This design note describes why the SMPTE serial digital interface (SDI) form of digital video is
difficult to transmit over fiber optic links, and how
the MAX3656 laser driver can be used to
successfully overcome this problem.
Early digital fiber-optic transmission systems were
designed to deliver digitized audio (voice) over large
telecommunication networks. For example,
Synchronous Optical Network (SONET) is an
optical transmission standard that uses scrambling to
make clock and data recovery easier. Scrambling can
help randomize the data to make it easier for the
receiver to recover the clock without increasing
overhead data.
Uncompressed digital video also uses scrambling to
help randomize the data being transmitted. However,
the problem with digital video is that it can be
created artificially by color generators, and digital
color generators can create video with no noise. In
particular, two colors do not always get adequately
randomized by the scrambler and can result in
repeating patterns that have either a large DC offset
or a low transition density. These difficult-totransmit
patterns are called “pathological patterns.”
The pathological pattern containing the large DC
offset is very troublesome for fiber optic
transmitters. Overcoming problems with
pathological patterns is the focus of this application note.