Data Sheet ADV3205
Rev. 0 | Page 17 of 20
The inputs are individually assigned to each of the 32 inputs of
the two devices, and the shunt 75 terminations are placed at
the end of the transmission lines. The outputs are wire-OR’ed
together in pairs. Only enable one of the outputs from a wire-
ORed pair at any given time. The device programming software
must be properly written to achieve this.
MULTICHANNEL VIDEO
The good video specifications of the ADV3205 make it an ideal
candidate for creating composite video crosspoint switches. These
switches can be made quite dense by taking advantage of the
high level of integration of the ADV3205 and the fact that
composite video requires only one crosspoint channel per system
video channel. There are, however, other video formats that can
be routed with the ADV3205, requiring more than one crosspoint
channel per video channel.
Some systems use twisted pair wiring to carry video signals.
These systems use differential signals and can lower costs
because they use lower cost cables, connectors, and termination
methods. They also have the ability to lower crosstalk and reject
common-mode signals, which can be important for equipment
that operates in noisy environments, or where common-mode
voltages are present between transmitting and receiving equipment.
In such systems, the video signals are differential; there are positive
and negative (or inverted) versions of the signals. These
complementary signals are transmitted onto each of the two
wires of the twisted pair, yielding a first-order zero common-
mode voltage. At the receive end, the signals are differentially
received and converted back into a single-ended signal.
When switching these differential signals, two channels are
required in the switching element to handle the two differential
signals that make up the video channel. Thus, one differential
video channel is assigned to a pair of crosspoint channels, both
input and output. For a single ADV3205, eight differential video
channels can be assigned to the 16 inputs and 16 outputs. This
effectively forms an 8 × 8 differential crosspoint switch.
Programming such a device requires that the inputs and outputs
be programmed in pairs. This information can be deduced through
inspection of the programming format of the ADV3205 and the
requirements of the system.
There are other analog video formats requiring more than one
analog circuit per video channel. One two-circuit format that is
commonly being used in video systems is S-Video or Y/C Video.
The Y/C Video format carries the brightness (luminance or Y)
portion of the video signal on one channel and the color
(chrominance, chroma, or C) on a second channel.
Because S-Video also uses two separate circuits for one video
channel, creating a crosspoint system requires assigning one
video channel to two crosspoint channels, as in the case of a
differential video system. Aside from the nature of the video
format, other aspects of these two systems are the same.
There are yet other video formats using three channels to carry
the video information. Video cameras produce RGB (red, green,
and blue) directly from the image sensors. RGB is also the usual
format used by computers internally for graphics. RGB can also
be converted to Y, R–Y, and B–Y format, sometimes called YUV
format. These three circuit video standards are referred to as
analog component video.
The analog component video standards require three crosspoint
channels per video channel to handle the switching function. In
a fashion similar to the two circuit video formats, the inputs and
outputs are assigned in groups of three, and the appropriate logic
programming is performed to route the video signals.
CROSSTALK
Many video systems have strict requirements for keeping the
various signals from influencing any of the others in the system.
Crosstalk is the term used to describe the coupling of the signals of
other nearby channels to a given channel.
When there are many signals in proximity in a system, as is the
case in a system that uses the ADV3205, the crosstalk issues can
be quite complex. A good understanding of the nature of crosstalk
and some definition of terms is required to specify a system that
uses one or more ADV3205 devices.
Types of Crosstalk
Crosstalk can be propagated by means of any of three methods.
These fall into the categories of electric field, magnetic field, and
sharing of common impedances. This section explains these effects.
Every conductor can be both a radiator of electric fields and a
receiver of electric fields. The electric field crosstalk mechanism
occurs when the electric field created by the transmitter propagates
across a stray capacitance (for example, free space) and couples
with the receiver and induces a voltage. This voltage is an unwanted
crosstalk signal in any channel that receives it.
Currents flowing in conductors create magnetic fields that circulate
around the currents. These magnetic fields then generate voltages
in any other conductors with whose paths they link. The undesired
induced voltages in these other channels are crosstalk signals.
The channels that crosstalk can be said to have a mutual inductance
that couples signals from one channel to another.
The power supplies, grounds, and other signal return paths of a
multichannel system are generally shared by the various channels.
When a current from one channel flows in one of these paths, a
voltage that is developed across the impedance becomes an input
crosstalk signal for other channels that share the common
impedance.
All these sources of crosstalk are vector quantities; therefore, the
magnitudes cannot simply be added together to obtain the total
crosstalk. In fact, there are conditions where driving additional
circuits in parallel in a given configuration can actually reduce
the crosstalk.
