The RB-ADDA A/D and D/A converter is a 1U rack unit that produces an AES/EBU or S/PDIF level digital audio output from a balanced XLR or unbalanced RCA stereo audio input. It also produces a balanced XLR or unbalanced RCA stereo output from an AES/EBU or S/PDIF digital input signal. The device operates in four modes: Master Mode In this mode, the device receives an analogue audio signal, which is digitised and formatted for digital serial transmission (IEC958). The necessary clock signals are generated internally from a built-in master clock at a selectable frequency (32kHz, 44.1kHz, 48kHz or 96kHz). Slave Mode In this mode, the device automatically detects the presence of a digital audio synchronisation signal, if present at the digital input, and synchronises the digital output to it. If no synchronisation is present, no output will be generated. Auto Mode Here, the device synchronises to the digital audio synchronisation signal if it is present at the digital input and uses the internal master clock only if no synchronisation input signal is detected. In this case, the internal master clock is used at the selected sampling frequency. Auto Lock Mode This mode operates like auto mode, except that if no synchronisation input signal is detected, it will use the internal master clock to synchronise to the sampling frequency that was last synchronised. When operating in synchronisation modes, the front panel power LED flashes whenever the unit is not synchronised to the incoming digital signal. The analogue inputs have left and right level controls using preset potentiometers and DIP switches allowing for a signal range of +9dBu to +27dBu. The RCA inputs have an additional 10dB gain built-in to give a total gain range of -1dBu to +17dBu for full-scale figures. The analogue outputs have an output level control, allowing for selectable full-scale settings of +12dBu, +18dBu or +24dBu. There are factory-set internal level controls for the analogue outputs allowing for ±1dB gain adjustment. There are buttons to select AES/EBU or S/PDIF input or output for the D/A and A/D sections respectively. The output bit depth can be selected from 16, 20 or 24 bits. Inputs of a different bit depth to the output are dithered using a noise filter. For the digital output, a switch is available to set the content of the channel status bits embedded in the digital audio stream. The status bits can be forced to Professional Mode or Consumer Mode. Additionally, if de-emphasis is selected, the RB-ADDA will decode 50/15µs emphasis when indicated by certain channel status bits in the incoming digital audio data. The RB-ADDA has a calibration routine for optimal performance, which allows for improvement in noise floor and dynamic range by 1-2dB. The calibration cycle works by calibrating the A/D converter's gain and zero reference.