LMH1981
Multi-Format Video Sync Separator
LMH1981 Multi-Format Video Sync Separator
December 2006
General Description
The LMH1981 is a high performance multi-format sync separator ideal for use in a wide range of video applications, such
as broadcast and professional video equipment and HDTV/
DTV systems.
The input accepts standard analog SD/ED/HD video signals
with either bi-level or tri-level sync, and the outputs provide
all of the critical timing signals in CMOS logic, which swing
from rail-to-rail (VCC and GND) including Composite, Horizontal, and Vertical Syncs, Burst/Back Porch Timing, Odd/
Even Field, and Video Format Outputs. HSync features very
low jitter on its leading (falling) edge, minimizing external circuitry needed to clean and reduce jitter in subsequent clock
generation stages.
The LMH1981 automatically detects the input video format,
eliminating the need for programming using a microcontroller,
and applies precise 50% sync slicing to ensure accurate sync
extraction at OH, even for inputs with irregular amplitude from
improper termination or transmission loss. Its unique Video
Format Output conveys the total horizontal line count per field
as an 11-bit binary serial data stream, which can be decoded
by the video system to determine the input video format and
enable dynamic adjustment of system parameters, i.e.: color
space or scaler conversions. The LMH1981 is available in a
14-pin TSSOP package and operates over a temperature
range of −40°C to +85°C.
Connection Diagram
14-Pin TSSOP
Top View
20174501
Features
Standard analog video sync separation for NTSC, PAL,
■
480I/P, 576I/P, 720P, and 1080I/P/PsF from Composite
Video (CVBS), S-Video (Y/C), and Component Video
(YPBPR/GBR) interfaces
Bi-level & tri-level sync compatible
■
Composite, Horizontal, and Vertical Sync Outputs
■
Burst/Back Porch Timing, Odd/Even Field, and Video
■
Format Outputs
Superior jitter performance on leading edge of HSync
■
Automatic video format detection
■
50% sync slicing for video inputs from 0.5 VPP to 2 V
■
3.3V to 5V supply operation
■
Applications
Broadcast and Professional Video Equipment
■
HDTV/DTV Systems
■
Genlock Circuits
■
Video Capture Devices
■
Set-Top Boxes (STB) & Digital Video Recorders (DVR)
■
Video Displays
■
Pin Descriptions
Pin No. Pin Name Pin Description
1 R
2, 5, 10 GND Ground
3, 6, 11 V
4 V
7 HSOUT Horizontal Sync Output
8 VSOUT Vertical Sync Output
9 VFOUT Video Format Output
12 CSOUT Composite Sync Output
13 BPOUT Burst/Back Porch Timing Output
14 OEOUT Odd/Even Field Output
EXT
CC
IN
Bias Current External Resistor
Supply Voltage
Video Input
PP
FIGURE 1. Pinout
Ordering Information
Package Part Number Package Marking Transport Media NSC Drawing
14-Pin TSSOP
© 2006 National Semiconductor Corporation 201745 www.national.com
LMH1981MT
LMH1981MTX 2.5k Units Tape and Reel
LMH1981MT
94 Units/Rail
MTC14
Absolute Maximum Ratings (Notes 1, 7)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
LMH1981
Distributors for availability and specifications.
Storage Temperature Range −65°C to +150°C
Lead Temperature (soldering 10 sec.) 300°C
Junction Temperature (T
Thermal Resistance (θJA)
ESD Tolerance (Note 2)
Human Body Model 3.5 kV
Machine Model 350V
Charge-Device Model 1.0 kV
Supply Voltage, V
Video Input, V
IN
CC
−0.3V to VCC + 0.3V
0V to 5.5V
Operating Ratings (Note 1)
Temperature Range (Note 3) −40°C to +85°C
V
CC
Input Amplitude, V
IN-AMPL
Electrical Characteristics (Note 4)
Unless otherwise specified, all limits are guaranteed for TA = 25°C, VCC = V
RL = 10 kΩ, CL < 10 pF. Boldface limits apply at the temperature extremes. See Figure 2 for Test Circuit.
