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BACKGROUND OF THE INVENTION
The present invention relates to a image data transfer method, i.e.,
coding, transmitting and decoding method of moving image digital data
which method is efficient, cost effective and capable of transferring
rapidly moving images clearly without a loss of recording time and image
quality.
Conventional transfer methods of moving image data to be used with image
coding transmission systems are disclosed in JP-A-No. 62-164391 and IEEE,
Trans. on Communications, Vol. COM-32, No. 3, March 1984, pp. 225 to 232.
FIG. 2 is a block diagram showing an image data transfer system according
to the above conventional techniques. In the figure, reference numeral 20
designates an image data transfer unit (moving image coding unit), 22 an
input video signal, 23 an A/D converter, 24 image data, 25 a frame memory
whose capacity allows storage of image data of at least one frame, 26 an
image coding circuit, 27 coded data, 28 a buffer memory, 29 a coded data
amount control circuit, and 30 a coded data amount control signal.
Reference numeral 31 represents a transmission medium, 21 an image data
reproduction unit (moving image decoding unit), 32 coded data, 33 a buffer
memory, 34 an image decoding circuit, 35 image data, 36 a frame memory, 37
a D/A converter, and 38 an output video signal. FIG. 3 is a graph showing
an example of a change with time of the amount of coded data (hereinafter
called coded data amount) generated by the image coding circuit 26 shown
in FIG. 2. Generally, the coded data amount becomes small as the frame
correlation becomes large, and large as the latter becomes small.
In operation of the system shown in FIG. 2, the input video signal 22 is
transformed into digital image data 24 by the A/D converter 23, one frame
after another every 1/30 second, and is stored in the frame memory 25. The
image data 24 stored in the frame memory 25 is subjected to high
efficiency coding by the image coding circuit 26 to be stored in the
buffer memory 28. The coded data 27 stored in the buffer memory 28 is read
out therefrom at a speed of, e.g., 1M bits per second (hereinafter
represented by 1 Mbps) and is outputted to the transmission medium 31. If
the content of an image in the input video signal 22 changes slowly, the
coded data amount 40 generated by the image coding circuit 26 is small,
such as in the case of frames a, b, c and d shown in FIG. 3. Namely, the
coded data amount, 40 is smaller than the data transfer capacity 41 or
level A per one frame determined by the data transmission speed of the
transmission medium 31 (in this case, the data transfer capacity per frame
is 1 Mbps.times.1/30 second which is approximately 32K bits). In such a
case, the coded data with dummy data inserted therein is sometimes used
for synchronous data transmission between the image data transfer unit 20
and the image data reproduction unit 21. On the other hand, if the content
of the image in the input video signal 22 changes rapidly, or the scene
represented by the input video signal 22 changes, the coded data amount 40
generated by the image coding circuit 26 may exceed the data transfer
capacity 41 or level A per one frame, such as in the case of the e and f
frames shown in FIG. 3. In such a case, in view of an increase of data
amount in the buffer memory 28, a frame such as g frame is deleted by
means of the coded data amount control circuit 29, or coding parameters
are changed to use a higher data compression rate, such as in the case of
the h and i frames. The coded data amount 40 is thus forcibly made small.
A constant data transmission speed has been used for the transmission
medium in the prior art techniques, so that frame deletion and/or a coding
parameter change has been used for rapidly moving images in order to
forcibly reduce the coded data amount. Therefore, the image quality of
moving images with rapid change deteriorates extremely, posing a problem
associated with information transfer and image reproduction.
In order to solve the above problem, the data transmission speed of the
transmission medium can be increased. However, according to the
conventional image data transfer method, the data transfer speed is fixed
at a constant value, so that, if a communication line is used as the
transmission medium, an excessive rise in communication cost occurs. Also,
if a package-based medium such as CD-ROM (Compact Disc Read Only Memory)
is used as the transmission medium, the total image recording time is
considerably reduced. Each of them poses another issue to be solved.
It has been desired therefore to realize an image data transfer method
which can solve the above problems and allow rapidly moving images to have
the same quality as that of gently moving images, with less reduction of
total image recording time, and with cost effectiveness.
SUMMARY OF THE INVENTION
It is an object of the present invention to realize an image data transfer
method which, in transferring image data including both gently and rapidly
moving images, it is possible to prevent quality deterioration of rapidly
moving images, with substantially no reduction of total image recording
time, and with high efficiency and cost effectiveness.
