Video Frame Display Synchronization

by Dan H. Nowlin,
Intel Corporation

Introduction

Discover how a simple algorithm can help synchronize content frame rate and display refresh rate to improve video playback quality.

Step by step, the digital home is becoming a reality. The last several years have seen increasing numbers of digital home devices become commercially available to consumers. From digital media adapters providing music and video remotely to complete entertainment systems within a single PC, the digital landscape is growing.

Media Center PCs - boasting the ability to watch and record television, as well as save, store, and render digital photos and music - are becoming a standard offering from PC companies. In addition, several vendors are providing kits that allow users to turn their PCs into Media Center PCs.

Unfortunately these Media Center PCs do not always provide high quality video. Several factors contribute to this poor showing - factors such as improper buffering and rendering of streaming content, failure to de-interlace interlaced content, and poor synchronization of video and audio all effect the quality of the video experience. Most of these problems and their solutions are well understood, and many products provide good support for them. However, there is also a less known and more subtle problem that can lead to small but noticeable video hiccups. This paper explains this problem in detail and shows a solution.

More consumers are watching television on their PCs, thanks to the growing numbers of Media Center PCs being sold. As this group expands beyond the hobbyist/enthusiast stage, the demand for better quality video will increase. In order to sustain the growth of the Media Center PC market these video hiccups must be addressed.

Things can be done to improve video playback quality on PCs, and many of the video independent software vendors (ISVs) are doing them. However, an often overlooked aspect of video playback is that software displaying video frames must take into account and synchronize around the refresh cycle of the display device. Whereas televisions synchronize with the video signal received from a broadcast studio, computer monitors refresh at a fixed rate set by the graphics adapter clock, which is totally unrelated to the video input. This major difference can cause a big problem when it comes to properly synchronizing video with a PC monitor. The rest of this paper is dedicated to describing the problem and a solution in detail. Before diving into the problem, some concepts will be introduced to give the reader a firm foundation.


Display Refresh Cycle

A PC monitor’s refresh timing synchronizes with the frequency of the graphics adapter clock. For example, a graphics card and a monitor may both support 60 Hz. This combination works out because the monitor can synchronize with the 60 Hz signal from the graphics card. In fact the monitor will be able to synchronize even when there are minor variations in frequency of the output from the graphics adapter (such as 60.06 Hz vs. 60 Hz).

During the refresh cycle, the display on the PC monitor is redrawn from the current display buffer in the PC graphics adapter&rs quo;s address space. One by one, each horizontal line on the display is updated with the new image from the graphics buffer. The current line being updated is known as the scan line. Since this refresh process is done 60 times per second for a 60 Hz graphics adapter, then the display on the PC monitor is updated 60 times per second as well (the same rate).


Figure 1 – Display Refresh

Tearing Artifact

There is a potential problem with updating the graphics adapter’s display buffer at the wrong time. If the video memory buffer is updated while the monitor is in the middle of being updated (going through its refresh cycle), then only the updated image after the scan line will be redisplayed on the screen in this refresh cycle (See Figure 2). This artifact, where the top part of the screen shows an older image and the bottom part of the screen shows a new image, is known as tearing. This is a pretty descriptive term, because the resulting image can appear torn in two.


Figure 2 – Tearing Artifact


The Flip Command

One way to prevent tearing is to always make sure that the video memory is updated just after one monitor refresh cycle has ended and just before the next cycle begins (in other words, updated during vertical retrace). However, this puts a tremendous burden on software to time the graphic updates very precisely.

For this reason, the Flip command was devised. The Flip command is quite simple - it allows software to update the image any time within the refresh cycle, but the update is not actually delivered to the graphics memory until the current refresh cycle ends. Thus, the image update is performed on the monitor on the next refresh cycle following the Flip command’s execution. Using the Flip method, tearing is never observed because the Flip command guarantees the whole new image is displayed in one refresh cycle (See Figure 3). However, in the next section, we will see that using Flip alone does not eliminate every problem.


Figure 3 – Flip Command Sequence


Potential Problems with Flip

While the Flip construct is a great innovation to easily allow software to prevent the tearing artifact, there is still one potential problem.

When using Flip, the rules that apply to software video rendering are now different. Before Flip, software had to make sure that the timing of the graphics frame update was performed at the proper frame time. However, the monitor’s refresh cycle frequency is now the only clock that the frame display timing can be based on. In other words, a new frame can now be displayed only at the beginning of a re fresh cycle - frame display times are essentially locked in step with the display’s refresh frequency.


Figure 4 –Frame Rate and Display Rate Mismatch

This insight reveals that unless the monitor’s refresh rate exactly matches, or is a multiple of the delivered content’s frame rate, a perfect rendition of the content on the display is not possible. Figure 4 shows an example of this problem. In this scenario, the content frame rate is running slower than the display rate. Because of the phase shift between the two frequencies, the Flip times for two frames will eventually span across a complete refresh cycle (see Flips for frames 3 and 4). This causes frame #3 to be displayed for twice as long as the other frames. This problem demonstrates why it is best to have a perfect “fit” between the frame rate and the display rate, although that is not always possible.

