Matrox Helios Series Installation And Hardware Reference page 62

62 Chapter 4: Matrox Helios hardware reference
Input voltage level and protection
The various amplification stages on Matrox Helios eA/XA are able to provide a
maximum peak signal of 3 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 0.5 A, 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. The signal coupling of each input can be either
capacitor coupled (AC coupling) or directly coupled (DC coupling), single ended.
A multiplexer allows the input source selection and the AC/DC coupling selection.
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 0.8 Hz -3dB cutoff frequency. The high-pass frequency
cutoff is 0.8 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 or DC coupling, the signal can be DC
restored. While AC-coupled signals require DC restoration to retrieve the DC
level of the original video signal, DC restoration can be useful for DC-coupled
signals to compensate for any DC offset (although not required). You can enable
or disable DC restoration independently of the input coupling selection.
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 either
0 or 1 V, depending on which of the two restoration levels has been specified. If
the synchronization pulse is negative with non-inverted video, you should set the