WO1988006390A1 - Codeur - Google Patents

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Publication number
WO1988006390A1
WO1988006390A1 PCT/GB1988/000087 GB8800087W WO8806390A1 WO 1988006390 A1 WO1988006390 A1 WO 1988006390A1 GB 8800087 W GB8800087 W GB 8800087W WO 8806390 A1 WO8806390 A1 WO 8806390A1
Authority
WO
WIPO (PCT)
Prior art keywords
mode
quantizer
subset
coder
codewords
Prior art date
Application number
PCT/GB1988/000087
Other languages
English (en)
Inventor
Graham R. Seabrook
David I. Crawford
Original Assignee
British Telecommunications Public Limited Company
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by British Telecommunications Public Limited Company filed Critical British Telecommunications Public Limited Company
Publication of WO1988006390A1 publication Critical patent/WO1988006390A1/fr

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Classifications

    • 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
    • H04N19/124—Quantisation
    • 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
    • H04N19/50—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding

Definitions

  • DPCM Differential Pulse Code Modulation
  • VLC Variable-Length Coding
  • Huffman D. "A Method for the Construction of Minimum Redundancy Codes," Proc. IRE, pp 1098-1101, September 1952].
  • VLC attempts to allocate shorter length codewords to high probability levels and longer length codewords to low probability levels: the VL Codebook is thus dependent on the statistics of the input signal. It can be shown [Jayant N.S. and Noll P., "Digital Coding of Waveforms", Prentice and Hall, 1984] that the minimum entropy achievable with a VLC/uniform quantizer combination is lower than that of a VLC/non-unifor quantizer combination. However, this is the case only if. the VL Codebook is perfectly matched to the statistics of the input signal - clearly a difficult constraint to fulfill if the signal is non-stationary.
  • a perfectly matched non-uniform quantizer aims to equalise the probability of all levels.
  • the ideal situation is in practice never reached and in a non-ideal case a combination of VLC/non-uniform quantization yields an average bit-rate very close to the minimum as given by:
  • N the number of quantization levels.
  • Any VLC system must employ a buffer to smooth the variable data rate for transmission over a fixed rate channel.
  • any buffer will, almost inevitably, eventually exceed its limits; so some form of buffer control is imperative.
  • the traditional way of preventing buffer overflow is to employ feedback from the buffer to control selection of a more, or less, coarse quantization characteristic: the system is thus adaptive.
  • the main penalties incurred by an adaptive system are: (a) increased complexity, and hence cost,
  • a coder comprising a differential coding loop including a quantizers variable length coder; a buffer for adapting the bit rate of the variable length coder output to the characteristics of a transmission channel; control means responsive to the condition of the buffer to control the switching of the quantizer between a first mode and a second mode, the quantizer and the variable length coder being; a) in the first mode responsive to the quantizer input signal lying within respective ranges defined by a first set of threshold values to generate respective codewords of a set of codewords; b) in the second mode responsive to the quantizer input signal lying within respective ranges defined by a second, smaller set of threshold values to generate respective codewords from a subset of the set of codewords; wherein; c) the subset is such that for each codeword of the subset, the value ranges giving rise to generation of that codeword in the two modes at least partially overlap; and d) the set and subset of codewords are so selected that the variable length coder output exhibits a lower
  • the thresholds employed by the quantizer in the second mode are a subset of those employed in the first mode and the quantizer outputs are correspondingly a subset of those used in the first mode. This advantageously simplifies the construction and operation of the quantizer.
  • the codebook is structured so that one or more relatively shorter codewords are assigned to quantizer output levels which, in the first mode, have a relatively lower probability of recurrence than a level or levels to which relatively longer codewords are assigned; the said shorter codeword(s) being members of the subset.
  • the second mode subset of codewords is an optimal subset for the second mode quantizer output levels, with an added bit to allow expansion of the set, and the remainder of the codebook is an optimal set for the first mode, non-second mode, quantizer output levels, each codeword of the remainder having a binary prefix to distinguish it from the subset.
  • the invention comprises a transmission system employing such a coder and a decoder with a single, fixed codebook.
  • Figure 1 shows a coder according to an embodiment of the invention
  • Figure 2 shows an example of a quantizer characteristic that may be used in an embodiment of the invention.
  • a coder comprising a Differential Pulse Code Modulation (DPCM) loop 1 having an input terminal 2 connected to a quantizer 4 via a subtractor 3; the output of the quantizer 4 is connected both to a variable length coder 5, and to a feed-back predictor 6 via a second adder 7.
  • the output of predictor 6 is connected to the inverting input of first adder 3 so as to complete a negative feed-back loop, and the output of the predictor 6 is also connected to a second input of the second adder 7.
  • the output of variable length coder 5 is connected via data buffer 8 to a fixed rate transmission channel 9.
  • the buffer 8 also has a feed-back connection 10 to the quantizer 4.
  • the coder processes the luminance (Y) component s(t) of a colour television signal in the DPCM loop 1, to produce a quantized innovation signal E g (t).
  • the quantizer 4 has a non-uniform quantization characteristic for example as indicated by a dotted line ll in figure 2. Columns 1 and 2 of Table 1, which follows below, indicate the quantized outputs obtained from inputs to a quantizer having this characteristic.
  • the quantized output levels (Table 1 column 2) are then fed to the variable length coder 5 where high probability levels are coded with shorter words and low probability levels are coded with longer words in order to produce an output data stream, of variable bit rate, having a low average bit rate.
