Setting Up and Operation
Sample Rate Converters
The sampling frequency of the audio data embedded in SDI video signals is 48 kHz. This is invariant, and is enshrined in the defining standards for all SDI formats. As with all operations involving digital audio, it will generally be desirable for the audio outputs of the SDI-MADI to be synchronised (in frequency and/or phase) with a master reference clock, which will also synchronise all other digital audio processing in the facility.
To achieve this, the SDI-MADI includes four 16-channel Sample Rate Converters (SRCs), one for each SDI input (see Rear Panel Description on page 8). Each SRC may be switched in or out by a front panel switch (see Front Panel Description on page 7) or by external control via the GPIO port. When each SRC is active, a red LED illuminates, and a tally output voltage is also available at the rear GPIO port. When enabled, the SRCs re-clock the audio data against a new reference, thereby changing the sampling frequency of the data to that of the reference. They also ensure that the word blocks in the data are phase-locked exactly to the reference clock. The SDI-MADI allows the user to select a reference clock source to suit the infrastructure of the facility and the nature of the transfer process.
See following section on “Synchronisation”
GPIO Control
As the SDI-MADI could be installed in a central control room or outside broadcast vehicle using automated system software applications, external control of the SRCs can be achieved via the GPIO port. Simple contact closure (grounding the GPI) enables any or all the SRCs, allowing them to be switched in or out from a remote point. Maximum recommended operating distance is 5 m (though this may be increased by the use of external relays or similar).
Refer to the pinout on page 12 for wiring details.
Synchronisation
The SDI-MADI offers a choice of wordclock synchronisation sources. The selection is made from the front panel, and the array of LEDs confirms the currently-selected source. The selected source is used as reference for all four output SRCs, thus ALL audio outputs, both in AES3 and MADI formats, will be locked to the selected reference. The various clock sources are discussed on the following page.
