Decoding Audio with FFmpeg

Oct 13, 2025 by wbrum.org 15 views

Introduction

In this article, we will delve into the world of audio processing using FFmpeg and XAudio2 on Windows. We will explore the process of decoding audio using FFmpeg and outputting it using the XAudio2 API. However, we will also discuss a common issue that arises when using this combination - the frequency ratio being wrong, resulting in a high-pitched audio output. We will provide a step-by-step guide on how to resolve this issue and achieve a correct frequency ratio.

Understanding the Problem

When using FFmpeg to decode audio and output it using XAudio2, the audio output may sound high-pitched or like chipmunks. This issue arises due to the frequency ratio being incorrect. The frequency ratio is the ratio of the output frequency to the input frequency. In this case, the output frequency is higher than the input frequency, resulting in a high-pitched audio output.

FFmpeg and XAudio2: A Brief Overview

FFmpeg

FFmpeg is a powerful, open-source multimedia processing tool that can be used for a wide range of tasks, including audio and video decoding, encoding, and processing. It supports a wide range of audio and video formats and can be used to perform various operations such as filtering, scaling, and format conversion.

XAudio2

XAudio2 is a low-latency, high-performance audio API developed by Microsoft. It provides a set of APIs for audio processing, including audio decoding, encoding, and playback. XAudio2 is designed to work with a wide range of audio formats and can be used to play back audio in real-time.

Decoding Audio with FFmpeg

To decode audio using FFmpeg, we can use the ffmpeg command-line tool. The basic syntax for decoding audio is as follows:

ffmpeg -i input_file -c:a codec_name output_file

In this command, input_file is the input file that contains the audio to be decoded, codec_name is the name of the audio codec to be used for decoding, and output_file is the output file that will contain the decoded audio.

Outputting Audio with XAudio2

To output audio using XAudio2, we need to create an XAudio2 engine and a buffer to hold the audio data. We can then use the XAudio2 APIs to play back the audio in real-time.

Here is an example of how to create an XAudio2 engine and a buffer:

// Create an XAudio2 engine
IXAudio2* pXAudio2 = NULL;
HRESULT hr = XAudio2Create(&pXAudio2, 0, XAUDIO2_DEFAULT_PROCESSOR);
if (FAILED(hr))
{
    // Handle error
}
// Create a buffer to hold the audio data
IXAudio2Buffer* pBuffer = NULL;
hr = pXAudio2->CreateBuffer(1024, &pBuffer, NULL);
if (FAILED(hr))
{
// Handle error
}

Resolving Frequency Ratio Issues

To resolve frequency ratio issues when using FFmpeg and XAudio2, we need to ensure that the output frequency is correct. We can do this by using the ffmpeg command-line tool with the -ar option to specify the output sample rate.

For example, to decode an audio file with a sample rate of 44.1 kHz and output it with a sample rate of 48 kHz, we can use the following command:

ffmpeg -i input_file -ar 48000 output_file

We can also use the ffmpeg command-line tool with the -ac option to specify the output channels. For example, to decode an audio file with 2 channels and output it with 2 channels, we can use the following command:

ffmpeg -i input_file -ac 2 output_file

Example Code

Here is an example of how to use FFmpeg and XAudio2 to decode audio and output it with a correct frequency ratio:

#include <Windows.h>
#include <XAudio2.h>
#include <iostream>
int main()

// Create an XAudio2 engine
IXAudio2* pXAudio2 = NULL;
HRESULT hr = XAudio2Create(&pXAudio2, 0, XAUDIO2_DEFAULT_PROCESSOR);
if (FAILED(hr))
{
std:cerr << "Error creating XAudio2 engine: " << hr << std::endl;
return 1;

// Create a buffer to hold the audio data
IXAudio2Buffer* pBuffer = NULL;
hr = pXAudio2-&gt;CreateBuffer(1024, &amp;pBuffer, NULL);
if (FAILED(hr))
{
    std::cerr &lt;&lt; &quot;Error creating buffer: &quot; &lt;&lt; hr &lt;&lt; std::endl;
    return 1;
}

// Decode audio using FFmpeg
std::string command = &quot;ffmpeg -i input_file -ar 48000 -ac 2 output_file&quot;;
system(command.c_str());

// Open the output file
FILE* pFile = fopen(&quot;output_file.wav&quot;, &quot;rb&quot;);
if (pFile == NULL)
{
    std::cerr &lt;&lt; &quot;Error opening output file&quot; &lt;&lt; std::endl;
    return 1;
}

