Specifications

6 ExtroNews 12.2 March/April 2001
The Serial Digital Interface-SDI (SMPTE
259M) grew out of the need for longer
distance connection of component digital
television equipment, the result being the
viability of a truly digital broadcast station. SDI
is capable of running hundreds of feet and
can run thousands of feet if properly
distributed. For additional information on SDI,
see my articles in the January – April 2000 and
the September – October 2000 ExtroNews.
Taking It One Bit At A Time
Digital component recording began in
1987 with the creation of the D1 format
(SMPTE 125M). The D1 interface is an 8/10
bit parallel system intended for close-in
connection between digital tape recorders
(19 mm tape). Its interface cabling is short
due to the difficulty in maintaining proper bit
timing over a byte-wide data channel.
Somewhat like DVI, D1 requires
management of differential signals over 8 or
10 twisted pairs. Bit skew, crosstalk, and
attenuation are adversarial to the task of
transmitting parallel D1 for long distances.
Getting the Most from SDI
TECHNICALLY SPEAKING...
by Steve Somers, Vice President of Engineering
S
DI does NOT stand for “Short Distance Interface.” But, a serial digital
interface run can revert to that status if some basic rules are not
followed.
The interface uses a 25-pin D-sub miniature
connector. As a result, termination is not
really easy, and the thought of managing
that many parallel bits through a router is
good for a migraine, not to mention the
hardware cost of 10 switching planes.
Therefore, parallel D1 connections are easily
managed over only a few meters.
Reformatting the byte-wide D1 data via a
serializer yields a very high-speed serial data
stream. Serializing a 10-bit data word results in
a data rate ten times faster. The 27 MHz D1 data
becomes serial data at 270 megabits per second
for standard component NTSC. See
Figure 1
(below) showing the basic conversion
methodology. Benchmark signal performance is
captured in
Figure 2 (on opposite page).
Why SDI?
Although SDI bit rates are very high,
distribution of serial data as a single cable
connection presents significant advantages.
First, it’s much easier (read cheaper) to route
and switch one cable than a parallel system
PARALLEL
4:2:2 DIGITAL
COMPONENT
VIDEO
PARALLEL
4:2:2 DIGITAL
COMPONENT
VIDEO
27 MHz
CLOCK
27 MHz
CLOCK
10 10
SHIFT
REGISTER
SCRAMBLER DESCRAMBLER
75-OHM
COAX
270 MHz
CLOCK
SHIFT
REGISTER
SAV, EAV
DETECT
270 MHz
PLL
DIVIDE
BY 10
10x
PLL
Figure 1. 4:2:2 Serial Digital Interface