  • the buffer 8 then smooths the bit rate prior to transmission. when the buffer 8 approaches an overflow condition a signal is sent via a feed-back line 10 to the quantizer 4 to alter the quantizer characteristic 11. Strictly speaking the quantizer is therefore adaptive, however, the characteristic is not in fact changed to a different characteristic, but a simplified sub-set, (the dash line 12 in figure 2) of the existing characteristic 11 is brought into play.
  • the simplified characteristic 12 uses only five of the original thirteen output levels, as indicated in Table 1 column 3, and makes use of (some of) the same quantization thresholds that the original characteristic used, namely those between input levels -14 and -13, -3 and -2, 2 and 3, and 13 and 14 (although it could use thresholds which differed slightly from those of the original without seriously impairing the operation of the invention).
  • the variable length coder 5 sees the quantized output signals, in the "overflow" mode, no differently from the signals received during normal operation and uses the same codewords to code the same levels as previously. This means that the same codebook is used for the same quantizer output levels in the overflow mode as in normal operation although, in practice, only a sub-set of the codewords present in the codebook are actually utilised in the second mode.
  • codewords of both normal and overflow transmissions are decoded by the same codebook, and since it is not even necessary for the receiver to know which mode of operation the transmitter is in, no overhead information need be sent.
  • the only effect on the decoded signal, when the coder is in overflow mode, is reduced resolution.
  • variable length coder In the second mode a set of quantizer output levels which is a subset of the set used in the first mode, and to arrange the variable length coder to respond in a fixed manner to these output levels, it would of course be possible to envisage other arrangements of the variable length coder and the quantizer in which in the second mode, codewords are assigned to quantizer output levels which correspond to input signals to which those codewords would have been assigned in the first mode; the essential thing is that the decoder at the receiver produces an output in response to a codeword which bears some resemblance to the input to the quantizer at the coder which produced that codeword.
  • the average bit rate in overflow mode should be below that of normal operation, or else the buffer will not empty.
  • such a "sub-optimum" codebook should preferably provide: an average bit rate for the majority of typical source material that is less than the effective video transmission bit rate (in bits per pel); as large a reduction in average bit rate as possible when operating in overflow mode; and synchronous code. For example if the available channel rate is 29 Mbit/s then the effective video transmission rate is given by:
  • the quantized levels produced in the overflow mode are taken and ranted by magnitude (0, +/- 5, +/- 28).
  • short codewords are assigned in order to these levels to form an 'optimal' Huffman codebook for Overflow 1 mode.
  • an extra bit is added t: the longest two codes (the +/- 28 codes), rendering the c debook slightly sub-optimal.
  • the remaining normal mofe levels, not used in the Overflow 1 mode (+/- 10, +/- 17, +/- 40, +/- 58) are ranked by magnitude and codewords are assigned to them to form an optimal (Huffman) codebcok.
  • the corresponding typical average luminance bit rates using this codebook are: normal mode 2.61 bits per pel; overflow mode (Table 2 column 3) 2.20 bits per pel.
  • the typical average chrominance bit rates are: U-band 1.4 bits per pel and V-band 1.2 rits per pel thus the typical average overall bit rates ie for all three Y, U and V bands are: normal mode 2.286 bits per pel; overflow mode 1.975 bits per pel, a reduction in bit rate of 0.311 bits per pel.
  • a second, and even a third, overflow mode can be provided. The second overflow mode will come into play when the buffer approaches the overflow condition in the first overflow mode.
  • the quantizer In the second overflow mode the quantizer outputs only three levels as shown in Table 1 column 4 (shown as Overflow 2 mode'); the variable length coder in this case will only output three different codes as shown in Table 2 column 4.
  • the third overflow mode ( 'Fallbac ' mode) which is brought into play when the buffer approaches overflow in second overflow mode is a fall-back position in which only one quantizer output level is used, as shown in Table 1 column 5, for all input signals; and in this case variable length coder outputs only one codeword one bit long (Table 2 column 5).
  • the corresponding typical luminance average bit rates in overflow mode 2 will provide 1.93 bits per pel; with the same typical average chrominance bit rates as before this will provide an average overall bit rate for all three bands of 1.776 bits per pel, a reduction in bit rate of 0.51 bits per pel.
  • the third preferred requirement of the sub-optimum codebook, that the output code is synchronous, requires that the total probability of obtaining a synchronising codeword must be as large as possible; there must therefore be as many synchronous codewords as possible each with as high probability of occurrence as possible.
  • the codebook as shown in Table 2 is very synchronous - any codeword ending in the sequence 100 is a synchronising codeword: the total probability of obtaining synchronization (when coding typical source material) is thus 0.26, ie approximately every fourth codeword.
  • the buffer may operate line by line, switching the quantization characteristic at the end of any particular line in which overflow occurs.
  • a buffer of 100 K bits will usually be sufficient.
  • a particular line causes a buffer to overflow (i ⁇ during an "busy" area) then since the number of subsequent frames are likely to have a very similar picture content (in particular the areas of busyness are likely to be spatially coincident) there is a high probability of overflow during contiguous fields.
  • the area of low resolution thus become sub ectively apparent as a rolling bar.of low resolution lines. It may therefore be preferable to initiate switching on a field by a field basis - clearly a much larger buffer would then be required, in the order of 3 Mbits. However in hardware this can easily be implemented at minimal cost with DRAMs. In terms of subjective quality the low resolution fields are hardly discernible. Table 1

