Sony showcased its new MDR-NC500D digital noise canceling headphones in January 2008 at the 2008 International CES. Held in Las Vegas, International CES is a trade show for consumer electronic products. Sony announced that these new headphones, the world's first (*) digital noise canceling headphones, would go on sale in Japan in late April 2008.
Conventional noise canceling systems have relied on analog signal processing. Sony has brought together acoustic analysis technology, digital signal processing technology and transducer technology to create a digital noise canceling system.
* Based on Sony research in March 2008
What are Digital Noise Canceling Headphones?
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.
Superb Noise Canceling Performance
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* .
