GB2276058A - Image data encoding and decoding - Google Patents
Image data encoding and decoding Download PDFInfo
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- GB2276058A GB2276058A GB9305090A GB9305090A GB2276058A GB 2276058 A GB2276058 A GB 2276058A GB 9305090 A GB9305090 A GB 9305090A GB 9305090 A GB9305090 A GB 9305090A GB 2276058 A GB2276058 A GB 2276058A
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- decoding
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N5/00—Details of television systems
- H04N5/76—Television signal recording
- H04N5/91—Television signal processing therefor
- H04N5/92—Transformation of the television signal for recording, e.g. modulation, frequency changing; Inverse transformation for playback
- H04N5/926—Transformation of the television signal for recording, e.g. modulation, frequency changing; Inverse transformation for playback by pulse code modulation
- H04N5/9261—Transformation of the television signal for recording, e.g. modulation, frequency changing; Inverse transformation for playback by pulse code modulation involving data reduction
- H04N5/9264—Transformation of the television signal for recording, e.g. modulation, frequency changing; Inverse transformation for playback by pulse code modulation involving data reduction using transform coding
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M7/00—Conversion of a code where information is represented by a given sequence or number of digits to a code where the same, similar or subset of information is represented by a different sequence or number of digits
- H03M7/30—Compression; Expansion; Suppression of unnecessary data, e.g. redundancy reduction
- H03M7/40—Conversion to or from variable length codes, e.g. Shannon-Fano code, Huffman code, Morse code
- H03M7/42—Conversion to or from variable length codes, e.g. Shannon-Fano code, Huffman code, Morse code using table look-up for the coding or decoding process, e.g. using read-only memory
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
- H04N19/30—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using hierarchical techniques, e.g. scalability
- H04N19/37—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using hierarchical techniques, e.g. scalability with arrangements for assigning different transmission priorities to video input data or to video coded data
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
- H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
- H04N19/102—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or selection affected or controlled by the adaptive coding
- H04N19/13—Adaptive entropy coding, e.g. adaptive variable length coding [AVLC] or context adaptive binary arithmetic coding [CABAC]
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
- H04N19/60—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using transform coding
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
- H04N19/90—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using coding techniques not provided for in groups H04N19/10-H04N19/85, e.g. fractals
- H04N19/91—Entropy coding, e.g. variable length coding [VLC] or arithmetic coding
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- Engineering & Computer Science (AREA)
- Multimedia (AREA)
- Signal Processing (AREA)
- Theoretical Computer Science (AREA)
- Compression Or Coding Systems Of Tv Signals (AREA)
Abstract
The encoding method comprises the steps of variable length encoding image data 70, storing n predetermined quantities of the encoded image data in n respective data buffers (where n/2) and generating n streams of encoded image data from the data stored in respective ones of the n data buffers. The data is split and stored in the n data buffers in accordance with the bit count 130 of the VLC encoder output. The complementary decoding method operates on the n streams of encoded image data to form a representation of an image by decoding each of the n streams of encoded image data to generate n respective streams of decoded image data, storing each of then streams of decoded image data in a respective one of n data buffers and concatenating the decoded image data from each of the n data buffers, to form the representation of the image. <IMAGE>
Description
IMAGE DATA ENCODING AND DECODING
This invention relates to image data encoding and decoding.
Some data compression systems produce compressed data comprising successive variable bit length code words. For example, in an image data compression system proposed by the Joint Photographic Experts
Group (JPEG), image data are subjected to frequency separation, quantization and entropy encoding. The entropy encoding stage includes a process of variable length encoding, in which variable bit length data words are generated in such a way that more commonly occurring data patterns (such as runs of zero-valued data) are encoded as shorter data words.
Figure 1 is a schematic block diagram of part of a previously proposed data compression apparatus, in which frequency separated and quantized image data are subjected to entropy and variable length (VLC) encoding by an entropy and VLC coder 10. Data comprising successive variable bit length code words are output by the entropy and VLC coder 10 and are stored temporarily in a buffer 20 before being passed to a data packer 30. The data packer 30 concatenates the variable bit length data words to generate encoded output data. The encoded data are then stored or transmitted as appropriate.
