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How to detect and fix variable frame rate video uploads before they break your pipeline

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Screen recording tools like OBS, QuickTime, Loom, and phone capture apps produce variable frame rate (VFR) video, which causes audio drift when processed by transcoders that assume a constant frame rate (CFR). The drift is subtle at first but becomes clearly noticeable around the four-minute mark, making it a silent but serious problem for platforms that accept user-uploaded tutorials, demos, or gameplay footage. A two-tier detection approach using ffprobe can identify VFR files: a cheap header comparison between r_frame_rate and avg_frame_rate serves as a first-pass filter, followed by a per-frame timestamp analysis to confirm irregular gaps. Once a VFR file is confirmed, FFmpeg 8's -fps_mode cfr flag can normalize it at ingest before it causes downstream issues. The article provides Python 3.11 scripts and ffprobe commands to automate both detection tiers efficiently.

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