This is only a preview of the October 1996 issue of Silicon Chip. You can view 24 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Send Video Signals Over Twister Pair Cable":
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Send video signals
over twisted pair cable
Use this Video Transmitter and Video
Receiver to wire your home or business
with remote video for entertainment or
for CCTV security systems.
By JOHN CLARKE
What’s a twisted pair cable? For all
intents and purposes, it is equivalent
to a pair of telephone wires and you
can’t send video over telephone wires,
can you? Well, with this new chipset
you could, although that is not why
we are presenting this article.
With the recent introduction of
12 Silicon Chip
low-cost monochrome video cameras,
(less than $200 retail) and this project,
you can now easily monitor your front
door, your swimming pool or any other
part of your home that needs watching
from a central location.
Video signals are normally sent
through 75Ω coax cable and for short
runs over several tens of metres there
is very little loss in signal level. But
over long distances the losses in the
cable reduce the signal to an unacceptably low level for the receiving
equipment.
One way of overcoming the signal
loss is to modulate it onto a carrier in
the UHF or VHF range. Any signal loss
in the cable can then be made good
by wideband distribution amplifiers.
However, at the receiving end, the
signal must be demodulated before it
can be displayed on a monitor.
This approach is well-proven but
coax cable and distribution amplifiers
are expensive. Wouldn’t it be nice to be
able to send video over ordinary twist
ed wires? Using the video transmitter
Fig.1: the general arrangement of the MAX435 & MAX436 ICs. The MAX435 has a differential
output while the MAX436 only has a single ended output. These are transconductance amplifiers
so the outputs produce a current that’s proportional to the applied differential input voltage.
Features
•
•
1.5km range (expected)
Video transmitted over low cost
twisted pair
•
Audio transmitted in stereo over
50m using op amp transmitter
•
Up to 1.5km range for mono
audio using video transmitter
and receiver described here, video can
be sent over distances up to 1.5km.
Transconductance amplifiers
The heart of this project is a pair of
ICs made by Maxim Integrated Products, the MAX435 and MAX436. These
two ICs are classified as high speed,
wideband transconductance amplifiers (WTAs) with true differential,
high impedance inputs. The unique
architecture of these amplifiers provides accurate gain without negative
feedback. Without the feedback, the
possibility of spurious oscillation is
virtually eliminated.
Fig.1 shows the general arrangement of the ICs. The MAX435 has a
differential output while the MAX436
only has a single ended output. The
outputs produce a current that’s proportional to the applied differential
input voltage, providing inherent
short circuit protection for the outputs. The circuit gain is set by the
ratio of the output impedance “RL”,
the user connected transconductance
network “ZT” and an internally set
current gain factor, K. In the case of
the MAX435, the current gain is nomi
nally 4 (±2.5) and for the MAX436 this
figure is 8 (±2.5).
Inside the video transmitter. The top view shows the audio board “hinged” back
to reveal the video board, which sits on the bottom of the case. The view above
shows the audio board in place.
The MAX435 has a 275MHz bandwidth and 800V/µs slew rate, while
the MAX436 has a 200MHz bandwidth
and 850V/µs slew rate. The common
mode rejection ratio for both is -53dB
at 10MHz and -90dB at DC.
While MAX ICs could also be used
to transmit audio signals, we have
taken a cheaper approach and used
dual op amps to produce a balanced
October 1996 13
Fig.2: the transmitter circuit takes composite video and provides a balanced
output to the twisted pair. The unbalanced audio signals are converted to
balanced outputs by the LM833 op amps.
audio transmitter and receiver. These
produce high quality stereo results
over short runs of less than 50m. This
will allow you to pipe composite video
and stereo audio signals around your
house. For many applications though,
we envisage that the video transmitter
and receiver boards will be all that are
required.
Transmitter circuit
Fig.2 shows the transmitter circuit.
Unbalanced video signal is applied
to the IN+ input of the MAX435. The
inverting input IN- is connected to
ground. The transconductance element impedance between pins 3 and
14 Silicon Chip
5 is set at 220Ω, while the output impedance is a nominal 50Ω. The 4.7kΩ
resistor sets the supply current for the
IC. Supply decoupling for the ±5V
rails, provided by the 0.1µF capacitors,
is necessary for best performance at the
high frequencies involved.
