Model 5601MSC
Model 5601MSC Master SPG/Master Clock System
Page - 48
Revision 2.2
OPERATION
If the frequency reference of the 5601MSC is set to GPS, the time reference will also be forced to GPS.
The system phase reference is derived from GPS time, but the
system clock
is set to UTC time, which
is offset from GPS time by the total number of leap seconds that have occurred since the year 1980.
Because the PAL color frame to timecode relationship runs over 4 seconds, this produces a unique
problem when aligning UTC-locked timecode to the PAL color frame.
The
system clock
of the 5601MSC is always set to UTC time. The PAL reference phase is tied to GPS
time which is not affected by leap seconds. Whenever a leap second occurs, UTC time counts an extra
second, slowing it down. At the next
jam
event, the PAL timecode clock could
jam
to the system clock
causing it to shift backwards by one second. If done this way, it would break the timecode to color
frame relationship. However, the 5601MSC must adjust the time that is jammed into the 25Hz
LTC/VITC clocks in order to maintain color frame alignment. This means is that a static, positive or
negative, 1 or 2 frame offset may be introduced between real time and 25Hz timecode even when the
5601MSC is locked to GPS. This static offset will change every time a leap second occurs.
2.3.5. IRIG Timecode
When equipped with the
IRIG
option, the 5601MSC is capable of generating IRIG-B timecode from the
LTC outputs and can also read IRIG-B timecode on the LTC input. The IRIG-B formats supported are
shown in Table 2-8. All IRIG-B formats are based on 100-bit timecode that is amplitude modulated with
a 1kHz sine wave carrier. The differences between the four formats are in what additional information is
carried besides the Time-of-Year timestamp. All control flag bits, if present, are set to zero.
Format Name
Information Carried
IRIG-B120
Time-of-Year, Control Flags, Straight Binary Seconds
IRIG-B122
Time-of-Year
IRIG-B123
Time-of-Year, Straight Binary Seconds
IRIG-B124
Time-of-Year, Year, Control Flags, Straight Binary Seconds
IRIG-B127
Time-of-Year, Year, Straight Binary Seconds
Table 2-8: Supported IRIG Timecode Formats
The IRIG-B outputs are aligned to the IRIG Datum pulse as shown in Figure 2-20. For an overall view of
the entire IRIG waveform, refer to Figure 2-2. The output waveform consists of a 1kHz carrier that is
amplitude modulated to the IRIG pulse waveform. The frame reference bit (P
R
) leading edge coincides
with the IRIG Datum pulse output from the 5601MSC. When locked to GPS, IRIG will be aligned within
20 microseconds of the 1pps pulse from the GPS receiver (with no global phase offset).
Figure 2-20: Detail of IRIG Alignment to IRIG Datum Pulse
Position Identifier P
0
Reference Bit P
R
IRIG-B12X - Modulated 1kHz Carrier
IRIG1 Datum Pulse
Содержание 5601MSC
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