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Changing Video Bit Rate

Adjust the bit rate of a video file using FFmpeg to change its size and quality, useful for optimizing videos for various bandwidths or storage constraints.

VideoAlchemy Compose File

version: 1  # Schema version of viddo-compose

generate_path: "./generated"  # Directory of log and command files

tasks:
  - name: Changing Video Bit Rate
    command: ffmpeg
    inputs:
      - id: input_4
        source: 'input.mp4'
    outputs:
      - id: output_4
        overwrite: true
        source: 'output.mp4'
    codecs:
      - constant_bitrate:
          video: 1M

Command

ffmpeg -i input.mp4 -b:v 1M output.mp4

Parameters

  • -i input.mp4: Specifies the input video file. Replace input.mp4 with the path to your source video file.
  • -b:v 1M: Sets the video bit rate to 1 Mbps. Replace 1M with your desired bit rate (e.g., 500k for 500 kbps).

Possible Errors

  • File not found: Occurs if FFmpeg cannot locate the input file. Ensure the path to the file is correct.
  • Invalid bit rate value: Occurs if the specified bit rate value is not recognized or is inappropriate for the video resolution or format. Ensure the bit rate value is specified in a valid format (e.g., k for kbps, M for Mbps).
  • Permission denied: Occurs if FFmpeg does not have the necessary permissions to read the input file or write to the output file. Ensure that the files and directories have the correct permissions.

GPU Acceleration Command

Changing the video bit rate usually involves re-encoding, which can benefit from GPU acceleration. Here’s how you can do it for Nvidia GPUs:

ffmpeg -i input.mp4 -c:v h264_nvenc -b:v 1M output.mp4

Replace h264_nvenc with your GPU's specific hardware encoder (e.g., hevc_nvenc for HEVC) and adjust the bit rate as desired.

Additional Information

  • Quality vs. Size: A higher bit rate generally results in better video quality but larger file sizes. Conversely, reducing the bit rate can significantly decrease file size at the cost of quality.
  • Audio Bit Rate: To change the audio bit rate, use the -b:a option followed by the desired audio bit rate (e.g., -b:a 128k for 128 kbps). This can further reduce file size or adjust audio quality.
  • Two-Pass Encoding: For more efficient bit rate utilization and consistent quality, consider using two-pass encoding. This process analyzes the video in the first pass and then adjusts the encoding to achieve the target bit rate more accurately in the second pass.
  • Compatibility: Ensure the chosen video and audio bit rates are compatible with your intended playback devices or platforms, especially if you are optimizing for streaming or mobile devices.