Specifications

6 ExtroNews 14.1 January – March 2003
The correct answer depends on a number
of factors. This is like asking if you could
drive your car 100 miles per hour through
your city’s downtown. First, we know it is
illegal based on speed limit postings; but,
aside from that, could you do it successfully?
Well, sure, if all the lights are in your favor,
there are not any pedestrians, no one violates
a traffic signal, and police are not around.
You just might pull it off. The probability of
success, however, is hard to calculate. It
would certainly be a hot story to tell over a
cold beer. Most likely, some cold beers have
set the stage for some similar
bar bets.
So, here we are with the question. The SDI
police are not going to fine you for routing
through an analog system. Moreover, since
this is an engineering-oriented column, you
SDI Through Analog Video Distribution
Equipment: A Good Idea?
TECHNICALLY SPEAKING...
by Steve Somers, Vice President of Engineering
A
/V applications for digital video over the serial digital interface, or SDI as it’s
commonly called, are on the rise. Originally just the realm of television production
and broadcast operations, SDI is used more often to convey professional quality component
video among systems. Many system designers are aware that the nominal SDI data signal
level is just under one volt, 0.800 volt to be exact. So, a question that often arises is: “May
I use analog hardware [switcher, router, DA] for SDI?”
probably can guess that I’m not simply
going to say “YES” or “NO.” I’ve still got a
few pages to fill here and, besides, when
have you ever gotten a simple answer from
an engineer? We are too busy specifying all
the variables before qualifying an
answer…to several decimal places. In fact,
did you ever ask an engineer a question and
note a very slight reciprocating motion in
his eyes, whilst he is calculating an answer?
This is the brain encountering a momentary
1/2 LSB rounding error.
Serial Digital Road Course
The impetus for serial digital coding and
transmission of video heightened with the
introduction of the first component digital
production video tape recorder in the mid-
‘80s, known as D1 or CCIR 601. (For more
background on the origin of SDI, see my
article “Getting the Most from SDI” from the
March/April 2001 issue of ExtroNews or
download it from the Technologies section of
our Web site under the Digital Video icon.)
SMPTE 259M supports four SDI
transmission rates and SMPTE 292M
supports 1.485 Gbps for HD SDI. (Refer to
Table 1). Currently, most serial digital
application situations involve standard
definition television, so I’m going to use the
most popular of the four standard definition
rates, NTSC serial digital component, as the
typical example.
Component serial digital requires 10 times
the clocking rate of the parent parallel
system, or 270 megabits per second. The SDI
encoding algorithm ensures enough signal
transitions to embed the clock within the
data and minimize any DC component. SDI
coaxial cable drivers AC-couple the serial
data into the transmission cable, thus
providing DC isolation between the source
and receiver (See Figure 1). Minimizing any
DC component is important for recovery of
the clock and data as will be discussed later.
The receiving device contains the same
algorithm for proper decoding and recovery
of clock and data.
Maintaining Distance
First of all, it should be noted that devices
designed for handling the SDI signal usually
Table 1. SMPTE 259M and SMPTE 292M transmission rates and specifications.