The 8816's optional A-D card is a high-quality stereo design based on a Burr Brown PCM1804 converter chip, supporting sample rates up to 192kHz. The DSD format is also supported. When the A-D card is installed, the analogue signal feeding it is normally taken from the mix buss output, but a pair of TRS sockets on the rear panel allow an external balanced line-level signal to be over-plugged to feed the A-D separately. The nominal input level is +26dB for 0dBFS, but this can be adjusted with a pair of recessed headroom switches to give 14 or 18 dB of headroom above +4dBu (in other words, +18 or +22dBu for 0dBFS). A button on the front panel allows the sample rate to be selected from any of the six standard rates between 44.1 and 192 kHz, followed by the DSD mode. Normal operation is based on internal crystal clocks, but an external AES or word clock reference can also be accepted - both inputs having a bi-colour LED associated with them. The unit selects the best reference automatically, and if no external clock is present, or if an external clock is at a different rate to the selected sample rate, the internal clock is used. If both AES and word clock references are present and at the correct sample rate, the word clock source is given priority. Word clock in and out are catered for with BNC sockets, and a recessed button allows a 75(omega) input termination to be applied, if required. Three XLR connectors provide an external AES reference input and two AES3 outputs. These can be used as separate identical outputs, or for dual-wire operation, although this mode requires an external clock reference. If an external clock is provided, and the sample rate is selected to twice the external rate, the AES output is automatically formatted for dual-wire operation. For example, a 48kHz clock can be provided when the unit is switched to a 96kHz sample rate, or a 96kHz clock for a 192kHz sample rate. In this mode the first XLR carries the left channel data and the second XLR carries the right channel data. The DSD mode is always referenced to a 44.1kHz clock, and the DSD output is provided on two BNC connectors, switchable from the rear panel for SDIF 2 or SDIF 3 formats. The former carries only the audio data and a separate word clock reference has to be routed along with it, while the latter has embedded clock signals.
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