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by JIM ROWE
S-video to composite
video converter
Some digital TV set-top boxes provide only
S-video and component video outputs, which
can pose a problem if your TV set only has
a composite video input – or you’ve used up
the S-video and component video inputs.
The same can happen with video tuner cards
for PCs. Here’s an easy-to-build adapter to
get you out of trouble.
Y
OU CAN BUY CHEAP S-video
to composite video adapters in
bargain stores but the unit described
here will do a much better job.
Although those really cheap bargain-store adapters do work, if you
examine the pictures critically, you’ll
find that their quality leaves quite a
bit to be desired. In particular, you’ll
find that wherever the image has large
areas of fine detail – like a shirt with
a fine striped or check pattern, or an
exterior panning shot of a multi-storey
Fig.1: adding a low-value capacitor in series with the
chrominance signal at the S-video input can reduce
cross-colour interference but also softens the picture.
Fig.2 (right): the unit described here uses an LC circuit
to notch out a narrow band of frequencies centred on
the 4.433MHz colour subcarrier frequency. This reduces
cross-colour interference while leaving a sharp picture.
84 Silicon Chip
siliconchip.com.au
Low-cost “bargain-store” adapters commonly produce pictures that suffer from cross-colour interference, as shown
in the photograph at left. By contrast, the SILICON CHIP converter dramatically reduces cross-colour interference – see
photo at right.
building – then you’ll see a very obvious coloured Moire interference
pattern, usually in shades of yellow
and purple.
This effect is called “cross-colour
interference” and it’s caused by heterodyne beats between the higher frequencies in the luminance (Y) signal
and the chrominance (C) subcarrier in
the receiver’s decoder. In effect, the
higher luminance frequencies tend
to behave as if they were part of the
chrominance signal and as a result,
produce fake colour patterns.
This happens when the two signals
are simply mixed together in the video
adaptor – which is what commonly
happens in the bargain store units.
This interference pattern can’t happen
when the Y and C signals are kept separate, which is why S-video produces
much better image quality.
Reducing the interference
Some of the better low-cost adaptors
try to reduce this cross-colour interference by adding a small capacitor
in series with the chrominance input
signal, as shown in Fig.1. The capacitor’s value is chosen so that it passes
most of the chrominance (C) information (it’s in a band about 2.5MHz wide,
centred on 4.43361875MHz) while at
the same time attenuating the higher
frequency luminance signals – ie, by
shunting the luminance output into
the chrominance output of the S-video
signal source.
This reduces the cross-colour interference although it also removes some
of the fine detail from the images, so
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Fig.3: the circuit is based on a MAX4451 dual-wideband video amplifier,
with each stage wired as a non-inverting amplifier with a gain of 2.0. RFC1
and VC1 provide the 4.43361875MHz notch in the luminance signal.
September 2006 85
Par t s Lis t
1 PC board, code 02109061, 76 x 46mm
1 UB5 jiffy box, 83 x 53 x 31mm
1 68mH RF choke (RFC1)
1 mini 4-pin DIN socket, PC-mount (CON1)
1 RCA socket, yellow, PC-mount (CON2)
1 2.5mm concentric power connector, PC-mount
(CON3)
4 M3 x 10mm machine screws, countersink head
4 M3 star lockwashers
10 M3 nuts
2 M3 x 6mm machine screws, round/pan head
This screen shot shows the Snell & Wilcox moving plate
test pattern as fed through an “el-cheapo” S-video to
composite video converter. Note the obvious colour Moire
patterns on the moving plate section and in other areas
where there are fine lines.
Semiconductors
1 MAX4451ESA dual video op amp (IC1)
1 7805 +5V regulator (REG1)
1 7905 -5V regulator (REG2)
2 1N4004 diodes (D1,D2)
Capacitors
2 470mF 16V RB electrolytic
1 220nF MKT metallised polyester
2 100nF multilayer monolithic
1 6-30pF trimcap, green (VC1)
Resistors (0.25W 1%)
1 680W
4 510W
4 75W
This is the same test pattern as above but this time fed
through the SILICON CHIP converter. As can be seen, the
cross-colour interference effects have been dramatically
reduced.
they become softer – ie, the chrominance output is also
shunted to some extent.
By contrast, the unit described here does a better job
of reducing cross-colour interference without sacrificing the higher frequencies in the Y signal nearly as
much. As a result, the images stay reasonably sharp.
It’s admittedly a bit more complex than the “el-cheapo”
adaptors but it’s still low in cost and very easy to build
and get going.
How it works
By way of comparison, this is the direct S-video signal.
It’s completely clear of cross-colour interference effects
and clearly demonstrates the advantages of S-video.
86 Silicon Chip
The approach taken here to reduce cross-colour
interference is to use a simple LC trap circuit to notch
out a fairly narrow range of frequencies in the incoming
luminance (Y) signal, centred on the 4.43361875MHz
colour subcarrier frequency. This removes most of
the higher luminance frequencies that cause obvious
cross-colour patterning, while leaving the luminance
frequencies below about 3.5MHz and above 5.4MHz
untouched. You can see the resulting luminance response in Fig.2.
