Eye Pattern and Re-Clocking

Signal Quality and Jitter

SDI and / HDMI / DVI signals must be recovered after being transmitted over long cable runs. This is generally performed by adding equalization to the receiver to return the signal level to its nominal value. But with active equalization systems not only the signal but also noise is amplified.

First step in recovering the signal is to extract the clock information to retime the signal. The noisier the signal is the more difficult it is to recover the clock and the signal jitters more.

Jitter is defined as the variations of the significant instants of a timing signal. Rising and falling edges of the SDI signal are moving forward and backward in time. Digital system jitter is measured in UI (Unit Interval = the period of one clock cycle) or by period (pS or nS). Signal quality is measured with waveform monitors there the 'eye pattern' is visualized. The more accurate the 'eye pattern' (see below) is displayed, the lower is the jitter and the more precise the signal is in variations of time.

Jitter Tolerance:
Jitter being present at the signal input that a system can sustain by recovering the input signal with no errors. It can be specified as a function of frequency.

Jitter Transfer Function
The ratio of input jitter to output jitter.

Residual Jitter
The amount of jitter present at the output of a device, partly random jitter and partly deterministic jitter.

Random Jitter
Random noise, caused for instance by cable equalization.

Deterministic Jitter
Can be generated by an internal transmit clock that is not uniform of if bit cells are short in relation to the rise/fall time of the signal. It can also be caused by serializers with unequal clock cycles etc.

Intrinsic Jitter
Output jitter from a device whose input has no jitter at all.

Alignment Jitter and Timing Jitter
Specified by the Society for Motion Picture and Television Engineers (SMPTE) alignment jitter defines the variations relative to a recovered clock, and timing jitter defines the variation in the edge transitions of the recovered clock relative to a stable clock, both dependent on the bandwidth of the PLL used to recover the clock.

Eye Pattern and Eye Pattern Measurement

The eye pattern presents the voltage waveform of the received video data signal. Because of higher jitter the leading and trailing edges of the signal (individual bits) are 'moving' forwards and backwards in time, the lines in the eye pattern become thicker. As soon as the voltage is varying too the top and bottom line will thicken also. This thickening is caused by several repeats of the signal laid on top of each other showing the 'sum' of the individual lines.

As any AC voltage the eye pattern is displayed via an oscilloscope by triggering the signal with its own data rate (wavelength / half wavelength). This way each bit of the incoming signal is 'put' on top of each other. All variances, jitter (backwards and forwards movements of the leading and trailing edges, movements up and down) can be seen as thickened lines. The thicker the line and respectively the smaller the open eye in the middle, the poorer is the signal quality.

typical rise and fall slope  (clock rate = unit interval)

real eye pattern measurement
jitter shown as shifting with time

Signal Recovery, Pre-Emphasis and Equalization

With AC currents flowing through cables at higher frequencies, the skin effect is coming more and more into account the higher the frequency is. Cables carrying high frequency signal act as low pass filters. The resulting loss in the high frequency part is called insertion loss.

typical insertion loss curve

Because of the insertion loss of the cable the digital video signal must be recovered after being transmitted over longer cable runs. This can be performed by pre-emphasis at the start point of the cable or by adding equalization to the receiver at the end of the cable. For the use of pre-emphasis the cable characteristics and the cable length must be known. Equalization can be done with adaptive cable equalizers that is controlling the amount of equalization by a feedback loop through the cable. With active equalization systems not only the signal but also noise is amplified.
Pre-Emphasis and Equalization is done by increasing the high frequency part according the insertion loss of the cable to finally get a more or less linear signal again.


First step in recovering the signal is to extract the clock information to retime the signal. The noisier the signal is the more difficult it is to recover the clock and the signal jitters more. Reclocking is done by extracting the clock information from the input signal and to retime the signal. The receiver actually 'knows' the data-rate of the incoming signal and it generates a stable clock at that frequency. By re-shaping the eye-pattern, the accumulated jitter is reduced.
The incoming signal is fed into the re-clocking unit and the phase of the data transitions is compared to the phase of the internal oscillator clock. Most PLLs have a constant clock as their input reference signal. The relative phase of the input signal is then compared to the clock. The polarity of the input data is not relevant because the input signal is data without a 0-1 or 1-0 pass through at each clock transition. Only the position of any transitions versus the position of the rising edge of the VCO clock are compared. In many discrete re-clockers the timing is done with an expensive voltage controlled crystal oscillator (VCXO).

Unit Interval

The Unit Interval is the inverse of the bandwidth, the time duration of one bit.

Format Standard Bandwidth Unit Interval
SD-SDI 525i/575i NTSC/PAL SMPTE 259M 270 Mbps 3700 picoseconds
HD-SDI 1080i SMPTE 292M 1,485 Gbps 670 picoseconds
3G-SDI 1080p SMPTE 424M 2,970 Gbps 340 picoseconds