Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
2000-05-04
2002-03-05
JeanPierre, Peguy (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C341S139000
Reexamination Certificate
active
06353404
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 owing to 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 is lowered 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.
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 which are of a floating type and capable of further minimizing the adverse influence of noise to thereby achieve an increased dynamic range.
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 analog signals have a level corresponding to a level of the input digital data, and then adds together all of the level-converted analog signals, wherein the digital signal processor carries out the switching over a predetermined cross-fading time period when an amplitude level of the input digital data crosses a predetermined threshold level from a larger side than the predetermined threshold level to a smaller side than the predetermined threshold level, insofar as the amplitude level of the input digital data does not cross the predetermined threshold level from the smaller side to the larger side for a predetermined hold time period, and wherein the predetermined threshold level comprises a plurality of threshold levels, the predetermined hold time period and the predetermined cross-fading time period being set in a manner corresponding to each of the plurality of threshold levels such that as the predetermined threshold level is smaller, the predetermined hold time period and the predetermined cross-fading time period are set to respective shorter time periods.
According to this D/A conversion apparatus, (
1
) results of a plurality of digital data obtained by D/A conversion are added for analog output, (
2
) switching of outputs of D/A conversion is carried out by switching attenuation degrees of digital data, (
3
) cross-fading is carried out for switching between converted digital data, and (
4
) the switching to selected digital data is carried out when a level (amplitude level) of the input digital data crosses a predetermined threshold level from a larger side than the predetermined threshold level to a smaller side than the predetermined threshold level, insofar as the level o
Jean-Pierre Peguy
JeanGlaude Jean Buner
Pillsbury & Winthrop LLP
Yamaha Corporation
LandOfFree
D/A conversion apparatus and D/A conversion method does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with D/A conversion apparatus and D/A conversion method, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and D/A conversion apparatus and D/A conversion method will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2878457