Pitch shifter

Details Pitch shifter Pitch shifter

2000-12-28

2001-10-09

Fletcher, Marlon T. (Department: 2837)

Music

Instruments

Electrical musical tone generation

C084S604000, C084S622000, C084S659000, C084S661000, C084SDIG009, C381S098000, C381S101000

Reexamination Certificate

active

06300553

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to pitch shifters and, more specifically, to a pitch shifter for shifting an acoustic signal in pitch to an arbitrary level.
2. Description of the Background Art
Pitch is a sense of sound, which means the value of frequency. A pitch shifter is a device for shifting an acoustic signal in pitch to a desired level. One well-known example of such pitch shifter is a key controller provided in a karaoke CD (compact disk) player or the like.
FIGS. 16
a
to
16
c
are diagrams in assistance of explaining the principle of shifting an acoustic signal in pitch to a desired level.
As shown in
FIGS. 16
a
to
16
c,
an original acoustic signal shown in
FIG. 16
a
is compressed to become an acoustic signal as shown in
FIG. 16
b
of higher frequencies and pitch, and is extended to become an acoustic signal as shown in
FIG. 16
c
of lower frequencies and pitch.
For example, if the acoustic signal is compressed to half along the time axis, the acoustic signal becomes double in frequency, and thus increases in pitch by one octave. On the other hand, if the acoustic signal is extended double along the time axis, the acoustic signal becomes half in frequency, and thus decreases in pitch by one octave.
In general, if the acoustic signal is compressed or extended by k
−1
(where 0<k, and 1<k for compression, 0<k<1 for extension) along the time axis, the acoustic signal becomes shifted in frequency by k, and thus in pitch by (log
2
k) octave.
Hereinafter, the above-stated k representing a ratio in pitch of the original acoustic signal to the shifted acoustic signal is referred to as a “pitch shift ratio”.
As such, by compressing or extending the acoustic signal along the time axis by k
−1
, the acoustic signal can be changed in frequency by k. Such compression or extension, however, also changes a time length (reproduction time) of the acoustic signal by k
−1
if no other measures is taken together. Therefore, so-called “crossfading” is further carried out on the acoustic signal to prevent changes in time length.
FIG. 17
is a diagram in assistance for explaining the principle of a crossfading process for smoothly connecting two insuccessive sound frames.
As shown in
FIG. 17
, consider a case in which a frame B is deleted, and a frame A and a frame C are connected together. In this case, if the frame A and the frame C are connected without any change, discontinuity occurs in signal value at their connecting point, and therefore noise may occur at signal reproduction.
Thus, these frames are connected together with the frame A being faded-out and the frame C being faded-in. Thus, continuity is kept in signal value at their connecting point, and therefore noise is prevented at signal reproduction.
However, if the frame A and the frame C are connected together by crossfading, reproduction time is shortened compared with the case where these frames are connected together without any change. Therefore, a combination of compression/extension along the time axis and crossfading enables shift in pitch of the acoustic signal without any other change.
FIGS. 18
a
and
18
b
are diagrams in assistance for explaining the principle of shifting the acoustic signal in pitch without any change in reproduction time.
FIG. 18
a
shows a case in which a signal is increased in pitch, that is, compressed along the time axis (time-axis compression).
FIG. 18
b
shows a case in which a signal is decreased in pitch, that is, extended along the time axis (time-axis extension).
In
FIGS. 18
a
and
18
b,
a time length of a frame after time-axis compression/extension, that is, an output frame length, is first determined. Then, an input frame length based on the pitch shift ratio is determined. Here, assume that the pitch is multiplied by k, the output frame length is 2, and the input frame length is 2 k.
Next, input frames of each frame length “2k” are sequentially extracted from the original signal as successive two frames overlap each other. The length of an overlapping part is (2k−1). In
FIGS. 18
a
and
18
b,
three input frames represented by A
1
and B
2
, A
2
and B
3
, and A
3
and B
4
, respectively, are shown.
Next, each extracted input frame is compressed/extended by k
−1
along the time axis with reference to the head of each frame (alternatively, with reference to the midpoint or end thereof). Thus, output frames of each frame length “2” can be produced. Among the output frames, successive two output frames overlap each other in half of each frame length.
Specifically, in
FIG. 18
a,
(A
1
H and B
2
H), (A
2
H and B
3
H) and (A
3
H and B
4
H) are the output frames, and (B
2
H and A
2
H), (B
3
H and A
3
H) are the overlapping parts. In
FIG. 18
b,
(A
1
L and B
2
L), (A
2
L and B
3
L), (A
3
L and B
4
L) are the output frames, and (B
2
L and A
2
L), and (B
3
L and A
3
L) are the overlapping parts.
Next, all these output frames are connected together by crossfading. The crossfading process may be carried out over the whole or part of the overlapping parts.
In
FIG. 18
a,
two cases are shown, one in which the crossfading process is carried out over the whole of the overlapping parts B
2
H and A
2
H, and B
3
H and A
3
H, and the other over 25% thereof. Also in
FIG. 18
b,
two cases are shown, one in which the crossfading process is carried out over the whole (that is, 100%) of the overlapping parts B
2
L and A
2
L, and B
3
L and A
3
L, and the other over 25% thereof.
Thus, the acoustic signal can be changed in frequency by k times while being unchanged in reproduction time.
Described below is a conventional pitch shifter for carrying out a pitch shifting process on discrete sound data through crossfading compression/extension.
FIG. 19
is a block diagram showing one example of structure of the conventional pitch shifter.
FIG. 20
is a block diagram showing one example of structure of a conventional CD player equipped with the pitch shifter of FIG.
19
.
In
FIG. 20
, a CD
20
has discrete sound data {x(
0
), x(
1
), x(
2
), x(
3
), . . . } produced by sampling an acoustic signal in every predetermined cycle T and recorded thereon in advance. The CD player includes a reader
21
, a reproducer
22
, a sound pitch shift ratio setting unit
23
, a pitch control signal generator
24
, and a sound data output terminal
25
, a pitch control signal output terminal
26
, and a sound data input terminal
27
.
The pitch shift ratio setting unit
23
includes a selector for selecting any of a plurality of predetermined pitch shift ratios or an adjustment control for specifying an arbitrary pitch shift ratio. The pitch shift ratio setting unit
23
sets the pitch shift ratio selected or arbitrarily specified by a user in the CD player. The pitch control signal generator
24
generates a pitch control signal indicating the pitch shift ratio set by the pitch shift ratio setting unit
23
. The pitch control signal generated by the pitch control signal generator
24
is outputted from the pitch control signal output terminal
26
.
The reader
21
sequentially reads sound the data from the CD
20
. The sound data read by the reader
21
is sequentially outputted from the sound data output terminal
25
in every cycle T.
The pitch shifter receives the sound data {x(
0
), x(
1
), x(
2
) x(
3
), . . .} sequentially outputted from the sound data output terminal
25
and the pitch control signal outputted from the pitch control signal output terminal
26
, and then sequentially produces sound data after shifted in pitch {out(
0
), out(
1
), out(
2
), out(
3
), . . . } in the cycle T.
The sound data after shifted in pitch sequentially produced by the pitch shifter is outputted from the sound data input terminal
27
. The reproducer
22
receives the sound data after shifted in pitch {out(
0
), out(
1
), out (
2
), out(
3
), . . . } outputted from the sound data input terminal
27
, and reproduces the acoustic signal. The acoustic signal reproduced by the reproducer
2

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