SMPTE Standard and Maximum Applicable Cable Length of Coaxial Cable

For baseband digital signal transmission, as the used cable length becomes longer,

  1. Amplitude of pulse shape is reduced caused by attenuation inside through cable
  2. Rise time of pulse shape is increased caused by phase distortion through cable
therefore, it becomes not possible to discriminate transmitted digital signal by receiver finally. We would work out reasonable prediction method for maximum applicable cable length of coaxial cable, referring to SMPTE standard as an example.

1. Basic characteristic of cable

The following phenomena can be proved for all of high frequency cable made of metal conductor:

  1. Attenuation increases proportionately to the square root of frequency
  2. Phase distortion is equivalent to attenuation
Both of above facts are originated in the skin effect.

In other words, cable affection can be determined by one attenuation value at any one specific frequency point in the high frequency range.

2. SMPTE Standard

2.1 Cable characteristics

Coaxial Cable Interface by SMPTE defines cable characteristics by the following two points:

The first rule is necessary condition for an automatic cable equalizer of a receiver to be able to work certainly, and it becomes necessarily like this in case of coaxial cable as explained already (basic characteristics of cable).

The second rule defines accuracy of characteristic impedance of cable.

2.2 Characteristic of receiver

Characteristic of receiver that difines maximum acceptable attenuation of cable by SMPTE is a little bit vague.

SMPTE 259143/270/360 Mb/s Typical loss amounts may be in the range of 20 dB to 30 dB at one-half clock frequency with appropriate receiver equalization. Receiver design to work with lesser signal attenuation are acceptable, but are not recommended for new designs.
SMPTE 2921.5 Gb/sReceivers operating with input cable losses in the range of up to 20 dB at one-half the clock frequency are nominal; however, receivers designed to work with greater or lesser signal attenuation are acceptable.
SMPTE 424M3 Gb/sThe receiver operating with input cable losses in the range up to 20 dB at half-the clock frequency are typical; however, receivers designed to work with greater or less signal attenuation are acceptable.

In SMPTE 259, receiver must work in the range of 20 dB to 30 dB as general rule, however, it also accepts receiver that works only with lesser signal attenuation, so that maximum applicable cable length depends on specification of receiver afterall.

Receiver that can work only with lesser signal attenuation is assumed for past design receivers, and receiver that can work with over 30 dB attenuation was beyond expectation at that time.

However, in the time of SMPTE 292 and SMPTE 424M, standard attenuation is assumed to be 20 dB, but both of any better or worse receiver are accepted.

This vagueness lies in the progress in the cable equalizer technique.

The idea of cable equalizer is that attenuation distortion and phase distortion of cable can be canceled with an additional circuit in front of receiver that increases amplitude proportionally to the square root of frequency. It was hard to realize such characteristic circuit at wide frequency range in the past, however, because of progress of LSI technique this type of circuit could be realized, and moreover, advanced LSI was realized that can adjust compensation volume automatically, calculating backwards the cable length from attenuated wave shape. Then, thereafter, those receivers and equalizers became to be realized that can work far beyond the original limit of applicable cable length.

Those products (LSI) are available on the market today from many semiconductor manufacturers such as Gennum, National Semiconductor, Maxim and so on. For example, in case of Gennum GS2994, it makes it possible to transmit 2.97 Gb/s SMPTE signal up to 200 m with RG-6/U cable whose attenuation at 100 MHz is 6.0 dB per 100 m, which is almost three times longer length of its originally applicable length of about 65 m.

3. Yardstick for maximum applicable cable length of coaxial cable

Thus, maximum applicable cable length based on SMPTE standard cannot be determined only by clock rate (transfer rate / transmission speed) and characteristic of cable, but by specification of receiver and or quality (characteristic) of equalizer. For the present, following idea would be a good yardstick.

  Cable length that becomes 20 dB attenuation at 1/2 clock rate
However, we should never forget that there are devices that won't work at this condition and or others that can work even at three times longer cable length available on the market.

Also, please remember that even though attenuation value per unit length of cable at 1/2 clock rate is unknown, it can be easily calculated if any value at specific frequency is known (given), as attenuation through cable is proportional to the square root of frequency.