AN1249: Using the EL4342 and ISL59446 Triple Video MUX-Amps as A/V Source Selectors

Using the EL4342 and ISL59446 Triple Video
MUX-Amps as A/V Source Selectors
®
Application Note
Overview
The increasing number of A/V sources (VCR, DVD, set top
box, etc.) in a typical home entertainment system presents
the need for a simple way to select the desired source.
Add-on mechanical switches available today are bulky, and
prone to contact wear resulting in degraded performance
over time. Solid state analog switches solve this problem,
but passive switching of AC coupling capacitors in audio
sources can produce an annoying thump as they are
charged and discharged during the make or break action of
the switch. This application note describes how to use the
Intersil EL4342, fixed gain of 1, and ISL59446, fixed gain of
2 Triple 4:1 MUX amps to design simple audio-video source
selection multiplexers that provide excellent performance
while greatly reducing the annoying effects due to audio
switching transients.
Circuit Description
The basic A/V multiplexer shown in Figure 1 uses the 4:1
Triple Video MUX amp to select any 1 of 4 external audiovideo sources consisting of a composite video signal and
stereo L-R audio channels. The circuit uses Channel A to
select the 1 of 4 composite video sources, and channels B
and C to select the stereo left and right audio channels.
Channel select logic inputs S0 and S1 are binary coded and
perform the input source selection according to the table in
Figure 1. The HIZ logic input disables all 3 channels by
placing their outputs into a high impedance state which
disconnects the audio sources, and places the composite
video output at the 0VDC black level. This feature is useful if
externally controlled blanking is desired during channel
switching. This HIZ disable function can also be used to
expand the number of A/V input sources from 4 to higher
multiples by enabling additional A/V MUX amps to share the
same output channels.
July 14, 2006
AN1249.0
gain of 1 would be a suitable choice. If the application
requires 75Ω back-terminated cable driving at the output, or
the input, the ISL59446 with it’s internal gain of 2 may be the
better choice. In the latter case, the higher gain of 2 would
also appear in the audio channels, which might require a
simple voltage divider at the audio outputs to maintain the
correct audio levels.
Adding capacitive coupling (C1b, c through C4b, c) to the
audio input isolates the amplifier from unwanted DC that
may be present in the audio source. Resistors R1b, c
through R4b, c set the desired input impedance and form the
input high pass filter. A 4.7µF capacitor and a 10k resistor
produce a 20Hz, -3dB cutoff frequency.
The usual approach of using AC coupling in this way has the
drawback that low impedance mechanical or solid state
switches produce an audible thump, as they charge and
discharge the AC coupling capacitors during channel to
channel switching. The high impedance input buffers in the
MUX amp solve this problem by maintaining a constant
impedance on the coupling capacitance during switching.
Video performance is exceptional owing to the 500MHz -3dB
bandwidth and 60MHz 0.1dB gain-flatness of the MUX
amps. The high video performance also benefits audio
performance resulting in a very flat (<±0.02dB) frequency
response from DC to well beyond 80kHz. The audio
frequency response is tailored by the input and output R-C
filters. THD+N vs. amplitude measured on an Audio
Precision series 2500 over the 20Hz to 20kHz frequency
range is less than 0.015% from 1Vrms down to 100mVrms,
increasing to 0.1% down to 4mVrms.
Operating the circuit from dual ±5V supplies enables the
video input and output to be DC coupled so that the
composite video DC and sync levels are preserved at the
output.
Choosing MUX Amp Gain
The EL4342 and ISL59446 are pin to pin replacement parts.
The only difference is in their internal gain settings. The
EL4342 has an internally set gain of 1, and the ISL59446
has an internally set gain of 2. The choice of gain depends
on the video channel input and output requirements. If the
composite video inputs and output display are in close
proximity to the MUX amp, input terminating resistors R1A
through R4A can be large so that the overall video channel
gain is preserved at 0dB. In this case, the EL4342 with a
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright © Intersil Americas Inc. 2006. All Rights Reserved.
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Application Note 1249
AV Source
Video 1
Video 1
IN0 A
R1A
Video 2
Audio 1 - L
EL4342
IN1 A
Out A
Audio 1 - R
Video 3
R2A
IN2 A
Video 4
R3A
IN3 A
R5A
75
+1
MUX
R4A
Video 2
Audio 1 L
C1B
Audio 2 - L
Audio 2 L
C2B
Audio 2 - R
Audio 3 L
C3B
C4B
Audio 4 L
IN0 B
R1B
IN1 B
Out B
R2B
R3B
R7B
47
Video Out
Audio L - Out
+1
IN2 B
C5B
4.7uF
R5B
500
IN3
R6B
10k
Audio R - Out
R4B
Video 3
Audio 1 R C1C
IN0 B
Audio 3 - L
Audio 2 R C2C
R1C
Audio 3 R C3C
R2C
IN2 B
Audio 4 R C4C
R3C
IN3
Audio 3 - R
Video 4
IN1 B
Out C
C5C
4.7uF
R7C
47
+1
R5C
500
R6C
10k
R4C
Audio 4 - L
S0
Source 3
Audio 4 - R
S1
+5V
Control
Logic
+5V
Ground
Source2
Source 4
Blank/Mute
Source 1
NC
-5V
-5V
Hi Z
Input Source
Select
Logic Truth Table
S0
S1
HIZ
OUTA,B,C
0
0
0
Source 1
1
0
0
Source 2
0
1
0
Source 3
1
1
0
Source 4
X
X
1
All Off
Figure 1: Basic A/V MUX
FIGURE 1. BASIC A/V MUX
Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to
verify that the Application Note or Technical Brief is current before proceeding.
For information regarding Intersil Corporation and its products, see www.intersil.com
2
AN1249.0
July 14, 2006
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