Microphones for Speech Recognition

(Compiled by Ray Grott, 06/28/2000)


Types of Microphones

Electret (using an electrically-charged diaphragm) is more sensitive than dynamic (magnet moving within a coil) and is used in most SR microphones.

Noise Canceling is a major issue:

Pressure Gradient - most common low-cost approach. The microphone has openings in front and back, allowing noise from other sources to enter. But if it enters both sides it cancels itself out, whereas the speech only enters one side. (VXI, Plantronics)

Active Noise Canceling - (found in Andrea ANC 600-700, and the NCT NoiseBuster mics). It uses two microphones facing in opposite directions. The signal from the outward microphone is electronically processed and used to cancel out the same sound coming from the inward mic.

Active Noise Reduction - (found in Andrea QW 1000 and NCT NoiseBuster). Similar to noise cancellation but uses a secondary mic (in the earpiece in Andrea‘s case) to reduce noise ambient noise in the earphone speakers.

High Frequency Suppression - (found in VXI Parrott 10 & 20‘s Translator Box). The box provides additional voltage to the microphone, if required, through its batteries and clips high frequencies to suppress noise.

USB mics - have their own sound card and send the signal digitally direct to the software, eliminating noisy sound cards, especially in laptops.

Digital Array Microphones - allow high-quality reception at a distance (+/- 2 feet). The principle is that the array of microphone elements will pick up your speech at the same time, but they will pick up more remote, directional sounds at different times. Complex digital signal processing then eliminates the noise without distorting the speech.

Super Directional Desktop - (Telex M-60 Aria) Utilizes "Acoustic Directional Processing" technology. No noise canceling.

What to look for in a microphone

Best rated headset microphones:
Evaluation reported in Dragon NaturallySpeaking Complete by Parmod Gandhi et al. (limited review):

More comprehensive test reported by Martin Furey in July 1999, comparing 15 brands for noise cancellation, recognition, comfort, etc.

Mute Switch Feature:
This is very helpful for demonstrations and training. Found in ANC 600 & 700 series, Plantronics LS1 and HS1, among others.

Vendors and vendor sites tend to promote the makes and models they carry. For basic functioning, all the higher-end noise-canceling models appear to be reasonably good. It then comes down to particular needs regarding particularly noisy environments, desire to move about, physical limitations, telephony, comfort, etc.

Alternatives to headsets

Wireless Microphones:

Desktop
Generally not preferred to headsets, as user has to stay within a couple of inches.
This has recently changed with the Digital Array Mic, listed below.

Digital Array Mics

USB Mics

Hand Held

Stub or Stalk Mics
(for plugging into handheld digital recorders such as the VoiceIt bundled with NaturallySpeaking NaturallyMobile or the Olympus D-150 bundled with VoiceExpress.

Macintosh

Computer / Telephone Switch Boxes:

What you get in the box

Company websites

DON'T FORGET: The quality of the sound card is critical as well. Sound cards make a difference in the amount of signal noise. SoundBlaster, Ensonic, and Turtle Beach are all generally good, but individual cards vary and products keep changing.

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