
        in.read((char *)data, length);
        return ;
    }
};

int main()
{
    ifstream in(path, ios :: binary);
    RiffHeader riff;
    FormatHeader format;
    riff.GetHead(in);
    ans.push_back(riff.id);
    format.GetHead(in);
    ans.push_back(format.id);
    while (!in.eof()) {
        DataHeader data;
        data.GetHead(in);
        ans.push_back(data.id);
    }
    cout << "All chunks : " << endl;
    for (auto i : ans) cout << "Chunk id : " << i << endl;
    return 0;
}

got the answer:

All chunks : 
Chunk id : RIFF
Chunk id : fmt 
Chunk id : data

There are no optional data blocks in this file.

references

WAV file format detailed
analysis of WAVE file format