Electronic Design

Triple Video Mux-Amp Doubles As A/V Source Selector

The increasing number of audio and video sources (VCR, DVD, set-top box, etc.) in a typical home-entertainment system requires a simple way to select the desired source. The 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 can produce an annoying thump as ac-coupling capacitors get charged and discharged by the make-and-break action of the switch. Instead, engineers can use a triple 4:1 multiplexer to design a simple A/V source selection multiplexer that produces excellent performance while greatly reducing the annoying switching transients.

The basic 4:1 A/V multiplexer shown in Figure 1 uses the Intersil EL4342 4:1 Video MUX amp to select any of up to four external audio-video sources consisting of a composite video channel and stereo L-R audio channels.

The circuit uses channel A to select one of four composite video sources and channels B and C to select the stereo left and right channels. Channel controllogic inputs S0 and S1, which are binary coded, perform the input selection. Logic inputs are TTL-compatible (transistortransistor-logic) and provide 20-ns switching times, suiting them for highspeed digital control in sophisticated multiplexer functions.

The HiZ logic input can disable all three channels by placing the output amplifiers in a high-impedance state. Besides supplying an audio mute and video blanking function, the HiZ state lets additional multiplexer amps share the same output, enabling input source expansion well beyond the basic 4:1 configuration.

Operating the multiplexer from dual 5-V supplies allows the video input and output to be dc-coupled so the video source composite-video, dcsync, and black levels are preserved at the output. Resistors R1a through R4a are input termination resistors. For precise video cable terminations, 75-Ω input terminations and output back terminations can be used, provided that the ?6-dB throughput loss can be tolerated.

If the ?6-dB loss can't be tolerated, the 75-Ω input termination resistor can be increased to 10 kΩ. Many video cables don't have controlled 75-Ω characteristic impedance, so increasing the termination resistor to 10 kΩ restores the gain to unity without degrading the video signal.

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, and c set the desired input impedance and form a high-pass filter. A 4.7-μF capacitor and a 10-kΩ resistor result in a 20-Hz, ?3-dB low-frequency cutoff. The main drawback to using large ac-coupling capacitors is the possibility that an audible "thump" may be heard as the channel is connected to the selector output. This can occur whenever a dc voltage difference exists between the source and the receiver input.

A low-impedance multiplexer switch can cause a transient to appear at the output when the accoupling capacitor is charged and discharged during switching. In this case, high-impedance input buffers present constant impedance to the accoupling capacitors, eliminating any charge-discharge transient.

The A/V multiplexer can be adapted to single +5-V supply operation using the circuit shown in Figure 2. The 10-kΩ/10-kΩ resistor voltage dividers at the input establish a 2.5-V input dc offset. This centers the amplifier output-voltage swing to 2.5 V 1 V. The 10-kΩ /10-kΩ voltage divider at the output of the audio channels keeps the bias on the output ac-coupling capacitor at 2.5 V. Therefore, the dc doesn't change as the mulitplexer is switched into (and out of) the highimpedance state.

The video channel input must be accoupled, which may require a dcrestore circuit if the TV or monitor isn't equipped with one. Frequency response and total harmonic distortion plus noise (THD+N) performance is unchanged from the dual ±5-V circuit.

However, the reduced amplifier supply voltage will cause a corresponding reduction in the amplifier output swing—from 5.5 V pp with the dual-supply version to 1.5 V p-p. There's sufficient margin for the composite video channel, and THD in the audio channels will increase at levels of approximately 2 V p-p and higher.

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