Time Code Display Discrepancy with External Time Code Source
The 702T, 744T, and 788T include powerful time code functionality. One time code feature is the ability for the recorder's time code clock to chase an external time code source. This is called "reader" mode. The time code value displayed by the recorder will indicate a +1 frame offset from the external master time code source when the master is in stop mode. This is normal behavior.
Why the One Frame Difference?
NOTE: The actual time code value stamped in recorded WAV files are correct.
This behavior occurs when a 702T, 744T and 788T recorder is set to any one of the following time code modes:
- Ext TC
- Ext TC/Cont
- Ext TC – Auto Record
- Ext TC/Cont – Auto Record
A typical set-up where the offset would be observed i,
- a camera, used as time code master, is set to record run mode,
- 7-Series set to auto-record based on incoming time code.
In order to understand where the one frame offset is introduced, it is necessary to know a little about Longitudinal Time Code (LTC).
LTC is transmitted serially in 80-bit word lengths where each 80-bit word represents a single time code frame in HH:MM:SS:FF format. A time code decoder (or reader) must receive all 80 bits before it is able to decode the time code value. The next frame is already on its way by the time the decoder has received and decoded the previous frame’s 80-bits. This inherent one frame decoding delay of LTC is depicted below.
To compensate for this processing delay, the time code reader must add 1 frame to its decoded output. Unfortunately, this has the side effect that the reader will show a +1 frame offset for a stationary time code source such as a ‘stopped’ camera in record run mode. The ‘stopped’ time code value is repeated over and over again as shown below.
Some readers automatically remove the +1 frame compensation when they detect stationary incoming time code. The 7-series recorders do not do this, but users should not be concerned since, as has already been mentioned, actual start time codes stamped in the recorded WAV files are correct.