U.S. patent application number 09/094317 was filed with the patent office on 2001-12-06 for method for regenerating the original data of a digitally coded video film, and apparatus for carrying out the method.
Invention is credited to ADOLPH, DIRK, OSTERMANN, RALF.
Application Number | 20010048805 09/094317 |
Document ID | / |
Family ID | 7833943 |
Filed Date | 2001-12-06 |
United States Patent
Application |
20010048805 |
Kind Code |
A1 |
ADOLPH, DIRK ; et
al. |
December 6, 2001 |
METHOD FOR REGENERATING THE ORIGINAL DATA OF A DIGITALLY CODED
VIDEO FILM, AND APPARATUS FOR CARRYING OUT THE METHOD
Abstract
A method for regenerating the original data of a digitally coded
video film is described, in which a specific picture type of a
number of picture types (I, B, P) has been allocated to each of the
individual pictures during coding and the coding order (DEO) does
not correspond, at least in part, to the display order (DIO). This
method provides for an independent time base to be used in order to
recover the display order, which time base is obtained from
internal information items in the decoder, the information items
having been stored during the preceding decoding of a number of
pictures. This method has the advantage that even with the omission
of a header of a Group of Pictures (GOPH), the correct display
order can still be achieved, even if the otherwise customary
temporal reference codes in the bit stream signal an incorrect
display order. An apparatus for carrying out the method is
furthermore proposed.
Inventors: |
ADOLPH, DIRK; (RONNENBERG,
DE) ; OSTERMANN, RALF; (HANNOVER, DE) |
Correspondence
Address: |
JOSEPH S TRIPOLI
G E & RCA LICENSING MANAGEMENT OPERATION
CN 5312
PRINCETON
NJ
085430028
|
Family ID: |
7833943 |
Appl. No.: |
09/094317 |
Filed: |
June 9, 1998 |
Current U.S.
Class: |
386/329 ;
375/E7.276; 375/E7.279; G9B/27.019; G9B/27.033; G9B/27.034 |
Current CPC
Class: |
H04N 7/56 20130101; G11B
27/105 20130101; H04N 19/89 20141101; G11B 27/3027 20130101; G11B
27/3036 20130101; G11B 2220/2562 20130101 |
Class at
Publication: |
386/95 ; 386/111;
386/112 |
International
Class: |
H04N 005/92 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 28, 1997 |
DE |
197 27 542.7 |
Claims
1. Method for regenerating the original data of a digitally coded
video film, a specific picture type of a number of picture types
(I, B, P) having been allocated to each of the individual pictures
during coding and the coding order (DEO) not corresponding, at
least in part, to the display order (DIO), characterized in that in
order to recover the display order, use is made of a time base
which is independent of a temporal reference code of the respective
picture and is obtained from internal information items in the
decoder (FIG. 7) which have been stored during the preceding
decoding of a number of pictures.
2. Method according to claim 1, in which in particular the
information items regarding the memory location of the picture in a
frame memory (A, B, C) or regarding the picture type and whether or
not the picture has already been displayed are evaluated for the
recovery of the display order.
3. Method according to claim 1 or 2, in which the information items
are evaluated retroactively until an unambiguous display order
(DIO) can be identified.
4. Method according to claim 3, in which, given a sequence of
pictures of the type IBBPBBPBBPBB . . . , the information items of
at least four pictures are evaluated retroactively with regard to
the next picture to be displayed.
5. Apparatus for carrying out the method according to one of the
preceding claims, having decoding means (18), having a number of
frame memories (A, B, C), having a control unit (19), characterized
in that a table (20) is provided, which the control unit (19)
evaluates, in which the information items for recovering the
display order are stored for a number of decoded pictures.
6. Apparatus according to claim 5, in which a GOP counter (GOP) is
provided which is incremented on identification of the start (GOPH)
of a Group of Pictures.
7. Apparatus according to claim 5 or 6, in which an IP counter (IP)
is provided which is incremented on identification of an I picture
or P picture and is reset on identification of the start (GOPH) of
a Group of Pictures.
8. Apparatus according to one of claims 5 to 7, in which a B
counter (B) is provided which is incremented on identification of a
B picture and is reset on identification of the start (GOPH) of a
Group of Pictures.
