Lexicon Converter - Manual (Page 18)

Lexicon
14
Using the 20/20 AD
The 20/20 AD can synchronize to its internally derived sample clock at 44.1kHz
or 48kHz, or to an external AES or Word Clock signal, in the range of 41-51kHz.
The EXT SYNC switch allows you to select external Word Clock (WC), external
AES (AES), or internal source (WC and AES off).
Before switching between Internal and External modes, be sure to reduce
monitor volume as some D/A converters do not mute when they lose lock.
The AES SYNC IN signal can be any AES3-compatible signal running at the
correct sample rate — It does not require a Digital Audio Reference Signal
(DARS). The 20/20 AD is shipped with a 110 ohm termination resistor across
pins 2 and 3 of the XLR connector, as required by the AES3 specification. If you
intend to daisy chain several 20/20 ADs from the same AES source, you will have
to open jumper W17 on all units except the last one in the chain.
Jumper W18 connects the cable shield to system ground. The 20/20 AD is
shipped with W18 open, assuming that ground is supplied from the generator of
the sync signal. Note that, unlike the analog inputs, the AES SYNC input is
transformer isolated; it is not necessary to have a ground connection between
the 20/20 AD and the sync source.
All AES connections require twisted -shielded pair cable with a controlled
impedance between 100 and 125
Ω
, and less than 15 pF/ft of capacitance.
Microphone cable will not work reliably. Refer to Page 7 for recommended
cables.
The WORD CLOCK IN signal should be a logic-level (logic high=3V-5V) square
wave. When shipped, the 20/20 AD terminates the signal with a 75
Ω
resistor
through the leftmost jumper in J13. If the clock source is not capable of driving
high through 75
Ω
(most TTL and CMOS drivers are not), it may be necessary
to open this jumper. In this case, it is important that the cable be kept as short
as possible. As with AES SYNC, if you intend to daisy chain several 20/20 ADs
from the same Word Clock, you should open the leftmost jumper on J13 on all
units except the last one in the chain. If an alternative ground path exists between
the word clock generator and the 20/20 AD,W16 can be opened to prevent a
ground loop.
Note that, unlike the analog and AES SYNC inputs, the Word Clock input is not
transformer isolated, and requires a ground connection between the 20/20 AD
and the signal source.
The Word Clock input requires a 75
Ω
RG-59/U coaxial cable. Use of ordinary
audio-quality cable may distort the edges of the Word Clock. This will add jitter
to the sample rate, which will, in turn, add noise and distortion to the analog/
digital converter. Refer to Page 7 for recommended cables.
The WORD CLOCK OUT signal is a logic-level square wave at the sample
frequency. It is capable of driving a 75
Ω
load, and is always available, even when
an internal sync source is selected. This output may be used to synchronize one
or more slave 20/20 ADs to a master. As with the Word Clock input, use only 75
Ω
RG-59/U or similar cable.
Synchronization
and Sample Rate
AES Sync
Word Clock