Digi Cart II Plus
Synchronizing Digital Audio to an External Reference

When DigiCart/II or DigiCart/II Plus are used in an all digital studio or with a digital audio router, it can be necessary to synchronize the digital audio output with the sample clock rate of other equipment, or to a master reference or "house sync". This ensures that the timing of digital audio signals are the same in both the transmitting and receiving equipment, preventing noises or dropouts.

Situations where External Synchronization is necessary
Normally, DigiCart/II uses its internal clock to generate the sample rate for playback and recording. When the Digital Output is used in simple dubbing operations, the input of the receiving unit synchronizes to the digital audio data sent by the DigiCart/II. In situations such as digital mixing and routing, a single receiver is called on to receive digital audio from more than one output. This means that it can choose only one input as its sync source, and the other inputs will not conform exactly to this timing. Some systems solve this problem by using sample rate converters on their inputs, so that the unreferenced signals are made to conform to the internal clock. This approach requires sample rate conversion on each input, and adds cost and complexity to the system, in addition to the possibility of signal degradation in the conversion process.

The solution is the use of a single reference to clock all devices in the studio. In small installations using a digital mixing console, this reference may be the console itself. In larger multi-room installations, this reference is referred to as "house sync", which is fed to all devices in the entire facility. The use of a referenced AES/EBU signal is detailed in the Audio Engineering Society publication "AES11-1997 AES Recommended Practice for Digital Audio Engineering -- Synchronization of Digital Audio Equipment in Studio Operations".

Requirements
1. DigiCart/II must be set to receive external sync. Press SETUP MENU and select SYNC SOURCE using the SELECT knob. Press the RIGHT ARROW key to set this to DIGITAL INPUT. Rotate the SELECT knob to select the DIGITAL INPUT menu item and select the input that you will apply the external sync signal to.
2. D-NET receive must be OFF, or the D-NET Transmitter must be referenced. See the D-NET Applications Manual for more information.
3. The sync signal must be applied continuously to the input selected as the Digital Audio Input.
4. The sample rate of the sync signal must match that of the Cuts to be played back! When recording using the analog inputs in external sync the sync signal must match the selected Recording Format. Mismatches are not detected, and will result in playback at incorrect speed, or recordings that can only play back correctly by using the same erroneous sync frequency.

In most cases the sync signal can be the same one that is used as the digital audio input to the DigiCart/II. For example, if an AES output of a digital console is used to supply input to the DigiCart/II, that same signal will be suitable to supply sync as long as it is never interrupted during playback. Similarly, if the DIGITAL INPUTS and DIGITAL OUTPUTS of DigiCart/II are connected to an AES router, as long as the input to the DigiCart/II is always present it will supply sync. It makes no difference whether or not there is any audio data present in the signal; the presence of the signal will supply the necessary clock reference. If the console or router are referenced to house sync, these signals will also be referenced. If the DigiCart/II is not used to record, then connect its DIGITAL INPUT directly to the house sync feed.

Operation
Operation of the DigiCart/II does not change in any obvious way. When PLAY is pressed, if the external sync signal is not present the display will show "SYNC INPUT BAD". Recording from the ANALOG INPUTS will also require presence of a sync signal. Remember to select the proper sampling rate in the Record Format menu to match the incoming sync signal. When using a 48K sync signal, you can select either AC-2 or 48K LINEAR. (Recording using the DIGITAL INPUTS always references to the clock embedded in the digital audio signal and automatically selects the correct sample rate.)

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