Splitting and Processing Large Audio Files with Xabe.FFmpeg

Welcome back! In the last lesson, you learned how to extract audio from video files using C# and the Xabe.FFmpeg library. We discussed why it is often better to work with audio files instead of video, especially when preparing content for transcription or speech recognition. You also saw how to use the right FFmpeg parameters to create audio files that are compatible with most APIs and services.

Today, we will build on these skills by addressing a common challenge: handling long-form or large audio files. Many real-world recordings — such as interviews, podcasts, or meeting recordings — can be much longer than what most transcription APIs will accept in a single upload. Even if you have already normalized and extracted the audio, you may still need to split it into smaller, manageable chunks before you can process or transcribe it.

By the end of this lesson, you will know how to split large audio files into smaller pieces using Xabe.FFmpeg in C#. This is a crucial step in any workflow that deals with long recordings, and it will help you avoid errors, stay within API limits, and keep your applications running smoothly.

Let’s take a moment to understand why splitting long audio files is so important. Most transcription APIs and cloud services have strict limits on the size or duration of audio files they will accept. For example, some services only allow files up to 25 MB or 30 seconds in length per request. If you try to upload a file that is too large or too long, you will likely get an error, or the service may simply reject your request.

Besides API limits, there are also performance reasons to split large files. Processing a long audio file in one go can be slow and may use a lot of memory. By breaking the audio into smaller chunks, you can process each piece independently, which is faster and more reliable. This approach also makes it easier to retry failed chunks without having to redo the entire file.

In summary, splitting long-form audio is not just about meeting technical requirements — it is also about making your workflow more efficient and robust.

Now, let’s look at how you can split an audio file into smaller chunks using Xabe.FFmpeg in C#. In your AudioProcessor class, you have a method called SplitAudioIntoChunksAsync. This method takes the path to your audio file and splits it into smaller files, each with a specified duration (for example, 30 seconds).

Here is the relevant code:

public async Task<List<string>> SplitAudioIntoChunksAsync(string audioPath, int chunkDuration = 30)
{
    var mediaInfo = await FFmpeg.GetMediaInfo(audioPath);
    double totalSeconds = mediaInfo.Duration.TotalSeconds;
    int chunkCount = (int)Math.Ceiling(totalSeconds / chunkDuration);

    List<string> chunkPaths = new();
    for (int i = 0; i < chunkCount; i++)
    {
        double start = i * chunkDuration;
        string chunkPath = Path.Combine("Assets", $"chunk_{i + 1}.wav");

        await FFmpeg.Conversions.New()
            .AddParameter($"-ss {start} -t {chunkDuration} -i \"{audioPath}\" -acodec copy \"{chunkPath}\"", ParameterPosition.PreInput)
            .Start();

        chunkPaths.Add(chunkPath);
    }

    return chunkPaths;
}

Let’s break down what is happening here. First, the method gets the total duration of the audio file using FFmpeg.GetMediaInfo. It then calculates how many chunks are needed by dividing the total duration by the desired chunk length. For each chunk, it uses FFmpeg’s -ss parameter to set the start time and -t to set the duration of the chunk. The -acodec copy parameter tells FFmpeg to copy the audio stream without re-encoding, which is faster and preserves quality.

For example, if you have a 90-second audio file and you want 30-second chunks, this method will create three files: chunk_1.wav (0-30s), chunk_2.wav (30-60s), and chunk_3.wav (60-90s). If the audio cannot be evenly divided into 30-second chunks — for example, if the total length is 95 seconds — the method will create a final chunk that contains the remaining audio. In this case, you would get three 30-second chunks and a fourth chunk with the last 5 seconds. This ensures that no part of the original audio is lost, even if the total duration is not a perfect multiple of the chunk size.

After running this method, you will get a list of file paths for all the chunks created. Here is what the output might look like:

Assets/chunk_1.wav
Assets/chunk_2.wav
Assets/chunk_3.wav

Each of these files can now be processed or transcribed separately.