9. Apparatus according to one of claims 5 to 8, in which cells are
provided in the table (20) for a decoded picture, to which cells
the information items regarding the frame memory (A, B, C) in which
the decoded picture is stored and whether or not the picture has
already been released for display can be written.
10. Apparatus according to claim 9, in which cells are provided in
the table (20) for a decoded picture, to which cells information
items regarding the counter readings of the GOP, IP and B counters
during decoding of this picture can be written.
11. Apparatus according to claim 9, in which the size of the table
(20) is adapted in such a way that the information items of the
pictures of the picture sequence having the largest possible number
of successive B pictures, including the I or P pictures adjoining
these B pictures, can still be called up in the table (20).
Description
[0001] The invention relates to a method for regenerating the
original data of a digitally coded video film and an apparatus for
carrying out the method.
PRIOR ART
[0002] The invention is based on a method for regenerating the
original data of a digitally coded video film of the generic type
of the independent Claim 1. Apparatuses of this type have recently
become known for example as a result of the commercially available
DVD (Digital Versatile Disk) playback devices. These devices are
designed in accordance with the so-called DVD Standard (Version
1.0). In this standard, reference is made to the so-called MPEG
standards MPEG1 and MPEG2 with regard to the reproduction of video
films. The MPEG2 standard is also known under the reference ISO/IEC
13818: 1994 "Information technology--generic coding of moving
pictures and associated audio". The MPEG1 standard is also known
under the designation ISO/IEC 11172: 1993 "Information
Technology--coding of moving pictures and associated audio for
digital storage media up to about 1.5 Mbit/s". The video coding
type is defined by one of these two standards in the case of DVD.
According to these standards, the individual pictures of a video
film are converted into a coded digital data stream (bit stream). A
wealth of information is provided in the bit stream for each
picture. All the details of the digital bit stream cannot be
described in detail here. Reference is made in this respect to the
standards mentioned, from which the necessary details can be
obtained.
[0003] Today's DVD playback devices encounter the problem that they
cannot always operate without any errors in everyday operation, one
of the reasons being that on account of the drastic increase in the
storage density on the DVD itself, error-free reading cannot be
effected in every situation. Slight scratches as well as dust and
grease spots can then very easily prevent satisfactory reading of
information. The nature of the recording means that even when the
scratch/spot is relatively small, relatively large areas may be
affected on account of the high storage density, and are not
correctly evaluated. This results in poor picture and sound quality
from time to time, despite error concealment measures that are
present. The invention will address a specific problem with regard
to picture display, but not the decoding of audio information.
[0004] As will be explained in more detail below, the video
information items are stored in so-called sectors on the DVD. Such
a sector comprises 2048 bytes of digital video information. Since
the data in such a sector are compressed to a very great extent and
correspondingly little redundancy is present, it is often the case,
in spite of the fact that only a few items of data of a sector were
not able to be read correctly, that the entire sector is omitted as
video information because the data can no longer be completely
decoded when part of the information has been lost. Consequently,
it can happen that entire sectors are omitted during the
reproduction of a video film. The MPEG standards MPEG1 and MPEG2
provide virtually a hierarchical division of the video data
according to their importance. There are data which are important
for recovering the video information items of an entire sequence of
pictures; there are other data, in turn, only for recovering the
information of an individual picture; and again there are other
data which are absolutely necessary for recovering information
items of specific sections in a picture.
[0005] The present invention will consider in further detail
particularly those data which apply to a so-called Group of
Pictures. The digital data stream is therefore provided with a
section, called "Group of Pictures Header" GOPH, which also
contains important system time reference data. A system clock is
fed these data and synchronizes practically all of the operations
within the DVD playback device. The system clock in the DVD
playback device is from time to time readjusted by this system time
reference data, thereby ensuring synchronous operation between the
video decoding operation and the video encoding operation. Thus, in
the same way that there is a specific GOP header for a Group of
Pictures, there is also a header, called "Pictures Header", for an
individual picture. This header contains inter alia a so-called
temporal reference code (temporal_reference_code). This code
consists of a 10-bit-wide number and in practice indicates the
position of this picture within the Group of Pictures, which then
finally governs when this picture is displayed within the Group of
Pictures, that is to say the order of display of the pictures is
indicated thereby. The individual pictures of the Group of Pictures
are thus, as it were, consecutively numbered. However, since the
pictures are scrambled together in a specific manner during the
encoding operation, the coding order no longer corresponds to the
display order. This is in turn due to the fact that in accordance
with the MPEG1 and MPEG2 standards, three different picture types
are provided and the pictures are accordingly coded differently.
