702T/744T Digital Inputs and Time Code

When using the 702T or 744T AES/EBU digital inputs and the built in time code generator to sync the recorders from a mixer with digital outputs, care must be taken. The 702T and 744T internal time code generator can only run off of its internal high-accuracy crystal - it does not use word clock or AES/EBU to derive time code. For accurate Broadcast Wave time stamps to be written, it is essential that both the time code generator and the sampling clock (word clock) to be derived from the same clock source, since the time stamp in a Broadcast WAV file is generated from the sample count. Thus, in a WAV file, samples and time stamp cannot be separated. When using the digital input(s) and using the 702T and 744T's internal time code generator, significant drift will certainly occur because the time code and word clock will be referenced to two separate time bases,
  • the 7-Series recorder's crystal and
  • the external mixer's crystal.
There is presently no way for the 702T or 744T to be the source device for the mixer by outputting a word clock signal while accepting incoming AES/EBU inputs. The solution to this situation is to use an external Ambient Lockit (ACL202CT) or an Ambient Controller (ACC501), which generates both time code and wordclock outputs. Note that a Lockit can only operate at 48 kHz, while the Controller allows multiple sample rates as well as pull-up and pull-down.
  1. Connect the Lockit time code output to the 702T/744T time code input.
  2. Set the recorders to external time code mode.
  3. Connect the Lockit word clock output to the digital mixer word clock input.
  4. Run the AES/EBU outputs from the digital mixer into the 744T or 702T.
In this scenario, all clocks are syncronized and the 702T or 744T can record the output of the digital mixer with no drift and perfect timing accuracy.

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