According to one aspect of the image data transfer method of the present
invention, in transferring image data including both gently and rapidly
moving images, different methods of image data transfer are used
respectively for gently and rapidly moving images. Specifically, the image
data transfer amount to be sent from the image data transfer unit is
reduced for gently moving images and increased for rapidly moving images.
According to this method, the transfer data amount for rapidly moving
images can be made large so that even rapidly moving images do not undergo
image quality deterioration. Further, in transferring image data including
both gently and rapidly moving images, the total image recording time does
not increase substantially since the transfer data amount for gently
moving images is made small.
According to the image data transfer method of the present invention, three
different types of image transfer methods are used.
According to a first embodiment of the present invention, image data is
grouped in units of a predetermined plurality of frames to calculate an
average data amount of each group. The data transfer speed is changed in
accordance with the average data amount to perform high speed data
transfer for rapidly moving images and low speed data transfer for gently
moving images.
According to a second embodiment of the present invention, the transfer
speed of image data is fixed constant or it is arranged to take a few
steps, e.g. two steps of high and low speeds. However, the apparent data
transfer speed is changed through a skip, pause or seek operation. Image
data for rapidly moving images is transferred continuously at high speed,
whereas image data for gently moving images is intermittently transferred
through a skip or pause operation to thereby reduce an apparent data
transfer speed.
According to a third embodiment of the present invention, in coding image
data for a plurality of frames stored in a frame memory including both
rapidly and gently moving images, a fraction of the coded data for rapidly
moving images is filled in the remaining portion of the data transfer
capacity of gently moving images and pretransferred. Therefore, without
increasing the total transfer data amount, the transfer amount of image
data information for rapidly moving images can be increased.
It is obvious that a combination of the above three different image data
transfer methods may also be used.
The present invention realizes an image data transfer method which can
transfer rapidly moving images clearly, with substantially no reduction of
image, recording time, and with high efficiency and cost effectiveness.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing a first embodiment of an image data
transfer unit (moving image coding/recording unit) according to the image
data transfer method of the present invention;
FIG. 2 is a block diagram showing a conventional image data transfer
system;
FIG. 3 is a graph showing an example of a change in coded data amount in a
conventional image data transfer method;
FIG. 4 is a graph showing an example of a change in coded data amount in
the image data transfer unit of the first embodiment shown in FIG. 1, and
the corresponding change of data transfer capacity;
FIG. 5 is a block diagram of an image data reproduction unit (moving image
decoding/reproducing unit) used in combination with the image data
transfer unit of the first embodiment of the present invention shown in
FIG. 1;
FIG. 6 is a schematic diagram showing the structure of a CD-ROM which is
one type of optical disk;
FIGS. 7(a)-7(e) are schematic diagrams showing the data format of a CD-ROM;
FIG. 8 is a block diagram showing a second embodiment of the image data
transfer unit (moving image coding/recording unit) according to the image
data transfer method of the present invention;
FIG. 9 is a block diagram of an image data reproduction unit (moving image
decoding/reproducing unit) used in combination with the image data
transfer unit of the second embodiment of the present invention shown in
FIG. 8;
FIGS. 10(a)-10(e) illustrate an embodiment of a data transfer speed control
scheme used with the unit shown in FIG. 8 according to the image data
transfer method of the present invention;
FIGS. 11(a)-11(e) illustrate an embodiment of another data transfer speed
control scheme used with the unit shown in FIG. 8 according to the image
data transfer method of the present invention;
FIGS. 12(a)-12(e) illustrate an embodiment of a further data transfer speed
control scheme used with the unit shown in FIG. 8 according to the image
data transfer method of the present invention;
FIG. 13 is a block diagram showing a third embodiment of an image data
transfer unit (moving image coding/recording unit) according to the image
data transfer method of the present invention;
FIG. 14 is a block diagram of an image data reproduction unit (moving image
decoding/reproducing unit) used in combination with the image data unit of
the third embodiment of the present invention shown in FIG. 13;
FIG. 15 is a schematic diagram showing an example of a change in coded data
amount in the moving image coding unit of the third embodiment of the
present invention; and
FIGS. 16(a) and (b) are schematics diagram showing a flow of coded data in
the moving image coding unit of the third embodiment of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The embodiments of the present invention will be described in detail with
reference to the accompanying drawings.
