Actuator
Once you have a signal activated on ring, be it in software or hardware, you must use that signal to mute the television output. There are many ways to mute the sound. Here are three:
1. Transmit an infrared muting command to the television.
2. Interrupt the signal between the tuner and audio amplifier.
3. Interrupt the signal between the audio amplifier and speakers.
Another approach would be to interrupt the power to the audio amplifier, but that’s relatively hard on the equipment.We don’t recommend it.
Perhaps the easiest way to implement the actuator is with an infrared command to the television. You could couple the Javelin Stamp or Basic Stamp to your ring detect signal and to a serial port infrared transceiver, and use the processor to both learn the mute code and transmit it. If you have a digital video recorder (DVR), such as a TiVo or the system in Chapter 3, you could pause the playback the same way.
If you’re using the TAPI modem interface instead, you could use the USB infrared transceiver in much the same way, except you’d use a software interface instead of a control signal, letting you implement the entire project with just a PC, some plug-in hardware, and some software. Using the mute command, incidentally, solves a knotty design problem, which is how to decide when to restore normal sound. Using the built-in mute function lets you restore sound with your normal remote control, while using a direct electrical interface requires you implement the mechanism to restore sound yourself.
You can also combine a hardware ring detect with a PC-operated infrared transceiver. The USB or serial port infrared transceiver still works in that design; the issue becomes how to interface the ring detect circuit into the PC.
1. If you use one of the open collector signal devices, such as the ELK-930, a pull up resistor completes a circuit from the positive power line, through the resistor and open collector driver, to ground. You can tie a standard digital input to the open collector/resistor junction.
2. If the digital input you connect to the junction is that of an RS-232 transmitter, you can tie the RS-232 signal into the ring detect or data set ready line of a serial port.
3. Alternatively, if you use the junction signal to drive a standard TTL logic buffer, you can toggle the paper out signal line on a standard parallel port.
A USB interface is much more complex, but there are interface chips to help you there too, such as those from Future Technology Devices International. If you provide external power to the USB interface, you can probably avoid needing an embedded microprocessor (USB has very strict power management requirements if you’re drawing power from the bus).
You don’t, however, have to use infrared and mute the TV. Assuming a stereo television, a pair of relays wired into the audio signal path will do the job too. Your design has to connect the ring detect signal to the relays, and has to maintain the integrity of the audio circuits to preserve sound quality. Here are some ideas:
1. If you don’t have a telephone line connection near the television, you’ll need to remote the signal. You could use the same radio technology as for the Wireless RS-232 Link or the Remote Control Finder, because all you need to do is carry one signal from one point to another.
2. Instead of a radio connection, you could use X-10 power line technology. In its simplest form, you’d use the PF284 Powerflash to create a power line transmission when the ring detector fires, and the UM506 Universal Module to open and close the audio circuits. You’ll need two UM506 modules for stereo, one for each channel.
Figure 5 shows the X-10 approach. If the PF284 sends the same house code and unit code (that is, address) as is received by both UM506 modules, the ring detector will control the television directly, so you’ll only get a brief sound interruption when there’s really a ring sounding. If you set them to different addresses, you can use software in the control PC, such as HomeSeer (Chapter 9), to decide how long to keep the sound turned off.
FIGURE 5: X-10 television muting
Modifying the inputs to the audio amplifier within a television is likely to be difficult. A better alternative is to open the connection to the speakers. Keeping a proper load on the amplifiers is a good idea, so you could use a double pole, double throw relay with load resistors. Figure 6 shows the idea; make sure the load resistors are sized for the amplifier power output levels. Be very careful working inside a TV (or don’t do it) — televisions with picture tubes contain lethally high voltages that persist well after the TV is shut off or unplugged.
FIGURE 6: Speaker muting
You could use a similar circuit, and one that’s far easier to install, to interrupt the line-level signals between the television and your stereo or home theater pre-amplifier, although in that case (instead of load resistors on the amplifier output) you’d want to connect a 600–1000 ohm resistor across the pre-amplifier inputs when muted to make sure the amplifier stays silent and doesn’t pick up noise or hum.