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8 ExtroNews 12.2 March/April 2001

TECHNICALLY SPEAKING...

fails, ungracefully. For this reason, allow at

least 10% margin in your cable length

calculations to account for other connection

changes, connector resistances, connector

and termination reflections, etc. Most pre-

calculated cable charts build in this

allowance just for good practice. See

Table 2

for calculated cable lengths for Extron cables.

There’s Clocking, And Then There’s

Re-clocking

All digital data is derived and managed by

a repetitive pulse train called a clock… the

literal heartbeat of the machine. Without it,

data transitions could not be identified in a

coherent way. Either, digital data somehow

contains the clock information embedded

within it, or the clock signal accompanies the

data separately. Since SDI is a singular wire

transmission scheme, the clock is embedded.

Therefore, not only does cable attenuation

affect recovery of data, it seriously affects the

receiver’s ability to recover the clock signal

such that the system can stay synchronized.

This is where basic cable attenuation

comes in. The maximum cable distance is

governed by the receiver’s ability to recover

clock and data reliably. As the digital cliff is

approached, bit errors typically appear and

escalate rapidly toward transmission failure.

But, (you say) I need to transmit SDI over

1,000 feet and the best cable I have available

is not guaranteed much beyond that. What

will I do?

The solution is straightforward. Position an

SDI receiving device in the line at a point

where reliable communication is maintained.

Make sure this device is a true SDI receiver

that can equalize and re-clock the signal. In

analog signal transmission systems there is

no really good way to reform and transmit

the signal while still maintaining good

linearity. With digital data streams, however,

the data can be captured and reconstructed

by a “squaring” circuit that restores the

original risetime of the signal. Because we

aren’t concerned about linearity in digital

data, the data can be indefinitely

reconditioned as long as good signal

conditioning practices are followed. When

SDI is reconditioned for retransmission, the

data edges are sped up and the original

timing accuracy restored. This operation is

referred to as “re-clocking.” Now, we can

run the additional distance our cabling

system will allow.

To Re-clock Or Not To Re-clock

Yes, that is a question. Properly re-clocking

SDI calls for additional circuit complexity and

cost. Good SDI matrix routers include re-

clocking systems. Further, it’s a good idea to

consider the location of a router in a new

installation such that cabling distances can

work to your advantage toward

minimizing the number of repeating

stations, or re-clocking points, required.

Routers are typically the focal point for re-

clocking data. SDI distribution amplifiers

can involve a receiver/re-clocker circuit as

well. In major installations having large

routers, it is common for the router to have

re-clocking ability at the input and at the

output as well. Why? The actual

propagation distance and loss effects

through some large routers represent a

serious impairment to SDI signals.

Therefore, re-clocking may be included at

the output to ensure signal quality for the

next long cable run.

Now, back to the question. Do I need to

always re-clock SDI signal runs? No.

Suppose you have a relatively small set of

cable runs involving a small matrix router

and good receivers at each destination.

Good SDI receivers can recover the signal

under some surprising signal degradation

conditions. In some cases, adding the

wrong equipment into the line (or

something having a poor re-clocking

system) may actually increase signal jitter,

which makes recovery more difficult. As

long as you carefully maintain signal

quality and are not in danger of exceeding

cabling distances, you do not necessarily

need to re-clock. Re-clocking is primarily

intended to clean up long run losses, allow

easier decoding, and redrive additional

cable runs.

An analog matrix router under the right

conditions can handle SDI nicely. The key

issue is whether the router will introduce

crosstalk or other noise that may affect the

signal jitter performance. If the router is

bandwidth limited, performance will be

directly affected as high frequencies are

attenuated rapidly. You must look at router

bandwidth specs to see if the SDI risetime

can be accommodated without significant

effect. Since the SDI signal is about 800

Table 2. Recommended Serial Digital (SDI) Transmission

Distances through Coaxial Cable

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