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AUDIO
&VIDEO
Video to S-VHS
Converter
From composite (CVBS) to C/Y
Design by W. Foede
foede.koeln@web.de
Having problems connecting standard video to your S-VHS equipment?
You need this neat circuit. It takes a composite video signal and converts
it into the C/Y input signals used by S-VHS. To simplify construction a PCB
design is supplied.
A standard video signal (also known as Com-
posite Video Baseband Signal or CVBS) con-
tains the black/white and picture synchroni-
sation information together with the colour
information in one signal. S-video signals
(also known as Y/C) are slightly different in
that these two parts of the picture signal are
supplied as separate signals. Y is the lumi-
nance signal containing both black/white and
sync information while C is the chrominance
signal containing colour information.
Equipment using Y/C signals has the
advantage of better picture resolution
(greater bandwidth), less cross-colour inter-
ference and fewer filters for the signals to
squeeze through.
The circuit described here allows a stan-
dard CVBS signal to be connected to the
input of S-VHS equipment. It will however not
improve picture quality. The circuit
requires a +5 V power source sup-
plying approximately 50 mA.
The bandwidth of a composite
video signal extends from 50 Hz up
5 MHz. Colour information is mixed
into the signal on a sub-carrier at
4.43 MHz with a bandwidth of –1.2
to +0.6 MHz. The job of our circuit is
to try to cleanly separate the Y and
C signals from each other (a stan-
dard TV needs to be able to do this
as well). The method chosen here is
one of the simplest. A notch (or
bandstop) filter tuned to the sub-car-
rier frequency is used to remove the C
from the Y information. A disadvan-
tage of this method is that the pic-
ture information suffers a bandwidth
reduction from 5 MHz to around
4 MHz thereby giving poorer picture
resolution. The C signal is produced
by passing the composite signal
through a 4.43 MHz high-pass filter
to remove most of the lower fre-
quency Y components however
some Y remains and causes visible
cross-colour effects on the picture.
Modern TVs achieve this Y/C sep-
aration by using a comb filter, and a
delay line. The advantage is a
greater bandwidth of the Y signal
and no cross-colour effects. S-VHS
video recorders also use this tech-
R1/ L1/ C9
T1
Y
x 1
4,43 MHz
CVBS
4,43 MHz
T2
C
x 1
R2 /C1/ L2
010056- 12
Figure 1. Block diagram showing separation of
the CVBS signal into its Y and C components.
32
Elektor Electronics
9/2001
AUDIO
&VIDEO
was in or out. In this case you could simply
bypass the Y circuitry and just use the com-
posite video signal as the Y signal.
Finally a reality check, as mentioned
before, the quality of the S-VHS compatible
output signal generated here cannot be any
better than the composite video input signal.
To get real S-VHS picture quality, you do of
course need a ‘genuine‘ S-VHS signal source
connected directly to the Y/C input of the
monitor, TV or recorder.
5V
R5
C5
C6
C2
100µ
T1
R1
2k2
100n
4µ7
35V
C8
100n
10V
BC550B
L1
R6
47µH
C7
470µ
10V
C
Y
C9
25p
(010056-1)
K2
K1
CVBS
4
3
R7
SVHS
2
1
T2
R2
2k2
C1
C3
5
33p
100n
C4
BC550B
COMPONENTS LIST
R3
R4
100n
L2
Resistors:
R1,R2 = 2k
47µH
2
R3,R4,R6 = 120
Ω
R5,R7 = 47k
Ω
010056 - 11
Ω
Figure 2. Circuit diagram.
Capacitors:
C1 = 33pF
C2 = 100
µ
F 10V radial
C3,C4,C6,C8 = 100nF ceramic
C5 = 4
µ
F7 35V tantalum bead
C7 = 470
nique so that the Y component of the
recorded signal will have a better
bandwidth.
The circuit block diagram is
shown in
Figure 1
. The C part of the
signal is cleanly filtered out from the
composite signal using an RLC high
pass filter circuit while the Y part
uses an RLC notch or bandstop filter
to remove the C information. Tran-
sistors T1 and T2 act as buffers to
produce low impedance Y and C out-
puts.
The circuit diagram shown in
Fig-
ure 2
closely follows the block dia-
gram layout. R2, C1 and L2 form a
high pass filter to recover the C sig-
nal. The filter output is coupled via
C3 to the base of transistor T2. This
transistor is a common-emitter con-
figuration and buffers the filter while
providing a low impedance output
via C4 to the S-VHS connector.
The bandstop filter used in the Y
path is made up of R1, L1 and C9. L1
is again a standard 47
nals C and Y are output on pins 4
and 3 respectively of the S-VHS con-
nector K2. Two supply decoupling
capacitors C5 and C6 complete the
circuit.
The input and output impedance
of this circuit deviate from the opti-
mum value but were chosen to
ensure that the output signal levels
are correct. At 75
F 10V radial
C9 = 25pF trimmer capacitor (3-25 p)
µ
Inductors:
L1,L2 = 47
µ
H
Semiconductors:
T1,T2 = BC550B
the signal levels
should be 0.3 V
pp
(burst-amplitude)
for C and 1.0 V
pp
for Y.
The layout of the PCB is shown in
Figure 3
. As in all RF circuits it is
important to keep the component
leads as short as possible when fit-
ting them to the PCB. An earth plane
surrounds the tracks so a little extra
care is required when soldering to
ensure that no solder bridges are
formed. In place of a genuine Hosi-
den S-VHS socket we have specified
a standard mini DIN socket for PCB
mounting. It will happily accept a
Hosiden plug as long as the small
plastic stop pins are first removed
from the plug.
During set-up, trimmer C9 should
be adjusted to produce minimum
Moiré (colour patterning) effect on
the picture. The circuit was success-
fully tested on many different S-VHS
inputs. Generally the bandstop filter
in the Y path was necessary but
with a WinTV card it did not make
any noticeable difference if the filter
Ω
Miscellaneous:
K1 = Cinch socket, PCB mount, e.g., T-
709G (Monacor/Monarch)
K2 = 6-way Mini-DIN socket, PCB mount,
angled pins
H15
010056-1
(C) ELEKTOR
H fixed
inductor while C9 is an adjustable
trimmer, this allows some optimisa-
tion of the picture. The Y signal con-
tains frequency components going
down to 50 Hz so the signal coupling
capacitors C2 and C7 need to have a
larger value than in the C signal
path, otherwise low frequency parts
of the signal would be attenuated.
The ac-coupled low impedance sig-
µ
T1
T2
OUT1
H17
Figure 3. PCB layout. The groundplane is typical
of an RF layout (board not available ready-
made).
9/2001
Elektor Electronics
33
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