1st Embodiment
FIG. 1 is a block diagram showing a first embodiment of an image data
transfer unit (moving image coding/recording unit) for coding and
recording data in an optical disc according to the image data transfer
method of the present invention. In FIG. 1, reference numeral 1 represents
a moving image coding unit, and reference numeral 2 represents an optical
disc manufacturing unit. Reference numeral 3 represents a moving image
source such as a film player, VTR or the like, 4 a moving image signal, 5
an A/D converter, 6 image data, 7 a frame memory whose capacity
corresponds to at least n frames, 8 an image coding circuit, 9 coded data,
10 a data transfer speed calculation circuit, 11 data transfer speed
information, and 12 a buffer memory. Reference numeral 13 represents a
mastering apparatus which reads coded data at a constant speed and forms
an original optical disc, 14 an original optical disc, 15 a press machine
for manufacturing an optical disc from the original optical disc, and 16
an optical disc.
FIG. 4 is a graph showing an example of a change of coded data amount
generated in the image coding circuit 8 shown in FIG. 1, and FIG. 5 is a
block diagram of an image data reproduction unit (moving image
decoding/reproducing unit) for reproducing data from an optical disc
formed by the image data transfer unit shown in FIG. 1. Referring to FIG.
4, reference numeral 45 represents a coded data amount generated by the
image coding circuit 8, and reference numerals 46, 47 and 48 data transfer
capacities to be changed in n frames unit. Referring to FIG. 5, reference
numeral 50 represents an optical disc reproducing unit, 51 a moving image
decoding unit, 52 an optical disc on which coded moving images have been
recorded, 53 a servo control circuit for control of a motor and an optical
pickup device, 54 a motor for rotating an optical disc, 55 a motor
rotation control signal, 56 an optical pickup device, 57 a preamplifier
for amplifying a signal from the optical pickup device, 58 a data
processing circuit for reproducing data, 59 reproduced coded data, 60 data
transfer speed information, 61 a buffer memory for storage of reproduced
coded moving images, 62 an image decoding circuit for decoding coded
images, 63 decoded image data, 64 a frame memory, 65 a D/A converter, and
66 an output video signal.
The operation of the image transfer unit shown in FIG. 1 will be described.
In coding and recording images of the moving image source 3 into an
optical disc, first moving image signals 4 for n frames are transformed
into image data 6 by the A/D converter 5, and stored in the frame memory
7. The image data 6 for n frames in the frame memory 7 are coded by the
image coding circuit 8. The data transfer speed calculation circuit 10
calculates an average data amount of the coded data 9 to output it as the
data transfer speed information 11. Specifically, the data transfer speed
calculation circuit 10 determines V(t) such that it becomes:
##EQU1##
where V(t) is a calculated data transfer speed, and D(i) is a coded data
amount for each frame. For example, as shown in FIG. 4, V(t) takes a
value:
##EQU2##
The data transfer speed information 11 as well as the coded data 9 are
stored in the buffer memory 12. The stored data is edited into an optical
disc format by the mastering apparatus 13 to be recorded in an original
optical disc 14 from which an optical disc 16 is formed by the press
machine 15.
The operation of the image data reproduction unit shown in FIG. 5 will then
be described. Data stored in the optical disc 52 is read with the optical
pickup device 56, amplified and waveform-shaped by the preamplifier 57.
The data is then demodulated by the data processing circuit 58 and, is
thereafter, subjected to a data error correction process in accordance
with a predetermined procedure. The reproduced data is separated into the
coded data 59 and data transfer speed information 60. In accordance with
the data transfer speed information 60 reproduced by the data processing
circuit 58, the servo control circuit 53 outputs the motor rotation
control signal 55 for control of the motor 54. In this case, a servo
operation is performed so as to make the data transfer speed outputted
from the preamplifier 57 constant. Specifically, a constant line velocity
(CLV) rotation control is performed so as to obtain the data transfer
speed determined by the data transfer speed information 60. For example,
as described previously, the data transfer speed takes one of three speeds
.alpha., .beta. and .gamma. (bps). The coded data 59 of the moving image
reproduced by the data processing circuit 58 is stored in the buffer
memory 61 and decoded into the image data 63 by the image decoding circuit
62 to be written in the frame memory 64. The image data 63 is sequentially
read from the frame memory in accordance with the designated scan scheme,
and is transformed into the output video signal 66 by the D/A converter 65
and outputted therefrom. Assuming that the maximum data transfer speed is
.gamma. (bps), then the capacity of the buffer memory 61 becomes
sufficient if only it takes a value of .gamma..times.n.times.1/30 (bit).