Figure 2 is a schematic block diagram of part of a previously proposed data decompression apparatus. Encoded data are received by a variable length code (VLC) decoder 40 which performs an inverse operation to that of the variable length encoding performed by the entropy and VLC coder 10. Data output by the VLC decoder 40 are temporarily stored in a buffer 50 before being subjected to further entropy decoding by an entropy decoder 60. The data output by the entropy decoder 60 are then subjected to stages of de-quantization and frequency combination (not shown) which are complementary to the frequency separation and quantization processes described above.
The quantity of encoded data output by the entropy and VLC coder 10 varies in inverse relation to the degree of compression applied by the data compression system. This means that, paradoxically, greater processing demands are placed on the VLC decoder 40 when a low (or mild) degree of compression is employed than when a high degree of compression is employed.
This invention provides a method of encoding image data from a representation of an image, the method comprising the steps of: variable length encoding image data from the representation; storing n predetermined quantities of the encoded image data in n respective data buffers, where n 2 2; and generating n streams of encoded image data from the data stored in respective ones of the n data buffers.
Viewed from a second aspect this invention provides a method of decoding n streams of encoded image data to form a representation of an image, where n 2 2, the method comprising the steps of: decoding each of the n streams of encoded image data, to generate n respective streams of decoded image data; storing each of the n streams of decoded image data in a respective one of n data buffers; and concatenating the decoded image data from each of the n data buffers, to form the representation of the image.
In the complementary data encoding and data decoding methods according to the invention, the decoding of the data is divided between n separate decoders, where n 2 2, thus reducing the processing requirements of each decoder by a factor of n. This means that the decoders can operate with lower clock speeds, even when low (mild) compression ratios are employed.
Preferably the representation comprises frequency separated image data.
In a preferred embodiment, n = 2.
Viewed from a third aspect this invention provides image data encoding apparatus for encoding image data from a representation of an image, the apparatus comprising: n data buffers, where n 2 2; a variable length encoder for variable length encoding image data from the representation; means for storing n predetermined quantities of the image data in respective ones of the n data buffers; and means for generating n streams of encoded image data from the data stored in respective ones of the n data buffers.
Preferably the image data encoding apparatus comprises a data counter for counting the quantity of encoded image data; and data switching means, responsive to the data counter, for controlling the storage of the encoded image data in the n data buffers.
The image data encoding apparatus is particularly usefully employed in a digital video storage apparatus comprising: image data encoding apparatus as defined above; and means for storing the a streams of encoded image data.
Although the means for storing may comprise, for example, a magnetic tape medium, it is preferred that the means for storing comprises a random access memory.
Viewed from a fourth aspect this invention provides image data decoding apparatus for decoding n streams of encoded image data to form a representation of an image, where n 2 2, the apparatus comprising: n data decoders for decoding respective ones of the n streams of encoded image data, to generate n respective streams of decoded image data; means for storing each of the n streams of decoded image data in a respective one of n data buffers; and concatenating the decoded image data from each of the n data buffers, to form the representation of the image.
The image data decoding apparatus is particularly usefully employed in a digital video reproduction apparatus comprising: means for reproducing n streams of encoded image data from a storage medium; and image data decoding apparatus as defined above.
Again, it is preferred that the storage medium comprises a random access memory.
The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which:
Figure 1 is a schematic block diagram of part of a previously proposed data compression apparatus;
Figure 2 is a schematic block diagram of part of a previously proposed data decompression apparatus;
Figure 3 is a schematic block diagram of a data encoding apparatus according to an embodiment of the invention;
Figure 4 is a graph showing the quantity of data output by an entropy and VLC coder during compression of each field of image data; and
Figure 5 is a schematic block diagram of a data decoding apparatus complementary to the apparatus of Figure 3.
Figure 3 is a schematic block diagram of a data encoding apparatus, forming part of a data compression system for use in, for example, a digital video recording/reproduction apparatus. Image data representing successive video fields are subjected to frequency separation and quantization before being supplied to an entropy and VLC coder 70. The output of the entropy and VLC coder 70 is stored in one of two data buffers, buffer A 80 and buffer B 90, under the control of a data switch 100. The data stored in the two buffers are supplied to two respective data packers, data packer A 110 and data packer B 120, to generate two respective streams of encoded data comprising successive 2-bit output data words.
