Rainbow Electronics MAX9656 - Manual (Page 9)

MAX9655/MAX9656
Low-Power Video Switches
for Dual SCART Connectors
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When the MAX9656 is in shutdown, its inputs are
biased at the same voltage and present the same input
resistance as unselected inputs.
Video Filter
The MAX9655/MAX9656 video filter features ±1dB
passband out to 9.5MHz and 47dB attenuation at
27MHz, making the filter suitable for standard-definition
video signals from all sources (e.g., broadcast and
DVD). Broadcast video signals are channel limited:
NTSC signals have 4.2MHz bandwidth, and PAL sig-
nals have 5MHz bandwidth. Video signals from a DVD
player, however, are not channel limited; so the band-
width of DVD video signals can approach the Nyquist
limit of 6.75MHz. Recommendation: ITU-R BT.601-5
specifies 13.5MHz as the sampling rate for standard-
definition video. Therefore, the maximum bandwidth of
the signal is 6.75MHz. To ease the filtering require-
ments, most modern video systems oversample by two
times, clocking the video current DAC at 27MHz.
Outputs
The video output amplifiers can both source and sink
load current, allowing output loads to be DC- or AC-
coupled. The amplifier output stage needs approxi-
mately 300mV of headroom from either supply rail. The
devices have an internal level-shift circuit that positions
the sync tip at approximately 300mV at the output.
If the supply voltage is greater than 3.135V (5% below
a 3.3V supply), each amplifier can drive two DC-cou-
pled video loads to ground. If the supply is less than
3.135V, each amplifier can drive only one DC-coupled
or AC-coupled video load.
Applications Information
Audio Switch for Dual SCART Connectors
In addition to video signals, SCART connectors also
support left and right audio signals that are full duplex.
Figure 1 shows a matching audio switch for the
MAX9655. Notice that it can be made from low-cost,
discrete components. It is assumed that the set-top box
chip generates the left and right audio signals directly,
or the set-top box chip sends an I
2 S stream to a stereo
audio DAC that generates the left and right audio sig-
nals. In both cases, the audio signals are filtered and
amplified by a dual audio op amp before they are pre-
sented to the audio switch.
Figure 2 shows a matching audio switch for the
MAX9656. Similar to how the MAX9656 handles video
signals, the audio signals from the set-top box chip or
the audio signals from the TV SCART are routed to the
VCR SCART.
AC-Coupling the Outputs
The outputs can be AC-coupled since the output stage
can source and sink current as shown in Figure 3.
Coupling capacitors should be 220µF or greater to
keep the highpass filter, formed by the 150
Ω equivalent
resistance of the video transmission line, to a corner
frequency of 4.8Hz or below. The frame rate of PAL
systems is 25Hz. The corner frequency should be well
below the frame rate.
INPUT
INPUT CIRCUIT
(TV_SEL = LOW)
INPUT CIRCUIT
(TV_SEL = HIGH)
ENC_B_IN Sync-tip clamp Bias
ENC_G_IN Sync-tip clamp Bias
ENC_R_IN Sync-tip clamp Bias
ENC_CVBS_IN
Sync-tip clamp
(VCR_SEL = 0)
Sync-tip clamp (VCR_SEL = 1) Sync-tip clamp (VCR_SEL = 0)
Bias
(VCR_SEL = 1)
TV_CVBS_IN
Bias
(VCR_SEL = 0)
Sync-tip clamp
(VCR_SEL = 1) Bias
(VCR_SEL = 0)
Sync-tip clamp
(VCR_SEL = 1)
VCR_B_IN Bias Sync-tip clamp
VCR_G_IN Bias Sync-tip clamp
VCR_R_IN Bias Sync-tip clamp
VCR_CVBS_IN Bias Sync-tip clamp
Table 5. MAX9656 Input Circuit of Input as Determined by State of TV_SEL
Note: VCR_SEL = X (don’t care), except where noted.