Sync Jitter

In the following article, we attempt to explain a phenomena called "sync jitter" which some HDTVs' YPbPr video inputs are very sensitive to. In extreme cases, this can cause a distracting visual effect on the screen. A couple of examples of what a display which is very sensitive to this type of jitter are shown below. The first is on a static screen containing only text.  The other 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 when 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: 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 lasts approximately 4.7µs (microseconds). 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 "negative-going edge".


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 negative-going edge of the sync pulse. All 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. In some TVs, only a very small amount of jitter is all that is required to cause visual artifacts. As an example, the waveform on the right might cause problems on sensitive TVs while the waveform on the left doesn't, even though the amount of additional jitter present is on the order of a few nanoseconds.


Sensitivity

So what causes additional sync jitter? In the case of our SNES and Genesis YPbPr cables, the circuitry inside the cables performs an operation on the composite video signal that allows the sync signal on the composite video line to be recovered and used for the YPbPr output. The main reason for using this type of circuit is to maximize compatibility across various consoles and console revisions. Although the circuitry only introduces a small amount of additional jitter, it has been discovered to be enough to causes visual effects on some HDTVs.

It's unclear why these HDTVs have this problem on their YPbPr video inputs, while the same signal connected to their composite video input results in no visual disturbance at all. Our best guess is that most YPbPr video inputs typically support both standard definition & high definition video signals. High definition signals have a stricter sync jitter requirement, and also have a much different pulse shape (google: tri-level sync). Perhaps the universal detection method of both types of pulses in certain video processing equipment causes the extra sensitivity to jitter.

Solutions

NOW:  If you are plagued with a TV that's sensitive to sync jitter, there are a couple things you can do immediately to alleviate it. The first is to try different TVs which might not be as sensitive to jitter. Another option is to use an intermediary device in between the cable and the TV, such as an A/V receiver or a video processor. We're continually testing affordable converter boxes and video processors and will update our lists on our 240p page with compatible models.

FUTURE:  We are currently in the process of researching a cost effective method to reduce sync jitter within our cable circuitry for the SNES and Genesis YPbPr cables. The intent is to reduce the jitter enough to make it unaffected by sensitive display devices. At the moment, we are unsure if this a definite possibility.