Power is derived from a 12VAC
plugpack. This is rectified with halfwave rectifier diodes D1 and D2 to
supply the ± rails before regulation.
The 470µF capacitors filter the raw DC
to produce a relatively smooth voltage.
REG1 and REG2 regulate the supplies
down to ±5V for IC1.
Audio input is applied to a single
ended to balanced output amplifier
comprising IC2a and IC2b for the
left channel and IC3a and IC3b for
the right. IC2a is a unity gain buffer
which is non-inverting. The output at
pin 1 is therefore the + output which
drives the positive twisted pair line
via a 680Ω resistor. IC2b is connected as an inverting amplifier and the
resulting output at pin 7 drives the
negative twisted pair line via its 680Ω
resistor. The 22pF capacitor across the
feedback resistor of IC2b prevents high
frequency oscillation.
The right channel audio amplifier
operates similarly to the left channel
circuit.
Receiver circuit
Fig.3 shows the receiver circuit.
IC4 is a MAX436 which accepts the
Fig.3: the receiver circuit uses a MAX436 to convert the balanced input from the twisted
pair to an unbalanced video output. Similarly, the balanced audio signals are converted
to single-ended signals by op amps IC5 and IC6.
balanced input from the twisted pair
and produces an unbalanced output.
The 51Ω resistors at the IN+ and INinputs at pins 2 and 6 provide the
correct loading for the twisted pair
line. A 100Ω resistor in series with
trimpot VR1 is connected between
pins 3 and 2.
VR1 sets the gain of IC4, to compensate for losses in the twisted pair
line. A second 100Ω resistor from pin
3 is connected in series with a 56pF
capacitor and VC1. The capacitance
corrects for the loss of high frequency
signal through the line. In practice the
capacitance is adjusted until the colour burst signal is at its correct level.
The 4.7kΩ resistor at pin 11 sets the
current for IC4.
The power supply circuit is iden-
Specifications
Video transmitter/receiver pair
Frequency response ����������������� typically -3dB at 200MHz
Common mode rejection ����������� typically -53dB at 10MHz; -90dB at DC
Audio transmitter/receiver pair
Signal to noise ratio ������������������ -102dB unweighted (20Hz to 20kHz) with
respect to 1V RMS
Common mode rejection ����������� -62dB at 50Hz and 1kHz
Harmonic distortion ������������������� less than .016% from 20Hz to 20kHz
Frequency response ������������������ -0.25dB at 20Hz and 20kHz
Clipping level ���������������������������� 1.7V RMS at input
Crosstalk ����������������������������������� -80dB (20Hz to 20kHz) with 20m twisted
pair alongside video pair
October 1996 15
Fig.4: the component
overlays and wiring details
for the transmitter boards.
tical to that used in the transmitter.
The audio signal is converted from
the balanced twisted pair signal to an
unbalanced output using op amps IC5a
& IC5b for the left channel and IC6a &
IC6b for the right channel.
The balanced signal is applied to
the non-inverting inputs of IC5a and
IC5b. The 330Ω resistors tie the inputs
to ground and provide a load for the
twisted pair line. A .001µF capacitor
is included across the input terminals
to remove high frequency noise from
16 Silicon Chip
the line. Both IC5a and IC5b are set for
a gain of two due to the 1kΩ feedback
resistors.
Signals which are common to each
input are rejected at the output and
this is due to the feedback for IC5a
being connected to the output of IC5b.
Difference signals are amplified at the
pin 1 output of IC5a. The 100Ω output
resistor prevents oscillation in IC5a
due to capacitive loading.
The right channel audio amplifier
operates in exactly the same manner
as the left channel audio amplifier.
Construction
The Video Transmitter and Video
Receiver are housed in separate plastic
cases measuring 130 x 68 x 42mm. The
video transmitter (using the MAX435)
is built onto a PC board measuring
60 x 102mm (coded 02306961). The
audio transmitter PC board (coded
023069623) is piggy-backed onto the
video transmitter.