The so-called I pictures corresponding to intra-coded pictures are
known. These pictures contain only information items of one picture
and can therefore be decoded independently of other pictures. The
so-called P pictures, corresponding to unidirectionally predicted
pictures, are provided as a further picture type. These pictures
have been predicted on the basis of a preceding I or P picture,
with the result that their data volume is reduced. They are thus
dependent on a preceding I or P picture, however. The B pictures,
corresponding to bidirectionally predicted pictures, are provided
as the third picture type. These pictures are dependent on two
neighbouring I or P pictures. They are predicted from the
neighbouring I or P pictures. The average value is formed and the
difference from the original picture is calculated. Since only a
fraction of video information items remain in the case of the
difference, the B pictures are considerably reduced in terms of
their data volume. The provision of such different picture types
also necessitates the scrambling of the display order for the
purposes of coding. Specifically, a B picture can only be coded
when the preceding and succeeding I or P picture is present in
coded form. Consequently, in the data stream the chronologically
succeeding I or P picture will always appear first before the B
pictures which are actually to be displayed chronologically
earlier. The original display order can be recovered during the
decoding operation using the concomitantly transmitted temporal
reference codes. A problem arises whenever, on account of an error,
a GOP header is omitted and suddenly pictures of another Group are
decoded which have a smaller temporal reference code than pictures
that have already been decoded and are located in a memory.
Specifically, it can then happen that the pictures with the higher
temporal reference code which are actually to be displayed earlier
are only displayed after the pictures with the lower reference
codes which are actually to be displayed later. This, of course,
leads to an incorrect display of the pictures, which is clearly
discernible to the viewer. Since the picture with the higher
temporal reference code remains continually in a frame memory, the
display order within the new Group of Pictures can continue to be
scrambled. This leads to undesirable "jitter" of the picture, which
is extremely disturbing. The effect will be explained in more
detail below with reference to pictures.
INVENTION
[0006] The object of the invention is to prevent in particular the
undesirable scrambling of the display order after an omission of a
GOP header.
[0007] The object is achieved according to the invention by means
of the method specified in Claim 1. According to the invention,
namely, an independent time base is used to recover the display
order in the DVD playback device. In this case, a few information
items of the decoded data stream are buffer-stored over a specific
period of time in order to ensure the independent time base. The
particular advantage of these measures consists in the fact that
scrambling of the display order after the omission of a GOP header
does not arise.
[0008] Claim 2 specifies advantageous information items which can
be evaluated for the correct recovery of the display order.
[0009] Claim 3 specifies an advantageous measure which can
advantageously be used particularly when, in the coding operation,
variable sequences of picture types are permitted and a defined
sequence of picture types for the pictures to be coded does not
have to be rigidly complied with.
[0010] Claim 4 specifies an advantageous measure for a frequently
selected sequence of picture types during a coding operation.
[0011] For an apparatus for carrying out the method according to
the invention, it is very advantageous if a table is stored in
which the information items for recovering the display order are
stored for a number of decoded pictures and which is evaluated by a
control unit in order to ensure the independent time base. Such a
table can also be realized in a simple manner by software, with the
result that the circuitry outlay remains low.
[0012] The measures in Claims 6 to 8 are likewise advantageous
since, using the counter readings of the claimed counters, it is
easy to ascertain which type of picture a preceding picture was.
Since the counter readings are a matter of simple numerical values,
these can easily be compared with one another in order rapidly to
obtain the picture type. This information combined with the
information regarding whether or not the picture has already been
released for display easily permit, given a joint consideration of
a plurality of preceding pictures, a conclusion to be drawn
regarding which picture must subsequently be released for display.
According to Claim 11, the size of the table is advantageously
adapted such that the picture sequence having the largest number of
successive B pictures, including the I or P pictures adjoining
these B pictures, can still just be called up in the table.
