Digital-to-analog converter

Abstract: A PWM digital-to-analog converter is disclosed in which a differential component between the pulse width modulated waveform consistent with input digital data and the 2's complement data of the input digital data is taken at a differential amplifier (46) and freed of high frequency components to produce an analog ouput signal (S out) with improved distortion factor.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a PWM digital-to-analog converter for converting input digital data into a pulse width modulated waveform for conversion into corresponding analog signals. Prior Art There are known in the art various types of digital-to-analog converters for converting input digital signals into corresponding analog signals, such as pulse width modulation or PWM system of converting input digital data into a pulse width modulated waveform, or pulse amplitude modulation or PAM system of converting input digital data into a pulse amplitude modulated waveform, for conversion into corresponding analog signals. On the other hand, in digital audio signal processing systems, such as CD or DAT, there are proposed, as a technique of reducing quantization noises at the time of decoding analog audio signals from digital data, an oversampling technique of converting digital data with a sampling frequency fs according to the Nyquist's theorem into an n-number multiple sampling frequency n·fs, or a noise shaping technique of changing frequency distribution characteristics of the noise for improving the S/N ratio. For example, as disclosed in the Japanese Patent Publication 61-177819 (1986), digital data which are previously subjected to the above mentioned oversampling or noise-shaping are converted into an analog form by a digital-to-analog converter to produce analog audio signals with a reduced quantization noise level. Meanwhile, although it is possible in principle with the above mentioned PAM digital analog converter to produce converted analog output signals with excellent linearity and reduced distortion, it is beset with a drawback that high accuracy resistance summation or current summation circuits, accurately registering with the weights to the input digital data bits, are necessitated, and that, if it is desired to improve the resolution, the circuit scale is increased and the circuit as a whole need be designed with high accuracy. On the other hand, although the above mentioned PWM digital-to-analog converters are simplified in circuit organization, the conventional PWM digital-to-analog converters in which, as shown in Fig. 1, a pulse width modulated waveform having pulse widths W 7 , W 6 , W 5 , W 4 , W 3 , W 2 and W 1 consistent with 3-bit 7-value input digital data D 7 , D 6 , D 5 , D 4 , D 3 , D 2 and D 1 are generated and freed of high frequency components in a low-pass filter to produce analog output signals, is beset with an inconvenience that, as shown in the spectral distribution diagram of Fig. 2, which illustrates the results of the FFT analyses of the pulse width modulated waveform, generated from the 10kHz sinusoidal signal, the even-number order distortion components, above all, the second order distortion components HD, are increased. US Patent 4 542 371 describes a digital-to-analog converter according to the pre-characterising portion of claim 1, adapted to reduce second harmonic component distortion in the output analog signal. OBJECTS AND SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a PWM digital-to-analog converter in which the even-number order distortion components may be reduced. It is another object of the present invention to provide a digital-to-analog converter of a simplified circuit organization, which is low in distortion factor and excellent in conversion characteristics. It is a further object of the present invention to provide an arrangement in which a decoded output signal with reduced distortion may be produced in a decoding section of a digital audio signal processing system. In accordance with the present invention, there is provided a digital-to-analog converter, comprising: first pulse width modulated waveform generating means for generating a pulse width modulated waveform consistent with input digital data; second pulse width modulated waveform generating means for generating a second pulse width modulated waveform consistent with the complement of said input digital data; differential amplifying means for deriving a differential component of the first and second pulse width modulated waveforms output by said first and second pulse width modulated waveform generating means; and filter means for outputting an analog output signal freed of high frequency components of the differential output waveform output by the differential amplifying means; wherein a differential component between the pulse width modulated waveforms consistent with input digital data and with complement data of said input digital data is output by the differential amplifying means and freed of high frequency components to produce analog output signals; characterised in that: the second pulse width modulated waveform generating means is adapted for generating the second pulse width modulated waveform consistent with the 2's complement of the input digital data; a first low pass filter is provided between the first pulse width modulated waveform generating means and the differential amplifying means; and a second low pass filter is provided between the second pulse width modulated waveform generating means and the differential amplifying