However, in practice, it is necessary to have some additional memory
capacity in consideration of a delay of the optical disc 52 in responding
to a change of the data transfer speed. Although the embodiment processes
image data in n frames as a unit, the value of n may be changed in inverse
proportion with the data transfer speed to reduce the capacity of the
buffer memory 61.
In the above embodiment, the data transfer speed has been changed by
controlling the rotation speed of an optical disc. An optical disc
reproducing unit having a fixed data transfer speed can be used. For
example, using a commonly employed CD-ROM reproducing unit, it is possible
to make the data transfer speed equivalently variable through a devised
data transfer scheme. An example of such method will be described in
detail below.
A CD-ROM records in an optical disc similar to that of an audio CD, not
digital audio data but computer data. The structure of an optical disc is
shown in FIG. 6. An optical disc is made of a circular disc having a
diameter of 12 cm and thickness 1.2 mm with one spiral track 70 of about 5
km formed thereon. On the track 70 there are recorded pits of width 0.4
micron and length 0.9 to 3.3 micron (which length varies with recorded
data). A lead-in area 71 starts at the diameter 46 mm, a program area 72
follows from diameter 50 mm to 116 mm, and a lead-out area 73 up to
diameter 120 mm. Digital audio data and computer data are recorded in the
program area 72. A TOC (Table Of Contents) indicating the directory of the
whole disc is recorded at the lead-in area.
The format of data recorded in a CD-ROM disc is shown in FIG. 7. The
minimum data record unit is a frame 80 similar to the case of an audio CD,
as shown in FIG. 7(a). As shown in FIG. 7(b), the frame 80 made of 588
bits in total includes frame sync data 81, sub-code 82 (attribute
information freely usable), digital data 83 of 24 bytes, and an error
correction code ECC 84 according to double coded Reed Solomon Code. Frames
80 of about 26 millions are assigned on the track 70 of the disc. Data is
not sequentially recorded in the disc but is properly interleaved over a
fairly long region and recorded therein in order to improve the error
correction ability for burst errors to be caused by defects or stains on
the disc. In the case of an audio CD, 16 bit, 2 channel digital audio data
totaling in amount 24 bytes and sampled from an audio signal at 44.1 KHz
is recorded in one frame 80. In the case of a CD-ROM, instead of such
audio data, computer data is recorded.
One sector 85 of the CD-ROM is constructed of, as shown in FIG. 7(c), 2352
bytes of 98 frame digital data 83. The 2048 byte computer data are
recorded and blocked therein. One sector 85 includes 12 byte sync data 86
for synchronization purposes, 4 byte header data 87 for address/mode
information, 2048 byte computer data 83, a 32 bit CRC, a 4 byte error
detection code EDC 89, an 8 byte reserved region 90 for future expansion,
and a 276 byte error correction code ECC 91 according to a double coded
Reed Solomon Code. A computer recognizes if computer data 88 of 2 kB are
recorded sequentially one after another as shown in FIG. 7(e).
An enhanced error correction is applied to audio CDs as previously
described. In the case of CD-ROMs which are directed to use by computers,
data integrity is improved through a further enhanced error correction.
The data transfer speed of a CD-ROM is 1.2 Mbps, i.e., 150 kB/sec,
allowing computer data of about 40 MB to be recorded.
2nd Embodiment
FIG. 8 is a block diagram showing an image data transfer unit (moving image
coding/recording unit) for recording moving images into a CD-ROM, and FIG.
9 is a block diagram showing an image data reproduction unit (moving image
decoding/reproducing unit) for reproducing moving images from a CD-ROM.
Referring first to FIG. 8, reference numeral 100 represents a moving image
coding unit, 111 a CD-ROM manufacturing unit, and 112 a CD-ROM. In the
moving image coding unit 100, reference numeral 101 represents an input
video signal, 102 an A/D converter, 103 image data, 104 a frame memory
having a one or more frame capacity, 105 an image coding circuit, 106
coded data, 107 data transfer speed information, 108 a data multiplexer,
109 a buffer memory, and 110 a data transfer speed information generation
circuit. Referring next to FIG. 9, reference numeral 120 represents a
moving image decoding unit, 121 a CD-ROM, and 122 a CD-ROM reproducing
unit. In the moving image decoding unit 120, reference numeral 123
represents a data separator, 124 coded data, 125 data transfer speed
information, 126 a data transfer control circuit, 127 a buffer memory, 128
an image decoding circuit, 129 image data, 130 a frame memory having a one
or more frame capacity, 131 a D/A converter, and 132 an output video
signal.