Each field of image data (before compression) comprises 768 x 248 pixels, ie. 190464 pixels. In the image's uncompressed form, each pixel is represented by an eight bit word. In the present embodiment, 2:1 compression is performed, so that, on average, each pixel is represented by four bits of the encoded data which is output by the apparatus of Figure 3.
The level of quantization applied to the data supplied to the entropy and VLC coder 70 is varied from field to field in such a way that an overall compression by a factor of 2:1 is maintained. The variation in the degree of quantization is achieved by performing a test compression of the data and adjusting the degree of quantization in response to the quantity of data generated by the test compression.
This arrangement is described in detail in the British Patent
Application No GB 9119985.1 (publication number GB-A-2 259 324).
Accordingly, for each field processed by the data compression system, 190464 x 4 bits are output by the entropy and VLC coder 70.
The quantity of data output by the entropy and VLC coder 70 is counted by a bit counter 130, which controls the data switch 100. At the start of the processing of data from one field, the data switch 100 is set to direct the output of the entropy and VLC coder into the buffer A 80. When the bit counter 130 detects that half the data corresponding to one compressed field has been output by the entropy and VLC coder 70 (i.e. 190464 x 2 bits), the bit counter 130 controls the data switch 100 to direct the remainder of the data corresponding to that field to the buffer B 90. In this way, the data output by the entropy and VLC coder 70 corresponding to one field is divided equally between buffer A 80 and buffer B 90.
Figure 4 is a graph showing the quantity of data output by the entropy and VLC coder 70 during compression of each field of image data. As described above, the degree of quantization applied to the data is varied from field to field in order that an overall compression ratio of 2:1 is maintained. This means that the quantity of data generated by the entropy and VLC coder 70 for a complete field is constant from one field to the next. This constant total quantity (190464 x 4 bits per field) is indicated by the value 100% on the vertical axis of Figure 4. Although the total amount of data generated for each field is a constant, the rate at which the entropy and VLC coder 70 outputs data during processing of a single field varies according to the information content of different parts of that field. Three examples are shown in Figure 4, namely the curves 140, 150 and 160.The three curves 140, 150 and 160 illustrate the processing of three fields for which the information distribution within those fields is very different. However, in each case, when the quantity of data output by the entropy and VLC coder reaches 50% of the total data for one field (indicated by a broken horizontal line in
Figure 4) the bit counter 130 controls the data switch 100 to direct the output of the entropy and VLC coder 70 into the buffer B 90 rather than the buffer A 80.
The two streams of encoded data output by the two data packers 110, 120 are then stored (e.g. in a large random access memory or on a magnetic recording medium - not shown) or transmitted (e.g. via a video distribution cable - not shown) as appropriate.
Figure 5 is a schematic block diagram of a data decoding apparatus complementary to the data encoding apparatus of Figure 3.
Two streams of encoded data (as stored or transmitted by the apparatus of Figure 3) are received by respective VLC decoders 200, 210. The output of each VLC decoder is passed to a respective buffer 220, 230.
An entropy decoder 240 receives the data stored in buffer A 220 via a data switch 250 and performs entropy decoding on that data. The quantity of decoded data input to the entropy decoder 240 is counted by a bit counter 260. When this quantity reaches one half the quantity corresponding to a complete field, the bit counter 260 controls the data switch 250 to connect the output of buffer B 230 to the input of the entropy decoder 240.
The data output by the entropy decoder 240 are supplied to a dequantizer and a frequency combiner (not shown), in order that the original image data can be regenerated.
In the apparatus described above, the entropy and VLC encoded data are divided into two separate streams of data for transmission or storage. VLC decoding is then performed separately on each of the two data streams. For operation in real time, this means that each of the two VLC decoders 200, 210 operates on (190464 x 4)/2 bits of data in each field period. In other words, this is equivalent to each VLC decoder operating on two bits per pixel, at an effective data compression ratio of 4:1. The two bit counters 130, 260 control the respective data switches 100, 250 to divide and reconstruct the data in an exactly complementary manner.
Claims (22)
1. A method of encoding image data from a representation of an image, the method comprising the steps of:
variable length encoding image data from the representation;
storing n predetermined quantities of the encoded image data in n respective data buffers, where n 2 2; and
generating n streams of encoded image data from the data stored in respective ones of the a data buffers.
