Datavideo DAC-07 Manuale di istruzioni - Pagina 10

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SD-SDI Cabling Advice

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 120m should be possible with this unit before

the signal would need to be re-clocked or repeated.

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.