GS4901B/GS4900B Preliminary Data Sheet
5. For blanking-based input references, the device will advance all line-based
output timing signals by 1 line if the value programmed in the H_Offset register
is greater than the number of output video clock cycles from the start of H
Sync to the end of active video (Hsync_to_EAV) + 20. The value of
Hsync_to_EAV is reported in register 51h and changes according to the output
VID_STD selected. The user may compensate for this advance by adding 1
line to the desired vertical offset before loading this value into the register. In
addition, the internal V_lock and F_lock signals reported in bits 3 and 4 of
register 16h will be LOW when H_Offset = Hsync_to_EAV + 21 only, although
the device will remained genlocked. The user may choose to mask these lock
signals such that the device will continue to report genlock under this
condition.
6. The offsets that occur as described in notes 1-5 are independent of one
another and must be accounted for as such.
3.2.1.2 Freeze Mode
When the device is in Genlock mode and the input reference is removed, the
GS4901B/GS4900B will enter Freeze mode. The behaviour of the device during
loss and re-acquisition of an input reference signal is described in Section 3.5.3 on
page 44.
In Freeze mode, the frequency of the output clock and timing signals will be
maintained to within +/- 2ppm. This assumes a loop bandwidth of 10Hz. Also, if the
frequency of the 27MHz reference crystal shifts while in Freeze mode, the
frequency of the output clock and timing signals will shift as well.
3.2.2 Free Run Mode
The GS4901B/GS4900B will enter Free Run mode when the GENLOCK pin is set
HIGH by the application layer. In this mode, the occurrence of all frequencies is
based on the external 27MHz reference input. Therefore, the frequency of the
output clock and timing signals will have the same accuracy as the crystal
reference.
If operating in Free Run mode, using a more accurate crystal (e.g. 10ppm) ensures
more accurate clock and timing signals are generated.
NOTE: In Free Run mode, the audio clocks of the GS4901B will remain genlocked
to the video clock.
Figure 3-2 summarizes the differences in output accuracy in each mode of
operation. Assuming a crystal reference of +/-100ppm, in Free Run mode the
frequency of the output clock and timing signals will be as accurate as the crystal.
In Genlock mode the frequency will be as accurate as the input reference
regardless of the crystal accuracy. In Freeze mode, the frequency of the output
clock and timing signals will be maintained to within +/- 2ppm.
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