Datavideo DAC-7 Kullanım Kılavuzu - Sayfa 11

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SD-SDI Cabling Advice

4.1

How to avoid problems with SD-SDI

Start with good cabling. BNC cables and connectors are not all made to the same standard. From

the thickness and grade of the centre conductor to the type of dielectric, all these factors

contribute to its impedance, attenuation and Return Loss (RL).

RL is the portion of the transmitted signal that is not admitted to the receiver and is then

reflected back along the cable toward the transmitter (DAC-07). Reflections in the transmission

path will cause attenuation as well as distortion of the signal received. The signal is attenuated

because part of it is reflected back and does not make it to the receiver; it is also distorted

because the reflected signal mixes with the original signal causing it to distort as well as adding

to the noise floor.

A 75 Ohm (Ω) impedance must be maintained throughout the DAC-07 SD-SDI signal/cable path.

Always use patch bays, hubs and repeaters that match the data rate and impedance you are

working with.

Care must also be taken with cable length, as it will affect the error rate in the SD-SDI signal.

Cable lengths beyond acceptable limits will see error rates reach a tipping point and this will

bring about a total loss of picture also known as the Cliff Effect.

Always check with your supplier for cabling advice on the maximum distance for a particular

signal and data rate, in this case an SD-SDI signal that complies with the SMPTE259M-C standard

at 270 Mbps. Datavideo have determined that single BNC SD-SDI cable runs of up to 100m

should be possible with this unit before the signal would need to be re-clocked or repeated.

4.2

Here is a basic list of other things to avoid

•

Do not step on or rest equipment on the cables.

Do not roll dollies or trolleys over the cables.

•

Do not put kinks or sharp bends in the cables.

Do not exceed the minimum bend radius of the cable.

•

The general rule for an acceptable bend radius is 10 times the diameter of the cable.

Cable pulls should be done carefully – do not jerk or stretch the cable(s).

•

Do not exceed the cable's maximum pulling tension. (Call the cable supplier /

manufacturer for this information.)

Do not pull cable ties too tight on cable looms.

•

If you cannot move any cable inside a loom tie, the tie is too tight.

Put an extra piece of cable in when tightening loom ties - then remove it afterwards

for space.

Do not put cable management fixings at standard distances apart.

•

This can lead to deformity at a given wavelength, which can increase RL.

Place your loom cable ties at random distances for the same reason.