DRAWINGS
[0013] Exemplary embodiments of the invention are illustrated in
the drawings and are explained in more detail in the description
below. In the figures:
[0014] FIG. 1 shows a detail of data stored on a DVD and their
division between the sectors of the DVD;
[0015] FIG. 2 shows the structure of a GOP header;
[0016] FIG. 3 shows the structure of a Picture header;
[0017] FIG. 4 shows the chronological sequence of a decoding
operation of a sample video picture sequence;
[0018] FIG. 5 shows the chronological sequence of the processing of
a video picture sequence according to the conventional method in
the event of an error occurring;
[0019] FIG. 6 shows the chronological sequence of the processing of
a video sequence by the method according to the invention in the
event of an error occurring;
[0020] FIG. 7 shows a rough block diagram of the apparatus
according to the invention, and
[0021] FIG. 8 shows the entries in the table at selected instants
of the video picture sequence illustrated in FIG. 6.
DESCRIPTION OF THE INVENTION
[0022] The invention is explained in more detail using the example
of a video picture decoding unit of a DVD playback device. In FIG.
1, a header for a video picture sequence is designated by the
reference symbol SH. The reference symbol GOPH designates a GOP
header. I0 designates the data of the first video picture. The
latter is an I picture. P15 designates the data of the sixteenth
video picture. The latter is a P picture. B13 and B14 accordingly
designate video data of the 14th and 15th video pictures. The
latter are both B pictures. P16 designates the 17th video picture.
The latter is accordingly a P picture. The lower part of FIG. 1
illustrates sectors which are designated by the reference symbols
VP.sub.1 to VP.sub.j. All of the sectors have an identical length
of 2,048 bytes. Accordingly, the first video picture I0 occupies a
larger number of sectors. The video picture P16 occupies the last
three sectors VP.sub.j-2 to VP.sub.j illustrated. The header GOPH
from FIG. 1 is illustrated in more detail in FIG. 2. The start code
of the Group of Pictures is designated by the reference symbol GSC.
This is an unambiguous code within the data stream. A time code
information item (system time reference) is designated by the
reference symbols TC. As described, this time code information item
TC serves to readjust the system clock in the DVD playback device.
The reference symbols CG and BL designate the information items
closed_gap and broken_link which are provided in the MPEG2 standard
but whose meaning is not necessary for a deeper understanding of
the invention and so is not explained in more detail here.
[0023] In the Picture header illustrated in FIG. 3, the reference
symbol PSC designates the picture start code. This picture start
code, too, can be identified unambiguously in the bit stream. The
reference symbol TR designates a temporal reference code for the
picture. As already explained, this temporal reference code
specifies the position of the picture within the Group of Pictures.
With this information, the order of the pictures is determined in
conventional DVD playback devices. Since a 10-bit data word is
involved, a total of 1,024 pictures which might occur in practice
within an individual Group of Pictures can be consecutively
numbered. In the DVD standard, however, it is recommended to
establish groups of pictures with at most 15 or 18 pictures. The
reference symbol PCT specifies the picture type of the picture. The
further details in FIG. 3 relate to the parameters
[0024] vbv_delay, full_pel_forward_vector, forward_f_code,
full_pel_backward_vector, backward_f_code, extra_bit_picture and
extra_information_picture
[0025] defined in the MPEG2 standard. These information items, too,
are not of any major importance for a further understanding of the
invention and so are not explained any further here.
[0026] The chronological sequence of the decoding and displaying of
pictures of a Group of Pictures is illustrated in more detail in
FIG. 4. The sequence of pictures, in the way that it is ultimately
intended to be displayed, is indicated in the top line in FIG. 4.
This line is designated by the reference symbol DIO. Virtually a
random sequence of I, B, P pictures has been chosen in FIG. 4.
Although such a sequence of pictures is permitted according to the
DVD standard, it does not constitute the optimum sequence as is
recommended for DVD playback devices. This will be discussed in
more detail below. The line of FIG. 4 which is designated by the
reference symbol DEO indicates the decoding order for the sequence
of pictures that is located in the line above. As previously
described, this differs from the display order by the fact that the
B pictures, which are arranged chronologically before a P or I
picture in the display order, are decoded only after the P or I
picture. The exact chronological sequence when a picture of this
sequence is decoded and when it is released for display or is
displayed can be discerned in the other two parts of FIG. 4. In
this case, the reference symbol FM indicates an abbreviation for a
frame memory. Three frame memories A, B, C are provided for the
example. The I picture I0 is written first of all to the frame
memory A after the decoding operation. Next, the I picture I1 is
decoded and written to the frame memory B. An individual picture is
in this case divided in two, which is indicated by the hatching of
one half of the picture. The hatched area represents the first
field of the respective picture, while the non-hatched area
indicates the second field of the respective picture. According to
the MPEG2 standard, there are two different ways of coding
pictures. In the first case, a picture is coded with the two fields
together as a single picture (frame pictures). In the second case,
the individual fields are each coded separately (field picture).
