Rainbow Electronics MAX9595 - Manual (Page 12)

MAX9595
Video Inputs
All video inputs are AC-coupled with an external 0.1µF
capacitor. Either a clamp or bias circuit sets the DC
input level of the video signals. The clamp circuit posi-
tions the sync tip of the composite (CVBS), the compo-
nent RGB, or the S-video luma signal. If the signal does
not have a sync tip, then the clamp positions the mini-
mum of the signal at the clamp voltage. The bias circuit-
ry is used to position the S-video chroma signal at
midlevel of the luma (Y) signal. On the video inputs that
can receive either a chroma or a red video signal, the
bias or clamp circuit is selected through I
2 C. See Tables
3–12 for loading register details.
The MPEG decoder and VCR uses the RGB format and
fast switching to insert an on-screen display (OSD), usu-
ally text, onto the TV. The MAX9595 supports RGB as an
input from either the VCR or the MPEG decoder and as
an output only to the TV. The red video signal of the
RGB format and the chroma video signal of the S-VHS
format share the same SCART connector pin. Therefore,
RGB and S-video signals cannot be present at the same
time. Loop-through is possible with a composite video
signal but not with RGB signals because the RGB
SCART pins are used for both input and output.
In SCART, there is the possibility of a bidirectional use
of the red/chroma pin. When using the red/chroma pin
as an input port, terminate the red/chroma output with a
75Ω resistor to ground. Thus, a ground state is provid-
ed by an active pulldown to GNDVID on the red/chroma
output to support the bidirectional chroma or red I/O,
turning the output source resistors into terminations
(see Figure 2). The active pulldown also provides the
“Mute Output” function, and disables the deselected
video outputs. The “Mute Output” state is the default
power-on state for video.
For high-quality home video, the MPEG decoder, VCR,
and TV use the S-video format. The MAX9595 supports
S-video signals as an input from the VCR, the MPEG
decoder, and the TV, and also as a separately switch-
able output to the TV and VCR. Because S-video sup-
port was not included in the original specifications of
the SCART connector, the luma (Y) signal of S-video
and the CVBS signal share the same SCART connector
pins. If S-video is present, then a composite signal
must be created from the Y and C signals to drive the
RF_CVBS_OUT pin. For S-video, loop-through is not
possible since the chroma SCART port is used for both
input and output.
The MAX9595 supports composite video (CVBS) for-
mat, with inputs from the VCR, MPEG decoder, and TV.
Full loop-through is possible to the TV and VCR only,
since the MPEG decoder SCART connector has sepa-
rate input and output pins for the CVBS format.
Audio/Video Switch for Dual SCART Connector
12 ______________________________________________________________________________________
CLAMP
N
CLAMP/BIAS
TV_R/C_IN
VIDEO INPUT
TV_R/C_IN
VIDEO INPUT
75Ω
0.1μF 0.1μF
75Ω
PIN 15
PIN 13
PIN 15
PIN 13
MAX9595 MAX9595
TV_R/C_OUT
PULLDOWN
SCART
CABLE
SCART
CONNECTORS
PULLDOWN
CLAMP
N
TV_R/C_OUT
CLAMP/BIAS
Figure 2. Bidirectional SCART Pins