As our smartphones get smaller and thinner, one of the consequences is the loss of bass or low frequency sounds we are accustomed to hearing naturally. The miniaturization of all components, including the loudspeaker, leads to voice or audio reproduction from the gadget seem unnatural. This is mainly because handset manufacturers have been slow to improve the audio performance, except in high-end handsets, leading to a lack of low-frequency audio.
However, the situation is changing now. A technology called smart amplifier is available to extract the maximum performance from the micro-speaker of a cell phone. Where the coupling between a traditional amplifier and its speaker is unidirectional, a smart amplifier senses the loudspeaker’s operation while playing. It also applies advanced algorithms to drive the loudspeaker to its maximum without hurting your ears.
To discuss the operation of a smart amplifier, it is important to understand that a loudspeaker is a vital component in the audio reproduction chain. If the design of the loudspeaker is not up to the mark, no amount of amplification or audio processing will overcome its shortcomings. However, if you even have a reasonable loudspeaker to start with, a smart amplifier can turbo charge it and push it to its limits.
Speakers contain a frame, voice coil, magnet, and diaphragm. Electrical current from an amplifier coursing through the voice coil magnetizes it, making it react with and move against the fixed magnet of the speaker. The movement causes the membrane or diaphragm attached to the coil to also move back and forth, and emanate audible sound waves. The movement of the diaphragm is called excursion, and it has its limits – audible distortions can occur when an amplifier exceeds the limits of this excursion – leading to failure in extreme cases.
Traditionally, amplifiers have used simple equalization networks at their outputs to limit this excursion. Because there can be large varieties of speakers, and different operating conditions including extreme audio signals, the filters are generally conservative. They actually limit the capability of the amplifier to push the speaker to its true limit. Additionally, current through the voice coil generates heat to some extent, and this factor limits the extent to which an amplifier can drive the speaker.
With micro-speakers commonly used in smartphones, smart amplifiers use feedback when driving them. A common method with Class-D amplifiers is to add IV or current and voltage sense to the DAC or digital to analog converter that provides a feed-forward solution. With IV-sense, the system receives feedback about the speaker’s voice coil temperature, its loading, and variations from unit to unit. The algorithm in the system uses this information to extract the maximum SPL or sound pressure level from the speaker without damaging it.
However, before a smart amplifier can drive a loudspeaker safely, a few steps are necessary. These include thermal characterization, excursion characterization, and SPL measurements for the speaker. Usually, data plots are necessary of excursions versus frequency and safe operating area limits.
Smart amplifiers such as the TAS2555 from Texas Instruments have a DSP or digital signal processor integrated. That reduces the time required for software development tremendously.
