For some 35 million users in the U.S. (by the end of 2017), “I’m sorry, I didn’t get that” might feel like a familiar response when calling from one room to another, where a voice-controlled personal assistant (such as Amazon Alexa or Google Home) resides.

The problem arises when you are far from the unit and it cannot clearly understand your request. This can often lead to frustration, and you may ultimately give up on asking your smart home to perform a given task, reverting to your phone or even performing the task manually. However, all that is about to change. There is a trend beginning for a whole new network of smart devices to listen and respond to requests for all rooms of your smart home (Figure 1).

 

Figure 1. Smart home example nodes.

 

Applications include a new thermostat that integrates your personal assistant’s algorithm to now act as a node that will listen and respond to your requests. Better yet, a smart light switch that can now act as your personal assistant in every room of your home. This improves the ability for your smart home to clearly hear your requests regardless of where you are in the home. These nodes will continue to come on the market with innovative industrial designs that push the envelope on size.

All of this sounds really exciting, but there’s a catch; you’re now at risk of being the one left saying "I'm sorry, I didn't get that."  As these smart devices innovate in size, so do the speakers inside them as shown in Figure 2. Simply put, a smaller speaker typically translates to lower volume and audio quality.  

 

Figure 2. Voice-enabled smart switch.

 

As these new nodes move to smaller speakers, the speakers also become more fragile and susceptible to damage. In order to ensure safe operation, classic Class-D audio amplifiers attenuate the entire signal (Figure 3) so as to not over-drive or damage the speaker. These amplifiers have no feedback to monitor the speaker health so conservative attenuation is the only choice to ensure the speaker reliability over time. The signal attenuation coupled with the smaller speaker size results in degradation of audio loudness and quality. This means the emerging nodes will now be able to understand your commands, but you will not be able to hear their response confirming your request or answering your question. Because of this, only half of the smart home equation has been solved; leaving full autonomy slightly out of touch.

 

Figure 3. Standard Class-D audio amplifiers. 

 

There is a new technology that allows for innovative industrial design and great audio to coexist in one solution. Class-D audio amplifiers with IV sense monitor current and voltage feedback from the speaker in real-time to track temperature (heat generated by the speaker coil) and excursion (how much the speaker diaphragm moves in and out). The current and voltage information feeds back into the device and is controlled by a dual-core DSP integrated into the amplifier running a speaker protection algorithm. The algorithm is calibrated around the model of the speaker being used. Each speaker has a unique model based on its mechanical properties. Below are common speaker parameters – found in the speaker manufacturer’s datasheet – used by the algorithm to ensure temperature and excursion protection:

  • Xmax (mm): excursion limit
  • Tmax (˚C): temperature limit
  • Tcoeff (1/K): speaker coil temperature coefficient
  • Sd (cm2): speaker diaphragm surface area
  • Bl (N/A): force factor
  • Re (Ω): nominal impedance
  • f0 (Hz):  resonant frequency

The speaker protection algorithm uses IV sense information to update the speaker model in real-time. This allows the amplifier to safely drive the speaker to its limits (without going over) with higher energy peaks, compared to a standard Class-D audio amplifier, while keeping the speaker under safe operating limits Tmax and Xmax, as seen in Figure 4. For the end user, this translates to an additional 6-10dB increase in sound pressure level (SPL) or roughly double the loudness which is important when driving a small form-factor speaker. The DSP also enables the ability to tune the amplifier through features such as input gain control, equalization, dynamic range compression and psycho-acoustic bass. These features can be used to optimize tuning for different modes such as voice or music.  

 

Figure 4. IV sense Class-D audio amplifiers. 

 

The audio amplifier with IV sense solves the full equation for a truly automated home by enabling great audio quality and keeping the speaker safe over its many years of operation. Devices such as the TAS2557 Class-D audio amplifier with IV sense are helping to drive this trend and growth within the home automation space today.

 

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