Tektronix SPG8000 Technical Reference page 18

System timing
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Offset to TAI. TAI is the International Atomic Time, represented as the number of
seconds since the epoch of 0:00:00 January 1, 1958. Proposed standard SMPTE
404M defines the SMPTE Epoch at that same moment. There are exactly 8040
days and 19 leap seconds between the SMPTE Epoch (TAI) and the GPS Epoch.
Epoch Calculations. Option GPS is required to use Epoch Calculations feature.
The number of seconds since the SMPTE Epoch is used to precisely align video
frames with the Master Time clock. For example, an NTSC system operates at
29.97 frames per second (30 fps/1.001), so there are precisely 30,000 frames every
1001 seconds. By knowing the total number of seconds since the SMPTE Epoch,
when all video frames were perfectly aligned, the instrument can determine
"where" the GPS pulse-per-second signal is located with respect to video framing.
Frame Engine. The frame engine supplies frame pulse signals based on the
SMPTE Epoch to each video output. This ensures that they are properly aligned
with respect to the calculated phase for the selected video format of each output.
Output Timing. Each video output has an independent offset that can be specified
to shift that output in order to align the video and LTC signals in the studio. This
time adjustment does not affect the timecode that is applied to that output; the
signal and timecode stay together as the timing is changed. The timing offset range
and resolution depend on the signal. See the specific output menu for details.
Leap Second Information. Option GPS is required to use the Leap Second
Information feature. The GPS signal indicates the number of leap seconds
between GPS time and UTC. As of December 31, 2008, a total of 34 leap seconds
are required for the adjustment between TAI and UTC. This information only
stays current when a GPS signal is connected. On instrument boot up, stored leap
second information is used, and an asterisk appears next to the time in the status
screen until the leap second information from the GPS signal is received.
Time Zone Offset *. The SPG8000 has a selectable offset between UTC and the
local time, usually representing the local time zone. This offset can be specified
in hours+minutes+seconds, allowing specification of time zones that do not
align with the usual hours-only offset, or for other applications not related to
time zones. You can also use this setting to set the internal time to calculate
"backwards" to the Master Time.
When the input time source is set to VITC, the instrument works similar to how it
works in internal mode, pre-correcting the master time of day by the time zone
offset. This pre-correction assumes that the incoming time code has the correct
local time, and causes the outgoing time codes to nominally match the incoming
ones, regardless of time-zone offset. This means that when switching from GPS to
VITS mode, the time-zone offset usually does not need to change. However, if
the time code synchronization mode is set to "Synchronize now" or "Synchronize
once", and the time-zone offset is changed after the time-code sync has occurred,
then the time-zone change offsets the incoming and outgoing time codes until the
user reasserts the time synchronization.
SPG8000 Technical Reference