What is the significance of the values '0' and 'auto' for keyframe interval (GOP size) in OBS, and why should you steer clear of them?

In the dynamic world of content creation, OBS Studio has emerged as a powerhouse, empowering creators with a vast array of tools for streaming and recording videos. Amidst the myriad settings available, the "keyframe interval" stands out as a critical element that significantly impacts the quality and efficiency of your streaming experience. In this comprehensive guide, we unravel the intricacies of the "0 auto" keyframe interval setting in OBS. We'll explore its nuances, shed light on its relationship with streaming platform transcoding, and unveil why it may not always be the best choice for pristine streaming quality.

Understanding Keyframe Intervals

Before we delve into the heart of the matter, let's establish a solid foundation by understanding keyframe intervals. In the realm of video compression, a keyframe (or I-frame) serves as a vital reference point. It's a complete frame that aids in decoding and displaying subsequent frames. The interval at which these keyframes are inserted into the video stream during encoding can profoundly impact seeking efficiency, playback, and compression.

Decoding the "0 Auto" Keyframe Interval in OBS

To comprehend the implications of the "0 auto" keyframe interval setting, let's dissect the underlying code snippet in OBS Studio:



Here's the essence:

The code retrieves the "keyint_sec" setting from OBS Studio's configuration, representing the desired interval between keyframes in seconds.

The "gop_size" variable is computed based on the retrieved keyframe interval. If a non-zero interval is specified, the program calculates the Group of Pictures (GOP) size using the given interval and the video's frame rate. In the case of a keyframe interval set to 0 (i.e., "0 auto"), the GOP size defaults to 250 frames.

Unveiling the Illusion of "0 Auto"

While the "0 auto" keyframe interval may appear as a streamlined option for streamlining your streaming experience, its true implications warrant deeper exploration. This default "auto" behavior might not seamlessly align with genuine optimization, particularly when it comes to streaming efficiency and video quality enhancement. It's worth noting that when developers initially set the default keyframe interval to 250 frames, they likely had offline recording in mind, possibly catering to a default 25 fps PAL video format. However, when applied to real-time streaming scenarios, especially with varying frame rates and transcoding processes, this default can inadvertently introduce complexities and compromises.

The Interplay with Streaming Platform Transcoding

Here's where the plot thickens. Streaming platforms often transcode incoming video streams to various quality profiles and resolutions, catering to diverse devices and network conditions. The keyframe interval of your OBS output plays a pivotal role in this transcoding process.

Mismatched Keyframe Intervals: The "0 auto" keyframe interval setting, by default, prescribes a 250-frame interval. However, streaming platforms tend to adhere to fixed, integer second keyframe intervals for optimal transcoding efficiency.

Quality Degradation: When your OBS output's keyframe intervals don't align with those set by the streaming platform during transcoding, it can lead to quality degradation. Transcoding may involve additional keyframes or adjustments to conform to the platform's requirements, potentially compromising visual quality.

Bandwidth and Latency Implications: The misalignment of keyframe intervals can impact bandwidth consumption, with the transcoding process necessitating more bits to accommodate extra keyframes. Furthermore, this mismatch may introduce latency in the transcoded output, affecting the playback experience for viewers.

Choosing Keyframe Intervals Wisely

The "0 auto" keyframe interval setting, while seemingly convenient, demands thoughtful consideration. To optimize your streaming experience and uphold superior video quality, it's prudent to manually configure keyframe intervals that harmonize with the transcoding processes of your chosen streaming platforms. By doing so, you facilitate a smoother transcoding journey, mitigate unnecessary adjustments, and ultimately deliver a premium viewing experience to your audience.

Real Keyframe Intervals with 0 Auto in OBS

Video Frame Rate (fps)	Keyframe Interval (seconds)
23.976	10.42
24	10.42
25	10.00
29.97	8.34
30	8.33
50	5.00
59.94	4.17
60	4.17
120	2.08

Conclusion

As you navigate the labyrinth of keyframe intervals in OBS Studio, remember that streaming quality hinges on strategic decision-making. The "0 auto" keyframe interval may hold allure, but its compatibility with streaming platform transcoding is paramount. By embracing manual configuration and synchronization with transcoding requirements, you lay the foundation for a streaming masterpiece that marries efficiency, seamless playback, and uncompromised video quality.

Article written by
Sylvain Corvaisier

Independent streaming and iOS engineer

Last modified: January 15th, 2025