December 09, 2019

Three-phase video multiplexer plus channel for audio/video source selector

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With the increasing number of audio and video (A/V) sources in typical home entertainment systems (such as VCRs, DVDs, set-top boxes, etc.), one needs an easy way to select the desired audio or video source. However, the additional mechanical switches currently in use are both cumbersome and prone to wear, resulting in a gradual decrease in switching performance over time.

Solid-state analog switches can solve this problem, but when the switch is turned on and off to charge and discharge the AC-coupling capacitor, the passive switch can produce annoying hum. With a three-phase 4:1 multiplexer, engineers can design an A/V source-select multiplexer that is simple in construction, greatly reduces switching transient noise, and has excellent performance.

The basic 4:1 A/V multiplexer shown in Figure 1 utilizes Intersil's EL4342 4:1 video multiplexer to select any of four external audio/video sources, including composite video channels and stereo left and right channels. One.

The circuit uses channel A to select one of the four composite video sources, and channels B and C to select the stereo left and right channels. Input selection is accomplished by channel control logic inputs S0 and S1 using binary encoding. The logic inputs are TTL-compatible and have a switching time of 20ns for high-speed digital control in complex multiplexer functions.

The high-impedance (HiZ) logic input disables all three channels by placing the output amplifier in a high-impedance state. In addition to providing audio mute and video blanking, the HiZ state allows other multiplexers to share the same output, extending the input source to a basic configuration of more than 4:1.

Figure 1: There is no mechanical switch in the path of the A/V source selector, which can select any of the four external audio/video signals.

The multiplexer operates from a dual 5V supply and will allow the video input and output to be dc-coupled to maintain a composite video source (video, dc sync, and black level) on the output. Resistor R1a to resistor R4a are input termination resistors. For accurate video cable termination, a 75Ω input termination and an output backward termination can be used, provided that a 6dB throughput loss is allowed.

If you can't ignore the 6dB loss, you can increase the 75Ω input termination resistance to 10kΩ. Many video cables do not have a controlled 75Ω characteristic impedance, so increasing the termination resistance to 10kΩ restores the gain to unity gain without degrading the quality of the video signal.

Adding capacitive coupling (from C1b, c to C4b, c) to the audio input channel isolates the amplifier from unwanted DC signals that may be present in the audio source. The resistors R1b to R4b and R1c to R4c set a desired input impedance and constitute a high-pass filter. The 4.7μF capacitor and the 10kΩ resistor form a high-pass filter with a cutoff frequency of 20Hz (3dB). The disadvantage of using a large-capacity AC-coupling capacitor is that an audible "click" can occur when the channel is connected to the selector output. This can happen when there is a DC voltage difference between the audio source and the receiver input.

Figure 2: Using this circuit allows the circuit in Figure 1 to operate from a single supply.

When the AC coupling capacitor is charged and discharged during the switching process, the low impedance multiplexer can cause a transient signal at the output. In this case, the high-impedance input buffer provides a constant impedance to the ac coupling capacitor, eliminating transient signals during charge and discharge.

Using the circuit in Figure 2, this A/V multiplexer is suitable for operation with a single +5V supply. The input 10kΩ/10kΩ resistor divider provides an input DC bias of 2.5V and places the center of the amplifier's output voltage range between 2.5V and 1V. At the output of the audio channel, a 10kΩ/10kΩ resistor divider maintains the bias voltage on the output ac coupling capacitor at 2.5V. Therefore, the DC component does not change as the multiplexer enters/exits the high impedance state (and vice versa).

The video channel input must be AC-coupled. If the TV or monitor is not equipped, it may be necessary to add a DC recovery circuit. At this time, the frequency response and total harmonic distortion plus noise (THD+N) performance are the same as for the double ±5V circuit.

However, a reduction in the amplifier supply voltage will cause the amplifier's output range to drop, from 5.5V (peak-to-peak) at dual supplies to 1.5V (peak-to-peak). At this point, the composite video channel still has sufficient margin and the THD in the audio channel will increase to approximately 2V (peak-to-peak) or higher.