Jitter in Professional Video Systems

Understanding the causes of deterministic and random jitter in SDI and other digital video signals, and how to minimize jitter in the design of pro video equipment meeting SMPTE specs.

By Mark Sauerwald, Applications Engineer, MTS, National Semiconductor Corp.

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Video/Imaging DesignWire
(9/11/2009 7:00:13 PM)

The recent business downturn has forced many managers to tighten budgets, but for those of us who work with serial video data transmission, this is nothing new — our jitter and timing budgets get tightened every time the data rates are increased (and Congress has never offered us a bailout!). Sometimes all that is needed is a few picoseconds to help you meet your jitter budget.

In this article, I’ll show how jitter is measured and specified for the Serial Digital Interface (SDI) links used in broadcast video applications. Armed with this information, you’ll be able to craft your own jitter “bailout.”

What is Jitter?
Serial interfaces send data at a fixed rate. Some interfaces might have a variety of different rates at which they operate, but once the transmitter and receiver have settled on a particular data rate, it remains constant for a long period of time. The ideal transmitter has a perfectly stable reference clock oscillator, with one clock tick happening at the time each bit is transmitted. The exact placement of this clock edge is perturbed each time the signal goes through a driver, cable or a receiver.


Figure 1: Block diagram of a data transmission system

Jitter is the variation in the placement of the edges of the signal with respect to the ideal location of the edges. Jitter is important because the receiver operates by setting up its own clock — running at the same rate as the transmitter — to sample the incoming signal.

If the signal has sufficient jitter to make its transition from one state to another after the sampling point, a bit error will occur. This means your jitter budget has been exceeded and is now the equivalent to an overdraft on your bank account. It is not a good thing. Jitter is a major concern because it leads to bit errors in the transmission system (see Figure 1).

NEXT: How does SMPTE define and measure jitter?

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