Symbol Parameter Conditions Min
I
CC
Supply Current No input signal VCC = 3.3V 9.5 11.5
Video Input Specifications
V
IN-SYNC
Input Sync Amplitude Amplitude from negative sync tip to video
blanking level for SD/EDTV bi-level sync
(Notes 8, 9, 11)
Amplitude from negative to positive sync tips
for HDTV tri-level sync
(Notes 8, 10, 11)
V
IN-CLAMP
V
IN-SLICE
Input Sync Tip Clamp Level 0.7 V
Input Sync Slice Level Level between video blanking & sync tip for
SD/EDTV and between negative & positive
sync tips for HDTV
Logic Output Specifications (Note 12)
V
OL
Output Logic 0 See output load conditions
above
V
OH
Output Logic 1 See output load conditions
above
T
SYNC-LOCK
Sync Lock Time Time for the output signals to be correct after
the video signal settles at VIN following a
significant input change. See Start-Up Time
section for more information
T
VSOUT
Vertical Sync Output Pulse
Width
See Figures 3, 4, 5, 6, 7, 8 for SDTV, EDTV
& HDTV Vertical Interval Timing
= V
CC2
= V
CC3
CC1
(Note 6)
VCC = 5V 11 13.5
0.14 0.30 0.60
0.30 0.60 1.20
VCC = 3.3V 0.3
VCC = 5V 0.5
VCC = 3.3V 3.0
VCC = 5V 4.5
JMAX
) (Note 3)
+150°C
52°C/W
3.3V −5% to 5V +5%
= 3.3V, R
(Note 5)
140 mV to VCC–V
= 10 kΩ 1%,
EXT
Typ
Max
(Note 6)
IN-CLAMP
Units
mA
V
PP
50 %
V
V
2 V
periods
3 H periods
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see the Electrical Characteristics
Tables.
Note 2: Human Body Model, applicable std. MIL-STD-883, Method 3015.7. Machine Model, applicable std. JESD22-A115-A (ESD MM std. of JEDEC)
Field-Induced Charge-Device Model, applicable std. JESD22-C101-C (ESD FICDM std. of JEDEC).
Note 3: The maximum power dissipation is a function of T
PD = (T
Note 4: Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very limited self-heating
of the device such that TJ = TA. No guarantee of parametric performance is indicated in the electrical tables under conditions of internal self-heating where TJ >
TA.
Note 5: Typical values represent the most likely parametric norm at the time of characterization. Actual typical values may vary over time and will also depend
on the application and configuration. The typical values are not tested and are not guaranteed on shipped production material.
Note 6: Limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlations using the Statistical Quality
Control (SQC) method.
Note 7: All voltages are measured with respect to GND, unless otherwise specified.
Note 8: V
Note 9: Tested with 480I signal.
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- TA)/θJA . All numbers apply for packages soldered directly onto a PC board.
J(MAX)
IN-AMPL
plus V
should not exceed VCC.
IN-CLAMP
, θJA. The maximum allowable power dissipation at any ambient temperature is
J(MAX)
Note 10: Tested with 720P signal.
Note 11: Maximum voltage offset between 2 consecutive input horizontal sync tips must be less than 25 mVPP.
Note 12: Outputs are negative-polarity logic signal, except for odd/even field and video format outputs.
LMH1981 Test Circuit
LMH1981
20174502
FIGURE 2. Test Circuit
The LMH1981 test circuit is shown in Figure 2. The video generator should provide a low-noise, broadcast-quality signal over
75Ω coaxial cable which should be impedance-matched with a 75Ω load termination resistor to prevent unwanted signal distortion.
The output waveforms should be monitored using a low-capacitance probe on an oscilloscope with at least 500 MHz bandwidth.
See the PCB LAYOUT CONSIDERATIONS section for more information about signal and supply trace routing and component
placement.
