Download Subject: Video Surveillance Systems Topic: VGA Video Cable Driver

Subject: Video Surveillance Systems
Topic: VGA Video Cable Driver/Receiver (Part 2) (Continued from Part 1 from last
week)
The ability to transmit and receive video over a local network is useful for
security/surveillance and in-house video entertainment systems. Due to its low cost
and predictable electrical characteristics, standard un-shielded CAT5e twisted-pair
cable is a good choice for long-distance transmission of electrical information. CAT5e
usually consists of 4 separate twisted-pair channels, usually 24 AWG wire. For a
video system, this allows 3 channels to carry RGB video information while allowing
an additional channel to carry audio and/or control information. A typical video
graphics interface uses the standard VGA video format, originally designed to drive a
640x480 pixel array. Advances in technology have allowed for higher and higher
resolution formats. The VGA type interface consists of three video channels and
two timing or synchronization channels. The 3 video channels convey separate true
RED, GREEN and BLUE information. The timing signals are for the standard horizontal
(Hsync) and vertical (Vsync) pulses.
Twisted-pair transmission allows for differential signaling. This provides better
quality video by allowing a larger signal swing and suppression of even-ordered
harmonic distortion. Single-ended RGB video information is first converted to a
differential signal before transmission. At the receiver, the signal is then converted
back to single ended in order to drive a standard video monitor. To run the 5
separate signal information over three channels, the digital Hsync and Vsync
information is encoded onto the common-mode level of the three video channels.
This is done in a manner in which the encoded levels cancel EMI radiation which may
corrupt the video signal. Common-mode levels are then detected at the receiver and
de-coded into the original Hsync and Vsync information.
Vertical and Horizontal sync pulses are encoded into three separate common-mode
levels to drive each of the channels using the weighting functions:
RED CM = K*(Vsync-Hsync) + Vcm
GREEN CM = K*(-2Vsync) + Vcm
BLUE CM= K*(Vsync+Hsync) + Vcm
Where Vcm is the output common-mode voltage.
Vsync and Hsync are expected to be standard logical levels. The K factor limits the
logic swing of the Vsync and Hsync allow for a +-0.5V common-mode range. This
creates the following weighting table for the 4 possible combinations. As can be seen
from the table, all four combinations create the same levels of 3.0, 2.0 and 2.5
among the various channels. This allows for cancellation of EMI interference
between channels during switching.
Table 1 – Common-mode levels for encoded Hsync and Vsync
For long distance transmission, the cables electrical characteristics will attenuate
higher frequency components of the signal. For good quality video, this must be
corrected for at the receiver by incorporating a frequency response equalization
function to selectively boost higher frequency components back to their original
values.
Another effect of long distance cabling is drop in DC gain due to the finite resistance
of the cable. This will affect contrast levels of the reproduced video picture.
Plot 1 below shows typical CAT5 attenuation characteristics over various cable
lengths.
Plot 1 – Cat5 Cable Attenuation Characteristics
The CADEKA solution addresses all of these issues in a low cost, effective circuit
consisting of both a Transmitter (Tx) (see last weeks article) and a Receiver (Rx)
board (following). For computer graphics applications, it will handle SVGA (800x600)
and XVGA(1280x720) formats.
RECEIVER BOARD
Figure 5 shows the schematic of the receiver board. It provides the following
functions:
1. Provides cable termination.
2. Performs differential to single conversion and output drive capability.
3. Allows cable equalization from 0M to 300M.
4. Receives and decodes Hsync and Vsync from input common-mode voltage.
5. Regulated Supply Voltage.
6. Generates required DC bias voltages.
7. 50Ω, DC coupled inputs. 75Ω, DC coupled outputs.
8. ESD protection on inputs and outputs.
Fig.5 -Receiver Board Schematic
Receiver Board Specifications
Receiver Channel Schematics
Fig.6 – Receiver Channel
All three receiver channels are identical, as shown in figure 6.
Resistors R1 and R2 provide a high impedance common-mode sense point for
extraction of the common-mode sync signals. An additional network (shown in figure
5) sums the RED and BLUE common-mode for comparison against the GREEN for
Vsync extraction. Resistors R3 to R6 along with capacitor C3 form the cable
termination network. It allows low DC loading while providing proper cable
termination at higher frequencies. One of the two CLC2000 amplifiers performs
differential to single ended conversion, while the second performs both the DC boost
and high frequency equalization function.
Ability to adjust both DC gain and frequency equalization for different CAT5e cable
lengths is incorporated through a binary switch mechanism. Three DIP switches
allow for 8 different settings to allow for cable lengths ranging from 0M to 300M.
This allows for a single, digital control which will adjust for both DC (contrast) and
high frequency equalization for varying cable lengths.
Fine tuning adjustment of individual channels is available with an analog
potentiometer. Through a combination of coarse adjustment using the DIP switch
settings and fine adjustment using the channel potentiomenter, each individual
channel can be tuned to a gain flatness within ± 1.5dB from DC to 16MHz. In most
cases however, the fine tune potentiometers can be replaced with fixed value
resistors and cable adjustment by the DIP switches will provide adequate quality
video.
Table 2 below provides suggested DIP switch settings for cable lengths ranging from
0M to 300M in 25M increments. Data is provided for both 10Mhz and 18Mhz
bandwidth which covers requirements for both SVGA and XVGA formats.
Output series resistors RO are set to 75 ohms for proper driving a doubly-terminated
video load.
Table 2 – Suggested DIP switch settings for various cable lengths.
Fig.7 - Receive Support Circuits
Figure 7 shows the receiver support circuitry. It uses 3 low-cost CLC2005 dual
operational amplifiers for required functions and to drive the required output logic
levels.
Three amplifiers provide 2X gain equalization for the RED, BLUE and the RED+BLUE
signals. GREEN common-mode levels are already 2X of the RED and BLUE, so no
further equalization is required at 300M cable length. Two amplifiers extract and
drive the Hsync and Vsync signals from these signals. To allow for single supply
operation, a low impedance common-mode reference level is needed to drive all
three channels. The reference generates a mid-supply from a filtered resistor divider
placed between the regulated power rails.
When designing a high performance video surveillance system, it is important to
simply break down the system into the various functional blocks that make up the
system and address each performance limiting factor. Depending on the overall
system specification, such as the video data transmission rate and resolution, these
numbers will determine many of the required analog performance specifications of
the system including simple layout geometries, amplifier bandwidths, slew rates, and
required gains. The number one passive component within a high performance video
surveillance system is the cable and understanding the positive and adverse effects
of this single component will greatly enhance your ability to design the system.
Kai ge from CADEKA
(www.cadeka.com