AD8112-EVALZ Analog Devices Inc, AD8112-EVALZ Datasheet - Page 23

• Manufacturer Part Number: AD8112-EVALZ
• Manufacturer: Analog Devices Inc

Specifications of AD8112-EVALZ

• Lead Free Status / Rohs Status: Compliant

MULTICHANNEL VIDEO AND AUDIO
The video specifications of the AD8112 make it an ideal
candidate for creating composite video crosspoint switches.
These can be made quite dense by taking advantage of the
AD8112’s high level of integration 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 AD8112, requiring more than one crosspoint
channel per video channel.
Some systems use twisted pair cables to carry video or audio
signals. These systems utilize differential signals and can lower
costs because they use lower cost cables, connectors, and termi-
nation 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 audio or video signals are differential;
there are positive and negative (or inverted) versions of the
signals. These complementary signals are transmitted through
each of the two cables 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 compose the video or audio channel. Thus, one dif-
ferential video or audio channel is assigned to a pair of crosspoint
channels, both input and output. For a single AD8112, eight
differential video or audio channels can be assigned to the 16
inputs, and four differential video or audio channels can be
assigned to the eight outputs. This effectively forms an 8 × 4
differential crosspoint switch.
Programming such a device requires that inputs and outputs be
programmed in pairs. This information can be deduced through
inspection of the programming format of the AD8112 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 systems such as satellite TV, digital
cable boxes, and higher quality VCRs is called S-video or
Y/C video. This format carries the brightness (luminance or
Y) portion of the video signal on one channel and the color
(chrominance, chroma, or C) portion 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
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format, other aspects of these two systems are the same. Stereo
audio can also be routed in a paired-channel arrangement
similar to a two-channel video system.
There are yet other video formats using three channels to carry
the video information. Video cameras produce RGB (red, green,
blue) directly from the image sensors. RGB is also the usual for-
mat used internally by computers for graphics. RGB can also be
converted to Y, R–Y, B–Y format, sometimes called YUV format.
These three-circuit video standards are referred to as compo-
nent analog video.
The component video standards require three crosspoint channels
per video channel to handle the switching function. 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 systems, such as studio audio or broadcast video, that
handle numerous analog signal channels have strict require-
ments for keeping the various signals from influencing other
signals 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 close proximity in a system,
as undoubtedly is the case in a system that uses the AD8112, the
crosstalk issues can be quite complex. A good understanding of
the nature of crosstalk and some definition of terms is required
in order to specify a system that uses one or more AD8112s.
Types of Crosstalk
Crosstalk can be propagated by one 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 then
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 conductor whose paths is linked.
The undesired induced voltages in these other channels are
crosstalk signals. The channels with crosstalk 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 into 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.
AD8112