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Switch no 4:
When this switch is on, 525- line standards are assumed to have no
pedestal.
Switch no 3:
Set this switch on to disable the insertion of EDH information in the
output signal.
Switch no 2:
Set this switch on to blank VBI and off to pass VBI. Passing VBI is the
normal mode.
Switch no 1:
Set this switch on to select remote control via BO/S and off for control
via the local front switches on the PCB.
Front Switches (SW2)
Switch no 7:
Set this switch on to select complementary decoding, off to select anti
diff phase decoding.
Switch no 6:
Decoding adaption selector. See Notes below
Switch no 5:
Decoding adaption selector. See Notes below
Within SW2, switches 5 and 6 select the different modes of decoding adaptation
available from the DEC-6002 card. Switch 5 acts as the least significant bit and switch 6
as the most significant bit. So 00 (off,off) is decoding mode 0 and 10 (on,off) is decoding
method 2. Some further explanation follows.
Comb-Filter Decoding - Adaption modes
The DEC-6002 comb-filter decoder applies a 3 line comb-filter aperture where-ever the
three lines contain consistent colour information. By compensating for the change in
subcarrier phase over these three lines it is possible to combine the signals in such a
way that the luminance information is separated from the chrominance. This allows the
full luminance bandwidth to be recovered. Where the three lines do not contain
consistent colour information it is no longer appropriate to combine them in this way. The
decoder must ADAPT into an alternative decoding mode. The four adaption modes
affect the methods used to detect vertical changes in colour (vertical chrominance
transitions).
Luminance information close to the subcarrier frequency may change phase over the
three lines. This is mathematically identical to a signal which has resulted from a colour
change over the three lines. Where this signal has resulted from high frequency
luminance information comb-filtering is appropriate. However if a colour change has
produced the signal the decoder should adapt to an alternative decoding mode. As a
result there is a trade-off between detecting chrominance transitions and incorrectly
switching out the comb-filter in the presence of high frequency luminance.
Adaption mode 3 (maximum adaption) is the least likely to miss a vertical chrominance
transition. It will produce minimum dot crawl in the decoded picture.
At the other extreme adaption mode 0 is the least likely to incorrectly adapt in areas of
the picture which contain luminance detail. It will produce higher levels of dot crawl but
will offer some increase in luminance detail.
6
Document Number: DEC-6002UM2.doc Issue 2
24th March 1998
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