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SDTV Vertical Interval Timing (NTSC, PAL, 480I, 576I)
LMH1981
FIGURE 3. NTSC Odd Field Vertical Interval
20174503
FIGURE 4. NTSC Even Field Vertical Interval
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20174504
EDTV Vertical Interval Timing (480P, 576P)
LMH1981
20174505
FIGURE 5. 480P Vertical Interval
HDTV Vertical Interval Timing (720P, 1080P)
FIGURE 6. 720P (1080P) Vertical Interval
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20174514
HDTV Vertical Interval Timing (1080I)
LMH1981
20174506
FIGURE 7. 1080I Field 1 Vertical Interval
20174507
FIGURE 8. 1080I Field 2 Vertical Interval
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SD/EDTV Horizontal Interval Timing
LMH1981
20174508
FIGURE 9. SD/EDTV Horizontal Interval with Bi-level Sync
SDTV Horizontal Interval Timing Characteristics (NTSC, PAL, 480I, 576I)
VCC = 3.3V , TA = 25°C
Symbol Parameter Conditions Typ Units
td
td
td
CSOUT
HSOUT
BPOUT
Composite Sync Output Propagation Delay from Input
Sync Reference (OH)
Horizontal Sync Output Propagation Delay from Input
Sync Reference (OH)
Burst/Back Porch Timing Output Propagation Delay from
See Figure 9 NTSC, 480I 475
PAL, 576I 525
See Figure 9
(See note below)
NTSC, 480I 40
PAL, 576I 60
ns
ns
See Figure 9 300 ns
Input Sync Trailing Edge
T
HSOUT
T
BPOUT
Horizontal Sync Output Pulse Width See Figure 9 2.5 µs
Burst/Back Porch Timing Output Pulse Width See Figure 9 3.2 µs
EDTV Horizontal Interval Timing Characteristics (480P, 576P)
VCC = 3.3V , TA = 25°C
Symbol Parameter Conditions Typ Units
td
CSOUT
Composite Sync Output Propagation Delay from Input
See Figure 9 450 ns
Sync Reference (OH)
td
HSOUT
Horizontal Sync Output Propagation Delay from Input
See Figure 9 35 ns
Sync Reference (OH)
td
BPOUT
Burst/Back Porch Timing Output Propagation Delay from
See Figure 9 500 ns
Input Sync Trailing Edge
T
HSOUT
T
BPOUT
Horizontal Sync Output Pulse Width See Figure 9 2.3 µs
Burst/Back Porch Timing Output Pulse Width See Figure 9 350 ns
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HDTV Horizontal Interval Timing
LMH1981
20174509
FIGURE 10. HDTV Horizontal Interval with Tri-level Sync
HDTV Horizontal Interval Timing Characteristics (720P, 1080I)
VCC = 3.3V , TA = 25°C
Symbol Parameter Conditions Typ Units
td
CSOUT
Composite Sync Output Propagation Delay from
See Figure 10 150 ns
Input Sync Leading Edge
td
HSOUT
Horizontal Sync Output Propagation Delay from
See Figure 10 30 ns
Input Sync Reference (OH)
td
BPOUT
T
HSOUT
T
BPOUT
Burst/Back Porch Timing Output Propagation Delay
from Input Sync Trailing Edge
Horizontal Sync Output Pulse Width See Figure 10 720P 525
See (Note 10) 720P 400
1080I, 1080P 300
1080I, 1080P 475
ns
ns
Burst/Back Porch Timing Output Pulse Width See Figure 10 350 ns
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LMH1981
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, advanced features and easy application make
it ideal for broadcast and professional video systems where
low jitter is a crucial parameter. The device can operate from
a supply voltage between 3.3V and 5V. The only required external components are bypass capacitors at the power supply
pins, an input coupling capacitor at pin 4, and a precision
R
resistor at pin 1. Refer to the test circuit in Figure 2.
EXT
R
Resistor
EXT
The R
rent and precise reference voltage for the LMH1981. For
optimal performance, R
resistor with a low temperature coefficient to ensure proper
operation over a wide temperature range. Using a R
sistor with less precision may result in reduced performance
(like worse jitter 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
“R
older LM1881, the R
different input line rates. For the LMH1981, the R
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 external
programming via a microcontroller or R
vice outputs will respond correctly to video format switching
after a sufficient start-up time has been satisfied. Unlike other
sync separators, the LMH1981 does not require the power to
be cycled in order to guarantee correct outputs after a significant change to the input signal. See the Start-up Time section
in page 10 for more details.