2. A method of decoding fl streams of encoded image data to form a representation of an image, where n 2 2, the method comprising the steps of:
decoding each of the 11 streams of encoded image data, to generate n respective streams of decoded image data;
storing each of the n streams of decoded image data in a respective one of n data buffers; and
concatenating the decoded image data from each of the n data buffers, to form the representation of the image.
3. A method according to claim 1 or claim 2, in which the representation comprises frequency separated image data.
4. A method according to any one of claims 1 to 3, in which n = 2.
5. A method according to any one of the preceding claims, in which the n predetermined quantities are of equal size.
6. Image data encoding apparatus for encoding image data from a representation of an image, the apparatus comprising:
n data buffers, where n 2 2;
a variable length encoder for variable length encoding image data from the representation;
means for storing n predetermined quantities of the image data in respective ones of the n data buffers; and
means for generating n streams of encoded image data from the data stored in respective ones of the a data buffers.
7. Apparatus according to claim 6, in which a = 2.
8. Apparatus according to claim 6 or claim 7, in which the a predetermined quantities are of equal size.
9. Apparatus according to any one of claims 6 to 8, comprising:
a data counter for counting the quantity of decoded image data; and
data switching means, responsive to the data counter, for controlling the storage of the decoded image data in the n data buffers.
10. Apparatus according to any one of claims 6 to 9, in which the representation comprises frequency separated image data.
11. Digital video storage apparatus comprising:
image data encoding apparatus according to any one of claims 6 to 10; and
means for storing the n streams of encoded image data.
12. Digital video storage apparatus according to claim 11, in which the means for storing comprises a random access memory.
13. Image data decoding apparatus for decoding n streams of encoded image data to form a representation of an image, where a 2 2, the apparatus comprising:
n data decoders for decoding respective ones of the a streams of encoded image data, to generate n respective streams of decoded image data;
means for storing each of the n streams of decoded image data in a respective one of n data buffers; and
concatenating the decoded image data from each of the a data buffers, to form the representation of the image.
14. Apparatus according to claim 13, in which n = 2.
15. Apparatus according to claim 13 or claim 14, in which the a predetermined quantities are of equal size.
16. Apparatus according to any one of claims 13 to 15, in which the representation comprises frequency separated image data.
17. Digital video reproduction apparatus comprising:
means for reproducing n streams of encoded image data from a storage medium; and
image data decoding apparatus according to any one of claims 13 to 15.
18. Digital video reproduction apparatus according to claim 17, in which the storage medium comprises a random access memory.
19. A method of data encoding, the method being substantially as hereinbefore described with reference to the accompanying drawings.
20. A method of data decoding, the method being substantially as hereinbefore described with reference to the accompanying drawings.
21. Data encoding apparatus substantially as hereinbefore described with reference to the accompanying drawings.
22. Data decoding apparatus substantially as hereinbefore described with reference to the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9305090A GB2276058B (en) | 1993-03-12 | 1993-03-12 | Image data encoding and decoding |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9305090A GB2276058B (en) | 1993-03-12 | 1993-03-12 | Image data encoding and decoding |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB9305090D0 GB9305090D0 (en) | 1993-04-28 |
| GB2276058A true GB2276058A (en) | 1994-09-14 |
| GB2276058B GB2276058B (en) | 1997-05-28 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9305090A Expired - Fee Related GB2276058B (en) | 1993-03-12 | 1993-03-12 | Image data encoding and decoding |
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| Country | Link |
|---|---|
| GB (1) | GB2276058B (en) |
Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5138315A (en) * | 1990-01-30 | 1992-08-11 | U.S. Philips Corporation | Arrangements for variable-length encoding and decoding of digital signals |
-
1993
- 1993-03-12 GB GB9305090A patent/GB2276058B/en not_active Expired - Fee Related
Patent Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5138315A (en) * | 1990-01-30 | 1992-08-11 | U.S. Philips Corporation | Arrangements for variable-length encoding and decoding of digital signals |
Also Published As
| Publication number | Publication date |
|---|---|
| GB2276058B (en) | 1997-05-28 |
| GB9305090D0 (en) | 1993-04-28 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| 730A | Proceeding under section 30 patents act 1977 | ||
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 20110312 |