Data processing: generic control systems or specific application – Specific application – apparatus or process – Digital audio data processing system
Reexamination Certificate
1997-05-27
2002-08-20
Mei, Xu (Department: 2644)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Digital audio data processing system
C381S119000
Reexamination Certificate
active
06438434
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an improved mixing device and a mixing method, and coding and decoding devices and methods employing such an improved mixing device or method.
The present invention also relates to a improved mixing device for mixing audio signals which minimizes signal deterioration resulting from multi-stage connection of mixing circuits, as well as an audio system using such an improved mixing device.
As known in the art, the CODEC (abbreviation of Coder-Decoder) is an indispensable means to enable information communication between an analog signal system and a digital signal system, and generally comprises an A/D converter for converting an analog signal into a digital signal and a D/A converter for converting a digital signal into an analog signal. In recent years, CODECs employing a so-called “oversampling” technique have been attracting the attention of many people who desire to simplify the analog circuitry contained therein. In these CODECs, analog signals are converted, via an A/D converter based on a delta sigma modulation scheme, into bitstream data of a much higher bit rate than the frequency of the analog signals. The bitstream data are then subjected to a filtering process, “decimation” process, etc. to provide high-quality PCM data of minimized noises.
Also, with today's technological diversification, there is an increasing demand that the CODECs should have various other functions than the above-noted signal format conversion, among which is a mixing function to synthesize a plurality of input signals. To meet such a demand with the CODEC using the oversampling technique, it may be useful to first mix or synthesize a plurality of analog signals and then convert them into bitstream data, or to first convert a plurality of analog signals into bitstream data and then subject the data to a “decimation” process (i.e., process to convert the data into low-sampling-frequency and multiple-bit data), followed by mixing of these data.
However, the above-noted mixing technique has various problems as follows.
(1) Problem of Electrical Characteristics:
When the required mixing is performed on analog signals, unwanted entry of noises into the signals and distortion of the signals would unavoidably result during the mixing. Such noises and signal distortion may be reduced by use of a good-performance analog circuit, which, however, makes it necessary to apply relatively high electric current to the analog circuit, thus leading to in increased overall electric current consumption by the CODEC.
(2) Problem of Costs:
The mixing analog circuit has to have high resistance to adjust the mixing level, which would require a great space to accommodate a level adjusting resistor in an IC circuit implementing the CODEC, thereby increasing the size or area of the IC circuit chip. In addition, it is generally difficult to reduce the size of analog circuit. Therefore, in cases where the mixing circuit is analog one, reduction in size of the CODEC IC circuit is hard to achieve.
(3) Problem of Productivity:
Normally, after the CODEC IC circuit is manufactured, a test is carried out to check the performance of the IC circuit, in which it is necessary to determine whether or not the analog circuit properly performs a desired mixing function. However, this determination would be time-consuming because it has to be done via analog measurement using an analog tester.
(4) Problem of Quality:
The analog circuit is more susceptible to manufacturing unevenness or variations than the digital counterpart. Thus, in cases where the mixing circuit is analog, it is more likely that mixing circuits unable to provide desired performance will be produced.
The problems as set forth above are found not only in CODECs but also various other devices that require a function to mix bit stream data.
There are known two audio-signal mixing methods: one of the two is to synthesize a plurality of audio signals in analog form; and the other is to synthesize a plurality of audio signals in the form of digitized PCM data. The first-said method allows the necessary mixers to be constructed at a relatively low cost but is susceptible to noises, so that intolerable signal deterioration would often occur at the last-stage mixing output if the mixing circuits are connected in a multi-stage fashion. Further, the second-said method can avoid the problems of unwanted noises and signal deterioration because digitized PCM data are synthesized in this method; however, the overall cost of a device employed would substantially increase because channel-by-channel synchronization is required.
SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to provide a mixing device and a mixing method which are capable of mixing bitstream data without having to convert the data into analog representation, as well as coding and decoding devices and methods employing such a mixing device or method.
It is a second object of the present invention to provide an audio signal interface device which is simple in construction and yet permits appropriate input of analog audio signals of plural channels.
It is a third object of the present invention to provide a mixing device which can perform high-quality mixing at low cost with minimized influence of noises and signal deterioration, as well as an audio system employing such a mixing device.
In order to accomplish the above-mentioned objects, the present invention provides a mixing device for mixing plural bitstream data, which comprises: a weighting circuit that weights a plurality of bitstream data for conversion thereof into respective plural-bit digital data; an adder circuit that adds together the respective plural-bit digital data outputted from the weighting circuit, so as to output added digital data; and a converter circuit that converts the added digital data into bitstream data.
The “bitstream data” is pulse-density-modulated (PDM) data and has a logical value “1” or “0” at each of time slots corresponding to a predetermined bit rate. A plurality of bitstream data to be mixed are weighted for conversion into respective plural-bit digital data. That is, by being weighted with plural-bit digital coefficients of optionally selected values, the bitstream data to be mixed are converted into respective plural-bit digital data, i.e., data in PCM representation (pulse-code-modulated data). The weighting digital coefficients perform not only a function of converting the bitstream data into PCM data but also a function of setting a desired mixing ratio. Thus, the weighting degrees for the individual bitstream data may be variably set as desired. Then, the converted PCM data are added together, and the resultant added PCM data is reconverted into bitstream data (PDM data). In the above-mentioned manner, the plurality of bitstream data are mixed together through full-digital processing without having to use analog circuitry, and the mixed result is provided as one bitstream data. The converter circuit for converting the PCM data into the bitstream data (PDM data) may comprise a &Dgr;&Sgr; (delta sigma) modulator. The delta sigma modulator includes an integrator circuit in its input section, which is advantageously capable of removing shaping noise components from the original bitstream data (PDM data).
The mixing device according to the present invention is applicable not only to mixing between bitstream data but also to mixing between bitstream data and PCM data.
Namely, the mixing device of the present invention may comprises: a weighting circuit that receives bitstream data of a predetermined bit rate and weights the bitstream data for conversion thereof into plural-bit digital data; a PCM data supply circuit that supplies PCM digital data of a sampling rate corresponding to the bit rate; an adder circuit that adds together the digital data outputted from the weighting circuit and the digital data supplied from the PCM data supply circuit, so as to output added digital data; and a converter circuit that converts the added di
Mei Xu
Morrison & Foerster / LLP
Yamaha Corporation
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