Pulse or digital communications – Pulse code modulation – Differential
Reexamination Certificate
1999-11-04
2003-09-30
Chin, Stephen (Department: 2634)
Pulse or digital communications
Pulse code modulation
Differential
C341S077000
Reexamination Certificate
active
06628720
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a transmitting apparatus and a reproducing apparatus whereby a one-bit digital audio signal acquired through delta-sigma modulation is converted to a multi-bit audio signal to be transmitted.
This application has already proposed a system whereby an analog audio signal is subjected to delta-sigma modulation at a sampling frequency significantly higher than that of the conventional compact disc (CD) in order to transmit, record and reproduce a one-bit digital audio signal.
The one-bit digital audio signal above is radically different from a multi-bit digital audio signal exemplified by the compact disc (CD) having a sampling frequency (fs) of 44.1 kHz and 16 quantization bits. That is, a high-quality audio standard offering a wide band and a wide dynamic range is implemented by means of the one-bit digital signal system that involves the quantization bits being sampled at a sampling frequency 64 times the sampling frequency (fs) of the conventional CD. This high-quality audio standard is called the DSD (Direct Stream Digital) standard, and discs having data recorded thereon as per the DSD Standard comply with what is known as the Super Audio CD Standard.
A one-bit digital audio signal obtained by delta-sigma modulation may be converted to a conventional multi-bit digital audio signal through down-sampling by a converter such as a decimation filter. Before the one-bit digital audio signal is output to a conventional digital audio interface that complies with IEC 958 (International Electrotechnical Commission 958), the signal needs to be converted to the conventional multi-bit digital audio signal through down-sampling.
In a setup where an analog audio signal is converted to a one-bit digital audio signal through delta-sigma modulation, there is defined a degree of modulation corresponding to the input conversion level in effect. That is, the modulation degree represents level information that is possessed by the one-bit digital audio signal in question. A maximum modulation degree, i.e., a maximum allowable input level defined by a delta-signal modulator in use, is generally established in accordance with an optimum point of modulator characteristics. If an analog audio signal level is input so as to obtain a maximum modulation degree, the rise of the level seldom results in an abrupt deterioration of level and distortion characteristics specific to an output one-bit digital audio signal.
Meanwhile, the modulation degree of an input one-bit digital audio signal and the level of an output multi-bit digital audio signal are generally determined in a unique manner for each decimation filter. For example, where a decimation filter is designed to have a maximum modulation degree of 50 percent, the filter outputs a multi-bit digital audio signal at its maximum value (i.e., full bit level) when a one-bit digital audio signal with the modulation degree of 50 percent is input. Upon input of a one-bit digital audio signal exceeding the maximum modulation degree, the multi-bit digital audio signal is kept fixed to the maximum value, i.e., brought into a clipped state.
The process above is explained below in more detail with reference to
FIG. 1
which shows a conventional digital signal processing apparatus
50
. In the digital signal processing apparatus
50
, an analog audio signal A
1
input through an input terminal
51
is fed to a one-bit delta-sigma modulator
52
for delta-sigma modulation into a one-bit digital audio signal D
1
. The one-bit digital audio signal D
1
from the modulator
52
is supplied to both a one-bit D/A converter
53
and a decimation filter
55
.
FIG. 2
illustrates a typical constitution of the one-bit delta-sigma modulator
52
mentioned above. The one-bit delta-sigma modulator
52
comprises an adder
71
, an integrator
72
, a quantizer
73
, and a delay circuit
75
. An added output of the adder
71
is fed to the integrator
72
. An integrated output of the integrator
72
is sent to the quantizer
73
. A quantized output of the quantizer
73
is led out of an output terminal
74
and fed through the delay circuit
75
back to the adder
71
in which the quantized output is added to an analog audio signal coming from an input terminal
70
. The added output of the adder
71
is integrated by the integrator
72
whose integrated output is in turn quantized by the quantizer
73
per sampling period. After the processing, one-bit quantized data, represented by the one-bit digital audio signal D
1
, are output from the output terminal
74
.
The one-bit D/A converter
53
converts the one-bit digital audio signal D
1
to an analog audio signal A
0
that is sent to an output terminal
54
. Generally, the audio signal composed of one-bit digital audio signals is converted back to the analog audio signal by the D/A converter as described.
The decimation filter
55
turns the one-bit digital audio signal D
1
into a multi-bit digital audio signal D
M
that is fed to an output terminal
56
. The decimation filter
55
converts the one-bit digital audio signal having the sampling frequency of 64×fs into a multi-bit digital audio signal of 16 bits with the sampling frequency of fs, whereby sound quality equivalent to that of CD is acquired. Typically, the decimation filter
55
has the maximum modulation degree of 50 percent.
There is a problem when the decimation filter
55
having the maximum modulation degree of 50 percent constitutes part of a converter that converts the one-bit digital audio signal D
1
through down-sampling into the multi-bit digital audio signal D
M
. That is, if the level of an input one-bit digital audio signal D
1
exceeds the maximum modulation degree, the output multi-bit digital audio signal D
M
is fixed to a clip level.
Below is a detailed description of the deficiency outlined above.
FIG. 3
is a graphic representation of characteristics indicating levels of an analog audio signal A
0
output by the one-bit D/A converter
53
and levels of a multi-bit digital audio signal D
M
furnished by the decimation filter
55
with respect to modulation degrees (input signal levels) of a one-bit digital audio signal D
1
from the one-bit delta-sigma modulator
52
in the digital signal processing apparatus
50
. The levels of the analog audio signal A
0
are plotted by a broken line characteristic
61
while the levels of the multi-bit digital audio signal D
M
are depicted by a solid line characteristic
62
.
The broken line characteristic
61
indicates that the levels of the analog audio signal A
0
remain approximately linear when the modulation degree of the input signal exceeds 50 percent. This is what is known as a soft clipped state, not perfectly linear, but almost. On the other hand, as indicated by the solid line characteristic
62
, the multi-bit digital audio signal D
M
is clipped to a full bit level when the modulation degree exceeds 50 percent. The decimation filter
55
is designed to effect output at the full bit level illustratively when the modulation degree reaches 50 percent. Thus an input with the modulation degree of 50 percent or higher causes the output signal to be clipped as exemplified by the characteristic
62
.
FIG. 4
is another graphic representation of characteristics plotting distortion factors of the analog audio signal A
0
output by the one-bit D/A converter
53
as well as distortion factors of the multi-bit digital audio signal D
M
furnished by the decimation filter
55
with respect to modulation degrees of the one-bit digital audio signal D
1
in the digital signal processing apparatus
50
. The distortion factors of the analog audio signal A
0
are denoted by a broken line characteristic
63
while the distortion factors of the multi-bit digital audio signal D
M
are represented by a solid line characteristic
64
.
The broken line characteristic
63
indicates that the distortion factor of the analog audio signal A
0
is at a minimum point when the modulation degree is about 50 percent. This means that the best characteristic is obt
Chin Stephen
Lugo David B.
Maioli Jay H.
Sony Corporation
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