50% Sync Slicing
The LMH1981 features 50% sync slicing on HSync to provide
accurate sync separation for video input amplitudes from 0.5
VPP to 2 VPP, which enables excellent HSync jitter performance even for improperly terminated or attenuated source
signals and stability against variations in temperature. The
sync separator is compatible with SD/EDTV bi-level and
HDTV tri-level sync inputs. Bi-level syncs will be sliced at the
50% point between the video blanking level and negative sync
tip, indicated by the input's sync timing reference or “OH” in
Figure 9. Tri-level syncs will be sliced at the 50% point between the negative and positive sync tips (or positive zerocrossing), indicated by OH in Figure 10.
external resistor establishes the internal bias cur-
EXT
should be a 10 kΩ 1% precision
EXT
resistor serves a different function than the
EXT
resistor” used in the LM1881 sync separator. In the
SET
value was adjusted to accommodate
SET
EXT
resistor. The de-
SET
re-
EXT
value is
VIDEO INPUT
The LMH1981 supports sync separation for CVBS, Y (luma)
from Y/C and YPBPR and G (sync on green) from GBR with
either bi-level or tri-level sync, as specified in the following
video standards.
•
Composite Video (CVBS) and S-Video (Y/C):
SDTV: SMPTE 170M (NTSC), ITU-R BT.470 (PAL)
—
•
Component Video (YPBPR/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)
The LMH1981 does not support RGB formats that conform to
VESA standards used for PC graphics.
Input Termination
The video source should be load terminated with a 75Ω resistor to ensure correct video signal amplitude and minimize
signal distortion due to reflections. In extreme cases, the
LMH1981 can handle unterminated or double-terminated input conditions, assuming 1 VPP signal amplitude for normal
terminated video.
Input Coupling Capacitor
The input signal should be AC coupled to the VIN (pin 4) of the
LMH1981 with a properly chosen coupling capacitor, CIN.
The primary consideration in choosing CIN is whether the
LMH1981 will interface with video sources using an AC-coupled output stage. If AC-coupled video sources are expected
in the end-application, then it’s recommended to choose a
small CIN value such as 0.01 µF as prescribed in the next
section. Other considerations such as HSync jitter performance and start-up time are practically fixed by the limited
range of small CIN values. It’s important to note that video
sources with AC-coupled outputs will introduce video-dependent jitter that cannot be remedied by the sync separator;
moreover, this type of jitter is not prevalent in sources with
DC-coupled input/output stages.
When only DC-coupled video sources are expected, a larger
CIN value can be chosen to minimize voltage droop and thus
improve HSync jitter at the expense of increased start-up time
as explained in the Start-up Time section. A typical CIN value
such as 1 µF will give excellent jitter performance and reasonable start-up time using a broadcast-quality DC-coupled
video generator. For applications where low HSync jitter is not
critical, CIN can be a small value to reduce start-up time.
START-UP TIME
When there is a significant change to the video input signal,
such as sudden signal switching, signal attenuation (i.e.: additional termination via loop through) or signal gain (i.e.: disconnected end-of-line termination), the quiescent operation
of the LMH1981 will be disrupted. During this dynamic input
condition, the LMH1981 outputs may not be correct but will
recover to valid signals after a predictable start-up time, which
consists of an adjustable input settling time and a predetermined “sync lock time”.
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Input Settling Time and Coupling Capacitor Selection
Following a significant input condition, the negative sync tip
of the AC-coupled signal settles to the input clamp voltage as
LMH1981
the coupling capacitor, CIN, recovers a quiescent DC voltage
via the dynamic clamp current. Because CIN determines the
input settling time, its capacitance value is critical when minimizing overall start-up time.
For example, a settling time of 8 ms can be expected for a
typical CIN value of 1 µF when switching in a standard NTSC
signal with no prior input. A smaller value yields shorter settling time at the expense of increased line droop voltage and
consequently higher HSync jitter, whereas a larger one gives
lower jitter but longer settling time. Settling time is proportional
to the value of CIN, so doubling CIN will also double the settling
time.
The value of CIN is a tradeoff between start-up time and jitter
performance and therefore should be evaluated based on the
application requirements. Figure 11 shows a graph of typical
input-referred HSync jitter vs. C
line. Refer to the Horizontal Sync Output section for more
values to use as a guide-
IN
about jitter performance.
20174517
FIGURE 12. Typical Start-Up Time for NTSC Input to
LMH1981 via 1 µF Coupling Capacitor
LOGIC OUTPUTS
In the absence of a video input signal, the LMH1981 outputs
are logic high except for the odd/even field and video format
outputs, which are both undefined, and the composite sync
output.
