1 MULTICHANNEL SPECTRAL MAPPING AUDIO APPARATUS AND METHOD
The present application is a continuation of U.S. patent application Ser. No. 11/515,400 filed on Sep. 1, 2006, which is a continuation of U.S. patent application Ser. No. 09/891, 941 filed on Jun. 25, 2001, now U.S. Pat. No. 7,164,769, which is a continuation of U.S. patent application Ser. No. 08/715,085 filed on Sep. 19, 1996, now U.S. Pat. No. 6,252, 965. Each of these applications is hereby incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to multichamiel audio systems and methods, and more particularly to an apparatus and method for deriving multicharmel audio signals from a monaural or stereo audio signal.
2. Description of the Related Art
Monaural sound was the original audio recording andplayback method invented by Edison in 1877. This method was subsequently replaced by stereo or two channel recording and playback, which has become the standard audio presentation format. Stereo provided a broader canvas on which to paint an audio experience. Now it has been recognized that audio presentation in more than two channels can provide an even broader canvas for painting audio experiences. The exploitation of multicharmel presentation has taken two routes. The most direct and obvious has been to simply provide more record and playback chamiels directly; the other has been to provide various matrix methods which create multiple channels, usually from a stereo (two channel) recording. The first method requires more recording chamiels and hence bandwidth or storage capacity. This is generally not available because of intrinsic bandwidth or data rate limitations of existing distribution means. For digital audio representations, data compression methods can reduce the amount of data required to represent audio signals and hence make it more practical, but these methods are incompatible with normal stereo presentation and current hardware and software formats.
Matrix methods are described in Dressler, “Dolby Pro Logic Surround Decoder-Principles of Operation” (http:-// www.dolby.com/ht/ds&pl/whtppr.-html); Waller, Jr., “The Circle Surround® Audio Surround Systems”, Rocktron Corp. White Paper; and in U.S. Pat. Nos. 3,746,792, 3,959,590, 5,319,713 and 5,333,201. While matrix methods are reasonably compatible with existing stereo hardware and software, they compromise the performance of the stereo or multichannel presentations, or both, their multichannel performance is severely limited compared to a true discrete multicharmel presentation, and the matrixing is generally uncontrolled.
The present invention addresses these shortcomings with a method and apparatus which provide an uncompromised stereo presentation as well as a controlled multichannel presentation in a single compatible signal. The invention canbe used to provide a multichamiel presentation from a monaural recording, and includes a spectral mapping technique that reduces the data rates needed for multichannel audio recording and transmission.
These advantages are achieved by sending along with a normally presented “carrier” audio signal, such as a normal stereo signal, a spectral mapping data stream. The data stream
comprises time varying coefficients which direct the spectral components of the “carrier” audio signal or signals to multichannel outputs.
During multichannel playback, the invention preferably first decomposes the input audio signal into a set of spectral band components. The spectral decomposition may be the format in which the signals are actually recorded or transmitted for some digital audio compression methods and for systems designed specifically to utilize this invention. An additional separate data stream is sent along with the audio data, consisting of a set of coefficients which are used to direct energy from each spectral band of the input signal or signals to the corresponding spectral bands of each of the output chamiels. The data stream is carried in the lower order bits of the digital input audio signal, which has enough bits that the use of lower order bits for the data stream does not noticeably affect the audio quality. The time varying coefficients are independent of the input audio signal, since they are defined in the encoding process. The “carrier” signal is thus substantially unaffected by the process, yet the multicharmel distribution of the signal is under the complete control of the encoder via the spectral mapping data stream. The coefficients can be represented by vectors whose amplitudes and orientations define the allocation of the input audio signal among the multiple output chamiels.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a digital signal processor (DSP) implementation of the invention’s multicharmel spectral mapping (MSM) decoder;
FIG. 2 is a block diagram illustrating the DSP multichannel spectral mapping algorithm structure;
FIG. 3 is a set of signal waveforms illustrating the use of apeiture fiinctions to obtain discrete transform representations of continuous signals;
FIG. 4 is a block diagram of a DSP implementation of a method for calculating the spectral mapping coelficients in the encoding process;
FIG. 5 is a block diagram illustrating the spectral mapping coelficient generating algorithm;
FIG. 6 is a block diagram illustrating a vector technique for representing the mapping coefiicients;
FIG. 7 is a diagram illustrating the use of the vector technique with decoder lookup tables; and
FIG. 8 is a diagram illustrating a fractional least significant bit method for encoding an audio signal with mapping coefficients.
DETAILED DESCRIPTION OF THE INVENTION
A simplified functional block diagram of a DSP implementation of a decoder that can be used by the invention is shown in FIG. 1. A “carrier” audio signal, which may be monaural or stereo for example, is input to an analog-to-digital (A-D) converter and multiplexer 2 via input lines 1. For simplicity singular term “signal” is used to include a composite of multiple input signals. In some applications the audio signal will already be in a multiplexed digital (PCM) representation and the A-D multiplexer will not be needed. The digital output of the A-D multiplexer is passed via line 3 to the DSP 5, where the signal is broken into a set of spectral bands in the spectral decomposition algorithm 4, and sent to a spectral mapping fiinction algorithm 6. The spectral bands are preferably the conventional critical (bark) bands, which have a roughly constant bandwidth of about 100 Hz for frequencies below 500 Hz, and a bandwidth that increases with frequency for higher frequencies (roughly logarithmically above 1 kHz). Critical