GS4901B/GS4900B Preliminary Data Sheet
3.6.3 Locking to Digital Timing from a Deserializer
As described in Section 3.4.1 on page 40, the GS4901B/GS4900B may be
genlocked to either an analog reference, such as a Black & Burst signal, or to an
SDI input via the digital H, V, and F blanking signals normally produced by a
deserializer. When locking to an SDI input, the user should consider the possibility
of a switch of the SDI signal upstream from the system.
If the GS4901B/GS4900B is locked to the digital H, V, and F blanking signals
produced by a deserializer, and the SDI input to the deserializer is switched such
that the phase of the H input changes abruptly, the REF_LOST output will remain
LOW and the GS4901B/GS4900B will not crash lock to the new H phase. Instead,
the clock and timing outputs will very slowly drift towards the new phase. During
this period of drift, the LOCK_LOST output will be LOW, even though the device is
not genlocked.
The user should clear the Run_Window bits [2:0] of register adress 24h to force the
device to crash lock should such a switch occur. This will cause the
GS4901B/GS4900B to crash lock whenever it sees a disturbance of the input H
signal.
NOTE: Any action that causes an abrupt phase change of the H input to the
GS4901B/GS4900B such that REF_LOST is not triggered will cause the device to
respond in the manner described above.
In addition to the slow drifting behaviour outlined above, there may also be a
random phase difference between the input VSYNC and output V Sync signals
occurring each time a switch in the SDI stream causes an abrupt phase change of
the H input to the GS4901B/GS4900B. This will only occur when attempting to lock
the 525-line SD output standards to the "f/1.001" HD input reference standards. All
line-based timing outputs are affected.
The only way to ensure a constant phase difference between the input VSYNC
signal and the line-based timing outputs is to reset the line-based counters after
such a switch occurs. This is acheived by toggling bit 15 of register address 83h in
the host interface. The device will then delay all line-based output timing signals by
ΔVsync lines relative to the input VSYNC reference, as described in NOTE 3 of
Section 3.2.1.1 on page 35.
37703 - 0 April 2006
51 of 95
GENNUM [ GENNUM CORPORATION ]