// Read the audio data from the file
char buffer[1024];
while (fread(buffer, 1, 1024, pFile) &gt; 0)
{
    // Write the audio data to the buffer
    hr = pBuffer-&gt;WriteData(buffer, 1024);
    if (FAILED(hr))
    {
        std::cerr &lt;&lt; &quot;Error writing data to buffer: &quot; &lt;&lt; hr &lt;&lt; std::endl;
        return 1;
    }
}

// Play back the audio
hr = pXAudio2-&gt;StartEngine();
if (FAILED(hr))
{
    std::cerr &lt;&lt; &quot;Error starting engine: &quot; &lt;&lt; hr &lt;&lt; std::endl;
    return 1;
}

hr = pXAudio2-&gt;PlayBuffer(pBuffer, 0, 0);
if (FAILED(hr))
{
    std::cerr &lt;&lt; &quot;Error playing buffer: &quot; &lt;&lt; hr &lt;&lt; std::endl;
    return 1;
}

// Wait for the audio to finish playing
while (pXAudio2-&gt;GetState() != XAUDIO2_STATE_STOPPED)
{
    Sleep(10);
}

// Clean up
pXAudio2-&gt;StopEngine();
pXAudio2-&gt;Release();
pBuffer-&gt;Release();
fclose(pFile);

return 0;

}

Conclusion

Introduction

In our previous article, we discussed the process of decoding audio using FFmpeg and outputting it using the XAudio2 API. We also discussed a common issue that arises when using this combination - the frequency ratio being wrong, resulting in a high-pitched audio output. In this article, we will provide a Q&A section to help you better understand the process and resolve any issues you may encounter.

Q: What is the difference between FFmpeg and XAudio2?

A: FFmpeg is a powerful, open-source multimedia processing tool that can be used for a wide range of tasks, including audio and video decoding, encoding, and processing. XAudio2, on the other hand, is a low-latency, high-performance audio API developed by Microsoft. It provides a set of APIs for audio processing, including audio decoding, encoding, and playback.

Q: How do I use FFmpeg to decode audio?

A: To use FFmpeg to decode audio, you can use the ffmpeg command-line tool. The basic syntax for decoding audio is as follows:

ffmpeg -i input_file -c:a codec_name output_file

Q: How do I use XAudio2 to play back audio?

A: To use XAudio2 to play back audio, you need to create an XAudio2 engine and a buffer to hold the audio data. You can then use the XAudio2 APIs to play back the audio in real-time.

Here is an example of how to create an XAudio2 engine and a buffer:

// Create an XAudio2 engine
IXAudio2* pXAudio2 = NULL;
HRESULT hr = XAudio2Create(&pXAudio2, 0, XAUDIO2_DEFAULT_PROCESSOR);
if (FAILED(hr))
{
    // Handle error
}
// Create a buffer to hold the audio data
IXAudio2Buffer* pBuffer = NULL;
hr = pXAudio2->CreateBuffer(1024, &pBuffer, NULL);
if (FAILED(hr))
{
// Handle error
}

Q: How do I resolve frequency ratio issues when using FFmpeg and XAudio2?

A: To resolve frequency ratio issues when using FFmpeg and XAudio2, you need to ensure that the output frequency is correct. You can do this by using the ffmpeg command-line tool with the -ar option to specify the output sample rate.

For example, to decode an audio file with a sample rate of 44.1 kHz and output it with a sample rate of 48 kHz, you can use the following command:

ffmpeg -i input_file -ar 48000 output_file

Q: What are some common issues that arise when using FFmpeg and XAudio2?

A: Some common issues that arise when using FFmpeg and XAudio2 include:

Frequency ratio issues: The output frequency may not match the input frequency, resulting in a high-pitched audio output.
Audio decoding errors: FFmpeg may not be able to decode the audio correctly, resulting in a distorted or corrupted audio output.
XAudio2 engine errors: The XAudio2 engine may not be created correctly, resulting in an error when trying to play back the audio.

Q: How do I troubleshoot issues when using FFmpeg and XAudio2?

A: To troubleshoot issues when using FFmpeg and XAudio2, you can try the following:

Check the FFmpeg and XAudio2 documentation for any known issues or limitations.
Use the ffmpeg command-line tool with the -v option to enable verbose mode and see any error messages.
Use the XAudio2 APIs to check the state of the engine and the buffer.
Use a debugger to step through the code and see where the issue is occurring.

Conclusion

In this article, we have provided a Q&A section to help you better understand the process of decoding audio using FFmpeg and outputting it using the XAudio2 API. We have also discussed some common issues that arise when using this combination and provided tips on how to troubleshoot them. By following the steps outlined in this article, you should be able to resolve any issues you may encounter and achieve a correct frequency ratio.