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64 Chapter 4: Matrox Helios hardware reference
signal up to 2.4 V in amplitude (active video) without distortion. If the active
input video (excluding the sync pulse) is greater than 1.2 V in amplitude, the
attenuator should be enabled.
The maximum gain factors and the recommended video signal amplitude are as
follows:
2:1 attenuator
Maximum gain
Off (bypassed)
4
On (active)
2
There are three ways to program the reference levels and the attenuator. You can
specify the actual black and white voltage levels of your input signal and have the
software calculate appropriate values for each element in the offset-gain topology.
You can have the software emulate an A/D with programmable black and white
references and specify the levels as a percentage of their possible values.
Alternatively, you can specify the required values for the offset and the gain
controls. If the signal amplitude is specified as greater than 1.1 V, the attenuator
will be enabled automatically. In all cases, you use the MIL-Lite function
MdigReference() or MdigControl().
Low-pass filter
The low-pass filtering stage is used to limit high frequency noise and aliasing effects
at the input of the A/D converter. Each acquisition path has two filters. The filters
used on Matrox Helios are 4th order Butterworth filters. The first has a -3 dB
cutoff frequency of 40 MHz. The second filter has a -3 dB cutoff frequency of 7.5
MHz, useful for RS-170 and CCIR video sources. All filters provide 80 dB/decade
or 24 dB/octave of attenuation for frequencies above the cutoff value. Enable the
low-pass filters using the MIL-Lite function MdigControl() with
M_INPUT_FILTER.
10-bit A/D converters
Matrox Helios eA/XA uses high-quality, high-speed, 10-bit analog-to-digital
(A/Ds) converters to sample the video signal. Therefore, Matrox Helios eA/XA
provides excellent digitization quality, even at the highest sampling frequency. The
A/D converters also feature a wideband input section, allowing full use of their
sampling speed.
Max recommended video signal
amplitude (terminated)
1.2 V
2.4 V
Min recommended video
signal amplitude
0.25 V
0.5 V
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