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MICROPHONE PREAMPLIFIER
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Instruction Manual
5. USING THE DIGITAL MODULE
BRIEF TUTORIAL
BRUTE FORCE AND IMPEDANCE MATCHED TRANSMISSION
BACKGROUND
Synchronization of digital audio equipment shouldn’t be a problem but it is. All the problems would
disappear if impedance matched transmission were used by every piece of gear. But, this just isn’t
the case, even though it is simple and cheap. “Brute Force” transmission (the kind that causes
trouble) is still found throughout the industry.
Brute Force Transmission
What’s “brute force”? It refers to driving the Word
Clock output jack directly from a line driver logic
chip. The driver’s output impedance is too low,
usually around 10 ohms. This assures a good and
clean unloaded output pulse at full logic level
(5Vp-p) but does not consider transmission line
characteristics. Over distances less than 6 feet this
method seems to work ok, but with longer lines,
heavy ringing and overshoot occur which causes
high jitter and in the extreme cases equipment
may not even be able to lock.
Impedance Matched Transmission
Impedance matching changes a few things, but
is not complicated. If a transmission line (like
75 ohm coax cable) is sourced and loaded by its
characteristic impedance, then there will be no overshoot or ringing of the word clock, even at great
distances. The only complication is that the pulse amplitude received will be half the amplitude of
the logic driver’s output. That means the clock receiver must have a greater sensitivity, or effectively
a lower logic threshold to detect the clock’s level
transitions.
PROBLEM:
A lot of word clock inputs use standard
logic chips that won’t work on a halved input level.
If they did, it would be easy to convert any brute
forced clock system to matched lines by adding a
resistor at each end.
SOLUTION: Aphex has made the 1788A capable
of receiving Word Clock down to 1/2 volt p-p
so you can safely run very long Word Clock
cables to its remote location while getting no
cable induced jutter. The 1788A’s Word Clock
output impedance can be switched from 10
ohms (brute force) to 75 ohms (matched)
in order to meet any requirement. We highly
recommend the use of impedance matching
whenever possible.
5Vp-p Output Level
Open Circuit
Still 5Vp-p But With Ringing
(Worse With Longer Cable)
Coax
Line
Brute Force
Clock Transmitter
Brute Force
Clock Transmitter
Clock Receiver
<10
Ω
Source
Impedance
<10
Ω
Source
Impedance
5Vp-p Open Circuit
2.5Vp-p With Termination
75
Ω
Load
Termination
75
Ω
Source
Termination
75
Ω
Coax
Line
Clock
Transmitter
Clock
Receiver
75
Ω
Source
Termination
Clock
Transmitter
