1. Field of the Invention
The present invention relates in general to digital signal processing and in particular to circuits and methods for noise filtering in 1-bit audio applications and systems using the same.
2. Description of the Related Art
Current digital audio systems record data on a compact disk (CD), digital audio tape (DAT) or digital video disk (DVD) in a 16-bit format. During playback, this data is typically passed through a 24-bit interpolation filter which smooths the incoming digital samples, increases the precision and increases the data sample rate. A delta-sigma modulator then quantizes the signal, shapes most of the noise power and moves the noise outside the audio band. The delta-sigma modulator also reduces the number of bits representing each sample, for example from 24-bit samples to 5-bit samples. The 5-bit data is then processed by dynamic element matching logic which insures that all bits in each sample are used equally by the following digital to analog converter by thermometer coding to 24 bits. The digital to analog converter which ultimately converts the digital data to analog for eventual presentation to the listener as audio is typically a switched-capacitor circuit that also provides filtering.
In the new Sony/Philips Super Audio Compact Disk (SACD) format, audio data is recorded on a high density optical disk using the Sony Direct Stream Digital (DSD) technology. Here, an oversampled delta-sigma modulator-based analog-to-digital converter (ADC) converts the analog audio streams being recorded into 64 fs (2.8224 MHz) 1-bit data streams, where fs, the sampling frequency, is the 44.1 kHz rate commonly used for CD recordings. Each 1-bit data stream is directly recorded as a channel on high density dual layer compact disk. One layer contains two-channel data to support existing CD players and the other contains six-channel data for surround sound applications. This is in contrast to existing PCM formats where the 1-bit resulting from A/D conversion is decimated into multiple bit PCM code and then recorded. The same 1-bit audio recording format can also be used with high density digital video disks (DVD).
A number of the intermediate steps can be eliminated in the DSD/SACD system at both the recording and playback stages. The system also proclaims of improved audio quality to the listener. As a result, techniques for converting data in the 1-bit digital format to analog in the corresponding CD players must be developed. This is a non-trivial problem since such factors as filtering out-of-band noise, gain control through the modulator, and hardware minimization must be considered. Moreover, it is usually a requirement that a dynamic range of -120 dB in the audio band be achieved.
Hence, given the potential for widely accepted use of the Sony/Philips 1-bit audio format, and the continuous demand for improved sound quality, circuits, systems and methods for digital to analog conversion of 1-bit audio data are required.