means. Thus, with the digital-to-analog converter according to the present invention, a differential component between the low pass filtered pulse width modulated waveform consistent with input digital data generated by first pulse width modulated waveform generating means and the low pass filtered pulse width modulated waveform consistent with the 2's complement data of the input digital data generated by second pulse width modulated waveform generating means, is taken out from differential amplifying means, and the differential output waveform produced by the differential amplifier means is freed of high frequency components by filter means to produce analog output signals. In this manner, the even-number order distortion components may be reduced to improve the distortion factor. The above and other objects and novel features of the present invention will become apparent from the following description and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a waveform diagram showing the PWM waveform in a conventional PWM digital-to-analog converter. Fig. 2 shows spectrum distribution showing the results of the FFT analyses of the PWM waveform shown in Fig. 1. Fig. 3 is a block diagram of a digital data decoding system to which the digital-to-analog converter according to the present invention is applied. Fig. 4 is a waveform diagram showing the pulse width modulated waveform from a first pulse width modulated waveform generator forming part of the digital-to-analog converter shown in Fig. 3. Fig. 5 is a waveform diagram showing the pulse width modulated waveform from a second pulse width modulated waveform generator forming part of the digital-to-analog converter shown in Fig. 3. Fig. 6 is a waveform diagram showing the differential output waveform by a differential amplifier forming part of the digital-to-analog converter shown in Fig. 3. Fig. 7 shows spectrum distribution showing the results of the FFT analyses of the differential output waveform of the differential amplifier shown in Fig. 6. EMBODIMENT By referring to the drawings, a preferred embodiment of a digital-to-analog converter of the present invention will be explained in detail. In an embodiment shown in Fig. 3, the present invention is applied to a decoding system in a digital audio signal processing system wherein 16-bit playback digital audio data D IN are supplied via an input terminal 1 to an oversampling section 2. The oversampling section 2 performs an oversampling on the playback digital audio data D IN for converting it into digital data having a frequency 64·fs, which is 64 times the sampling frequency fs, and transmits the oversampled digital data to a noise shaping section 3. The noise shaping section 3 performs a noise shaping on the oversampled 16-bit digital data from the oversampling section 2 for rounding it to a 3-bit 7-value digital data for removing the quantization error, and transmits the noise shaped 3-bit 7-value digital data to a digital-to-analog converter 4 of the present invention. The digital-to-analog converter 4 is constituted by a first pulse width modulated waveform generator 41 for generating a pulse width modulated waveform PWM 1 corresponding to the noise-shaped 3-bit 7-valued input digital data from the noise shaping section 3, a 2's complement converter 42 for converting the input digital data into 2's complement data, a second pulse width modulated waveform generator 43 for generating a pulse width modulated waveform PWM 2 , a differential amplifier 46 to the input of which pulse width modulated waveform outputs by the first and second pulse width modulated waveform generators 41, 43 are supplied via low-pass filters 44, 45 respectively, and a low-pass filter 47 for eliminating a high frequency component of the differential output waveform from the differential amplifier 46. The first pulse width modulated waveform generator 41 receives a 3-bit 7-value input digital data from the noise shaping section 3, and generates, in synchronism with a master clock CK MS , a pulse width modulated waveform PWM 1 having pulse widths W 17 , W 16 , W 15 , W 14 , W 13 , W 12 and W 11 , which are consistent with the input digital data and which are symmetrical with respect to a central timing ts of one sampling period Ts, as shown in Fig. 4. The second pulse width modulated waveform generator 43 generates, in synchronism with the master clock CK MS , a pulse width modulated waveform PWM 2 having pulse widths W 27 , W 26 , W 25 , W 24 , W 23 , W 22 and W 21 which are consistent with 2's complement data of the input digital data D 7 , D 6 , D 5 , D 4 , D 3 , D 2 and D 1 from the 2's complement converter 42 and which are symmetrical with respect to the central timing ts of one sampling period Ts. Thus the first and second pulse width modulating waveform generators 41, 43 receive the input digital data D 7 , D 6, D 5 , D 4 , D 3 , D 2 and D 1 , and generate, in synchronism with the master clock CK MS , pulse width modulated waveforms PWM 1 , PWM 2 having pulse widths defined by W 17 =W 21 , W 16 =W 22 , W 15 =W 23 , W 14 =W 24 , W 13 =W 25 , W 12 =W 26 and W 11 =W 27 and symmetrical with respect to the central timing ts of one sampling period Ts. The pulse width modulated waveform outputs PWM 1 , PWM 2 from the first and second pulse width modulated waveform generators 41, 43 are supplied via low-pass filters 44, 45 to the input of the differential amplifier 46, from which a differential output PWM 0 for the input digital data D 7 , D 6 , D 5 , D 4 , D 3 , D 2 and D 1 as shown in Fig. 6 is produced as a differential output