How to Pipe Raw Audio into FFmpeg on Windows

Piping raw audio from an external encoder or application into FFmpeg on Windows allows you to process, transcode, or stream audio in real time without generating temporary files. By using the standard command-line pipe operator (|) alongside FFmpeg’s stdin input reader (-i -), you can seamlessly bridge the output of your audio generator or encoder directly into FFmpeg.

The Basic Command Structure

To pipe raw audio, you must output the raw audio data to standard output (stdout) from your encoder and configure FFmpeg to read from standard input (stdin). Because raw audio (PCM) lacks header information, you must explicitly define the audio parameters (sample format, sample rate, and channels) in the FFmpeg command before the input flag.

The general syntax on Windows Command Prompt or PowerShell is:

external_encoder.exe --output-stdout-option | ffmpeg -f s16le -ar 48000 -ac 2 -i - output.mp3

Step-by-Step Configuration

  1. Configure the External Encoder: Ensure your external application is set to output raw PCM data directly to stdout. This option is often designated by flags like -o -, --stdout, or -.

  2. Define the Input Format (-f): You must tell FFmpeg the format of the incoming raw audio. Common formats include:

    • s16le (Signed 16-bit Little Endian PCM)
    • f32le (32-bit Floating Point Little Endian PCM)
    • u8 (Unsigned 8-bit PCM)

  3. Set the Sample Rate (-ar): Specify the sampling frequency in Hz to match the encoder’s output (e.g., -ar 44100 or -ar 48000).

  4. Set the Channels (-ac): Specify the number of audio channels (e.g., -ac 1 for mono, -ac 2 for stereo).

  5. Specify the Input Source (-i -): The dash - (or pipe:0) tells FFmpeg to listen to the standard input stream instead of looking for a physical file.

Real-World Example

If you are using a command-line tool like sox to generate a raw audio test tone and pipe it into FFmpeg to encode it as an AAC file, the command looks like this:

sox -n -t raw -r 44100 -c 2 -b 16 - synth 10 sine 440 | ffmpeg -f s16le -ar 44100 -ac 2 -i - output.aac

Handling Windows PowerShell Quirks

If you are using PowerShell instead of the classic Command Prompt (cmd.exe), standard piping can sometimes corrupt raw binary data due to character encoding attempts. To prevent this, force PowerShell to handle the pipeline as raw bytes, or run the command via cmd:

cmd /c "external_encoder.exe | ffmpeg -f s16le -ar 48000 -ac 2 -i - output.mp3"