FIG. 4 illustrates both types of picture coding. This situation has
utilized the special feature that in the event of decoding an I
picture with separately coded fields, it is possible to code the
first field as an I picture and to code the second field as a P
picture. Therefore, in each of the I pictures, one field is
designated as an I picture and the second field as a P picture. In
the case of the B and P pictures, both fields must be coded
identically.
[0027] FIG. 4 clearly reveals that a time delay of three fields
exists between the decoding of the first I picture, the I0, and its
release for display. The first I picture, the I0, is released for
display at the beginning of the decoding of the second field of the
I picture I1. The vertical synchronization pulses are used for
complying with this time offset. The instants at which the decoded
pictures are displayed can in each case be seen in the part of FIG.
4 below the dashed time axis. An additional special feature is that
in the case of B pictures, the decoding instant and the display
instant are in each case offset from one another only by one field.
Moreover, the B pictures are exclusively entered in the frame
memory C. The frame memory C is then organized in such a way that
the first part, that is to say the part in which the first field is
located, is in each case already being read for the display of the
picture while the second field is still being written to the second
part of the memory. The other frame memories A and B are also
designed in the same way.
[0028] FIG. 5 illustrates a frequently used sequence of pictures
which is also used for recording video data on a DVD. In this
sequence of pictures, an I or P picture is in each case followed by
two B pictures. A Group of Pictures is formed by 13 pictures
I0-P12. This is then followed by the next Group of Pictures, which
is constructed in the same way. The display order of the pictures
is illustrated in the line designated by DIO. The line bearing the
reference symbol DEO once again shows the decoding order of the
pictures, that is to say the order in which the pictures occur in
the bit stream. The line which is designated by the reference
symbol FM respectively specifies the frame memory to which the
decoded picture located underneath is written. The line designated
by TR in each case specifies the temporal reference code of the
respective picture.
[0029] FIG. 5 now shows the particular situation whereby a reading
error occurs at the instant t.sub.E, that is to say both the
succeeding picture B11 and the succeeding GOP header and the
picture I0 of the next Group of Pictures can no longer be read
correctly and, therefore, cannot be decoded correctly either.
Instead of this, the first thing that a detection circuit finds
next is the correct picture header of the picture p3 in the bit
stream and the decoder starts to decode from that point on. The
further part of FIG. 5 shows in detail what happens in that case in
the event of re-establishing the display order based on the
temporal reference code. The decoded picture p3 is displayed next
after the display of the picture B10. Specifically, this picture p3
has a temporal reference code of 3, which is smaller than the
temporal reference code of the picture p12 located in the memory A.
Consequently, the picture p3 of a following Group of Pictures,
which picture p3 is actually to be displayed later, is in fact
displayed earlier than the previously decoded picture p12 of the
preceding Group of Pictures. This keeping back of the decoded
picture p12 also propagates during the decoding of the further
pictures of the second Group of Pictures. Thus, all of the pictures
b1 and b2 and also p6, b4, b5, b9, b7, b8 are output first before
the picture P12. In this case, it may also be regarded as
particularly disturbing that the pictures, p6 and p9 are each
displayed before the pictures b4 and b5 and, respectively, b7 and
b8, even though they ought actually not to be displayed until after
these B pictures. The result of this is a scrambling of the picture
order, which becomes greatly apparent to the viewer in the form of
picture "jitter". This is due to the fact that virtually all the
pictures have to be output immediately because the picture P12
remains permanently in the frame memory A and therefore only a
frame memory B is available for the P and I pictures. At the
instant t.sub.G a new GOP header is correctly evaluated and the
sequence of pictures therefore settles down starting from this
instant, since now the temporal reference codes of the new Group of
Pictures are handled separately. The picture p12 would be displayed
as last picture of the preceding Group of Pictures. It should also
be mentioned that not only is the picture order scrambled, as
described, but also the decoded pictures p3 to p12 are displayed
with errors since, after all, they have actually been predicted on
the basis of other pictures, namely on the first I picture iO, in
particular, but this has been omitted. The prediction instead takes
place based on the last picture of the preceding Group of Pictures
I0-P12. The respective picture content then governs to a very great
extent whether these errors can be discerned to a very great extent
in the pictures. In the event of a scene change between picture P12
and iO, large areas of the picture will certainly be displayed
incorrectly. However, if a film sequence in which there is little
movement in the picture is concerned, it may well be that these
errors would not even stand out clearly to the viewer.
