11
GENx192 Studio Master Clock
Clocks, Cables and Termination
Clock Distribution: Parallel
In parallel clock distribution (word clock only), the clocks of multiple
devices are connected to the same signal in parallel using “T”
connectors. In a proper parallel setup, the device at the end of the
chain is terminated (low-impedance), and all devices in between are
unterminated (high-impedance) so as not to adversely load the
signal down.
Parallel distribution can be used effectively,
but it also has drawbacks:
Termination Problems:
Some manufacturers do not clearly mark
the termination (input) impedance of their word clock inputs. Thus
it is quite easy to have two or more devices in the chain which are
both terminated, causing a sharp drop in clock signal level leading
to synchronization failure in one or more devices. The GENx192
termination sensing LEDs are designed to aid in diagnosing problems
with multiple terminated inputs.
Reliability:
Having multiple cables and multiple BNC-T connectors in
the chain can lead to a higher incidence of mechanical failure.
Lack of Flexibility:
Many devices cannot be unterminated, and
therefore cannot be connected together in the same parallel chain.
Clock Distribution: Serial
In serial clock distribution, clocks are chained from the output of one
device to the input of another. Serial distribution has several drawbacks
and should be used only for devices that have self-clocking interfaces
(e.g. certain DAT machines and stand-alone CD burners, digital effects
processors, etc.) These drawbacks include:
Signal Degradation:
As the signal is passed from one device to
another, the signal quality can become degraded the farther down the
chain it gets.
Clock Skew:
Devices may add a small amount of delay to the output
signal causing the clock edge to become progressively misaligned
between each device in the chain.
Added Jitter:
Each device has an inherent amount of jitter in its
internal clocks. If the device regenerates the clock rather than passing
it through then the inherent jitter of the device will get added to the jitter
contributed by all previous devices. Any signal quality degradation will
only exacerbate the jitter problem.
CLOCK IN
MASTER
CLOCK OUT
CLOCK IN
SLAVE #1
CLOCK OUT
CLOCK IN
SLAVE #N
CLOCK OUT
BNC 75 Ohm
Terminator
CLOCK IN
MASTER
CLOCK OUT
CLOCK IN
SLAVE #1
CLOCK OUT
CLOCK IN
SLAVE #N
CLOCK OUT
BNC 75 Ohm
Terminator
CLOCK IN
MASTER
CLOCK OUT
CLOCK IN
SLAVE #1
CLOCK OUT
CLOCK IN
SLAVE #N
CLOCK OUT
CLOCK IN
MASTER
CLOCK OUT
CLOCK IN
SLAVE #1
CLOCK OUT
CLOCK IN
SLAVE #N
CLOCK OUT
