Matrox Solios eA, Установка и справочник по оборудованию, Страница: 74 / 196

74 Chapter 4: Matrox Solios hardware reference
Input voltage level and protection
The various amplification stages on Matrox Solios eA/XA are able to provide a
maximum peak signal of 2.4 V without saturation. Any positive video signal level
greater than this threshold will be distorted, so it is not recommended to feed a
signal above 3 V with termination (6 V unterminated).
Clamping diodes protect video inputs from overvoltage. The diodes clamp (clip)
the inputs if they go under -5 V or above +5 V. When there is overvoltage, the
maximum current flowing must be no more than 0.5A, otherwise damage can
occur to the board.
Selectable inputs
Each acquisition path can acquire data from one of two possible input sources (A
or B); each is 75-Ohm terminated and is capacitor coupled (AC coupling).
AC coupling transmits the varying (AC) characteristics of the signal while blocking
the static (DC) characteristics. This produces a signal that has an average DC level
of 0 Volts regardless of average picture level or DC offset of the incoming signal.
In effect, this ensures that the average surface area of the signal above 0 Volts is
the same as the average surface area below 0 Volts.
AC coupling is implemented using a series capacitor. The capacitor is a first order
high-pass filter with a 4 Hz -3dB cutoff frequency. The high-pass frequency cutoff
is 4 Hz to allow all frequency components of a frame with a 3 kHz line rate to
pass with less than 0.3% attenuation.
DC restoration
After the signal undergoes AC coupling, the signal is DC restored in order to
retrieve the DC level of the original video signal.
To perform DC restoration, the board samples the signal during each scanned line
in the specified region, and vertically shifts the signal so that this region is at 0.
Note that DC restoration requires a reference region in the video signal, usually
the back porch of the signal.