Handbook for Sound Engineers

Transmission Techniques: Wire and Cable 411

The IEEE 1394 system uses two shielded twisted
pairs and two single wires, all enclosed in a shield and
jacket, Fig. 14-2. Each pair is shielded with 100%
coverage foil and a minimum 60% coverage braid. The
outer shield is 100% coverage foil and a minimum 90%
coverage braid. Each pair is shielded with aluminum
foil and is equal to or greater than 60% braid. The
twisted pairs handle the differential data and strobe
(assists in clock regeneration) while the two separate
wires provide the power and ground for remote devices.
Signal level is 265 mV differential into 110:.

The IEEE 1394 specification cable length is a
maximum of 4.5 m (15 ft). Some applications may run
longer lengths when the data rate is lowered to the
100 Mbps level. The typical cable has #28 gage copper
twisted pairs and #22 gage wires for power and ground.
The IEEE 1394 specification provides the following
electrical performance requirements:

• Pair-to-pair data skew is 0.40 ns.


• Crosstalk must be maintained below 26 dB from
  1 MHz to 500 MHz.


• Velocity of propagation must not exceed 5.05 ns/m.

Table 14-14 gives details of the physical interface sys-
tem for IEEE 1394.

14.9.3.4 USB

The USB, universal serial bus, simplifies connection of
computer peripherals. USB 1.1 is limited to a communi-
cations rate of 12 Mbps, while USB 2.0 supports up to
480 Mbps communication. The USB cable consists of

one twisted pair for data and two untwisted wires for

powering downstream appliances. A full-speed cable

includes a #28 gage twisted pair, and an untwisted pair

of #28 to #20 gage power conductors, all enclosed in an

aluminized polyester shield with a drain wire.

Nominal impedance for the data pair is 90:. The

maximum cable length is determined by the signal prop-

agation delay which must be less than 26 ns from end to

end. Table 14-15 lists some common plastics and the

theoretical distance each could go based on 26 ns. With

an additional allowance of 4 ns, which is split between

the sending device connection and the receiver connec-

tion/response function, the entire one-way delay is a

maximum of 30 ns. The cable velocity of propagation

must be less than 5.2 ns/m and the length and twist of

the data pair must be matched so time skew is no more

than 0.10 ns between bit polarities. The nominal differ-

ential signal level is 800 mV.

14.9.3.5 DisplayPort

DisplayPort is an emerging protocol for digital video. Its

original intention was the transfer of images from a PC

or similar device to a display. It has some significant

advantages over DVI and HDMI. DisplayPort is by

design backward-compatible to single link DVI and

HDMI. Those are both severely distance-limited by the

delay skew of the three data pairs when compared to the

clock pair. With DisplayPort the clock is embedded with

Figure 14-2. IEEE 1394 cable and connector.

Copper or gold contacts

Metal shroud

Connector

Signal lines A

Shielded twisted pair

Signal lines A

Shielded twisted pair

Power supply

8 V–40 V, 1.5 Adc max.

Signal line shield

Outer jacket

Individual shield

Table 14-14. Critical IEEE 1394 Timing Parameters

Parameter 100 Mbps 200 Mbps 400 Mbps

Max Tr/Tf 3.20 ns 2.20 ns 1.20 ns

Bit Cell Time 10.17 ns 5.09 ns 2.54 ns

Transmit Skew 0.40 ns 0.25 ns 0.20 ns

Transmit Jitter 0.80 ns 0.50 ns 0.25 ns

Receive End Skew 0.80 ns 0.65 ns 0.60 ns

Receive End Jitter 1.08 ns 0.75 ns 0.48 ns

Table 14-15. Dielectric Constant, Delay, and

Transmission Distance of Various Plastics

Material Dielectric

Constant

Delay

ns/ft

Maximum

USB

Distance

Foam, Air-Filled Plastic 1.35 1.16 22.4 ft

Solid Teflon™ 2.1 1.45 18 ft

Solid Polyethylene 2.3 1.52 17 ft

Solid Polyvinyl Chloride 3.5–6.5 1.87–2.55 10–14 ft