Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
2000-07-20
2002-03-26
Wamsley, Patrick (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C341S118000
Reexamination Certificate
active
06362764
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to D/A conversion apparatus and method of a floating type which perform level conversion of input digital data by different conversion factors, converts the resulting data into analog data, then restores the analog data to an original level of the input digital data, and carries out addition of the analog data, thereby achieving an increased dynamic range of the reproduced sound.
2. Prior Art
In recent years, the conversion accuracy of A/D converters has been improved by virtue of delta-sigma modulators of higher order, and with this improvement, there is an increasing demand for further enhancement in the resolution and dynamic range of D/A converters. To meet the demand, a D/A conversion apparatus of a floating type has been conventionally developed which uses a D/A converter (hereinafter referred to as the “DAC”) having a limited number of bits for conversion, and is capable of realizing a resolution and a dynamic range exceeding respective levels attainable by the limited number of bits for conversion. In this type of converter, when an N-bit (e.g. 20-bit) DAC is used for carrying out D/A conversion of M-bit (M>N: e.g. 24-bit) digital data, if the digital data has P bits (M≧P>N) as effective bits, the digital data is directly subjected to D/A conversion without being further processed, and M−N less significant bits (e.g. four less significant bits) are truncated. On the other hand, if the input level of the digital data lowers so that the effective word length of the same is reduced to P′ bits (P′≧N), the digital data is converted into data obtained by multiplying the same by a conversion factor of 2
M−N
, i.e. by shifting the original data toward MSB (most significant bit) by M−N bits so that the M−N less significant bits have a value of zero, and then the resulting level-converted data is subjected to D/A conversion. Whether input digital data is to be subjected to D/A conversion without being further processed or after being multiplied by the conversion factor of 2
M−N
is determined depending on whether an overflow of data occurs when the input digital data is shifted by M−N bits.
In the D/A conversion apparatus constructed as above, when input data has P significant bits as effective bits, the length of word or bits for conversion is sufficiently large, so that the effect of the truncation of the M−N less significant bits is almost negligible (even if a problem occurs due to the truncation, it can be solved e.g. by additionally carrying out dithering as required). On the other hand, when the effective bit length of input data is P′ bits, the data is multiplied by the conversion factor of 2
M−N
, and the M−N less significant bits thereof are truncated during the D/A conversion. Therefore, in this case, M−N less significant bits of the data which would be truncated if the data were not multiplied by the conversion factor of 2
M−N
can be effectively D/A converted, whereby an increased resolution and an increased dynamic range are achieved. In the latter case, however, since an analog signal output from the DAC also has a magnitude multiplied by 2
M−N
, it is required to carry out a level adjustment by multiplying the analog output by 1/2
M−N
.
The D/A conversion apparatus of the floating type constructed as above includes one which employs a single DAC and the gain of an amplifier that amplifies an output from the single DAC is switched according to the conversion factor by which the level of input digital data is converted, and another which employs a plurality of DAC's that perform D/A conversion of plural pieces of digital data obtained through level conversion of input digital data by respective different conversion factors, and one of the outputs from the DAC's which has been subjected to the level conversion by the most appropriate conversion factor is selected (Japanese Patent Publication (Kokoku) No. 7-93579).
However, according to the former floating-type D/A conversion apparatus, since it is required to switch the gain of the analog amplifier instantaneously according to the level of the digital data, the output of the amplifier cannot follow up the switching, or DC offset of the amplifier can fluctuate, which can produce untoward noise which is audible. The latter floating-type D/A conversion apparatus also switches between analog signals output from the DAC's, so that transient noise occurs upon the switching. These problems are extremely serious particularly when the resolution of digital data to be subjected to D/A conversion covers even a low noise range e.g. an SN ratio of 120 to 140 dB which can be conventionally realized only by analog circuitry.
To solve these problems, the present assignee has already proposed, e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 11-308109, a D/A conversion apparatus of an advanced cross-fading algorithm type that is capable of effectively preventing generation of noise upon the switching between DAC's, by cross-fading the digital data in advance of the switching. This D/A conversion apparatus generates only a very small amount of noise upon the switching between the DAC's and is therefore capable of realizing a high-accuracy D/A conversion, as well as achieving an improved dynamic range.
In this D/A conversion apparatus, the advanced cross-fading operation is made possible by provision of a delay memory for delaying input digital data by a predetermined time period. To reduce the level of noise upon the switching between the DAC's, it is preferred that the cross-fading is carried out at a gentle cross-fading rate (by a small cross-fading step i.e., over a large number of cross-fading steps) or over a sufficiently long time period. However, to realize such a gentle advanced cross-fading without causing overflow and clipping of data even when the level of the digital data steeply changes, the delay memory is required to provide a large amount of delay, which leads to an increased size of hardware, and hence to an increased manufacturing cost. Further, such an increase in the amount of delay results in a perceivable auditory delay, particularly when the D/A conversion apparatus is applied to a mixer that processes data of live sounds, or the like.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a D/A conversion apparatus and a D/A conversion method of a floating type which are not only capable of low-noise and high-dynamic range D/A conversion but also capable of carrying out the advanced cross-fading at a cross-fading rate sufficiently gentle for practical use by using a delay memory having a small amount of delay.
To attain the above object, according to a first aspect of the invention, there is provided a D/A conversion apparatus comprising a digital signal processor that carries out level conversion of same input digital data by different conversion factors into a plurality of level-converted digital data, selects and outputs most appropriate data of the plurality of level-converted digital data based on a signal quality of each of the plurality of level-converted digital data, outputs other data of the plurality of level-converted digital data after attenuating the other data to or below a predetermined noise level, and switches between data previously selected as the most appropriate data and data newly selected as the most appropriate data by carrying out cross-fading between the previously selected data and the newly selected data, a plurality of D/A converters that carry out D/A conversion of the plurality of level-converted digital data output from the digital signal processor to respective analog signals and outputs the analog signals, and an analog adder device that carries out level conversion of the analog signals output from the plurality of D/A converters again based on respective corresponding ones of the conversion factors in a manner such that resulting a
Kuroiwa Kiyoto
Niimi Koji
Wakatsuki Ryuuji
Pillsbury & Winthrop LLP
Wamsley Patrick
Yamaha Corporation
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