Avid Technology FT730, Manual

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FT730 Guide 8
Technical Reference
Synchronization for Distances Over 800 ft (250 m)
MADI utilizes a polarity-free, serial NRZI (non-return-to-zero
inverted) protocol with encoding at 125 Mbit/s (±100 ppm).
Sample timing is controlled independently by distributing a
master synchronization signal. Originally, the transmission
medium was 75-W coaxial cable. There are two important syn-
chronization issues:
• The MADI signal’s transmitted frame start time must be
within 5% of the sample period time determined by the syn-
chronizing signal.
This requirement is usually met by the circuit design using
the distributed synchronization signal.
• The received frame start time must be within ±25% of the
sample period reference time.
This requirement is governed by physics: it is the time of
flight (delay time) difference between the sample start
and the start of the frame. The speed of light
(299,792,458 m/s) and the propagation speed of the
transmission medium govern this delay time.
Transmission mediums, such as fiber and coaxial cable, have
propagation speeds in the range 60–70% of the speed of light.
This does not present a problem when MADI and the synchro-
nization signal are sent together to a destination, such as a
MA703. The problem manifests when MADI is used to return
data from a source such as an AM713, where the delay time
difference for the MADI and synchronization signals exceeds
the received frame start time requirement (±25% of the sam-
ple period reference time).
The FT730 has two types of synchronization outputs:
AES/Sync or Word Out
Used for devices receiving MADI at the
remote end. These outputs are used if the signal meets the cri-
teria stated in the first bullet above.
Retimed AES or Retimed Word Sync Used for devices transmit-
ting MADI to the local end. The control/sync fiber pair is used
to measure at the local end by comparing the local sync tim-
ing with that returned from the remote end. This number is
sent to the remote end where the timing is adjusted in six-
teenth frame increments to keep it within the operational re-
quirements stated in the second bullet above.
Single Mode vs. Multimode Fiber Optic Cable
In optical fiber technology, multimode fiber is optical fiber
that is designed to carry multiple light rays (modes) concur-
rently. Each mode is carried at a slightly different reflection
angle within the optical fiber core. Multimode fiber transmis-
sion is used for relatively short distances (less than 6500
ft/2000 m) because the modes tend to disperse over longer
lengths (modal dispersion). For distances longer than 6500 ft
(2000 m), single mode fiber (sometimes called monomode) fi-
ber is used. Multimode fiber has a typical core diameter of
50–100 mm with a refractive index that is graded or stepped.
It allows the use of inexpensive LED light sources, and con-
nector alignment and coupling is less critical than for single
mode fiber. This translates into lower connection and elec-
tronics costs and consequently lower overall system costs. The
FT730 uses 62.5/125 mm multimode fiber with ST connectors.