Change video bitrate with FFmpeg

Changing the bitrate of a video is a common task for anyone working with video content. Whether you're a content creator, video editor, or a developer working on video applications, understanding how to adjust bitrate can help you optimize videos for different platforms, reduce file sizes, or improve video quality.

FFmpeg is a versatile command-line tool for handling multimedia files and can be used to adjust video bitrates efficiently. This guide will walk you through various methods of adjusting video bitrates using FFmpeg.

Why change video bitrate?

There are several reasons why you might want to change the bitrate of a video:

1. File size reduction: Lower bitrates result in smaller file sizes, useful for storage constraints or faster uploads.
2. Streaming optimization: Adjust bitrates to suit different internet connection speeds for smoother playback.
3. Platform requirements: Meet specific bitrate guidelines set by social media or video sharing platforms.
4. Bandwidth management: Reduce bitrate for more efficient use of network resources.
5. Archiving: Use higher bitrates for archival purposes to preserve quality.

Let's explore how to use FFmpeg to change video bitrates in various scenarios.

Basic bitrate adjustment

The simplest way to change the bitrate of a video is to use FFmpeg's -b:v option:

ffmpeg -i input_video.mp4 \
  -b:v 1M \
  -c:a copy \
  output.mp4

Breakdown of the command:

• -i input_video.mp4: Specifies the input video file
• -b:v 1M: Sets the video bitrate to 1 Mbps
• -c:a copy: Copies the audio without re-encoding
• output.mp4: Name of the output file

This will produce a video with a bitrate of 1 Mbps.

Setting maximum bitrate

For scenarios where you need to cap the maximum bitrate:

ffmpeg -i input_video.mp4 \
  -c:v libx264 -b:v 1M -maxrate 1.5M -bufsize 3M \
  -c:a copy \
  output.mp4

This command sets an average bitrate of 1 Mbps, with a maximum of 1.5 Mbps and a buffer size of 3 Mbps. Buffer size here relates to how much data will be buffered before being checked against your target bitrate, essentially encoding in 3Mb chunks at a time.

Advanced bitrate adjustment techniques

Maintaining a specific quality level

To maintain a certain level of quality while allowing FFmpeg to automatically adjust bitrates for you, you can use the Constant Rate Factor (CRF) method:

ffmpeg -i input_video.mp4 \
  -c:v libx264 -crf 23 \
  -c:a copy \
  output.mp4

In this command:

• -c:v libx264: Uses the H.264 codec
• -crf 23: Sets the Constant Rate Factor (0-51, where lower values are higher quality)

This method allows FFmpeg to adjust the bitrate dynamically while maintaining a given quality level. You should experiment with different CRF values to see what works for your content.

Two-pass encoding for more precise bitrate control

For more precise bitrate control, especially for longer videos, use two-pass encoding:

ffmpeg -i input_video.mp4 \
  -c:v libx264 -b:v 1M -pass 1 -an -f null /dev/null && \
  ffmpeg -i input_video.mp4 \
  -c:v libx264 -b:v 1M -pass 2 \
  -c:a aac -b:a 128k \
  output.mp4

This method analyzes the video in the first pass in order to better optimize encoding in the second pass, resulting in more consistent quality. This can result in slower encoding though.

Variable bitrate (VBR) encoding

This allows the bitrate to be adjusted dynamically across the stream so that less complex scenes can use lower bitrates and more visually complex scenes can use higher ones:

ffmpeg -i input_video.mp4 \
  -c:v libx264 -b:v 1M -minrate 500k -maxrate 2M -bufsize 2M \
  -c:a copy \
  output.mp4

This command sets both a minimum and maximum bitrate while targeting an average bitrate of 1 Mbps. This lets FFmpeg pick a different bitrates for different parts of the video.

Choosing the right bitrate adjustment method

Different bitrate adjustment techniques have various benefits and drawbacks:

Constant bitrate (CBR):

• Benefits:
  • Predictable file size
  • Suitable for live streaming
• Drawbacks:
  • May waste bits on simple scenes
  • Can result in quality fluctuations

Variable bitrate (VBR) with CRF:

• Benefits:
  • Maintains consistent quality
  • Efficient bit allocation
• Drawbacks:
  • Less predictable file size
  • May not meet strict bitrate requirements

Two-pass encoding:

• Benefits:
  • More precise bitrate control
  • Better overall quality for target bitrate
• Drawbacks:
  • Takes longer to encode
  • Not suitable for live encoding

