Use the command
ffmpeg -i input.mp4 to inspect video streams and extract frames per second (FPS) along with bitrate data. The output displays detailed stream metadata, allowing you to identify the specific codec, resolution, and average bit rate for each track in your media file.
Understanding FFmpeg Output Structure
When you run the basic input inspection command, FFmpeg reads the container format and decodes the header information without fully processing the video data. This makes it a fast and efficient way to gather statistics about your media files. The terminal output is divided into sections for each stream found within the file. You will see lines starting with
Stream #0:0,
Stream #0:1, and so on, which correspond to different tracks such as video, audio, or subtitles.
The most critical information for your needs appears in the line describing the video stream. Look for fields labeled
fps or
avg_frame_rate. These values indicate how many frames are displayed per second. Additionally, the
bitrate field shows the total bandwidth required to play the stream smoothly. Understanding this structure helps you quickly diagnose playback issues or prepare files for specific delivery platforms that have strict bandwidth limitations.
Locating Specific Stream Data
In complex files with multiple video or audio tracks, identifying the correct stream can be tricky. Always check the index number after
Stream #. For example,
Stream #0:0(und): Video: h264... tells you that the first video track uses the H.264 codec. If you have several video layers, ensure you are looking at the one intended for primary playback. The bitrate listed next to the codec name is usually the maximum or average bitrate allocated to that specific track.
Extracting Data Programmatically
While manual inspection works for single files, handling multiple videos requires automation. You can parse the JSON output from FFmpeg to integrate this data into web applications or backend services. By using the
-of json flag, FFmpeg outputs structured data that is easy for scripts to read. This approach eliminates the need to manually search through long text logs.
Here is an example of how to generate JSON metadata for a video file:
ffmpeg -v quiet -print_format json -show_format -show_streams input.mp4
This command suppresses standard logging noise and returns only the relevant JSON object. Inside the JSON response, look for the
streams array. Each object in this array represents a track. Find the object where
codec_type equals
video. Within that object, the
r_frame_rate field contains the frame rate as a fraction, such as
30/1 or
2997/100. To get the bitrate, check the
bit_rate field, which is provided in bits per second.
Integrating with Dash.js
If you are working with adaptive streaming formats like MPEG-DASH, tools like Dash.js often rely on accurate metadata to switch between quality levels. Ensuring your source files have correct FPS and bitrate tags is essential for smooth transitions. You can use the JSON extraction method described above to verify that your DASH manifests align with the actual encoded content. Mismatches here can cause buffering or incorrect quality switching.
For more details on managing bitrate consistency in your encoding pipeline, consider reading
How to Convert FFmpeg Constant Bitrate for Consistent Video Encoding. Properly setting constant bitrate profiles ensures that the reported bitrate matches the actual network usage, preventing player errors.
Common Troubleshooting Steps
Sometimes the reported FPS might appear as
0/0 or
n/a. This usually happens with corrupted files or certain proprietary containers that do not store frame rate headers explicitly. In these cases, try re-encoding the video with a fixed frame rate using the
-r flag. For instance, adding
-r 30 forces FFmpeg to output at 30 frames per second during conversion.
Another common issue is incorrect bitrate reporting. If the bitrate seems too low, check if the file was truncated or if the encoding process failed midway. Verifying the file integrity by playing it locally is a good first step. If the file plays fine but FFmpeg reports wrong stats, the metadata headers may be damaged. Re-muxing the file into a standard MP4 container often fixes header discrepancies.
Regarding connectivity issues with live streams, if you are dealing with RTSP sources, note that FFmpeg-recorded streams might behave differently in players like VLC. See
Troubleshooting FFmpeg-Recorded RTSP Video Stream Not Displayed in VLC Media Player for solutions related to stream compatibility and display errors.
Advanced Filtering Options
You can filter the output to show only video streams using the
-map option combined with other flags. However, for simple metadata extraction, the default behavior is usually sufficient. If you need to calculate the exact duration alongside bitrate, include the
-show_entries flag to specify exactly which fields you want returned. This reduces the size of the JSON payload and speeds up parsing in your application code.
ffmpeg -i input.mp4 -map 0:v:0 -f null -
This specific command maps only the first video stream and discards the output, forcing FFmpeg to print stream information to the error log. It is a lightweight way to get stats without writing temporary files.
Frequently Asked Questions
Why does FFmpeg show 0/0 for FPS?
This indicates that the frame rate is variable or not stored in the header. You should re-encode the video with a fixed frame rate using the
-r parameter to resolve this issue.
How can I get the bitrate in kilobits instead of bits?
FFmpeg always outputs bitrate in bits per second. You must divide the value by 1000 or 1024 in your script depending on whether you want decimal or binary kilobits.
Does changing the container format affect FPS info?
No, the frame rate is a property of the video codec, not the container. Changing from MKV to MP4 will not alter the FPS data, though it may update how the timestamp is stored.
Can I use this method on live streams?
Yes, you can pipe a live stream into FFmpeg and inspect its properties in real-time, though the bitrate may fluctuate significantly compared to static files.
What if my video has multiple audio tracks?
The command lists all streams separately. You will see separate entries for each audio track under
Stream #0:1,
Stream #0:2, etc., each with their own codec and bitrate info.