Active noise cancellation: Trends, concepts, and technical challenges
Active Noise Cancelling (ANC) headsets are an attractive proposition to consumers, since they offer a superior listening experience in conditions that are normally hostile to audio reproduction, such as trains, airplanes and busy urban areas. In fact, while the idea of silencing ambient noise is a simple one, its practical implementation is complex.
Nowadays the trend for music headsets goes to big over-the-ear headsets, whereas five years ago in-ear systems were the most popular. Consumer preferences have obviously changed in the last couple of years and besides the original use case (listening to music), headsets have turned into a fashion statement.
Developing an ANC headset requires considerable know-how, especially if you want to combine a piece of art with modern ANC technology. No matter in what kind of headset ANC is implemented (in-ear, on-ear or over-the-ear headsets) there are basically three different concepts to tackle the ambient noise.
ANC Concepts The most common is the feed-forward topology (see Figure 1), in which a micro-phone exposed to the exterior senses ambient noise, and the ANC circuit generates an anti-noise signal that the speakers reproduce (together with the user's audio playback signal). This headset type consists of four blocks: the speakers, battery, ANC circuit, and ANC microphones for the left and right channels.
The feed-forward topology is typically used in communication headsets like Blue-tooth headsets because of its wide ANC bandwidth. Such systems can cancel noise up to 3kHz with properly designed acoustics.
The higher frequencies in particular help to improve speech intelligibility if you are making phone calls since this is the typical frequency range of human voice. Another important advantage of a feed-forward ANC system is that there is absolutely no influence on the audio signal path.
Figure 2 shows the signal flow of a standard feed-forward ANC headset. The "Dff" block represents the feed-forward delay caused by the speaker due to the conversion from electrical impulses to air pressure waves. The noise signal that is picked up by the microphone is treated with a gain and phase compensation filter G(w) and mixed together with the sound signal. The music playback path is completely independent from the noise reduction path.
Another interesting mode that is commonly used in ANC headsets is the monitor mode. In this mode the ANC microphone is in turn being used to actively amplify the ambient noise. In this special mode the gain and phase compensation filter G(w) is bypassed and the microphone is connected directly to the speaker amplifier. This helps to overcome the passive attenuation of a headset when having a conversation with your neighbor or flight attended in a plane without removing the headset.
Typically this mode can be activated by pressing a push button on the headset. A disadvantage that comes with feed-forward systems is that they are susceptible to wind noise if the electronics and the acoustics are not properly designed.