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MC44140DW Datasheet PDF : 14 Pages
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FUNCTIONAL DESCRIPTION
The MC44140 has been designed and is intended as a
companion device to the MC440XX decoders. As such the
MC44140 is used as a baseband chroma correction circuit
with PAL, and as the one line delay for SECAM. The device
is also compatible with NTSC color difference signals which
do not require any correction, and has the facility for routing
external R–Y and B–Y signals (e.g., from satellite or from
field store feature systems).
The block diagram for the MC44140 appears at the begin-
ning of this document. The baseband color difference signals
are derived in the MC440XX by demodulation of the chromi-
nance part of the video signal. They are then ac coupled into
the MC44140 and black level clamped. Each of the channels
has a direct path and a path containing a delay of one line
(64 µs). The clamped signals from pins 14 and 15 are taken
from the input and A/D converted to a 8–bit wide digital data
stream; this then passes through shift registers containing
166 cells in order to realize the one line delay. After this, the
data is converted back into analog signals by means of D/A
converters. The clocks for the converters are derived by di-
viding by 6 the frequency obtained from the CHROMA 4
master oscillator, which consists of a crystal running at
17.734475 MHz for PAL and SECAM. Timing of the switches
and clamps in the circuit is achieved by means of a special
sandcastle pulse provided by the MC440XX. Mode selection
is also undertaken by the MC440XX by means of a 4 voltage
level output supplied to pin 5.
Color difference signals provided by the MC440XX may
originate from any one of PAL, SECAM, or NTSC, so the
treatment of these incoming signals is different according to
the mode selected; as determined by the SYSTEM SELECT
level emanating from the MC440XX. Referring to the block
diagram, it is possible to follow the procedure adopted for
each standard. The clamping action may be interpreted in
each case from Figures 1 thru 4.
In the case of PAL signals, black level clamps CL1 and
CL2 are in use every line. Considering only the R–Y channel,
it will be seen that the signal is ac coupled into the circuit at
pin 6 (direct) and pin 15 (delayed). CL1 clamps the input at
pin 15 to the BLACK LEVEL VOLTAGE, this signal then
passes through the delay line. The second CL1 clamp uses
the dc level present after the delay line to clamp the black
level of the direct path. The two signals are then buffered and
averaged together, and the result is switched through to the
output at pin 7.
For NTSC color difference signals, black level clamps
CL1, 2 and 3 are all in use on every line. Again considering
the R–Y channel, the inputs at pins 6 and 15 are both
clamped to BLACK LEVEL VOLTAGE by CL1 and CL3. The
delayed path is now switched out of circuit, however, as this
is not required with NTSC. The direct path only is then routed
to the output at pin 7.
The nature of the SECAM color difference signals is some-
what different from the other standards in that the signal is
only present on every other line as is indicated in Figure 3.
For the R–Y channel, the delayed path input is clamped to
BLACK LEVEL VOLTAGE by CL1 at pin 15. The direct path
signal is clamped by the other CL1 switch to the dc level of
the delayed path (including any offsets) to ensure there is no
difference in clamping level between the two paths. During
the period “SECAM 1”, the B–Y signal is present and this is
clamped to the delayed path dc level. Switch SECAM 1 is
closed during this time and so the direct signal passes
straight to the output. During the next line period (SECAM 2)
there is no direct path signal; now switch SECAM 2 is closed
and switch SECAM 1 is open. Therefore, the delayed path
signal is now switched through to the output. For the R–Y
channel the exact reverse process will occur as in this case
the direct path signal is present during the “SECAM 2” lines.
When EXT R–Y and B–Y signals are used, these are as-
sumed to be always “corrected” from whichever source they
originate. These signals are ac coupled into the circuit at pins
8 and 9 and are switched straight through to the outputs, us-
ing clamps CL4 to set the black level voltage of the two chan-
nels line by line.
SIGNALS SUPPLIED BY MC440XX DECODERS
Sandcastle Pulse
This is a multi–level line repetitive timing pulse input to
pin 4 of the IC. The signal provides timing commands to the
clamp circuits CL1, CL2, CL3, and CL4 and is also neces-
sary for the clock generator to indicate the beginning of ac-
tive signal storage. The pulse train contains a level changing
at half line rate which is used to control the switches SECAM 1
and SECAM 2 when the circuit is operating in the SECAM
mode. Tables 1 and 2 explain the meaning of the different
levels as used with the sandcastle pulse. It should be noted
that “level 1” of the pulse is only used for line by line switch-
ing in SECAM mode.
System Selection Signal
This input may have any one of four different dc voltage
levels and is used to command the functioning of the NTSC,
PAL and NTSC, SECAM 1, SECAM 2 and EXT switches of
the block diagram for the four possible modes of operation.
For the SECAM mode this signal together with the sand-
castle pulse command switches SECAM 1 and SECAM 2.
The significance of the different levels is given in Table 3.
Input Color Difference Signals
The general appearance of the baseband inputs as
derived from the MC440XX with a color bars input, is shown
in Figures 1 thru 4. Each of the color difference signals has
two ac coupled inputs to the MC44140. The line period is
64 µs for 625 line systems and 63.5 µs for 525 line systems.
Whichever line standard is in use, only about 52 µs of active
signal time needs to be stored and delayed for one line peri-
od for processing.
The PAL and NTSC inputs are both present at the same
time on every line and black level is provided during the whole
of the line blanking period (sandcastle periods 2 and 3) to
serve as a reference for the active signal. With SECAM only
one color difference signal is provided on any given line by the
MC440XX, while the other is replaced by a dc level for the
duration of that line period. On the following line the sequence
is then reversed. For the signal provided, black level is sup-
plied during blanking time minus the ident. gate period
(i.e., sandcastle period 3 only).
Output Color Difference Signals
Whatever the origin of the input signals, the two outputs
supplied at pins 7 and 10 are always corrected signals which
are then ac coupled back to the MC440XX. Black level is pro-
MC44140
2–6
MOTOROLA
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