Usually when we talk about electronic interference, the assumption is that the interference is caused by other electronic devices, power lines, radio waves, or lines that cross one another. However, not all of what appears to be electrical interference really is. In many cases, the problem isn’t interference at all, it’s caused by jitter.
Jitter occurs whenever a regular signal becomes irregular as compared to a regular source, such as a clock. This can happen for a number of reasons, but all of them have the same effect, causing problems with a transmission signal.
Basically the term jitter is only used in association with data that is transmitted from one place to another. As transmission ranges increase, the possibility of jitter does too, along with the frequency or amplitude of any jitter. This means that the effect of the jitter increases with transmission distance. If it increases enough, signal integrity is lost.
When we are talking about data transmission, it could refer to any sort of data. While we may be accustomed to applying that term to computers, it applies equally to video, audio and television signals being transmitted between various pieces of equipment.
How Jitter Works
For the purposes of explanation, we’re going to assume that we’re transmitting a simple square wave. This would be the same as transmitting a series of digital one’s and zero’s from one point to another. It really doesn’t matter what type of data it is, it all works the same.
The three waveforms in the diagram represent how jitter changes the data being sent. The top waveform is the original clock wave. The middle one represents data that is being compressed. If you follow the waveform from left to right, comparing it to the original, you see that each 1 is farther away from the correct time. Within a short while, the waveform will be showing 1s where there should be 0s, and 0s where there should be 1s. The bottom waveform shows irregular jitter, where one 0 is too short and another is too long. While the end result corrected itself. The data that is in between the two errors is out of sync.
Long transmission lines can act in a number of different ways, producing these types of jitter and others. One result can be like having a capacitor attached to the waveform, which would end up trying to smooth everything out, making it into a constant. While it may never reach that point, it can distort the data signal enough to make it unusable to the receiving equipment. (Please note that this waveform has been exaggerated for instructional purposes.)
Another way that long transmission lines can create jitter is like a coil or transformer would. In these cases, the timing of the waveform would be affected, rather than the shape of the waveform. In such a case, the result would be a waveform more like the second one shown in the first drawing.
All of these forms of jitter are considerably different than electronic interference, which appears like noise in the waveform.
This problem of jitter is one of the principal reasons why transmission lines for various types of connections, such as HDMI are limited to a certain length. Longer cables would tend to introduce more jitter into the transmission process. This can be mitigated somewhat by using larger diameter wire in the cables, which will transmit the signals over longer distances without these problems.
Affects of Jitter
Depending upon the severity of the jitter, the data being transmitted can be affected or totally corrupted to the point of being unusable. Generally speaking, specifications on systems and cabling are created which limit usage to a point where jitter is unlikely to occur. That doesn’t mean that it can’t occur, merely that it isn’t likely.
One way that we see jitter on a day-to-day basis is in random lines and blocks of pixels appearing out of place in a television screen. Another common result of jitter is delays in data transmission, resulting in packets of information to have to be resent. It also commonly shows up in CD playback of music. This can sound like miniscule skips or sound overlap where the same sound is played twice. These flaws are miniscule enough that one must listen closely to hear them.
Dealing with Jitter
Engineers have developed a number of strategies for dealing with jitter in equipment. With CD music, one common way is to read the data twice and compare it, “fitting” the two waveforms together to make a single waveform without jitter. While complex in theory, once the circuitry is created, it is an automatic process.
Many other types of electronic equipment which are subject to the effects of jitter have anti-jitter circuits as well. The most common of these is to realign the incoming signal to the clock signal in the receiving equipment. The same can be done with outgoing signals, checking and aligning them to a stable clock pulse to ensure that the data stream is going out without jitter.
Buffering is a common way of dealing with jitter in a network setting. This is why streaming audio and video from the Internet is always buffered. By receiving the data stream faster than it is needed, the data can be temporarily stored. The process of reading that data from the temporary storage eliminates any jitter, as it is done according to the clock pulse of the computer that is playing the audio and/or video files that were streamed.
All of these anti-jitter methods happen in the background, without the user seeing what is happening. They are automatic processes which make it possible to use the data being transmitted, even if it does contain jitter.