A Curious Case of Record Offsets

Alex Kaiser from ZDF contacted me with an sync problem recently - using a SR-4 and laying back (Recording) audio from ProTools 8.1 to HD VTR's as follows:


Sony A500 Digibeta -In Sync.
Panasonic  AJ-HD1800 - 7 frames late
Sony  HDW2000 - In Sync  .
Sony SRW5500 - 4 frames late


Alex eventually found the reason for the problem - ProTools was setup to use LTC for positional reference
Setup > Session > Positional Reference
This was set to "LTC", when changed to "Serial Timecode" the recording delay disappeared!


Analysis:-

On HD machines there are encode/decode delays. When insert edit is enabled the machines switch to confidence mode (Read after Write). The "LTCout Phase" is set by default to be in sync with playback. On Sony VTR's the menu for this is located as follows

SRW5500:  Menu 618

HDW2000: Menu 617

At ZDF the timecode input of ProTools was connected directly to the video machine.
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We recommend using the master timecode output of the SR-4/RM-6, this follows the selected master machine. The timecode is regenerated from the master Serial Timecode and is locked to video syncs.


When the same problem occurs with serial timecode it may be corrected on SR-4/RM-6 Software using the Edit Offset Menu
Setup | Root | IFACE | General | Menu 77 Edit Offset
The Edit Offset is added only when tracks on the VTR are armed and only needs to be set once.