Noise canceling headphones sample ambient (surrounding) noise through a microphone and use signal processing to generate signals that cancel ambient noise. Emitting these cancelling signals as sound through a driver unit provides the user with a quiet listening environment. The signal processing system at the heart of noise canceling headphones has conventionally been based on analog technology because of issues relating to processing speed. By introducing digital signal processing, Sony has succeeded in creating a system that offers unique advantages, as described below.

Figure 1 illustrates the MDR-NC500D system. The headphones operate as a feedback system. A detection microphone located in the housing continually monitors the sound reaching the ears. Output signals ( in Fig. 1) from this microphone are amplified, digitized by the A/D converter, and then sent to the DNC software engine (signal processor). Signals from the music source ( in Fig. 1) are digitized by the A/D converter and then processed by a digital equalizer to achieve the proper frequency characteristics. The signals then enter the DNC software engine, which subtracts the ambient noise from the music source signals and extracts the noise that needs to be cancelled (Signal in Fig. 1). Signal then undergoes phase reversal, and the result is played back through the driver together with the music signal, thereby canceling the noise before it can enter the ears.

However, if the noise signals that need to be cancelled are simply played back after phase reversal, oscillation will occur at relatively high frequencies because of delays in phase reversal as the sound moves along the microphone→noise canceling circuit→driver unit→air→microphone route. The result is a phenomenon known as "howling." To prevent this, it is necessary to use a filter circuit to eliminate the high frequencies that trigger oscillation. In Sony's MDR-NC500D, this is achieved through digital signal processing in the DNC software engine, which ensures extremely precise filtering and a dramatic improvement in performance* .

The noises that noise canceling headphones are designed to eliminate include many high-pressure bass sounds not included in normal music sources, such as CDs. To cancel such noise, headphones require the capacity to produce sound pressure strong enough to overwhelm powerful bass sounds. However, headphones are portable devices, and there is a limit to the amount of power that can be provided. To achieve maximum performance with a limited power supply, noise canceling headphones need tuning capabilities that are totally different from the tuning technology used in stereo headphones for normal hi-fi listening. Specifically, they must be able to provide excellent electro-acoustic conversion efficiency at the frequencies that require cancellation (Figure 2).

For this reason, when canceling performance is enhanced, there is a tendency for low frequencies to be over-boosted, resulting in sound that is less crisp and clear. Sony solved this problem by also using digital technology in the equalizer circuit positioned ahead of the cancellation circuit. This provides far better S/N performance and equalization precision than could be achieved with an analog equalizer, resulting in a more natural tone balance.

The history of digital noise canceling headphones has only just begun, and there is enormous potential. The advantages generally associated with digital signal processing are (1) the ability to handle non-linear signal processing, and (2) the capacity for complex processing. Sony will continue to target further improvements in noise canceling performance, as well as functional enhancement, power efficiency, and cost reduction.