FMS6407_05 Datasheet, PDF (10/17 Pages) Fairchild Semiconductor – Triple Video Drivers with Selectable HD/Progressive/SD/Bypass Filters

DATA SHEET
FMS6407
Functional Description
Introduction
The FMS6407 is a next generation filter solution from
Fairchild Semiconductor addressing the expanding filtering
needs for televisions, set top boxes, and DVD players including
progressive scan capability. The product provides selectable
filtering with cutoff frequencies of 30MHz, 15MHz, and
8.0MHz on the YPbPr, RGB and YC-CV channels. In addi-
tion, the filters can be bypassed for wider bandwidth applica-
tions. The FMS6407 allows consumer devices to support a
variety of resolution standards with the same hardware.
Multiplexers on the YPbPr / RGB / YC-CV channels provide
further flexibility. When the input multiplexer is changed
from YPbPr to RGB mode the sync tip clamp voltages are
changed appropriately. All three channels are set for 250mV
sync tips to reduce DC-coupled power dissipation for RGB
inputs. The lower output bias voltage is not suitable for the
PbPr outputs so for YPbPr inputs these signals are clamped
to 1.125V while Y is still clamped to 250mV. For systems
running YPbPr and YC-CV signals, the Y and CV signals
will be clamped to 250mV while C is clamped to 1.125V.
Sync tip clamping voltages are set by forcing the desired DC
bias level during the active sync period. For systems without
sync on green, an external sync input is provided. If sync
exists on the Y input signal but not on the G input signal, the
RGB and EXT_SYNC control inputs may be wired together
on the PCB to switch the sync source with the input source.
Both standard definition (bi-level) and high definition (tri-
level) sync are supported at YIN and SYNC_IN depending
on the FSEL[1:0] inputs.
not include sync, the FMS6407 can be used in External
SYNC Mode. When the FMS6407 is used in external sync
mode, (EXT_SYNC pin is high), a pulsed input must be
applied to the SYNC_IN pin. If there is no video signal
present, therefore no sync signal present, there must still be
an input applied to the SYNC_IN pin. When there is no
video signal on the video inputs SYNC_IN can be a sync
pulse every 60µs to mimic the slowest sync in a regular
video signal. The following two sections discuss the sync
processing and timing required in more detail.
SD and Progressive Scan
Video Sync Processing
The FMS6407 must control the DC offset of AC-coupled
input signals since the average DC level of video varies with
image content. If the input offset is allowed to wander, the
common mode input range of the amplifiers can be exceeded
leading to signal distortion. DC offset adjustment is referred
to as clamping or in some cases, biasing, and must be done at
the correct time during each video line. The optimum time is
during the sync pulse since it is the lowest input voltage.
This approach works well for 480i and 480p signals since the
sync tip duration is long enough to allow the DC-offset
errors to be compensated from line to line. The DC-offset of
the sync tip is adjusted as illustrated in Figure 1 by forcing a
current on the input during the sync pulse. The sync tip will
be clamped to approximately 250mV. Signals like Pb and Pr
with a symmetric voltage range (±350mV) will be clamped
to approximately 1.125V. Note that the following diagrams
indicate output voltage levels for both 0dB and 6dB gain
(1V_pp and 2V_pp video signals at the FMS6407 output pin).
Standard definition (480i) and progressive (480p) signals are
clamped by forcing the signal to the desired voltage during
the sync pulse. For signals with sync, the sync tip itself will
be forced to the clamp voltage (typically 250mV). When
high definition sync is present (tri-level sync) the sync tip
duration is too short to allow this approach. In order to accu-
rately clamp HD signals, the sync pulse starts a timer and the
actual clamping is done at the blanking level right after the
sync pulse. The sync tip will still typically be placed at 250mV.
All three outputs are driven by amplifiers with selectable gains
of 0dB or +6dB. These amplifiers can drive two terminated
video loads (75Ω) to 2Vpp with a 1Vpp input when set to 6dB
gain. The input range is limited to 1.5V_pp and the output range
is limited to 2.5V_pp.
All control inputs must be tied to V_SS or V_CC. Do not leave
them floating.
0dB Gain 6dB
1475mV 1875mV
1125mV 1125mV
775mV 425mV
1250mV 2250mV
550mV 850mV
250mV 250mV
0mV 0mV
Required Pb Offset
Av = 1 (0dB) or 2 (6dB)
Av*700mV
AAcctitvivee
Video
Av*300mV
Required Sync Tip Offset
Figure 1. Bi-Level Sync
Tip Clamping and Bias
In some cases, the sync voltage may be compressed to less
than the nominal 300mV value. The FMS6407 can success-
fully recover SD and Progressive Scan sync which is greater
than 100mV (compressed to 33% of nominal).
External Sync Mode
The FMS6407 can properly recover sync timing from video
signals that include sync. If the Y-input video signals does
The FMS6407 can properly recover sync timing from luma
and green which include sync. If none of the video signals
includes sync, the EXT_SYNC control input can be set high
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REV. 1I July 2005