LMH1981 Datasheet, PDF (9/13 Pages) National Semiconductor (TI) – Multi-Format Video Sync Separator

Application Information
GENERAL DESCRIPTION
The LMH1981 is designed to extract the timing information
from various video formats with vertical serration and output
the syncs and relevant timing signals in CMOS logic. Its high
performance, rich feature set, and easy application permit
use in critical systems where low jitter is a crucial parameter,
like in broadcast video and next-generation HDTV/DTV
equipment. The device can operate from a supply voltage
between 3.3V and 5V. The only required external compo-
nents are bypass capacitors at the power supply pins, an
input coupling capacitor at pin 4, and a R_EXT resistor at pin 1.
Refer to the application circuit in Figure 2.
Internal Reference Voltage and R_EXT
The R_EXT external resistor establishes the internal bias cur-
rent and precise reference voltage to assure proper opera-
tion of the LMH1981 over a wide temperature range. For
optimal performance, R_EXT should be a 10 kΩ 0.1% resistor
with a temperature coefficient under 100 ppm/˚C. See the
PCB LAYOUT CONSIDERATIONS section for more infor-
mation about the R_EXT component placement.
Note: The R_EXT resistor serves a different function than the
“R_SET resistor” used in older generations of sync separators
(i.e.: LM1881). Previously, the R_SET value was adjusted to
accommodate different input scan line frequencies. The
R_SET adjustment was outmoded for the LMH1981, which
automatically detects the input line rate to support various
video formats without electrical or physical intervention.
Automatic Format Detection and Switching
Automatic format detection eliminates the need for external
programming via a microcontroller or R_SET resistor. The
device outputs will respond correctly to video format switch-
ing after a hold-off period has been satisfied. Unlike other
sync separators, the LMH1981 does not require the power to
be cycled in order to guarantee correct outputs after switch-
ing between video formats. See the Sync-Lock Condition
for Output Accuracy sub-section for details on sync-
locking.
Advanced Video Sync Processing
The LMH1981 features 50% sync slicing to provide accurate
and robust sync separation, even in the presence of irregular
sync amplitudes from attenuated or improperly terminated
sources and noise. Adaptive sync slicing provides excellent
output timing jitter and stability against variations in input
signal amplitude and temperature. The sync separator also
supports SDTV, EDTV, and HDTV formats, and is compat-
ible with bi-level and tri-level syncs. Bi-level syncs will be
sliced at the 50% point between the sync tip and video blank
level. Tri-level syncs will be sliced at the positive zero-
crossing of the tri-level sync signal, which is the 50% point
between the negative and positive sync tips.
Macrovision Compatibility
The LMH1981 is compatible with the Macrovision Video
Copy Protection System commonly used in VHS and DVD
video sources, which inserts pseudo-sync pulses in the
video blanking signal. These Macrovision-embedded pulses
will be effectively ignored by the sync separator, and the
outputs will not be affected.
VIDEO INPUT
Video Standards Supported
The LMH1981 supports sync separation for the following
standard video interfaces and formats:
• Composite (CVBS) and S-Video (Y/C): NTSC, PAL,
SECAM
• Component (YP_BP_R):
– SDTV with bi-level sync: 480i /60, 576i /50
– EDTV with bi-level sync: 480p /60, 576p /50
– HDTV with tri-level sync: 720p /60, 1080i /50/60, 1080p
/24/25/30/50/60
• Computer Video: RGsB (Sync on green)
Video Input Requirements
The video input at pin 4 accepts Composite, Y from Y/C and
YP_BP_R, and G from Sync on green, with negative-going
bi-level sync or HD tri-level sync between 0.5 V_PP and 2 V_PP.
The video input signal should be AC coupled through a
coupling capacitor to minimize droop voltage and prevent
the signal at V_IN from going below the input sync clamp level.
The video source should be properly terminated with 75Ω to
ensure correct input amplitude and minimize video & sync
distortion due to reflections. In extreme cases, the LMH1981
can handle unterminated (2 V_PP) and double-terminated (0.5
V_PP) input conditions assuming a typical 1 V_PP video signal.
LOGIC OUTPUTS
Most of the logic outputs are held high in the absence of a
video input signal, except for the odd/even field and video
format outputs, which both have special logic signaling for
different video formats, and composite sync output.
Sync-Lock Condition for Output Accuracy
When a new input signal is detected, the outputs will begin to
produce timing signals; however, these signals may not be
accurate until a hold-off period or “sync-lock condition” has
been satisfied. The sync separator requires some time to
identify the new video format and process the sync informa-
tion before the outputs are accurate. T_SYNC-LOCK is the
maximum period from when the new input signal starts, to
when the syncs are locked and the output signals are valid.
It is recommended that the outputs are used only after
T_SYNC-LOCK condition has been satisfied.
Composite Sync Output
The composite sync output (pin 12) simply reproduces the
video input sync pulses below 0 mV (specified video blank-
ing level). This is obtained by clamping the video signal sync
tip to the internal clamp voltage at V_IN and using 50% sync
slicing to strip the sync signal. The resultant composite sync
logic signal is buffered out to pin 12. For both bi-level and
tri-level syncs, composite sync’s negative- and positive-
going edges are triggered from the 50% points of the input
sync’s leading negative- and positive-going edges, respec-
tively, with a propagation delay.
Horizontal Sync Output
The horizontal sync output (pin 7) produces an active low
horizontal sync logic signal with very low jitter on its leading
negative-going edge (reference edge). For bi-level sync sig-
nals, the horizontal sync leading edge is triggered from the
input sync leading edge reference with a propagation delay.
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