Digital MEMS mic put on single chip

Also unique about Akustica's design, besides being built in standard CMOS, is that it incorporates the preamplifier and analog-to-digital converter on the same chip. The AKU2000 packs a fourth-order sigma-delta modulator, the output of which is a pulse-density modulated single-bit digital-output stream that is insensitive to radio frequency and electromagnetic interference. By saving the time and expense of conditioning an analog audio signal from a traditional microphone and converting it to a digital format, Akustica claims to lower overall costs and to eliminate noise sources as well as the need for shielded cable to route analog signals.

Analysts think Akustica may have a winning combination of price, performance and ease of integration. Because its microphone has a digital output, it will probably be designed first into existing digital appliances, from PCs to PDAs and Bluetooth headsets, and, eventually, into cell phones.

"The incorporation of a delta-sigma analog-to-digital converter on a single-chip microphone is both clever and limiting," said Stephan Ohr, research director of analog semiconductors at Gartner Dataquest (Stamford, Conn.). "In principle, Akustica should have some ready-made slots in those space-constrained applications that rely on digitized voice and audio. Cell phones, voice-over-Internet phones and Bluetooth headsets come readily to mind. Bluetooth headsets, for example, shipped over 140 million [units] in 2005 and will be more than half the cell phone market--583 million units--by 2009."

But despite the potential, Ohr said, "Akustica's success won't be immediate." Startups may embrace a new microphone technology, "since they have little to lose. But the Nokias, the Motorolas and Philips will be slower to move on something like this. Today they pay only about 95 cents for an electret diaphragm mike, and just rely on the voice codec that is already on their baseband processor to clean up the signal." Akustica, he said, "has to convince these manufacturers that an integrated MEMS device offers a better way to design their voice-processing chain, and that could take a few years."

But by 2010, most analysts agree that Akustica and other MEMS microphones with digital outputs will be integrated not only into PCs and PDAs, but also into most cell phones.

"The most pertinent market for Akustica is obviously cell phones--a market that's closing in on a billion units per year," said Marlene Bourne, president and principal analyst at Bourne Research LLC (Scottsdale, Ariz.). "But it will take a number of years for MEMS microphones to find their way into most cell phones."

A secondary category, she said, is PCs, "particularly given the growth of VoIP--suddenly microphone quality is that much more important. Those are the key market segments. Of course, there are a myriad of others, but none with the volume potential that PCs and cell phones offer."

PC microphones are currently using a four-component solution--namely, a miniature electret condenser microphone, a discrete FET, a separate operational preamplifier chip and an analog-to-digital converter chip. The idea of condensing all four functions into a single digital-output MEMS microphone chip holds an obvious allure. Akustica's Yuknis said that AKU2000 samples are currently in the hands of all major laptop PC, PDA and Bluetooth headset manufacturers, and that Akustica will start to announce design wins later this year.

"The overall market is in the 1 billion unit per year range, when you combine subsegments such as PCs--roughly a quarter billion a year--and cell phone handsets, around three-quarters of a billion per year," said Dean McCarron at Mercury Research (Cave Creek, Ariz.). "Since virtually all of these microphones ultimately attach to analog-to-digital converters, I think it's reasonable to say that the entire market is potentially available to them. But the best fit at the moment is notebook PCs, due to Akustica's ability to deal with a harsh acoustic- and electrical-noise environments."

Analyst Cullen offered a two-phase analysis of what may be in store for Akustica, Knowles Electronics, Sonion MEMS and other microphone chip makers in the coming years. "Phase one of the MEMS microphone market was driven by Knowles with its analog MEMS microphone," Cullen said. "Akustica's digital MEMS microphone kicks off phase two, which will add more diverse and higher-value-added markets. One application that comes to mind is VoIP, which will benefit from higher-quality microphones for notebook and desktop PCs."

Ultimately, Cullen said, "the integration of MEMS and integrated circuits is necessary for the long-term commercial success of MEMS. A product like this that not only integrates them, but actually makes the MEMS device compatible with standard CMOS processing, is a big step forward for the MEMS industry."