[0030] FIG. 6 illustrates the same example as in FIG. 5, but in
this case the measures according to the invention are taken, which
is indicated by the lines bearing the reference symbols GOP, IP and
B. The other lines correspond to the first three lines of FIG. 5.
The reference symbol GOP designates a GOP error. The latter is
incremented whenever a GOP header has been correctly evaluated and
has thus been found in the bit stream. The reference symbol IP
designates an IP counter. The latter is incremented whenever an I
or P picture has been identified in the bit stream. This counter is
reset after a GOP header has been correctly found. The reference
symbol B designates a B counter. This counter is incremented
whenever a B picture has been found in the data stream. This
counter is also reset to zero when a GOP header has been correctly
found. After the occurrence of the reading error at the instant
t.sub.E, in this case the P picture P12 still located in the memory
is output next. The further pictures b1 to b12 are also displayed
in the correct order. This is due to the fact that the display
order is determined not on the basis of the temporal reference code
specified in the P picture header; rather an independent time base
on the basis of the counter readings of the GOP, IP and B counters
is used. The way in which each of these counter readings is
evaluated is explained in more detail below.
[0031] To that end, the block diagram according to FIG. 7 is
firstly described in more detail. The figure shows the functional
units and the signal flow in an MPEG2 decoder. The reference number
10 designates a detection circuit for the start codes of the
various headers GOP header, picture header, etc. The reference
number 11 designates a buffer memory for the read data of the data
stream. The reference number 12 designates a decoding unit which
reverses the variable length coding of the video data. The
reference number 13 designates a processing stage which carries out
inverse quantization. The reference number 14 designates a
computing circuit in which the DCT transform is reversed. In other
words, an inverse discrete cosine transform takes place. The
reference number 15 designates a circuit in which motion
compensation for the individual pictures is carried out. Finally,
the data decoded in this way are written to the memory unit 16,
which contains the three aforementioned frame memories, A, B and C.
The reference number 17, finally, designates a post-processing
stage. A series of parameters which characterize the coded signal
and can optionally be transmitted in the bit stream are then
processed here. These parameters include for example information
items for the matrixing of the chrominance and for PAL or NTSC
colour modulation. The finished video signal appears in the
YC.sub.BC.sub.R format at the output of the stage 17. The reference
number 19 designates a control unit which controls the diverse
components of the decoder. It contains the GOP, IP and B counters
already mentioned.
[0032] The method of operation of the individual components is
disclosed to the relevant person skilled in the art by diverse
publications and, therefore, does not need to be explained in any
further detail here. However, the way in which the control unit 19
determines the display order, even in the situation where a GOP
header in the bit stream is omitted, will be explained in more
detail below in connection with FIG. 8.
[0033] For this purpose, a table 20 is provided in the control unit
19. Information items regarding the last four decoded pictures of
the bit stream are continuously held in this table. FIG. 8
illustrates the entries in the table for the instants t.sub.1,
t.sub.2, t.sub.3, t.sub.4, t.sub.5 and t.sub.6 of FIG. 6. The table
20 comprises in practice five shift stores, in each of which the
entries are shifted across by one location after the decoding of
the first field of a picture. At this instant, it is then also
necessary to decide in each case which picture is to be displayed
next. Reference is once again made to FIG. 4 in this respect. In
the first example for the instant t.sub.1, the entries in the line
bearing the reference symbol DI in each case indicate for four
successive pictures whether or not the respective picture has
already been displayed at the last updating instant of the table.