The operation of the image data transfer unit shown in FIG. 8 will be first
described. A video signal outputted from a television, video recorder or
the like is separated from the NTSC composite video signal into three
components video signals (e.g., luminance signal Y, color difference
signals R-Y and B-Y). The input video signals 101 are sampled at a
predetermined sampling frequency (e.g., frequency four times as high as
the color subcarrier frequency 3.58 MHz) by the A/D converter 102 and
sequentially converted into the digital image data 103. Since the number
of bits processed at the A/D converter 102 is usually 8 bits, the data
amount of image data 103 per pixel is 8 bits for each component, totaling
24 bits. The frame memory 104 has a capacity capable of storing image data
of plural frames so that the digital image data 103 is sequentially stored
therein up to a predetermined number of frames. Although the number of
necessary frames depends on the type of coding method, if an inter-frame
coding is employed for example, image data of two frames at a minimum
becomes necessary. The image data 103 stored in the frame memory 104 is
subjected to high efficiency coding by the image coding circuit 105 to
generate the coded data 106 with less redundancy. Although there are known
various image coding methods, if for example a motion compensated
inter-frame coding method is employed, it is possible to compress image
data about 1/100 in average. Namely, the image data 103 having 24 bits per
pixel can be compressed to coded data having about 0.2 bit per pixel.
However, depending upon the ratio of moving image regions, the degree of
rapid motion, fineness and clearness of the image pattern, and the like,
the data compression ratio varies considerably, so that the data amount of
the coded data 106 changes with each frame.
Conventionally, a devised method has been generally adopted in controlling
the amount of the coded data 106 generated by the image coding circuit
105, whereby the coding parameters are changed in accordance with the
amount of coded data 106 stored in the buffer memory 109. The devised
method aims at maintaining the amount of the coded data in the buffer
memory 109 as constant as possible, and making the average value of data
amount generated during a unit time coincide with the data transfer speed
of a CD-ROM. Conventionally, since coding and decoding units which employ
a communication line with a constant data transfer speed have been
developed, such a devised method of controlling the coded data amount by
changing the coding parameters has been generally adopted. However, with
this method, the data amount is controlled by changing the coding
parameters at any time when necessary, independently from the information
amount or content of an inputted image, so that the coding error varies
greatly with each frame, resulting in considerable deterioration of an
image quality. In view of this, according to the present embodiment, a
method is employed wherein the data transfer speed information generating
circuit 110 is provided for generating the data transfer speed information
107 in accordance with the amount of the coded data 106 stored in the
buffer memory 109, to accordingly control the data transfer speed from the
buffer memory 109 to the CD-ROM manufacturing unit 111. With this method,
it becomes possible to improve the image quality while suppressing the
variation of coding error, and record moving images for a long period. The
method will be later described more particularly.
The coded data 106 outputted from the image coding circuit 105 is
multiplexed with the data transfer speed information 107 by the data
multiplexer circuit 108, and is stored in the buffer memory 109. The
stored data is sequentially read out from the buffer memory 109 and is
outputted to the CD-ROM manufacturing unit 111 while controlling the data
transfer speed. A CD-ROM is not such a medium in which users can record
data easily that the CD-ROM manufacturing unit 111 is used to form an
original disc and press it into a CD-ROM 112.
Next, the operation of the image data reproducing unit shown in FIG. 9 will
be described. Image data in the CD-ROM disc 121 is reproduced and read by
the CD-ROM reproducing unit 122. The read-out data is separated into the
coded data 124 and data transfer speed information 125 by the data
separator 123. In accordance with the data transfer speed information 125,
the data transfer control circuit 126 controls the data transfer speed to
the CD-ROM reproducing unit 122. This control scheme will be described
later in detail. The coded data 124 is temporarily stored in the buffer
memory 127. The amount of the coded data 124 to be stored in the buffer
memory 127 is maintained substantially uniform because the data transfer
speed control is being performed. The coded data 124 read out from the
buffer memory 127 is transformed into the image data 129 by the image
decoding circuit 128. The decoded image data 129 is stored in the frame
memory 130 and transformed into the analog video signal 132 by the A/D
converter 131 to be displayed on a CRT or the like.
