HDMI's Quick Media Switching

What is QMS?

Television displays are constantly rising to the challenge, delivering increasingly richer and higher-resolution content. However, it is still common to encounter a black screen for a few seconds while switching between videos, seemingly enough to ruin the entire viewing experience (as illustrated below).

What is frame rate?

While most of us as viewers might already be familiar with the term “frame rate”, do we really understand it? A video is made of a series of still images called “frames”. The frame rate is the speed at which the frames are generated, measured in frames per second (FPS). Thus, a 24 FPS video refers to 24 images present within a second of the video. Different types of videos have varying frame rates. Most of the movies are recorded at 24 FPS which is deemed the best for a cinematic experience. TV or sports shows use a slightly higher frame rate of 30 FPS. Video games or any video involving lots of motion detail utilize 60 FPS.
If you’re ever interested to know what the frame rate of a video playing on YouTube is, a simple right-click on the video and subsequent selection of the “Stats for Nerds” option from the video menu should reveal the FPS detail along with a hoard of other statistics (as illustrated below).

What is the refresh rate?

Although the terms “frame rate” and “refresh rate” are often used as synonymous with each other, they are not the same. Refresh rate is the number of times the display changes the image, and it is measured in Hertz (Hz). Modern TVs offer refresh rates of either 60 Hz which refreshes the display image 60 times per second, or 120 Hz catering to 120 times per second. A TV’s refresh rate is available for the user to see from its settings.

Why do black screens occur?

The frame rate and refresh rate do not always match. When they match, it is ideal hence allowing us to view an exact representation of whatever was transmitted, on the screen. However, when there is a difference between the frame rate and refresh rate, the display has additional work to do for reproducing the content properly. Most of us watch trailers or videos from YouTube or any other streaming service. These platforms might offer videos at different frame rates like 24 FPS, 30 FPS or 60 FPS. When a new video with a different frame rate begins streaming, the display either (i) compensates for the difference in frame rate by repeating the frames resulting in a motion judder, or, (ii) changes its clocking and re-syncs to the new frame rate resulting in an A/V blackout.
Let’s understand more about how HDMI 2.1 is equipped to eliminate the black screens.

How does QMS help?

The Source and the display must work hand in hand to avoid visual artifacts. Variable Refresh Rate (VRR) helps the display dynamically match its refresh rate with the changing frame rate. QMS allows swift transitions between videos having different frame rates, provided their resolution is the same. This helps in eliminating black screens and enables a smooth transition between two different video frame rates without having to affect a video mode change. QMS allows the Source to indicate the next frame rate to the display in advance. This enables the display to perform frame rate conversions ahead of time. In addition to this, the Source can also indicate that the frame rate shall be constant for a considerable amount of time. This helps the display to utilize video processing algorithms of the highest quality for a movie that streams with a constant frame rate. The Source passes the required information to the display using Video Timing Extended Metadata (VTEM).

QMS is however not intended for gaming. Since the display enables rendering high-quality video for QMS, the latency increases, as a result, slowing things down for gaming purposes. Moreover, even if the gaming application were to inform the Sink about a constant frame rate, it might last only for a frame or two and not for as long as expected by the Sink. The Sink might then end up utilizing video processing techniques that result in undesired effects on gaming. For QMS to work, both the transmitter and the receiver must support the feature. Both the set-top box and the television set must support QMS. However, the news that a QMS feature can be added to an existing device via a firmware update too is welcoming.

Synopsys continues to provide Industry’s first and most comprehensive Protocol Continuum solutions. Synopsys HDMI VIP, which supports HDMI 2.1a and VTEM, is a comprehensive solution for the verification of VRR for both QMS and Gaming. To learn more about Synopsys display and other VIPs, please visit www.synopsys.com/vip.