Viewers would rather watch video at slightly lower speeds than endure rebuffering, according to new research from Lancaster University.

A new study, led by researchers at Lancaster University, investigated how dynamically slowing down or speeding up playback is perceived by viewers, and how playback speed changes compare to other ways of overcoming viewer internet slowdowns – such as reducing picture quality and pausing for rebuffering.

Dr Tomasz Lyko, Senior Research Associate at Lancaster University’s School of Computing and Communications and the lead author of the study, said: “We found a safe range of slowed down playback – speeds that won’t be noticeable to most users.

“These speed ranges can be used by the video player to slow down the playback to avoid temporary rebuffering events. These are usually experienced by the viewer as paused playback, or with a spinning wheel – which basically means the player has run out of data and is in the process of downloading the next segment.

“By temporarily slowing down the playback speed it gives the player more time to download the next part of the content. However, we also found that slowing down too much is just as annoying to viewers as rebuffering itself.”

They discovered that slowing down playback to 90% of normal playback speed was largely imperceptible to viewers, and for half of content shown in the study up to 80% playback speed was also imperceptible. But dropping to 70% of normal playback speed was noticeable and negatively affected the viewing experience.

Benefits for On-Demand Streaming

Study participants also found temporary drops in quality from a high-quality bitrate to a medium quality are largely imperceptible to many viewers. The researchers believe that a combination of quality and speed changes could help to reduce the number of interruptions in playback, which many viewers find has the most frustrating impact on their viewing experience.

The researchers also found that speeded up playback of up to 30% quicker than normal was acceptable to many viewers.

Benefits for Live Events

This is useful for live streaming of video, for sports events for example. By dynamically and temporarily slowing down and then speeding up playback at acceptable or imperceptible rates, it enables streaming players to adjust to keep within a short timeframe of the live broadcast itself, known as latency.

“Imperceptibly speeding up is much better for the viewing experience than ‘seeking’, which is like an instant fast-forward,” said Dr Lyko. “Seeking can be jarring. The video jumps ahead to reduce latency, but that might mean that you missed a key event like a goal or booking.”

The research involved four studies exploring different research questions around perceptions of playback rate changes across different genres of content and compared to other adaptive video streaming factors. The overall study involved more than 200 viewer participants and 210 test sequences and covered a range of video content including sports, dramas, documentaries and computer game streaming.

The researchers, from Lancaster University, the University of Glasgow and BBC R&D, believe their findings can help inform the design of video players and enhance the streaming experience for viewers.

Dr Rajiv Ramdhany, Senior Research and Development Engineer at BBC R&D, said: “This study’s findings can be used to improve the design of modern media players like the BBC iPlayer for a better streaming experience, without causing any further disruptions, by establishing a safe range for slowing down or speeding up video playback.”

The study’s findings are outlined in the paper ‘Beyond Bitrate: Understanding the QoE Impact of Playback Rate and Seeking in Adaptive Video Streaming’, which has been published online by ACM Transactions on Multimedia Computing, Communications, and Applications.

The study was part of the AI4ME Prosperity Partnership, which is funded by UKRI’s Engineering and Physical Sciences Research Council (EPSRC) and the BBC.