Mux tips for effective bitrate adjustment

1. Consider content type: Different types of content (e.g., action vs. static scenes) benefit from different bitrate strategies.
2. Target your platform: Research recommended bitrates for your target streaming or distribution platform.
3. Balance quality and file size: Find the sweet spot between visual quality and file size for your specific needs.
4. Test thoroughly: Always test your adjusted videos on various devices and platforms to ensure quality and compatibility.
5. Don't forget audio: Adjust audio bitrate alongside video for a comprehensive optimization.
6. Use appropriate codecs: Modern codecs like H.264 or VP9 can provide better quality at lower bitrates.
7. Monitor performance: Use analytics to track video performance and adjust your bitrate strategy as needed.

Conclusion

FFmpeg provides powerful and flexible tools for changing video bitrates to suit a wide range of needs. Whether you're optimizing videos for streaming, reducing file sizes for storage, or improving quality for archival purposes, the techniques outlined in this guide will help you adjust video bitrates effectively.

Remember that the ideal bitrate depends on various factors including content type, target audience, distribution method, and quality requirements. Don't hesitate to experiment with different settings and techniques to achieve the best results for your specific use case. As you become more comfortable with FFmpeg's capabilities, you can develop more advanced workflows to streamline your video processing tasks.

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"Mux tips for effective bitrate adjustment"

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Video bitrate FAQs

What bitrate should I use for 1080p video?

For 1080p, typical bitrates range from 4-6 Mbps for good quality H.264 video. Use 8-10 Mbps for high-quality content with lots of motion (sports, action). For 720p, 2-4 Mbps works well, and for 4K, aim for 20-35 Mbps. These are starting points—the optimal bitrate depends on content complexity, codec efficiency, and quality requirements. Test different values to find the sweet spot for your specific content.

What's the difference between CBR and VBR encoding?

Constant Bitrate (CBR) maintains the same bitrate throughout the video, creating predictable file sizes but wasting bits on simple scenes. Variable Bitrate (VBR) adjusts dynamically—using higher bitrates for complex scenes and lower for simple ones—resulting in better quality per megabyte but unpredictable file sizes.

Should I use CRF or target bitrate for encoding?

Use CRF (Constant Rate Factor) when quality consistency matters more than file size—CRF 23 is a good starting point for H.264. Use target bitrate when you have strict file size requirements or platform limits. CRF produces better quality per megabyte since it allocates bits based on scene complexity, but file sizes vary. Target bitrate gives predictable file sizes but may compromise quality on complex scenes.

Why is my output file larger after reducing bitrate?

This typically happens when the encoding settings (codec, preset, profile) differ from the source. For example, encoding with -preset veryslow creates larger files than -preset ultrafast at the same bitrate due to different compression efficiency. Also, ensure you're not accidentally keeping multiple audio tracks or changing to less efficient codecs. Compare source and output codecs with ffprobe to diagnose the issue.

What's the difference between average bitrate and maximum bitrate?

Average bitrate is the target across the entire video. Maximum bitrate (-maxrate) caps peak bitrates in complex scenes, useful for streaming where bandwidth is limited. Buffer size (-bufsize) determines how much data can burst above average before being constrained by maxrate. For streaming, set maxrate to 1.5x average bitrate and bufsize to 2x maxrate for good results.

How does two-pass encoding improve quality?

Two-pass encoding analyzes the entire video in the first pass to understand complexity patterns, then uses that knowledge in the second pass to allocate bits optimally. This produces better quality at the target bitrate compared to single-pass, especially for long videos with varying complexity. The tradeoff is roughly double the encoding time. Use it for final production encodes, not for quick tests.

Can I change bitrate without re-encoding the video?

No. Bitrate is determined during encoding—you can't change it without decoding and re-encoding the video. Even "remuxing" (changing containers with -c copy) keeps the original bitrate since the actual video stream is unchanged. If the source bitrate is already acceptable, avoid re-encoding to preserve quality. For adaptive bitrate streaming, platforms like Mux automatically generate multiple bitrates from your source.

How do I choose between H.264 and H.265 for bitrate optimization?

H.265 (HEVC) achieves the same quality as H.264 at roughly 50% of the bitrate, making it better for bandwidth-limited scenarios. However, H.265 encoding is slower and not universally supported—older devices may struggle. Use H.264 for maximum compatibility and when encoding speed matters. Use H.265 when file size or bandwidth is critical and you know your audience has compatible devices. For adaptive bitrate streaming, many platforms support both.

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