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Posted 8 Dec 2018

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C++ audio mixing and WAV file & AudioFile classes

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8 Dec 2018CPOL7 min read
Mixing audio from multiple WAV files to a single WAV file. Includes a C++ class for reading and writing WAV audio files, derived from an AudioFile class for future support of other audio file formats.

Introduction

One of my interests and hobbies is music.  This project is able to mix multiple WAV audio files into a single audio file, so that the audio from each source WAV file will be heard simultaneously.  This project includes a C++ class that can be used to open, read, and write WAV audio files.  It is derived from an AudioFile class, and the mixing function uses the AudioFile interface.  That way, other audio formats (such as FLAC and AIF) could be added in the future and the mixing function would still work.  For mixing audio files, the source audio files will need to have the same sample rate and bit rate and same number of channels (mono or stereo).  No external libraries are required.

In addition to the WAVFile class and supporting code, I've included an application for mixing WAV audio files. The compiled application and the full source code are included in the link at the top of the article.

This project builds in Visual Studio 2017 (I used the free Community version).  Note that to build the project, you will need to have the C++ toolset installed in Visual Studio, along with MFC.  The code uses modern C++ features, such as std::thread, a lambda function, std::shared_ptr, and a modern way of iterating through a collection.

Years ago, I published a version of this in C#: https://www.codeproject.com/Articles/35725/C-WAV-file-class-audio-mixing-and-some-light-audio

Included Application

The source code includes a GUI application (written in MFC) for choosing and mixing WAV audio files.  The following is a screenshot of the application:

To add WAV files to the list, you can drag & drop files onto the GUI, or for each line in the list, there will be a "..." button that lets you browse and choose a WAV file to add.

Background

A WAV audio file consists of a header at the beginning of the file, which contains strings to identify the file type ("RIFF" and "WAVE"), as well as information about the audio contained in the file (number of channels, sample rate, number of bits per channel, size of the data, etc.). Following the header is all of the audio data. Digital audio data is numeric: each sample is an integer that represents the level of the audio signal at that point in time.

The general idea behind digital audio mixing is fairly simple: Until there are no more audio samples, the next audio sample from each audio file is read, then they're added together and saved to the output file.  A bit more needs to be done, though, to deal with digital audio clipping.  Digital audio clipping is caused by numeric range limitations of the values due to the sample size (i.e., 8 or 16 bits): When audio sample values are added together (or if the volume is increased), it's possible for the resulting values to go beyond the numeric range of the values. When that happens, the result is (often loud) pops and clicks in the audio, which is undesirable. So, in order to mix WAV files together, mixAudioFiles() (in AudioFileTools.h and .cpp) will first analyze each audio file to determine the highest audio sample, then reduce the volume of all the samples while mixing them to avoid digital audio clipping.

Wikipedia has an article on audio clipping that describes it in more detail.

Using the code

The AudioFile class is a parent class for working with audio files, with some pure virtual methods to be implemented in derived classes, such as the included WAVFile class.  The WAVFile class can open, read, and write WAV audio.  Most of the class methods return an AudioFileResultType, which can be used as if it was a bool (in an 'if' statement, for instance), and if it's false, it contains error messages in the form of std::string.

The following are some of the more important methods in the WAVFile class:

  • WAVFile(const std::string& pFilename): Constructor that takes a filename
  • WAVFile(const std::string& pFilename, const WAVFileInfo& pWAVFileInfo): Constructor that takes a filename and a WAVFileInfo objecct specifying the desired properties of the WAV file (sample rate, bit rate, number of channels, etc.)
  • WAVFile(const std::string& pFilename, AudioFileModes pFileMode): Constructor that takes a filename and a file mode (read, write, read/write)
  • template <class SampleType> AudioFileResultType getNextSample(SampleType& pAudioSample): A templatized method that reads the next sample from the audio file (if it's open in read or read/write mode).  For the template, the proper data type must be used (for instance, for 16-bit audio, you can use uint16_t.
  • template <class SampleType> AudioFileResultType writeSample(SampleType pAudioSample): A templated method that writes an audio sample to the WAV file (if it's open in write or read/write mode).  For the template, the proper data type must be used (for instance, for 16-bit audio, you can use uint16_t.