20174516
FIGURE 11. Typical HSync Jitter vs. CIN Values
Sync Lock Time
In addition to settling time, the LMH1981 has a predetermined
sync lock time, T
Once the AC-coupled input has settled enough, the LMH1981
SYNC-LOCK
, before the outputs are correct.
needs time to detect the valid video signal and resolve the
blanking & sync tip levels for 50% sync slicing before the output signals are correct.
For practical values of CIN, T
or 2 video fields in duration starting from the 1st valid VSync
SYNC-LOCK
is typically less than 1
output pulse to the valid HSync pulses beginning thereafter.
VSync and HSync pulses are considered valid when they
align correctly with the input's vertical and horizontal sync intervals
It is recommended for the outputs to be applied to the system
after the start-up time is satisfied and outputs are valid. For
example, the oscilloscope screenshot in Figure 12 shows a
typical start-up time of about 10 ms from when an NTSC signal is switched in (no previous input) to when the LMH1981
outputs are valid.
Composite Sync Output
CSOUT (pin 12) simply reproduces the video input sync pulses below the video blanking level. This is obtained by clamping the video signal sync tip to the internal clamp voltage at
VIN and extracting the resultant composite sync signal, or
CSync. For both bi-level and tri-level syncs, CSync's negative-going leading edge is derived from the input's negativegoing leading edge with a propagation delay.
Horizontal Sync Output
HSOUT (pin 7) produces a negative-polarity horizontal sync
signal, or HSync, with very low jitter on its negative-going
leading edge (reference edge) using precise 50% sync slicing. For bi-level and tri-level sync signals, the horizontal sync
leading edge is triggered from the input's sync reference,
OH, with a propagation delay.
HSync was optimized for excellent jitter performance on its
leading edge because most video systems are negative-edge
triggered. When HSync is used in a positive-edge triggered
system, like an FPGA PLL input, it must be inverted beforehand to produce positive-going leading edges. The trailing
edge of HSync should never be used as the reference or triggered edge. This is because the trailing edges of HSync are
reconstructed for the broad serration pulses during the vertical interval.
HSync's typical peak-to-peak jitter can be measured using the
input-referred jitter test methodology on a real-time digital oscilloscope by triggering at or near the input's OH reference and
monitoring HSync's leading edge with 4-sec. variable persistence. This is one way to measure HSync's typical peak-topeak jitter in the time domain. Figure 13 and Figure 14 show
oscilloscope screenshots demonstrating very low jitter on
HSync's leading edge for 1080I tri-level sync and PAL Black
Burst inputs, respectively, from a Tek TG700-AWVG7/AVG7
video generator with DC-coupled outputs and with LMH1981
VCC = 3.3V.
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20174513
FIGURE 13. Typical HSync Jitter for 1080I Input
Upper: Horizontal Sync Leading Edge (Reference)
Lower: Zoomed In — 400 ps/DIV, 25 mV/DIV
LMH1981
Burst/Back Porch Timing Output
BPOUT (pin 13) provides a negative-polarity burst/back porch
signal, which is pulsed low for a fixed width during the back
porch interval following the input's sync pulse. The burst/back
porch timing pulse is useful as a burst gate signal for NTSC/
PAL color burst synchronization and as a clamp signal for
black level clamping (DC restoration) and sync stripping applications.
For SDTV formats, the back porch pulse's negative-going
leading edge is derived from the input's positive-going sync
edge with a propagation delay, and the pulse width spans an
appropriate duration of the color burst envelope for NTSC/
PAL. During the vertical interval, its pulse width is shorter to
correspond with the narrow serration pulses. For EDTV formats, the back porch pulse behaves similar to the SDTV case
except that the shorter pulse width is always maintained. For
HDTV formats, the pulse's leading edge is derived from the
input's negative-going trailing sync edge with a propagation
delay, and the pulse width is even narrower to correspond
with the shortest back porch duration of HDTV formats.
