Signal Chain Basics #44: Get the best from your home audio system while maintaining low power and high-efficiency operation

(Editor's note : Click>here for a complete, linked list of all previous installments of the Signal Chain Basics series.)

My inspiration for this article comes from real home audio product users: namely, my wife and in-laws. They’re a good test group for any new product I bring into our home; they’re users, not geeky early adopters (like me!).

When I brought home a new soundbar to connect to our flat panel TV, I was met with two questions: 1) “Another remote control? Really?!”; and 2) the classic, “How do I turn it on and off?”

With the TV becoming the central point of the home audio experience, it makes sense for folks to use it as the central switching point for different media, such as plugging in their games console and/or Blu-ray Disc player.

With constant pressure on manufacturers to make TV panels ever thinner, the concept of outsourcing amplifiers and speakers is becoming stronger. This may be done by using wireless speakers, a soundbar, or even an audio video ( AV) receiver. Today, most of these products have their own remote controls, with no automatic shutdown method should the TV be switched off. Since the TV is usually on for only a few hours a day and many users don’t bother to place the system into shutdown mode, the result is a terrible waste of power.

One way to solve these issues is to make the amplifier intelligent enough to understand when its source is powered on and transmitting audio to it. It should also recognize when there isn’t audio content and mute its outputs, saving wasted power in the idle stages.

Most televisions have a headphone output that is subjected to the volume control. Using this connection to drive the AV equipment removes one of the remote control requirements (volume control) (Figure 1) .

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Figure 1: LCD TV and Soundbar, with headphone for audio transport.

(Click on image to enlarge)


Once we have some control over the signal, the next challenge is to teach the receiver to detect when there’s a real signal going into it. In the digital S/PDIF domain, this can be done by detecting either data input (anything other than a GND) or using the S/PDIF LOCK status pin found on many S/PDIF receivers (Figure 2).

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Figure 2: LCD TV and soundbar, with S/PDIF for audio transport and control.

(Click on image to enlarge)


In the analog domain, this is more difficult. In a purely analog system, the input signal should be rectified, smoothed, and then run into a comparator (Figure 3) . The output of the comparator should then be connected to an interrupt general purpose input/output (GPIO) (input) on a host processor.

Periodically, this detector input should be polled. After a fixed duration (number of polls) returns a zero, the system can be put into standby. This is assuming that a minute of silence (real GND silence, not just low-level noise) means the TV has been switched off.

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Figure 3: Analog input detection.

(Click on image to enlarge)


The GPIO input then can be switched to an interrupt mode and the amplifiers put into standby for minimum power consumption.

Once S/PDIF input, or the analog comparator is tripped, the system can wake up. Providing this is done within the group-delay time of the analog-to-digital converter (ADC) and digital signal processor (DSP), there should be no user-perceived delay in the sound.

A device that combines the ADC + S/PDIF transceiver is in a wonderful position to monitor both S/PDIF and analog inputs. Some combination transceivers like this also have the ability to set the trigger thresholds for the ADC in a register. This removes the requirement for the rectification and comparator circuit and allows the comparator level to be fine tuned in software (Figure 4) .

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Figure 4: Using a combination ADC + S/PDIF device to monitor all inputs.

(Click on image to enlarge)


Using these techniques immediately removes two primary uses of a remote control, and simplifies the user experience to one remote control they already use. This increases the wife acceptance factor and decreases on-the-phone technical support required by the in-laws.


For more information on digital audio transceivers and home audio solutions, visit

Join us next month for a discussion on high-speed data converters.

About the Author

Dafydd Roche is the home audio strategic marketing and systems engineer for the Audio Converter group at Texas Instruments. An avid musician in his spare time, Dafydd pours his passion and knowledge of audio and music-making into his work. If you have any questions or comments about this or any of the other Signal Chain Basics articles, email them to .

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