Lucid GENx192, Руководство пользователя

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10
User’s Guide
Clocks, Cables and Termination
In most cases such problems are immediately noticeable, and with
a little listening and a bit of deductive reasoning the source of the
problem can be quickly found. Some problems are less apparent.
Program material, listening levels and monitor systems as well as
certain types of digital interfaces can tend to mask the problem,
which although present, tends to be present at much lower levels in
the material. When the clock rate difference is dramatic (for example
when a system running at 44.1 kHz is interfaced to another running at
48 kHz), then discrete pops and clicks cannot be heard. Instead, the
‘resampled’ signal results in aliasing and severe harmonic distortion
that sounds grainy and ‘digital’, particularly in the high frequencies.
Skew – Skew is a misalignment of two clocks whose rates are
otherwise synchronized. Skew is almost always caused when devices
are synchronized by chaining synchronization signals serially from
one device to the other, instead of to all devices from a single device.
Skew can cause pops and clicks if the receiving device cannot receive
data from the transmitting device because of the particular phase
relationship of the clocks. Skew can also cause timing differences in
the A/D and D/A conversion processes which can cause slight high-
frequency phase cancellation and image smearing if phase coherent
material is distributed across the various sources. Skew should
not be a problem in properly designed equipment when the proper
interconnection topology is used.
Malfunction: At worst, some devices cease to pass audio altogether
when they cannot lock to an external clock. In other cases their PLLs
(Phase Locked Loops) may drift to invalid sample rates quite far from
the target sample rate and create all varieties of audible havoc both
internally and with other interconnected audio devices.
Fortunately, employing a low jitter, low skew, centralized master clock
/ distributor like the Lucid GENx192 can solve most, if not all, of the
above mentioned potential problems.
Cabling
Always use the correct cabling recommended for each type of clock
signal. Improper cabling can lead to several of the problems described
previously, such as clock signal degradation and jitter. The longer the
cable run, the more critical it is to use the correct cable.
Word Clock: Use 75Ω video coax (RG-59 or RG-6) with properly
matched 75Ω BNC connectors. In a pinch you may substitute 50Ω
coax (RG-58) for very short runs. Any use of 50Ω coaxial cable with a
properly terminated word clock input will cause signal reflections at the
connector that can induce clock jitter.
AES:
Use 110Ω shielded, twisted pair digital audio cable. Because
of the extremely high bandwidth of AES signals (up to 25 MHz),
microphone cable should never be used for AES signals, even for very
short runs.
S/PDIF: Use 75Ω video coax (RG-59 or or RG-6) with RCA male
connectors designed for attachment to coaxial cable. Unlike word
clock, 50 Ω coax should not be used.
Belden, Canare, and Mogami all make excellent coaxial and digital
audio cables that work well in these applications.
Clock Distribution: Three Common
Approaches
There are several types of sample clock signals commonly used today:
word clock, AES-3, AES-11, S/PDIF, and ADAT Optical. The GENx192
provides all of these except for ADAT Optical. AES-3, S/PDIF, and
ADAT Optical are known as self-clocking interfaces because they
carry both audio signals and clock data which receiving devices can
slave their sample clocks to. Word clock and AES-11 carry no audio
information – they are synchronization signals only.
There are three ways to distribute clock signals: in a serial chain, in a
parallel chain, or in a star configuration. Often times a setup may be a
combination of two or three distribution methods.
Clock Distribution: Star
In a star configuration, a single ‘master’ generator distributes a clock
signal to each device that requires it.
Without exception Star distribution should be used for every device
that has a word clock or AES-11 (i.e. non self-clocking) input. For
this reason the GENx192 was designed with a large number of clock
outputs to facilitate a proper star distribution within all but the
most demanding production environments. If at all possible, this is the
way to go!
CLOCK OUT 1
MASTER
CLOCK OUT 2
CLOCK IN
SLAVE #1
CLOCK OUT
CLOCK IN
SLAVE #N
CLOCK OUT
CLOCK OUT 1
MASTER
CLOCK OUT 2
CLOCK IN
SLAVE #1
CLOCK OUT
CLOCK IN
SLAVE #N
CLOCK OUT