PCVisionplus
Hardware Reference
Theory of Operation
2–15
Rev 02; February 8, 2002
75 ohm
Video Signal
1.428Vpeak
CCIR/RS–170
0.714 V
75 ohm cable
ADC
Video Input
termination
Gain
Figure 2–14. Input Voltage
2.5.3 Low-Pass Filters
Following the gain stage, the video passes through a selectable low-pass filter, optimized for standard video frequen-
cies. The filter values are 6.5 MHz and 12.5 MHz (no bypass or disable). The low-pass filter strips high frequency
signal content from the incoming video signal, to avoid sampling aliasing artifacts in the signal.
Standard video (RS170, CCIR, NTSC, PAL) has useful frequency content up to approximately 6.5 MHz. Frequen-
cies above this can be eliminated using the low-pass filters. Sampling rates for standard video are 10 MHz up to
14 MHz. If frequencies at or above the sampling rate are present they represent noise rather than useful video. These
frequencies can “alias” into the real video signal causing corruption. The low-pass filters can eliminate any high
frequency signal content before digitization. Figure 2–15 illustrates the 6.5 MHz filter response.
1MHz
6.5
10
20 MHz
1 MHz
6.5
10 MHz
+
=
0 0B
–3 dB
0 dB
–3 dB
Frequency Content of incoming Video
Low Pass Filter Response
Content of Filtered Video
High frequency noise
Useful video
High frequency noise removed
by Low Pass Filter
1 MHz
6.5
10 MHz
0 dB
–3 dB
Figure 2–15. Low-pass Filtering
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