By contrast, the incoming chrominance signal passes
through its channel largely untouched, so there’s no
degradation of colour detail. As a result, the image
quality of the composite video output signal is quite
good. Of course, it’s not as good as watching S-video
directly but it’s noticeably better than you get with an
“el-cheapo” adaptor.
siliconchip.com.au
Table 1: Capacitor Codes
Value μF Code EIA Code IEC Code
220nF 0.22µF 224
220n
100nF 0.1µF
104
100n
Now let’s take a look at the circuit
diagram – see Fig.3. As shown, the incoming S-video (Y/C) signals come in
via CON1, a standard 4-pin mini-DIN
socket. The C signal is then fed through
a 220nF coupling capacitor and is terminated by a 75W resistor to prevent
ringing due to cable reflections.
From there, the signal is fed to pin
5 of IC1b which is one half of a MAX4451 dual-wideband video amplifier
wired here as a non-inverting amplifier
with a gain of 2.0. This gain is necessary to allow for mixing and output
cable back-termination losses.
The incoming Y signal is treated a
little more harshly. After being terminated in the correct 75W impedance,
it’s then passed through the “notch”
circuit. This consists of a series 680W
resistor and a series LC tuned circuit
formed by a 68mH RF choke (RFC1)
and a 6-30pF trimmer capacitor (VC1).
When VC1 is adjusted to resonate
with RFC1 at 4.43361875MHz, this LC
circuit forms a low-impedance path
to earth at that frequency. This acts
together with the 680W series resistor
to produce the desired notch in the
response, as shown in Fig.2.
From there, the rest of the Y signal
is passed through IC1a, the other half
of the MAX4451 device which is also
wired as a non-inverting amplifier
with a gain of 2.0. The outputs of both
IC1b and IC1a are then mixed using
the two 75W output back-terminating
resistors, to produce the final composite video output signal at output
connector CON2.
Fig.4: install the parts on the PC board as shown here, making
sure that all polarised parts are correctly orientated. The leads
designated with a red dot must be soldered to both sides of the
board but only if your board doesn’t have plated-through holes.
Fig.5: the MAX4451 comes in an SOIC-8 package and is mounted
on the underside of the PC board as shown above and in the photo
below right. Be sure to mount it with its chamfer side towards
the bottom and use a soldering iron with a very fine chisel-tip to
solder its leads.
Power supply
The MAX4451 IC needs a DC supply of ±5V and this is provided using
the simple power supply shown at the
bottom of Fig.3.
This is the fully-assembled PC board, mounted on the lid of the
case. Power comes from an external 9V AC plugpack.
Table 2: Resistor Colour Codes
o
o
o
o
siliconchip.com.au
No.
1
4
4
Value
680W
510W
75W
4-Band Code (1%)
blue grey brown brown
green brown brown brown
violet green black brown
5-Band Code (1%)
blue grey black black brown
green brown black black brown
violet green black gold brown
September 2006 87
Fig.6: follow this diagram to mark out the holes to be drilled in the ends
of the box and the box lid. Alternatively, you can scan this diagram, print
it out and use it as a drilling template. The larger holes are best made by
drilling a small hole first and then enlarging them using a tapered reamer.
Power comes from a 9V AC plugpack supply and this feeds two halfwave rectifiers based on diodes D1 and
D2. Their outputs are filtered using two
470mF capacitors and fed to positive
and negative 3-terminal regulators
REG1 & REG2. REG1 then provides
the +5V rail while REG2 provides the
-5V rail.
Construction
All of the parts for the converter
fit on a small double-sided PC board
measuring 76 x 46mm and coded
02109061. This board has rounded
cutouts in each corner, so it will fit
snugly in one of the small plastic UB5
jiffy boxes. The S-video input socket
Fig.7: the PC board is mounted on
the lid using M3 x 10mm machine
screws, nuts and lockwashers.
88 Silicon Chip
CON1 is at one end of the board and
box, while the composite video output
and power sockets (CON2 and CON3)
are at the other end.
Fig.4 shows the assembly details.
Begin by checking the hole sizes for
the three connectors and enlarge
these if necessary. That done, start
the assembly by installing the resistors and the capacitors. Note that
the two 470mF electros are polarised
and must be fitted with their positive
leads towards the left, as shown in the
overlay diagram.
By the way, although the board is
double sided, it may not be supplied
with plated-though holes. In that
case, you must solder the component
leads to both sides of the PC board in
those locations marked with a red dot
on Fig.4. That way, the component
leads themselves make the necessary
connections between the two sides of
the board.
Trimmer capacitor VC1 can go in
next, noting that its flat side goes towards the bottom of the board. Follow
this with the 68mH RF choke (RFC1)
– the PC board can accept either an
axial-lead or “single-ended” choke, so
use whichever set of holes is the most
convenient for the part supplied.
Next, fit diodes D1 and D2, followed
by regulators REG1 and REG2. The latter are both installed with their leads
bent downwards by 90° about 6mm
from their bodies. Slip them into the
positions indicated, then fasten their
metal tabs to the PC board using M3 x
6mm machine screws and nuts before
soldering their leads.