Since only three pictures have been decoded at the instant t.sub.1,
in each case a zero is still entered everywhere at the first
location of the table. From the fact that of the three remaining
decoded pictures none has yet been displayed, and also from the
fact that the first and second decoded pictures are each a matter
of an I or P picture, which can been seen from the counter
readings, on the one hand, and, on the other hand, from the fact
that the pictures have been entered into the frame memories A or B,
it immediately follows that the third picture that arrived is to be
displayed between these two pictures, because it is a B picture. At
this instant, then, the first decoded picture from the frame memory
A must be displayed. This is indicated by the command DISP A
underneath the table.
[0034] The entries in the table at the instant t.sub.2 appear such
that the two B pictures which were each entered into the frame
memory C have already been displayed and a previously decoded
picture is still available in the frame memory A and, in addition,
a further I or B picture has been newly written to the frame memory
C. Since the B pictures have already been output, it follows that
the picture located in the memory A must be output next. The P
picture P6 is thus output next, which is still held in the frame
memory A at the instant t.sub.2.
[0035] A the instant t.sub.3, the following configuration is
produced in the table 20: the oldest picture is a B picture and has
already been displayed. The next picture entered is one which must
be an I or P picture, but this has not yet been displayed. There
then follows, finally, a B picture which has already been
displayed, followed by an I or P picture which has not yet been
displayed. This configuration reveals that the picture located in
the frame memory A must be displayed next since, after all, this
must represent a basis for the second B picture which, however, has
already been displayed. At the instant t.sub.3, therefore, the P
picture P12 still located in the memory A is released for
display.
[0036] The configuration in the table 20 at the instant t.sub.4 is
such that the memory B contains an I or P picture which has not yet
been displayed and B pictures are entered next which have already
been displayed, followed by an I or P picture which has not yet
been displayed. From this, it then follows for the same reasons as
at the instant t.sub.2 previously that the picture in the frame
memory B is to be displayed next.
[0037] The configuration of the table 20 at the instant t.sub.5
reveals that two B pictures have already been displayed and an I or
P picture has been entered next in the frame memory A, followed by
a further B picture. This reveals that the picture located in the
memory C must be output next because although the picture located
in the memory A has to be a reference for this B picture in the
memory C, the earlier reference must be located in the memory
B.
[0038] The instant t.sub.6 is considered as the last example. Here,
the configuration is the same as at the instant t.sub.2, with the
result, that in this case, too it follows that the picture located
in the frame memory A is to be output next. This is the P picture
p12 at the instant t.sub.6. All further configurations for the
sequence of FIG. 6 can be established and evaluated accordingly by
the person skilled in the art on the basis of the examples
explained.
[0039] It should also be mentioned that for a simplified exemplary
embodiment, the counters GOP, IP and B may also be dispensed with
if it is ensured during the decoding process that the I and P
pictures are in each case entered exclusively into the frame
memories A and B and the B pictures are entered exclusively into
the frame memory C. It is then possible to draw a conclusion
directly from this fact as to whether an I or P picture is involved
or whether a B picture is involved. The table would then, of
course, be distinctly simplified. As an alternative to the entries
in the table for the GOP, IP and B counter readings, provision may
also be made for entering a corresponding code for the respective
picture type of a decoded picture into the table. The table and the
evaluation method are preferably realized with the aid of a
corresponding computer program. The rules to be observed have been
explained comprehensively, so that this computer program can easily
be realized by a person skilled in the art.
[0040] In the event that arbitrary picture sequences are permitted
and have to be evaluated, the picture order can, nevertheless,
easily be ascertained by the method described. If a configuration
is produced which cannot be unambiguously decided by the said
rules, it would then be necessary, however, to enlarge the table
accordingly so that even further information items from preceding
decoded pictures are contained. As a result, even arbitrary picture
sequences can then be evaluated and the picture order be
recovered.
[0041] The method described and the apparatus described can be
practically employed not only in DVD playback devices but also in
all other devices in which "MPEG-similar" decoders are used, that
is to say in which it is likewise necessary to decode different
picture types and the decoding order does not correspond to the
coding order. Particular mention may be made of satellite receivers
or set-top boxes for terrestrial or cable-based reception of
digital television. Corresponding plug-in cards for computers may
also be mentioned.
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