Download Subject: Video Surveillance Systems Topic: Driving Video Signals

Subject: Video Surveillance Systems
Topic: Driving Video Signals Over Long Distances
Figure 1
Typical Video Surveillance System
Video surveillance systems are all about moving huge amounts of information between two
points as quickly, reliably and as economically as possible. Traditionally, the medium for
connecting these two points was coaxial cable. In the video world, 75 ohm coax cabling, usually
involved RG-59, RG-6 or RG-11. Amazing, RG-59, RG-6, and can transmit signals up to 3.5 GHz
which allows it to demonstrate minimal insertion losses (and interference) at signals below 200
MHz of less than 3 dB for lengths under 30 meters! Coaxial cable is simply an electrical cable
with an inner conductor surrounded by a flexible, tubular insulating layer which is also
surrounded by a tubular conducting shield which is then encased with another insulating layer.
Coaxial cable is used as a transmission line for high frequency signals that require minimal
interference. The one advantage of coax over other types of radio transmission line is that in an
ideal coaxial cable, the electromagnetic field carrying the signal exists only in the space
between the inner and outer conductors. This allows coaxial cable runs to be installed next to
metal objects such as a pipe, without the power losses that occur in other types of transmission
lines. Coax cabling is definitely preferable to most other types of cables (including CAT 5e) in
regards to electrical performance (especially interference), BUT, it is not usually the preferred
choice due cost and size. These day’s engineers are constantly developing new techniques and
ways to use cost (and size) effective cabling such as CAT 5e to run multiple video lines with
minimal distortion and interference (see Figure 2).
Figure 2 Differential CAT5e Video Signal Cable Driver (for CVBS Signals)
Figure 3 Composite Video Signal (CVBS)
Consider the composite video signal (CVBS) of Figure 2. This is a highly complex signal with
multiple levels of analog voltages, time intervals, edge rates, and differential phase and gain
relationships. Of course this signal has multiple frequency components and is highly susceptible
to interference pickup. Coax cable shields this interference, but is too expensive for building
video surveillance systems which can require thousands of meters of cabling AND if the video
format is VGA (which is similar to a CVBS signal but can also include individual Red, Green and
Blue signals) the cabling lengths can run 3X! Now, with simple CAT 5e cabling (with 4 pairs),
VGA format video can be transmitted over long distances inclusive of an encoded horizontal
and vertical sync signals overlaid on the RGB video signals (we will go over an implementation
of this in later issues.)
So let’s look into the issues in regards to driving the video cable that must be taken into account
when designing a video surveillance system and transmitting a signal like the one shown in
Figure 3.
First, setting some performance bounds is necessary when transmitted a highly complex signal
in regards to length the signal can be transmitted and the pixel rate/frequency that sets the
overall video resolution. As was previously discussed, insertion loss is a huge factor and is still
probably the most dominant issue when transmitting single line CVBS with multiple high
frequency components embedded within the signal. Having said this, keep in mind, when
running multi-signal VGA type video (which we will discuss later), insertion loss may not actually
be the dominant issue, and more than likely, data skew between signals (for higher pixel rates)
can set the upper bounds of the system. Above approximately 1MHz, the most dominant issue
for insertion loss (and data skew) is the skin effect of the transmission wire which is directly
proportional to the square root of the frequency. Per ANSI/TIA/EIA-568-B.2 Category 5e
standard, the insertion loss is mathematically modeled and you can approximate the loss for
any CAT 5e cable at any length and at frequencies according to Figure 4. You can use the data
from Figure 4 to estimate the maximum frequency component within your video data as a
function of cable length. Of course this estimate does not include the any errors that can be
introduced by the transmitter and receiver as well. The graph in Figure 4 can be used to
develop a model for a single line CVBS signal, point to point, with zero crosstalk, and no
external interference. Of course crosstalk and interference is very much an issue that must be
addressed.
Figure 4 CAT 5e Attenuation Over Frequency per ANSI/TIA/EIA-568-B.2
In regards to interference and crosstalk, another important issue that we have not discussed is
near-end crosstalk (NEXT). The NEXT is critical when driving multi-line VGA video data as well as
multi-channel CVBS data with the same cable. NEXT increases quickly with frequency and leaves
little room for cable attenuation given required signal-to-noise ratio limits, and therefore the
maximum cable length for a given video run is much shorter that it is for single line
transmission. By the way, cables with individually shielded or foiled twisted pairs (FTP) have
much better performance (see Figure 5) and allow for multi-line video (either VGA or multichannel CVBS) with minimal crosstalk and interference.
Figure 5 NEXT vs. Frequency for UTP (unshielded) and FTP (Foiled) Twisted Pair
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)