It’s important not solder the leads
before the metal tabs are secured. If
you do, the solder joints could fracture
(or the copper tracks could lift) as the
nuts are tightened. Take care also to
use the correct regulator type in each
position (REG1 is a 7805 type while
REG2 is a 7905).
Now for the dual video op amp (IC1).
This comes in a very small SOIC-8 surface-mount package and is mounted on
the underside of the board – see Fig.5.
Note also that it’s mounted with its
chamfer and notch side towards the
bottom edge of the board.
Because its leads are spaced just
1.25mm apart, you need to take great
care when soldering them to the copper pads. Be sure to use a soldering
iron with a very fine chisel-tip and
make sure the tip is very clean.
Soldering this type of device in
place is also much easier if all its board
pads are lightly tinned first. That done,
place the device and its leads carefully
over the pads and hold it in place with
a toothpick (or “crossover” tweezers)
while you just touch the tip of the iron
to one lead for a second or two, to melt
the solder underneath.
This should then be sufficient to
hold the device in place while you
solder all the other leads to complete
the job.
The board assembly can now be
completed by turning it back over and
fitting the three connectors (CON1CON3).
Final assembly
The PC board fits neatly into a UB5size plastic box but first you have to
drill the various holes in the box and
its lid.
Fig.6 shows the locations and sizes
of these holes. There are only seven
holes in all: four in the lid for the
PC board mounting screws and three
larger holes in the box ends for the
connectors.
Once these holes have been drilled,
mount the PC board on the inside of
siliconchip.com.au
the lid using four M3 x 10mm machine
screws with countersink heads – see
Fig.7. Note that each screw has an M3
star lockwasher fitted to it first, after
which a nut is fitted and tightened to
secure it in position.
In practice, these nuts act as spacers which raise the PC board about
3mm from the lid (which is used here
as the base). Once all the screws are in
position, slip the PC board into place
and secure it with the four remaining
nuts.
Fig.8: this full-size
artwork can be cut
out and attached to
the lid of the case. A
single layer of clear,
wide adhesive tape
will protect it from
damage. Alternatively,
you can download
both this and the PC
board artwork (in
PDF format) from the
SILICON CHIP website.
Notch adjustment
The luminance notch trap can either
be set visually or you can use an RF
signal generator and an oscilloscope.
If you have access to the required
test instruments, simply set the RF
generator to 4.43361875MHz (use a frequency counter to do this if necessary)
and feed its output into the Y signal
input – ie, pin 3 of CON1 (or the junction of the 75W and 680W resistors).
That done, use your scope to monitor
the signal level at the composite video
output of the converter (the centre
pin of CON2) and adjust trimcap VC1
carefully until you see the signal level
0
00
$10 I Z E
P R OL!
PO
drop down into a sharp null.
The correct setting for VC1 is right
at the bottom of that null.
Of course, you won’t be able to set
the notch frequency this accurately if
you don’t have access to test instruments. In that case, you’ll have to set it
visually, with the converter operating
on a suitable S-video signal from your
set-top box or a DVD player.
Try to pick a scene where there is
some cross-colour patterning visible
in the images. It’s then just a matter
of slowly and carefully adjusting VC1
with an alignment tool or jeweller’s
screwdriver until the cross-colour
“nasties” disappear.
Note that you may need to repeat
this procedure a few times, until you’re
confident that you’ve found the correct setting.
Finally, the box can be slipped over
the lid assembly and secured using the
small self-tapping screws provided.
That’s it – you’re now ready to connect
your set-top box or DVD player, etc to
your TV’s composite video input via
SC
your new adapter.
2006 SILICON CHIP
Excellence in Education Technology Awards
CLosing Soon
SILICON CHIP’S Excellence in Education Technology awards carry a prize pool of $10,000. Separate awards
will be made to students of secondary schools throughout Australia and to students of universities and
TAFE colleges throughout Australia.
The secondary school awards have three categories:
AWARD FOR
EXCELLENCE
(a) Best final year assignment of an individual student involving electronics technology.
(b) An award to the school sponsoring the winning individual student.
(c) Best school project involving electronics technology.
The university and TAFE college awards have three categories:
(a)
Best project from a student as part completion of a degree, diploma or certificate in electronics or
a related field (ie, mechatronics).
(b) Best research project from a post-graduate student working in an area of applied electronics.
(c) An award to the university faculty or school sponsoring the best research project.
Entries and judging
The awards will be judged by the editorial staff of SILICON CHIP, convened as a judges panel.
The decisions of the judges will be final. Entry requirements are as follows:
(1) A description of the project in no more than 1000 words.
(2) Full circuit and wiring diagrams, performance plots, etc.
(3) Good quality photographs to show all visual aspects of the project.
(4) Details of software.
Entries for the 2006 awards close on October 16th, 2006. All submissions will be confidential, until the
winners are announced, in the December 2006 issue of SILICON CHIP.
Each award will take the form of a cash prize and a commemorative plaque. All enquiries about these
awards should be directed to the editor via email to: awards<at>siliconchip.com.au
siliconchip.com.au
September 2006 89
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