Odd/Even Field Output
OEOUT (pin 14) provides an odd/even field output signal,
which facilitates identification of odd and even fields for interlaced or segmented frame (sF) formats. For interlaced or
segmented frame formats, the odd/even output is logic high
during an odd field (field 1) and logic low during an even field
(field 2). The odd/even output edge transitions align with
VSync's leading edge to designate the start of odd and even
fields. For progressive (non-interlaced) video formats, the
output is held constantly at logic high.
20174515
FIGURE 14. Typical HSync Jitter for PAL Input
Upper: Horizontal Sync Leading Edge (Reference)
Lower: Zoomed In — 1000 ps/DIV, 25 mV/DIV
Vertical Sync Output
VSOUT (pin 8) produces a negative-polarity vertical sync signal, or VSync. VSync's negative-going leading edge is derived from the 50% point of the first vertical serration pulse
with a propagation delay, and its output pulse width, T
spans approximately three horizontal periods (3H).
VSOUT
Video Format Output (Lines-per-Field Data)
The LMH1981 counts the number of HSync pulses per field
to approximate the total horizontal line count per field (vertical
resolution). This can be used to identify the video format and
enable dynamic adjustment of video system parameters,
such as color space or scaler conversions. The line count per
field is output to VFOUT (pin 9) as an 11-bit binary data
stream. The video format data stream is clocked out on the
11 consecutive leading edges of HSync, starting at the 3rd
HSync after each VSync leading edge. Outside of these active 11-bits of data, the video format output can be either 0 or
1 and should be treated as undefined. Refer to Figure 15 to
see the VFOUT data timing for the 480P progressive format
and Figure 16 and Figure 17 for the 1080I interlaced format.
See Table 1 for a summary of VFOUT data for all supported
formats.
A FPGA/MCU can be used to decode the 11-bit VFOUT data
stream by using HSync as the clock source signal and VSync
as the enable signal. Using the FPGA's clock delay capability,
a delayed clock derived from HSync can be used as the sampling clock to latch the VFOUT data in the middle of the
,
horizontal line period rather than near the VFOUT data-bit
transitions in order to avoid setup time requirements.
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TABLE 1. VFOUT Data Summary
LMH1981
TV Format
(Total Lines per Field)
NTSC/480I
(262.5)
PAL/576I
(312.5)
480P
(525)
576P
(625)
720P
(750)
1080I
(562.5)
1080P
(1125)
VFOUT Data
Field 1
00100000100b
260d
00100110110b
310d
01000001010b
522d
01001101110b
622d
01011101011b
747d
01000110000b
560d
10001100010b
1122d
VFOUT Data
Field 2
00100000011b
259d
00100110101b
309d
N/A
N/A
N/A
01000101111b
559d
N/A
FIGURE 15. Video Format Output for Progressive Format, 480P
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20174510
FIGURE 16. Video Format Output for Interlaced Format, 1080I Field 1
LMH1981
20174511
FIGURE 17. Video Format Output for Interlaced Format, 1080I Field 2
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20174512
OPTIONAL CONSIDERATIONS
Optional Input Filtering
An external filter may be necessary if the video signal has
LMH1981
considerable high-frequency noise or has large chroma amplitude that extends near the sync tip. A simple RC low-pass
filter with a series resistor (RS) and a capacitor (CF) to ground
can be used to improve the overall signal-to-noise ratio and
sufficiently attenuate chroma such that minimum peak of its
amplitude is above the 50% sync slice level. To achieve the
desired filter cutoff frequency, it’s advised to vary CF and keep
RS small (ie. 100Ω) to minimize sync tip clipping due to the
voltage drop across RS. Note that using an external filter will
increase the propagation delay from the input to the outputs.
In applications where the chroma filter needs to be disabled
when non-composite video (ie: ED/HD video) is input, it is
possible to use a transistor to switch open CF’s connection to
ground as shown in Figure 11. This transistor can be switched
off/on by logic circuitry to decode the lines-per-field data output (VFOUT). As shown in Table 1, NTSC and PAL both have
1 (logic high) for the 3rd bit of VFOUT. If the logic circuitry
detects 0 (logic low) for this bit, indicating non-composite
video, the transistor can be turned off to disable the chroma
filter.
FIGURE 18. External Chroma Filter with Control Circuit
AC-Coupled Video Sources
An AC coupled video source typically has a 100 µF or larger
output coupling capacitor (C
the DC bias of the amplifier output from the video signal.