The two receiver PC boards (coded
PARTS LIST
Video transmitter board
1 PC board, code 02306961, 60
x 102mm
1 plastic utility case, 130 x 68 x
42mm
1 self-adhesive front-panel label,
62 x 126mm
1 12VAC 500mA plugpack
1 SPDT toggle switch (S1)
2 3mm screws and nuts
1 DC socket
2 panel-mount RCA sockets
8 PC stakes
1 40mm length of 0.8mm tinned
copper wire
Semiconductors
1 MAX435CPD high-speed
transconductance amplifier
(IC1)
1 7805 3-terminal regulator
(REG1)
1 7905 3-terminal regulator
(REG2)
2 1N4004 1A silicon diodes
(D1,D2)
Video Receiver board
1 PC board, code 02306962, 60
x 102mm
1 plastic case, 130 x 68 x 42mm
1 front panel label, 62 x 126mm
1 12VAC 500mA plugpack
1 SPDT toggle switch (S2)
2 3mm screws and nuts
1 DC socket
2 panel-mount RCA sockets
8 PC stakes
1 40mm length of 0.8mm tinned
copper wire
1 500Ω horizontal trimpot (VR1)
Semiconductors
1 MAX436CPD high-speed
transconductance amplifier
(IC4)
1 7805 3-terminal regulator
(REG3)
1 7905 3-terminal regulator
(REG4)
2 1N4004 1A diodes (D3,D4)
Capacitors
2 470µF 16VW PC electrolytic
2 10µF 16VW PC electrolytic
3 0.1µF ceramic
Capacitors
2 470µF 16VW PC electrolytic
2 10µF 16VW PC electrolytic
3 0.1µF ceramic
1 56pF ceramic
1 3-60pF trimmer (optional)
Resistors (0.25W 1%)
1 4.7kΩ
1 75Ω
1 220Ω
2 51Ω
Resistors (0.25W, 1%)
1 4.7kΩ
1 75Ω
2 100Ω
2 51Ω
Audio transmitter board
Audio receiver board
1 PC board, code 02306964, 60
x 102mm
4 12mm spacers
4 6mm spacers
4 20mm x 3mm screws
4 3mm nuts
11 PC stakes
1 20mm length of 0.8mm tinned
copper wire
1 PC board, code 02306963, 60 x
102mm
4 12mm spacers
4 6mm spacers
4 20mm x 3mm screws
4 3mm nuts
11 PC stakes
1 20mm length of 0.8mm tinned
copper wire
Semiconductors
2 LM833 op amps (IC2,IC3)
Semiconductors
2 TL072 op amps (IC5,IC6)
Capacitors
4 10µF 16VW PC electrolytic
2 22pF ceramic
Capacitors
4 10µF 16VW PC electrolytic
1 .001mF MKT polyester
Resistors (0.25W, 1%)
6 x 10kΩ
2 330Ω
4 680Ω
Resistors (0.25W, 1%)
8 x 10kΩ
2 100Ω
4 330Ω
YOU CAN
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SYSTEM
SATELLITE ENTHUSIASTS
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1.8m solid ground mount dish
20°K LNBF
25m coaxial cable
easy set up instructions
regular customer newsletters
BEWARE OF IMITATORS
Direct Importer: AV-COMM PTY. LTD.
PO BOX 225, Balgowlah NSW 2093
Tel: (02) 9949 7417 / 9948 2667
Fax: (02) 9949 7095
VISIT OUR INTERNET SITE http://www.avcomm.com.au
YES GARRY, please send me more
information on international band
satellite systems.
Name: __________________________________
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____________________P'code:
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October 1996 17
Fig.5: the component
overlays and wiring details
for the receiver boards.
TABLE 1: RESISTOR COLOUR CODES
❏
No.
