How to Use FFmpeg Realtime Filter

This article explains how to use the FFmpeg realtime filter to slow down media processing to real-time speed. You will learn the basic syntax, practical command-line examples for both audio and video, and the key differences between the realtime filter and the global -re input option.

Why Use the Realtime Filter?

By default, FFmpeg processes files as fast as your CPU and GPU allow. While this is ideal for transcoding local files, it causes issues when streaming to a live server (like RTMP or UDP destinations). If you send data too fast, the server may drop packets or crash.

The realtime filter pauses the filtergraph processing to match the actual playback duration of the media, forcing FFmpeg to run at exactly 1x speed.

Basic Syntax

The realtime filter can be applied to video streams using -vf (video filter) and to audio streams using -af (audio filter).

Video Realtime Filter

To slow down video processing to real-time, add realtime to your video filter chain:

ffmpeg -i input.mp4 -vf realtime output.mp4

Audio Realtime Filter

To slow down audio processing, use the audio equivalent:

ffmpeg -i input.wav -af realtime output.wav

Combined Audio and Video

If you are processing both audio and video and want to ensure both are throttled, you can apply the filter to both streams:

ffmpeg -i input.mp4 -vf realtime -af realtime -f mpegts udp://127.0.0.1:1234

Advanced Filter Parameters

The realtime filter accepts an optional limit parameter. This parameter defines the maximum speedup delay in seconds. If the processing falls behind by more than this limit, the filter will pause to catch up.

-vf realtime=limit=2

In this example, if the processing speed deviates from real-time by more than 2 seconds, the filter adjusts the timing to prevent massive sync issues.

Realtime Filter vs. the -re Option

FFmpeg also has a global input option called -re (e.g., ffmpeg -re -i input.mp4...). While they achieve similar results, they work differently:

• -re (Input Option): Tells FFmpeg to read the input file at the native frame rate. It is simple but can sometimes be imprecise with variable frame rate files or complex filtergraphs.
• realtime (Filter): Regulates speed at the filtergraph stage (after decoding). This is much more precise when you are generating content dynamically within FFmpeg or using complex custom filter chains.