Digital Technology for Excellent Sound Quality

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.

Figure 2

Effective in Various Noise Environments
AI Noise Canceling

A unique feature of the technology used in Sony's MDR-NC500D is the use of artificial intelligence (AI) noise canceling. Sony engineers observed that the effectiveness of noise canceling varied according to noise environment. They identified aircraft, trains and buses and offices as the three most common noise environments in which noise canceling headphones are used, and succeeded in creating three noise canceling modes optimized for each environment. With AI noise canceling, the headphones analyze the ambient noise and automatically select the appropriate mode.

AI Operation

If the user presses the AINC Mode button when the power switch is on, the headphones will terminate normal noise canceling and switch to the analysis mode. In the analysis mode, the headphones analyze the ambient noise for three seconds before selecting one of the three noise canceling modes based on the noise spectrum and several other factors. The headphones then revert to noise canceling in the selected mode.

Figure 3:
AI Noise Canceling: Automatic Selection of Optimal Mode Based on Analysis of Ambient Noise

The Future of the Technology

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.