between the pulse width modulated waveforms PWM 1 , PWM 2 . The low-pass filters 44, 45 function to limit the tilt of rising and falling edges of the pulse width modulated waveform outputs PWM 1 and PWM 2 from the first and second pulse width modulated waveform generators 41, 43 to a tilt consistent with the response speed of the differential amplifier 46. These low-pass filters 44, 45 may be omitted in principle if the differential amplifier 46 having ideal operating characteristics is employed. This digital-to-analog converter 4 eliminates the high frequency component of the differential output waveform PWM 0 from the differential amplifier 46, by means of the low-pass filter 47 to output an analog output signal S OUT , consistent with the input digital data D 7 , D 6 , D 5 , D 4 , D 3 , D 2 and D 1 that is, the 16-bit playback digital audio data D IN supplied to the input terminal 1, at signal output terminal 5. With the digital-to analog converter 4 of the present embodiment, as shown in the spectrum distribution diagram of Fig. 7, which illustrates the results of FFT analyses on the differential output waveform PWM 0 obtained by the differential amplifier 45 from 10kHz sinusoidal digital data, waveform distortion of the even number order, especially the second order, may be reduced for realizing a digital-to-analog converter of a simplified structure which is low in distortion factor and superior in conversion characteristics. With the digital-to-analog converter 4 of the present illustrative embodiment, the input digital data are supplied via the 2's complement circuit 42 to the second pulse width modulated waveform generator 43. However, each of the first and the second pulse width modulated waveform generators 41, 43 may be constituted by a lookup table memory in which pulse width modulated waveform data PWM 1 , PWM 2 of the various pulse widths, having the input digital data D 7 , D 6 , D 5 , D 4 , D 3 , D 2 and D 1 as the readout addresses, as shown in Table 1, are stored in advance, while the pulse width modulated waveform PWM 1 , PWM 2 may be directly formed from the input digital data D 7 , D 6 , D 5 , D 4 , D 3 , D 2 and D 1 by the first and second pulse width modulated waveform generators 41, 43. Pulse width modulation output Address PWM 1 PWM 2 D 7 W 17 W 21 D 6 W 16 W 22 D 5 W 15 W 23 D 4 W 14 W 24 D 3 W 13 W 25 D 2 W 12 W 26 D 1 W 11 W 27

Claims: A digital-to-analog converter, comprising: a first pulse width modulated waveform generating means (41) for generating a first pulse width modulated waveform consistent with input digital data; a second pulse width modulated waveform generating means (42,43) for generating a second pulse width modulated waveform consistent with the complement of said input digital data; differential amplifying means (46) for deriving a differential component of the first and second pulse width modulated waveforms output by said first and second pulse width modulated signal generating means; and filter means (47) for outputting an analog output signal free of high frequency components of the differential waveform output by the differential amplifying means; whereby a differential component between the pulse width modulated waveforms consistent with the input digital data and with complement data of said input digital data is output by the differential amplifying means and freed of high frequency components to produce analog output signals; characterised in that: the second pulse width modulated waveform generating means (42,43) is adapted for generating the second pulse width modulated waveform consistent with the 2's complement of the input digital data; a first low pass filter (44) is provided between the first pulse width modulated waveform generating means (41) and the differential amplifying means (46); and a second low pass filter (45) is provided between the second pulse width modulated waveform generating means (42,43) and the differential amplifying means (46). The digital-to-analog converter according to claim 1, wherein said second pulse width modulated waveform generating means comprises a 2's complement converting circuit (42) for converting input digital data into 2's complement data and a pulse width modulated waveform generating circuit (43) supplied with the output of said 2's complement converting circuit (42) and generating a pulse width modulated waveform consistent with the 2's complement data of said input digital data. The digital-to-analog converter according to claim 1, wherein said second pulse width modulated signal generating means comprises a pulse width modulated waveform generating circuit including a look-up table memory for outputting pulse width modulated waveform data consistent with 2's complement data of input digital data. The digital-to-analog converter according to claim 1, further comprising: oversampling means (2) for subjecting playback digital data to an oversampling operation, and noise shaping means (3) for eliminating quantization errors of output data from said oversampling means (2); wherein said playback data is supplied to said first (41) and second (42,43) pulse width modulated waveform generating means as said input digital data by way of said oversampling means (2) and said noise shaping means (3).

Inventors: KANAI TAKASHI, MASUDA TOSHIHIKO, UEKI MASAAKI, KANAI, TAKASHI, MASUDA, TOSHIHIKO, UEKI, MASAAKI, KANAI, TAKASHI, SONY CORPORATION, MASUDA, TOSHIHIKO, SONY CORPORATION, UEKI, MASAAKI, C/O SONY CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1990
Main classification: H03M1
IPC codes: H03M1, H03M3