The situation can become much more pronounced when the difference between the frame rate and display rate is small. When the frame times begin to occur very close to the refresh start times, small inaccuracies in the software’s timer can cause several consecutive Flips to stutter back and forth across the refresh start time threshold. This means that some Flips are done too early and some Flips are done too late, resulting in several “over-displayed” and “under-displayed” frames. Figure 5 details this situation - the timer is inaccurate (has jitters) and this causes frames 2 and 4 to not be displayed, and frames 3 and 5 to be displayed twice.


Figure 5 – Flip with inaccurate timer

This case can occur even when the content frame rate and the display refresh rate are the same. Clearly, just a timer and the Flip command are not good enough to ensure quality video. As will be shown in the next section, to handle Flips properly, software must synchronize with the display’s refresh cycle.


Properly Timing Flips

As discussed earlier, using the Flip command brings the display’s refresh cycle squarely into play when timing video frame rendering. Each newly delivered frame is only displayed during one of the display’s complete refresh cycles. So when using the Flip command, software must accurately predict not only when each frame should be displayed, but it must also determine the specific refresh cycle to best meet the frame’s display time.

It is best to execute Flips early in the refresh cycle, immediately preceding the frame’s targeted refresh slot (See Figure 3 for an example). This gives the greatest chance for actually executing the Flip command before the target refresh cycle starts and ensures that the frame is displayed at the correct time. Note in the case where the content frame rate and the display refresh rate are not matched or an even multiple, then the “best-fit” refresh cycle for a particular frame may not produce acceptable video qual ity. There may be ways to perform frame-content generation or altering to get around these problems, but they are beyond the scope of this paper.

Some operating systems expose programming interfaces that allow software applications to synchronize with the display’s refresh cycle. For example, Microsoft’s DirectX* 9.0 contains some useful routines for doing just that. This paper will concentrate on the DirectX routines as an example of the techniques. Readers can use these examples to learn the methods, and then determine what support exists in other operating systems.

WaitForVerticalBlank() is a DirectDraw method (under IDirectDraw) that blocks the calling thread until the beginning of the next refresh cycle. This routine can be used to do synchronization, although it should only be done once or at a very low rate, due to the high overhead of calling this routine. However, this routine is useful for doing the initial synchronization with the refresh cycle.

GetScanLine() is a routine that can be used to retrieve which scan line is currently being updated on the display. If the total number of scan lines and current scan line is known, it is easy to determine where in the refresh cycle the display is. For example, if the total number of scan lines is 1024 and GetScanLine() returns 100, then the current refresh cycle is currently 100 divided by 1024 or about 10 percent complete. Using GetScanLine(), allows an application to track where the refresh cycle is and to use it to determine which refresh cycle to target the next frame for display and to set a timer to set the appropriate flip time. An example algorithm is given below:


Figure 6

Frame times are not chosen based solely on frame rate but by a combination of both the frame rate and the refresh rate. Since the frames can be displayed only at the display refresh times, it is important to target the best fit refresh cycle for each frame. Therefore, it is best to have a refresh rate that exactly matches the display rate. If this were the case, then each and every frame could be drawn at its actual frame time.


Another Option for Recorded Content

While these problems are applicable to all video playback scenarios, both live and recorded content, it may be possible to ease the problem while playing recorded content. If the difference between the content frame rate and the display refresh rate is small, it is possible to adjust the frame rate of the video (and adjust audio accordingly) to match the refresh rate without harming the quality of the content. An example of this is playing back standard definition television at 59.94 frames per second (de-interlaced with the Bob algorithm) played back on a 60 Hz monitor. By speeding up both video and audio to play at the 60 frames per second speed, the frame times match the refresh times and no artifacts occur.


Summary

This paper examines Flip and its intended use to prevent tearing artifacts. It discusses how the use of Flip can cause problems because new frames are displayed only on ref resh cycles of the display. With normal frame timing used to generate Flips, the results are that frame display times and durations differ from what the application expects. Finally, it shows that the proper technique for using Flips is to synchronize them with the display’s refresh and pick a best fit refresh cycle for each frame to be displayed in. Software can then time the Flips appropriately to meet the appropriate refresh cycle. Video quality is definitely best when the content frame rate and the display refresh rate are the same. However, since this rate matching is not always possible, algorithms to minimize the artifacts discussed here should be used.


Additional References

Microsoft DirectX*


Dan N. Nowlin
Insert Author Name
06-19-2009
06-19-2009
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Software displaying video frames must take into account and synchronize around the refresh cycle of the display device. Computer monitors refresh at a fixed rate set by the graphics adapter clock, which is totally unrelated to the video input. This causes problems with synchronizing video with a PC monitor. This article describes a solution using waiting on vertical blanks and adjusting strategy based on the current scan line.
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