Handbook for Sound Engineers

Transmission Techniques: Wire and Cable 421

In fact, Table 14-22 above is not entirely accurate.
The distances should be multiplied by the velocity of
propagation of the cable or other component, to get the
actual length, so they are even shorter still.

Since everything is critical at high frequencies, it is
appropriate to ask the manufacturers of the cable,
connectors, patch panels, and other passive components,
how close to 75: their products are. This can be estab-
lished by asking for the return loss of each component.
Table 14-23 will allow the user to roughly translate the
answers given.

Most components intended for HD can pass 20 dB
return loss. In fact, 20 dB return loss at 2 GHz is a
good starting point for passive components intended for
HD-SDI. Better components will pass 30 dB at 2 GHz.
Better still (and rarer still) would be 30 dB at 3 GHz.
There are currently no components that are consistently
40 dB return loss at any reasonable frequency. In
Table 14-22, it can be seen that 1080p/60 signals need to
be tested to 4.5 GHz. This requires expensive
custom-built matching networks. As of this writing,
only one company (Belden) has made such an invest-
ment.

Note that the number of nines in the Signal Received
column is the same as the first digit of the return loss
(i.e., 30 dB = 3 nines = 99.9%). There are similar tests,
such as SRL (structural return loss). This test only
partially shows total reflection. Do not accept values
measured in any way except return loss. The SMPTE
maximum amount of reflection on a passive line (with
all components measured and added together) is 15 dB

or 96.84% received, 3.16% reflected. A line with an RL

of 10 dB (10% reflected) will probably fail.

14.10.6 Video Triaxial Cable

Video triaxial cable is used to interconnect video cam-

eras to their related equipment. Triaxial cable contains a

center conductor and two isolated shields, allowing it to

support many functions on the one cable. The center

conductor and outer shield carry the video signals plus

intercoms, monitoring devices, and camera power. The

center shield carries the video signal ground or common.

Triax cable is usually of the RG-59 or RG-11 type.

14.10.7 S-Video

S-video requires a duplex (dual) coaxial cable to allow

separate transmission of the luminance (Y) and the

chrominance (C). The luminance signal is black or white

or any gray value while the chrominance signal contains

color information. This transmission is sometimes

referred to as Y-C. Separating signals provides greater

picture detail and resolution and less noise interference.

S-video is sometimes referred to as S-VHS™

(Super-Video Home System). While its intention was

for improved consumer video quality, these cameras

were also used for the lower end of the professional

area, where they were used for news, documentaries,

and other less-critical applications.

14.10.8 RGB

RGB stands for red-green-blue, the primary colors in

color television. It is often called component video since

the signal is split up to its component colors. When these

analog signals are carried separately much better image

resolution can be achieved. RGB can be carried on mul-

tiple single video cables, or in bundles of cables made

for this application. With separate cables, all the cables

used must be precisely the same electrical length. This

may or may not be the same as the physical length.

Using a vectorscope, it is possibly to determine the elec-

trical length and compare the RGB components. If the

cables are made with poor quality control, the electrical

length of the coaxes may be significantly different (i.e.,

one cable may have to be physically longer than the oth-

ers to align the component signals). Cables made with

very good quality control can simply be cut at the same

physical length.

Bundles of RGB cables should be specified by the

amount of timing error, the difference in the delivery

SD-SDI 135 MHz 405 MHz 2.43 ft 7.3 inches

HD-SDI 750 MHz 2.25 GHz 5.3 inches 1.3 inches

1080p/60 1.5 GHz 4.5 GHz 2.6 inches 0.66 inches

Table 14-23. Return Loss versus % of Signal
Received and Reflected

Return Loss % of Signal Received % Reflected

50 dB 99.999% 0.001%

40 dB 99.99% 0.01%

30 dB 99.9% 0.1%

20 dB 99.0% 1.0%

10 dB 90.0% 10.0%

Table 14-22. Wavelength and Quarter Wavelength
of Various Signals at Various Frequencies

Signal Clock

Frequency

Third

Harmonic

Wavelength Quarter

Wavelength