) for protection and to remove
OUT
When the video source is load terminated, the average value
of the video signal will shift dynamically as the video duty cycle
varies due to the averaging effect of the C
resistors. The average picture level or APL of the video con-
and termination
OUT
tent is closely related to the duty cycle.
For example, a significant decrease in APL such as a white-
to-black field transition will cause a positive-going shift in the
sync tips characterized by the source’s RC time constant, t
(150Ω*C
OUT
have difficulty stabilizing the input signal under this type of
). The LMH1981’s input clamp circuitry may
OUT
shifting; consequently, the unstable signal at VIN may cause
missing sync output pulses to result, unless a proper value
for CIN is chosen.
To avoid this potential problem when interfacing AC-coupled
sources to the LMH1981, it’s necessary to introduce a voltage
droop component via CIN to compensate for video signal shifting related to changes in the APL. This can be accomplished
by selecting CIN such that the effective time constant of the
LMH1981’s input circuit, t
, is less than t
RC-IN
RC-OUT
The effective time constant of the input circuit can be approximated as: t
RS = 150Ω, RI = 4000Ω (input resistance), T
= (RS+RI)*CIN*T
RC-IN
LINE/TCLAMP
LINE
20174518
RC-
.
, where
∼ 64 μs for
NTSC, and T
white-to-black field transition in NTSC video through C
exhibit the maximum sync tip shifting due to its long line period
(T
). By setting t
LINE
CIN can be calculated to ensure proper operation under this
= 250 ns (internal clamp duration). A
CLAMP
RC-IN
< t
, the maximum value of
RC-OUT
OUT
will
worst-case condition.
For instance, t
ensure t
ing CIN = 0.01 μF, the LMH1981 will function properly with AC-
< 33 ms, CIN must be less than 31 nF. By choos-
RC-IN
coupled video sources using C
is about 33 ms for C
RC-OUT
≥ 220 μF.
OUT
= 220 µF. To
OUT
PCB LAYOUT CONSIDERATIONS
LMH1981 IC Placement
The LMH1981 should be placed such that critical signal paths
are short and direct to minimize PCB parasitics from degrading the high-speed video input and logic output signals.
Ground Plane
A two-layer, FR-4 PCB is sufficient for this device. One of the
PCB layers should be dedicated to a single, solid ground
plane that runs underneath the device and connects the device GND pins together. The ground plane should be used to
connect other components and serve as the common ground
reference. It also helps to reduce trace inductances and minimize ground loops. Try to route supply and signal traces on
another layer to maintain as much ground plane continuity as
possible.
Power Supply Pins
The power supply pins should be connected together using
short traces with minimal inductance. When routing the supply traces, be careful not to disrupt the solid ground plane.
For high frequency bypassing, place 0.1 µF SMD ceramic bypass capacitors with very short connections to power supply
and GND pins. Two or three ceramic bypass capacitors can
be used depending on how the supply pins are connected
together. Place a 4.7 µF SMD tantalum bypass capacitor
nearby all three power supply pins for low frequency supply
bypassing.
R
Resistor
EXT
The R
sistor. Place R
connect to pin 1 and the ground plane using the shortest pos-
resistor should be a 10 kΩ 1% SMD precision re-
EXT
as close as possible to the device and
EXT
sible connections. All input and output signals must be kept
away from this pin to prevent unwanted signals from coupling
into this pin.
Video Input
The input signal path should be routed using short, direct
traces between video source and input pin. Use a 75Ω input
termination and a SMD capacitor for AC coupling the video
input to pin 4.
Output Routing
The output signal paths should be routed using short, direct
traces to minimize parasitic effects that may degrade these
high-speed logic signals. All output signals should have a resistive load of about 10 kΩ and capacitive load of less than
10 pF, including parasitic capacitances for optimal signal
quality. This is especially important for the horizontal sync
output, in which it is critical to minimize timing jitter. Each output can be protected by current limiting with a small series
resistor, like 100Ω.
www.national.com 14
Physical Dimensions inches (millimeters) unless otherwise noted
LMH1981
14-Pin TSSOP
NS Package Number MTC14
15 www.national.com
Notes
LMH1981 Multi-Format Video Sync Separator
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