10
NOTE:
The information concerning proper video termination and VITC acquisition procedures mentioned in the section on the
VITC reader also applies to the VITC-to-LTC translator mode of operation.
Controlling Direction of LTC Output
An internal jumper selection allows the choice of generating both forward and reverse direction LTC output, or forward direction
LTC only. Direction in this context refers to the direction information inherent in the VLR-100 LTC output waveform as it
would occur if this waveform was read directly off tape. For example, when normally reading LTC from a VCR's audio channel
or time code track with the tape moving in the forward direction, the time information contained in the LTC waveform is
received in the sequence of frames, seconds, minutes, and hours. In reverse tape direction, this sequence changes to hours,
minutes, seconds, and frames.
This direction information contained in the LTC waveform may be required by LTC-based audio synchronizers and edit
controllers for correct operation. If your downstream LTC-based equipment reads and processes bi-directional time code, then
the VLR-100 VITC-to-LTC translator should be setup to produce both forward and reverse LTC.
For downstream equipment which can only process forward LTC, then the VLR-100 should be setup to only produce forward
LTC output. In this mode of operation, the LTC waveform is always in the forward direction format even though the time code
numbers themselves may be counting down in reverse.
The VLR-100 is factory set to produce bi-directional LTC. See Chapter 6 for information on changing this setting.
Search Offset
When reading LTC at play speed, it takes the time of an entire video frame to read the LTC number for that frame of video, and,
as soon as this frame number has been read, that frame of video is history. Because of this situation, most (but not all) SMPTE
LTC equipment add one frame count to the LTC number so that the next frame of video will be known beforehand. This process
is generally referred to as "on time updating" or "+1 frame."
However, when the VCR is in search mode and is still framed, adding an extra count to the translated time code number causes
the frame number to be ahead by one. To compensate for this behavior, an internal jumper selection enables the VLR-100
VITC-to-LTC translator to apply a "-1 frame" search offset to the time code number contained in the output LTC waveform.
With search offset enabled, the VLR-100 subtracts one frame count from the translated value so that when downstream
equipment adds one frame, the correct frame number is produced.
The VLR-100 is factory set to enable search offset. See Chapter 6 for information on changing this setting.
Ending VITC-to-LTC Translation
The VITC-to-LTC translation mode is exited when any of the following operations take place:
* The MODE switch is changed back to its RDR position. The VLR-100 re-enters the VITC reader mode.
* The MODE switch is momentarily activated to SET. This action causes the VLR-100 to change from translation to
simply generating LTC. It immediately begins incrementing the LTC generator number starting from the most recently
translated VITC value.
* The SELECT switch is moved out of its VITC position. Again, the VLR-100 enters its LTC generator mode, but whether
it automatically starts incrementing depends upon the state of the input VITC. If play speed VITC was being successfully
read immediately before the SELECT switch was changed, then the VLR-100 jamsyncs to this input VITC and the LTC
generator enters its "run" mode and begins incrementing. On the other hand, if either the translated VITC input was not at
play speed or was not available at all, then the VLR-100 does not begin incrementing its time code number and enters its
"stop" mode.
5.4 Controlling the Display
The rear panel BRIGHT knob controls the display brightness. The DATA switch allows display of either time code or user bits.
The momentary DISPLAY switch toggles between display RUN and display HOLD modes. An initial press of this switch
"holds" the current display and prevents it from changing. The display HOLD mode is indicated by blinking the active colon
LEDs at a ¼-sec on/off rate. The DATA switch can be used to alternate between displaying the held time code number and its