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Maximum transmission distances for digital video cables
Resource Maximum transmission distances Digital Cables
The Society of Motion Picture and Television Engineers (SMPTE) publish a range of standards that differ by application. Each of these standards specify a maximum allowable loss at ½ the clock (signal) frequency being transmitted.

SMPTE 259M (PAL, widescreen): 30dB loss at ½ the clock frequency
SMPTE 292M (HD-SDI 1080i): 20dB loss at ½ the clock frequency
SMPTE 425M (3G-SDI single link): 40dB loss at ½ the clock frequency
SMPTE ST2081 (UHD 4K dual link): 40dB loss at ½ the clock frequency
SMPTE 2082 (UHD 4K single link): 40dB loss at ½ the clock frequency

The loss of a cable is directly related to its length.

As each standard has a different allowable loss, using the same cable, the nominal transmission length for SMPTE 2082 (UHD 4K single link) is greater than for SMPTE 292M (HD-SDI 1080i)

Determining Cable Lengths for 12G-SDI Applications

Once the attenuation of a given cable at a specific frequency is known, calculating the approximate transmission distance for that cable is a relatively straight forward process (with a few caveats mentioned later on).

For clarity, we will concentrate on SMPTE 2082 (UHD 4K single link) from here on in.

Taking Gigatronix 12G-SDI-6 Digital Video Cable as an example. This has an attenuation of 50dB per 100m at 6GHz. For a Single Link 12Gb/s application, ½ clock frequency = 6 GHz. Maximum allowable loss is 40dB.

Below is a plot showing insertion loss (attenuation) of 80 metres of Gigatronix 12G-SDI-6 Coaxial cable.

Plot for 12G-SDI-6

The marker "M2" marks the attenuation at the ½ clock frequency and can be seen to be -39.647dB.

Based on signal loss at half the clock frequency as defined by SMPTE 2082, SMPTE 2083 - Single Link 12Gb/s, the Transmission Distance Table shown below defines values for cables only. Any other components within the system (connectors / patch panels etc) will add loss.

Significant improvements in the development of chipsets and active equipment has resulted in much longer transmission distances for today's applications. Real world applications can often go considerably further than the recommended distances shown in the table below.

Recommended Maximum Transmission Distances by Cable Type

Maximum transmission distances for digital video cables

Bit error rate (BER) will vary significantly as the calculated distances are approached / exceeds, so receiver, distribution and routing equipment manufacturers should be consulted to verify their equipment supports -40dB (or better) loss at half clock frequency.

Remember, The SMPTE states a maximum value of -40dB for cable loss but they do not publish a standard requiring active equipment manufacturers to meet this value.

If the equipment supports -40dB, then you can confidently reach the distances stated in the Transmission Distance Table. If the equipment only supports up to -30dB loss, then the cable length must be reduced to support it.

Note - The calculations shown on this page assume that the cable (and a pair of connectors) are the only items between the transmitter and receiver.
In practice, modern equalisers are able to recover signals that fall below the levels defined by the standards.