❏ 6
❏ 2
❏ 8
❏ 4
❏ 6
❏ 2
❏ 2
❏ 4
18 Silicon Chip
Value
10kΩ
4.7kΩ
1kΩ
680Ω
330Ω
100Ω
75Ω
51Ω
4-Band Code (1%)
brown black orange brown
yellow violet red brown
brown black red brown
blue grey brown brown
orange orange brown brown
brown black brown brown
violet green black brown
green brown black brown
5-Band Code (1%)
brown black black red brown
yellow violet black brown brown
brown black black brown brown
blue grey black black brown
orange orange black black brown
brown black black black brown
violet green black gold brown
green brown black gold brown
02306962 and 02306964) are mounted
in a similar fashion to the transmitter.
On both cases, RCA sockets for video
input and output are mounted at the
sides of the box while the audio sockets are at one end of the box. The DC
socket is mounted at the opposite end
while the power switch is attached
to the lid.
Fig.4 shows the component overlay
and wiring details for the transmitter
boards while Fig.5 shows the wiring
details for the receiver boards.
Begin construction by checking
each PC board for breaks or shorts in
the copper pattern or any undrilled
holes. Fix any defects before proceeding further, then insert all the PC
stakes. These are located at the signal
input and output wiring points and
for power supply.
Next, solder in all the links and
resistors. Table 1 shows the resistor
colour codes and it is a good idea to
check each resistor value with your
multimeter before soldering it into the
board. The capacitors can be mounted
next, noting that the electrolytic types
must be oriented with the correct
polarity as shown. Finally, insert the
ICs, making sure that each one has the
correct orientation.
If you do not intend to use stereo
audio channels, IC3 on the audio transmitter board and IC6 on the receiver
board can be omitted.
The voltage regulators are each
mounted horizontally on the PC board
and secured with a screw and nut.
Bend the leads for each component
before insertion into the PC board
holes. Take care to orient the diodes
correctly.
Drill holes in the plastic cases for the
RCA and DC sockets using the front
panel as a guide to their location. A
hole is also required in the lid for the
power switch on each box.
Wire up the PC boards as shown in
Fig.4 and Fig.5. In each case, the audio
board is stacked on top of the video
board and the two are separated by
metal spacers. The integral side pillars
in each box will need to be removed
so that the PC board assembly can fit
comfortably within the case. Affix the
label to each lid and attach the power
switches.
Testing
The completed units are now ready
for testing. Apply power to the transmitter PC boards and check voltages.
These two photographs show the completed boards inside the receiver case. The
top view shows the video receiver board, while above is the audio board.
Fig.6: the top trace of the oscilloscope display shows a PAL colour bar as
the video source signal. Note the colour burst at the far lefthand side of the
trace. The lower trace shows the received signal after transmission over
20m of twisted pair. The gain has been compensated for signal loss and for
video colour burst level. Note the faithful reproduction of the signal.
October 1996 19
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20 Silicon Chip
ON
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VIDEO
TRANSMITTER
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R
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VIDEO OUT
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R
BALANCED AUDIO OUT
24/31 Wentworth St, Greenacre 2190
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VIDEO IN
Fig.7: actual size front panel & PC board artworks for the transmitter. Check
your PC boards carefully before installing any of the parts.
Connect the negative multimeter lead
to GND and touch the positive lead
on pins 1, 12 & 14 of IC1 where +5V
should be present in each case. Similarly, there should be -5V on pins 7, 8
& 10 of IC1. IC2 and IC3 should have
+5V on pin 8 and -5V on pin 4.
For the receiver PC boards, there
should be +5V on pins 1, 12 & 14 of
IC4 and -5V on pins 7, 8 & 10. IC5 and
IC6 should have +5V on pin 8 and -5V
on pin 4.
To transmit video you will require
one twisted pair while each audio
channel will require a separate twisted
pair. Use 75Ω coax cable from your
video source to the transmitter and
from the receiver to the video input of
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Fig.8: actual size front panel & PC board artworks for the receiver. All PC
boards measure 60 x 102mm.
your monitor or VCR. Audio connec
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shielded audio cable.
VR1 on the receiver is adjusted to
obtain best black and white levels as
seen on the monitor. The 56pF capacitor on IC4 should be satisfactory for
twisted pair up to 50m. A larger capacitor value can be used if the colour
burst signal is marginal. This can be
adjusted until the monitor provides a
solid colour picture.
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October 1996 21
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