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Microphone Arrays: Digital Microphones

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Old 11-10-2007
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Microphone Arrays: Digital Microphones
Below is part 2 of an audio series by Windows Vista Audio team Program Manager Richard Fricks, the first being Richard's piece on Using a microphone array to enhance sound capture. This follow-on article details how to get more out of your PC by using a digital microphone.
The digital microphone is a perfect fit for Windows Vista's microphone array technology. Digital microphones have been around for years, but until recently, the ability to integrate such technology into an everyday laptop computer at an affordable price has not. Did I mention they are incredibly compact? Here is a picture of a bottom-port Akustica digital microphone:

Akustica AKU2000
This is a great example of a high quality, cost-effective digital microphone that is easily integrated into a laptop PC.
There are some unique characteristics of this particular microphone that warrant mention. First of all, Akustica has a unique fabrication process that allows them to incorporate the entire design onto a single chip of silicon. This monolithic design places the sensor, microphone circuitry, amplifier and converter all together on one chip. This is a great step forward that provides significant advantages over analog microphones as well as other digital microphones that require multiple chips. One of the easiest advantages to articulate is its superior immunity to RF interference.
For comparison purposes, listen to the following audio clips. These clips were made using the same laptop equipped with both a traditional electric condenser microphone (ECM) and an Akustica digital microphone. They provide a clear example of how electrical noise can inject itself into the audio capture stream and how well the Akustica digital microphone is at rejecting this interference.
Wi-Fi Interference

GSM Noise

Any of those noises sound familiar? After I heard the GSM noise, I found myself thinking "Oh, so that's what was causing that strange sound on my PC speakers every time my mobile phone rang!"
Another advantage of the digital microphone is the flexibility it provides in placement. A good place to position a microphone is in the screen's bezel. However, this is also a location where there is a lot of RF noise. With its excellent immunity to such interference, the digital microphone can easily be placed in this area where it is not only less susceptible to keyboard, hard-drive and other physical noises, but also allows for a position that is more directly aligned with the talker's voice.
Single-chip digital microphones also have low manufacturing tolerances, which makes them more suitable for microphone-array applications where microphone matching is important.
If you are shopping for a computer equipped with a microphone array for use with Windows Vista, you need to keep in mind the various microphone array geometries that are supported. The array geometry refers to the number, type, and position of the microphones. The microphone array technology on Windows Vista was carefully tuned to provide the highest level of support for the following two and four microphone array geometries:
Small two-element Array: This geometry consists of two microphones, 100mm apart.

Big two-element Array: This geometry consists of two microphones, 200mm apart.

Linear four-element array: This geometry consists of four microphones with the far right and far left microphone 190 mm apart and the inner two microphones 55mm apart. A second geometry layout allows for the far right and left microphones to be 160mm apart and the inner two microphones to be 70mm apart.

L-Shaped four-element array: This geometry consists for four microphones mounted in the shape of the letter 'L'. It actually looks like a backwards upside down 'L', but that would make for too cumbersome of a name!

This design is targeted for a tablet PC where the screen can be flipped around. When writing on the table, this screen position can cause the hand to cover its lower right or left corners. By positioning the microphones in the manner described here, the hand will not interfere with the microphones' operation.
In general, the more microphones in the array the better, but here is a general rule of thumb:
  • If you will be recording in a quiet office and will be sitting no more than 2 feet away from the computer then a microphone array equipped with two microphones should be sufficient.
  • If you will be recording in an office or cubicle with normal noise levels and up to 6ft away from the computer then a microphone array equipped with 4 microphones should be sufficient.
In either case, I highly recommend finding a laptop equipped with digital microphones.
For those programmers out there who want to learn how to capture audio processed by Windows Vista's microphone array technology, I hope to be ready to share my programming experiences with you sometime in November.
If you'd like to dig deeper into the topics I have presented above, you can find some great white papers under the subject "Microphone Arrays" here.
Thanks for reading and happy recording!
- Richard Fricks, Program Manager, Windows Audio Team

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