LMH1980 Datasheet, PDF (9/14 Pages) National Semiconductor (TI) – Auto-Detecting SD/HD/PC Video Sync Separator

Application Information
GENERAL DESCRIPTION
The LMH1980 is designed to extract the timing information
from various video formats with standard and non-standard
vertical serration and output the syncs and relevant timing
signals in CMOS logic. Its advanced features and easy ap-
plication make it ideal for consumer, professional, and indus-
trial video systems where sync timing needs to be extracted
from SD, HD, and PC video signals. The device can operate
from a supply voltage between 3.3V and 5V. The only re-
quired external components are bypass capacitors between
the V_CC and GND pins, input coupling capacitor (C_IN) from the
signal source to the V_IN pin, and a fixed-value 1% precision
resistor between the R_EXT and GND pins. Refer to the test
circuit in Figure 2.
R_EXT Resistor
The precision external resistor (R_EXT) establishes the internal
bias current and precise reference voltage for the LMH1980.
For optimal performance, R_EXT should be a 10 kΩ 1% preci-
sion resistor with a low temperature coefficient to ensure
proper operation over a wide temperature range. Using a
R_EXT resistor with less precision may result in reduced per-
formance (like worse performance, increased propagation
delay variation, or reduced input sync amplitude range)
against temperature, supply voltage, input signal, or part-to-
part variations.
Note: The R_EXT resistor used with the LMH1980 serves a dif-
ferent function than the “R_SET resistor” used with other previ-
ous sync separators, like the LM1881. For the LM1881, the
R_SET value needed to be adjusted externally to support dif-
ferent input line rates. For the LMH1980, the R_EXT value is
fixed, and the device 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 adjusting
an external R_SET resistor or programming via a microcon-
troller. The device outputs will respond correctly to a switch
in video format after a sufficient start-up time has been satis-
fied, usually within 1 to 2 fields of video. Unlike other sync
separators, the LMH1980 does not require the power to be
cycled in order to produce correct outputs after a significant
change to the input signal. See the Start-up Time section for
more details.
Fixed-Level Sync Slicing
The LMH1980 uses fixed-level sync slicing for video inputs
with an amplitude from 0.5V_PP to 2V_PP, which allows for prop-
er sync separation even for improperly terminated or attenu-
ated input signals. The fixed-level sync slicing threshold is
nominally 70 mV above the clamped sync tip. This means that
for a minimum video input signal amplitude of 0.5V_PP, the
slicing level is near the mid-point of the sync pulse amplitude.
This slicing level is independent of the input signal amplitude;
therefore, for a 2V_PP input, the slicing level occurs at 12% of
the sync pulse amplitude.
INPUT CONSIDERATIONS
The LMH1980 supports sync separation for analog CVBS, Y
(luma) from Y/C and YP_BP_R, and G (sync on green) from
GBR/RGsB, as specified in the following video standards.
• Composite Video (CVBS) and S-Video (Y/C):
— SMPTE 170M (NTSC), ITU-R BT.470 (PAL)
• Component Video (YP_BP_R/GBR):
— SDTV: SMPTE 125M, SMPTE 267M, ITU-R BT.601
(480I, 576I)
— EDTV: ITU-R BT.1358 (480P, 576P)
— HDTV: SMPTE 296M (720P), SMPTE 274M
(1080I/P), SMPTE RP 211 (1080PsF)
• PC Graphics (RGsB):
— VESA Monitor Timing Standards and Guidelines
Version 1.0, Revision 0.8
• Non-Standard Video:
— Composite NTSC & PAL (or Component 480I & 576I)
without vertical serration & equalization pulses (i.e.:
from logical OR-ing of H & V signals)
Input Termination
The video source should be load terminated with a 75Ω re-
sistor to ensure correct video signal amplitude and minimize
signal distortion due to reflections. In extreme cases, the
LMH1980 can handle non-terminated or double-terminated
input conditions, assuming 1V_PP signal amplitude for normally
terminated video.
Input Filtering
An external filter is recommended if the video signal has large
chroma amplitude that extends near the sync tip and/or has
considerable high-frequency noise, so they do not interfere
with sync separation. A simple RC low-pass chroma filter with
a series resistor (R₉) and a filter capacitor (C₂) to ground can
be used to sufficiently attenuate chroma such that minimum
peak of its amplitude is above the slicing level and also to
improve the overall signal-to-noise ratio. To achieve the de-
sired filter cutoff frequency, it’s advised to vary C₂ and keep
R₉ small (i.e.: 100Ω) to minimize sync tip clipping due to the
voltage drop across R₉. Keep in mind that as the cutoff fre-
quency decreases, the LMH1980 output propagation delays
increase, which could affect the timing relationship between
the sync and video signals.
In applications where the chroma filter needs to be disabled
when HD video is input, it is possible to use a transistor switch
(Q1) controlled by the HD flag (pin 5) to open C₂’s connection
to ground as shown in Figure 11. When a HD tri-level sync
input signal is applied, HD will output logic low (following a
brief delay for auto format detection) and Q1 will turn off to
disable the chroma filter, which is intended for SD composite
video only. When a SD bi-level sync signal (i.e.: NTSC/PAL)
is applied, HD will output logic high and Q1 will turn on to
enable the chroma filter.
Important: If the filter cutoff frequency (f_CO) is set too low and
HD video is applied, the filter can severely roll off and atten-
uate the input's high-bandwidth tri-level sync pulses such that
the LMH1980 cannot detect a valid HD input signal. If the
LMH1980 cannot detect a valid HD input, then the HD flag will
never change from logic high to low and the switch-controlled
filter will never be disabled via Q1. In other words, f_CO should
not be set too low that the filter impairs the LMH1980's ability
to detect a valid HD input. The values of R₉ and C₂ shown in
Figure 11 give f_CO=2.79 MHz (about -4 dB at 3.58 MHz NTSC
subcarrier frequency) without impairing HD video format de-
tection.
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