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 125Ω , 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 75Ω resistor through the leftmost jumper in J13. If the clock source is not capable of driving high through 75Ω (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 75Ω 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 75Ω 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 75Ω RG-59/U or similar cable.