The description will now be directed to the scheme for control of data
transfer speed from the moving image coding unit 100 to the CD-ROM,
manufacturing unit 111 shown in FIG. 8, and to the scheme for control of
data transfer speed from the CD-ROM reproducing unit 122 to the moving
image decoding unit 120 shown in FIG. 9. The scheme for control of data
transfer speed will now be described in detail wherein the data transfer
speed takes two steps including a maximum data transfer speed and a lower
data transfer speed, the former being set at the CD-ROM data transfer
speed of 1.2 Mbps (i.e., 150 kB/sec).
FIG. 10 illustrates a first example of the data transfer speed control
scheme as mentioned with respect to the second embodiment. FIG. 10(a)
shows a sequence of coded data 106 recorded sequentially on the track of a
CD-ROM, FIG. 10(b) shows how data is transferred from the moving image
coding unit 100 to the CD-ROM manufacturing unit 111, FIG. 10(c) shows the
data transfer speed, FIG. 10(d) shows the amount of the coded data 106
generated by the image coding circuit 105, and FIG. 10(e) shows the amount
of the coded data 106 stored in the buffer memory 109.
The coded data amount 152 of the moving image subjected to high efficiency
coding varies greatly as shown in FIG. 10(d). Therefore, it is necessary
to control the data amount 153 within the buffer memory by changing the
data transfer speed. Specifically, while data transfer of coded data at
sector 140 is performed at the maximum data transfer speed, the data
amount 153 within the buffer memory falls to the lower limit threshold 154
at time A shown in FIG. 10(e). Then, coded data at sectors 141 to 149 are
transferred by changing to the lower data transfer speed at time A. During
this data transfer, the data amount 153 within the buffer memory goes to
the upper limit threshold 155 at time B. Therefore, coded data at sector
150 is transferred at the maximum data transfer speed again. In order to
allow data transfer at the lower data transfer speed, sectors 141, 143,
145, 147 and 149 are skipped whereas sectors 142, 144, 146 and 148 are
read, as shown in FIGS. 10(a) and 10(b) during the period from time A to
time B. Accordingly, the lower data transfer speed (indicated by a broken
line in FIG. 10(c), which is half the maximum data transfer speed, can be
realized equivalently.
During skipping of sectors, data transfer is not performed, or
alternatively data transfer is performed with the transferred data
neglected. Data other than coded data may be stored in sectors to be
skipped so that the memory capacity can be effectively utilized.
FIG. 11 illustrates a second example of the data transfer speed control
scheme as mentioned with respect to the second embodiment. FIG. 11(a)
shows a sequence of coded data, FIG. 11(b) shows how the coded data is
transferred, FIG. 11(c) shows the coded data amount, and FIG. 11(e) shows
the data amount within the buffer memory.
The data amount 168 within the buffer memory is controlled by changing the
data transfer amount in accordance with a change of the coded data amount
167. Specifically, during the period from time A when the data amount 168
within the buffer memory falls to the lower limit threshold 169 to time B
when it reaches the upper limit threshold 170, data transfer of coded data
at sectors 161 to 164 is performed at the lower data transfer speed. In
order to allow data transfer at the lower data transfer speed, data
transfer at sectors 161, 162, 163 and 164 pauses for a proper duration at
the start and end thereof, as shown in FIGS. 11(a) and 11(b) during the
period from time A to time B. Accordingly, the lower data transfer speed
(indicated by a broken line in FIG. 11(c), which is slower than the
maximum data transfer speed can be realized equivalently.
Instead of two steps of data transfer speeds, a larger number of steps may
be readily used by adjusting the pause duration. Instead of using a pause
for each sector, a pause may be used every plural sectors.
FIG. 12 illustrates a third example of the data transfer speed control
scheme as mentioned with respect to the second embodiment. FIG. 12(a)
shows a sequence of coded data, FIG. 12(b) shows how the coded data is
transferred, FIG. 12(c) shows the data transfer speed, FIG. 12(d) shows
the coded data amount, and FIG. 12(e) shows the data amount withi | | |