These are some important methods in the AudioFile class, which are pure virtual:

  • virtual AudioFileResultType getNextSample_int64(int64_t& pAudioSample): Reads the next audio sample from the audio file (if in read or read/write mode).  The audio sample is cast to an int64 so that it can be used in generic algorithms - Whether the audio data is 8-bit, 16-bit, or another bitness, the audio sample is cast to 64-bit.
  • virtual AudioFileResultType writeSample_int64(int64_t pAudioSample): Writes an audio sample to the audio file (if in write or read/write mode).  The sample parameter is an int64_t but will be cast down to the appropriate type when writing the sampel to the audio file.  This is to make generic audio algorithms simpler - Whether the audio data is 8-bit, 16-bit, or another bitness, the audio sample is cast down to the appropriate type.

AudioFileTools.h and AudioFileTools.cpp defines the following function (among others):

  • AudioFileResultType mixAudioFiles(const std::vector<std::string>& pFilenames, AudioFile& pOutFile): Mixes multiple audio files into a single audio file.  The pFilenames parameter is a vector of strings containing the source filenames, and pOutFile is an AudioFile object representing the audio file where the mixed audio file will be saved.  pOutFile is an AudioFile so that the calling code can decide what format the mixed file should be, and mixAudioFiles() will be able to work with it.  Currently, only WAV is implemented, but in the future, other audio file formats (such as FLAC, AIF, etc.) could be implemented.  Mixing audio files can take some time (due to the file I/O), so if you're using this in a GUI application, it's recommended to do the mixing in a separate thread so that the GUI doesn't freeze.

The WAVFileInfo and AudioFileInfo classes contain information about the audio files, such as bitness, number of channels, sample rate, etc.

An example of opening a WAV file and looping through to get each audio sample (assuming the audio file contains 16-bit audio):

C++
WAVFile audioFile("someAudioFile.wav");
AudioFileResultType result = audioFile.open(AUDIO_FILE_READ);
if (result)
{
    size_t numSamples = audioFile.numSamples();
    for (size_t i = 0; (i < numSamples) && result; ++i)
    {
        int16_t audioSample = 0;
        result = audioFile.getNextSample(audioSample);
    }
}
else
{
    cerr << "Error(s) getting audio samples:" << endl;
    result.outputErrors(cerr);
}
audioFile.close();

Points of Interest

One interesting thing to note is that data in a WAV audio file is always little-endian, per the specification. On big-endian systems, the byte order must be reversed before manipulating the audio data, and the byte order for a sample must be reversed before saving it to a WAV file. My WAVFile class handles this automatically; for example, if the system is big-endian, then when retrieving audio samples from a WAV file or adding a 16-bit sample to a WAV file, the bye order will be automatically reversed so that the data is in the proper order.

In creating the WAVFile class, it was necessary to look up the WAV file format specification. I found many web pages describing the WAV file format. Each page has basically the same information, but with different notes. I found the following four pages useful:

History

  • December 7, 2018: Submitted the first version of this article

Keep a running update of any changes or improvements you've made here.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


Written By
United States United States
I was born and raised (and currently still reside in) northwest Oregon, USA. I started using computers at a very young age, maybe 4 or 5 years old, using my dad's computers. I got my own computer when I was 12 (1992). Later, I realized that I really enjoy programming, and in college, I decided to get a degree in software engineering. In 2005, I earned my bachelor's degree in software engineering from Oregon Institute of Technology.

I have been working as a software engineer since October 2003. So far, I've had experience with C++, PHP, JavaScript, Perl, Bash shell scripting, C#, and HTML. I have developed GUIs with MFC, wxWidgets, and WinForms. I also have experience with open-source technologies such as Linux, Apache, MySQL, and PostgreSQL. My favorite Linux text editor is Vim.

In my personal time I enjoy tinkering with computers (building PCs, gaming, occasionally working on personal programming projects), playing music (I enjoy playing guitar and synthesizer/piano), and spending time with family & friends. I also enjoy reading the occasional book (pretty much anything with a good story, but usually sci-fi, drama, programming books, advice books, and scientific non-fiction, such as books written by physicist Michio Kaku). Currently, my favorite TV show is The Office; I also like watching the news when I can. I also enjoy the occasional movie (sci-fi, comedy, action, drama; pretty much anything with a good story).

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