A Comparison of Digital Video Standards - Digital Video and Audio Broadcasting Technology - page 174

10.3 Physical Interfaces for Digital Video Signals
Analog SDTV (Standard Definition Television) signals have a bandwidth
of appr. 4.2 to 6 MHz and are transmitted over 75 Ohm coaxial lines.
These cables, which in most cases are green-jacketed, are fitted with BNC
connectors both in professional applications and in high-end consumer ap-
plications. If they are terminated in exactly 75 Ohms, analog video signals
have an amplitude of 1 V PP . The first interfaces for digital TV signals were
designed as parallel interfaces, using the 25-pin Cannon connector known
from the PC printer interface. Because of its noise immunity, transmission
was conducted as low-voltage differential signaling via twisted pair lines.
Today, however, 75 Ohm technology is again being used in most cases.
Digital video signals are transmitted as a serial data signal with a data
rate of 270 Mbit/s via 75 Ohm coax cable fitted with the familiar, rugged
BNC connectors, making no distinction between uncompressed video sig-
nals according to the ITU 601 standard and MPEG-2 transport streams.
The distribution paths in the studio are the same, the cables are the same,
amplifiers and cable equalizers are also the same. Engineers often talk of
SDI or of TS-ASI. The physical interface is the same in both cases, only
the content differs. SDI stands for Serial Digital Interface, meaning the se-
rial digital uncompressed video signal of the 601 standard with a data rate
of 270 Mbit/s. TS-ASI stands for Transport Stream Asynchronous Serial
Interface, meaning the MPEG-2 transport stream on a serial interface, the
transport stream having a data rate which is distinctly lower than the data
rate on this serial transmission link. The data rate of the transport stream is
asynchronous to the constant data rate of 270 Mbit/s on the TS-ASI inter-
face. If, e.g., the transport stream has a data rate of 38 Mbit/s, stuffing in-
formation is used to fill up the data rate of 270 Mbit/s. The reason for
working with a constant 270 Mbit/s is clear: In the studio, it is desirable to
have uniform distribution paths for 601 signals and the MPEG-2 transport
streams.
10.3.1 Parallel and Serial CCIR 601
Uncompressed SDTV video signals have a data rate of 270 Mbit/s. They
are distributed either as parallel signals via twisted pair lines or serially via
75 Ohm coax cables. The parallel interface is the familiar 25 pin Cannon
socket also known as a PC printer interface. The signals are LVDS (low
volage differential signaling) signals which means that ECL levels and not
TTL levels are used as voltage levels (800 mV PP ). In addition, for each
data bit, the inverted data bit is also transmitted in order to keep the noise