Sync Jitter

Bottom Line Up Front: In order to help mitigate this issue, we have updated the circuitry in our SNES and Genesis YPbPr cables to reduce the amount of jitter on the rising edge of sync. Since equipment which triggers off this incorrect edge is still operating out of standard-definition video specifications, we cannot guarantee artifact-free operation in 100% of all cases. That being said, our testing with the new circuitry has shown promising results. There is more information regarding the SNS0-AH0 and GEN0-AH0 cable revisions on our SNES and Genesis product pages.


In the following article, we attempt to explain a phenomena called "sync jitter" which some HDTVs' YPbPr video inputs are more affected than others. In extreme cases, this can cause a distracting visual effect on the screen. A couple of examples of a display device which is sensitive to this type of jitter are shown below. The first is on a static screen containing only text. The second example contains motion and is more typical of what is seen during most gameplay.

 Display device sensitive to sync jitter. Static text screen.

Display device sensitive to sync jitter. Static text screen.

 Same display device showing a non-static image.

Same display device showing a non-static image.

If this is occurring with your TV or equipment, and is affecting your experience while using our SNES and/or Genesis YPbPr video cables, then please read the rest of this article to better understand the specific cause and possible solutions to help alleviate it.


Background

Before diving into what sync jitter is, we need to briefly explain what "sync" is in video terminology. Sync typically refers to reference timing signals present either alongside or within the video signals themselves, basically telling the display device where to place the video data. There are usually two sync signals in typical video systems: a horizontal sync which tells the display where each line of video begins, and a vertical sync which conveys where the top of the picture is. We're primarily concerned with horizontal sync for this article so we will focus on that. An example of what a horizontal sync pulse looks like on a standard-definition video signal is shown below.

The sync is a short downward going pulse that drops below the black level and lasts approximately 4.7µs (microseconds) before returning back to the black level. The most important thing to note here is that the beginning of a line of video occurs at the instant the pulse dips down, known as the "falling edge". This specific edge is the horizontal reference point of the video system and is explicitly defined in several standard-definition video standards, such as SMPTE-170M and CEA-770-2.D. The other edge, where the waveform goes back up to black level, is called the "rising edge" and is meant to be ignored in a standard-definition video system.


Line Shifting

So now that you know what horizontal sync is, then we can continue in explaining what sync jitter is and why it can cause the visual effects shown in the above images. Recall that the beginning of a line occurs at the falling edge of the sync pulse. All electrical pulses of this nature have a property called "jitter" which is how much that edge shifts around its nominal location.  An example of a line of video containing a sync pulse is shown below along with zoomed-in images highlighting different degrees of jitter behavior.

 Example line of video. Highlighted portion is zoomed-in below for two different cases.

Example line of video. Highlighted portion is zoomed-in below for two different cases.

 Zoomed-in sync pulse edge - low amount of jitter.

Zoomed-in sync pulse edge - low amount of jitter.

 Zoomed-in sync pulse edge - slightly more jitter.

Zoomed-in sync pulse edge - slightly more jitter.

If the position of the sync edge shifts around enough to cause the display to think that the beginning of different lines change periodically, then the display can show a resultant image where various lines appear shifted left or right from neighboring lines. This is exactly the effect shown in the example images at the beginning of this article. However, the original SNES (SNS0-AG0) and Genesis (GEN0-AG0) YPbPr cables do not introduce enough falling edge jitter to cause this shifting to occur in HDTVs, so there must be something else going on here.


High-Definition Sync

There is a final piece to this puzzle which is needed to explain why the visual artifacts manifest themselves only on HDTVs and other high-def-capable equipment. Unlike composite video and s-video, YPbPr is an analog video interface which is capable of supporting enhanced-definition (480p) and high-definition (720p/1080i) signals in addition to typical standard-definition content (240p/480i). Enhanced-definition video uses the same sync pulse structure as standard-definition signals, but high-definition sync pulses have a completely different structure known as "tri-level sync". These special pulses dip down below black, shoot up above black, and then finally settle back again at the nominal black level. An example waveform is shown below.

trilevel_sync.png

Because there are three levels to this sync waveform, it results in three edges: two short falling edges and one long rising edge. This symmetric structure is actually more immune to noise and is why it was chosen for the newer high-def analog signal standards. As indicated in the picture, the horizontal reference point is during the rising edge. This is in contrast with the "bi-level" sync pulse for standard-definition. And herein lies the problem. To guarantee proper operation, a piece of video equipment which supports both standard and high definition input signals must be able to use different sync edges for different input resolutions.


The Rising Edge

Unfortunately, we have discovered that many pieces of equipment, including several HDTVs, don't adhere to these sync edge rules. To simplify equipment design, it's easier to make a video input only look for one type of edge. In these cases the rising is used. However, using the rising edge as the horizontal reference point for standard definition sync causes a couple problems. Video displayed using this reference point will appear shifted too far to the left, possibly pushing some content off-screen. It will also be less stable since there is typically more jitter introduced onto the rising edge by intermediate equipment. In the specific case of the original SNES (SNS0-AG0) and Genesis (GEN0-AG0) YPbPr cables, they introduce a significant amount of jitter into the rising edge of the sync pulse. Properly designed equipment will not be affected, but anything that incorrectly uses the rising edge is at risk of producing horizontal shifting artifacts on the screen. The following is a snip from the hardware manual of a chip used in a very popular video processor, warning of the use of positive (rising) sync edges with non-HD signals.

 ADV7441A Hardware Manual

ADV7441A Hardware Manual


Solutions

If you are plagued with a TV or device that produces visual artifacts due to sync jitter on the rising edge, there are a couple things you can do to alleviate it. The first is to try and use different equipment which utilizes the correct falling edge of sync and doesn't exhibit this visual behavior. Another option is to use an intermediary device in between the YPbPr cable and the TV, such as an A/V receiver or a video processor. We are continually testing affordable converter boxes and video processors and will update our lists on our 240p page with compatible models as they are discovered.

In order to help mitigate this issue, we have updated the circuitry in our SNES and Genesis YPbPr cables to reduce the amount of jitter on the rising edge of sync. Since equipment which triggers off this incorrect edge is still operating out of standard-definition video specifications, we cannot guarantee artifact-free operation in 100% of all cases. That being said, our testing with the new circuitry has shown promising results. There is more information regarding the SNS0-AH0 and GEN0-AH0 cable revisions on our SNES and Genesis product pages.