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  • Engineering & Computer Science (AREA)
  • Multimedia (AREA)
  • Signal Processing (AREA)
  • Compression, Expansion, Code Conversion, And Decoders (AREA)

Abstract

Le codeur décrit comprend une boucle de modulation de code à impulsions différentielles (DPCM) (1) comportant un quantificateur (4), suivi par un codeur à longueur variable (VLC) (5) et par un tampon (8). Le VLC (5) affecte des mots provenant d'un livre de code aux différents niveaux de sortie du quantificateur (4) et le tampon (8) ralentit le débit binaire des signaux reçus en provenance du VLC avant la transmission le long d'un canal à débit fixe (9). Lorsque le tampon s'approche d'un état de dépassement de capacité, il en informe le quantificateur (4) pour que celui-ci réduise le nombre de seuils de quantification, ce qui a pour conséquence l'affectation d'un sous-ensemble seulement des mots du livre de codes et une réduction du débit binaire, si on utilise un livre de code dont la capacité maximale est sous utilisée en période de fonctionnement normale mais qui est capable d'une meilleure performance en condition de dépassement de capacité.
PCT/GB1988/000087 1987-02-13 1988-02-11 Codeur WO1988006390A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB8703354 1987-02-13
GB8703354A GB8703354D0 (en) 1987-02-13 1987-02-13 Coder

Publications (1)

Publication Number Publication Date
WO1988006390A1 true WO1988006390A1 (fr) 1988-08-25

Family

ID=10612258

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/GB1988/000087 WO1988006390A1 (fr) 1987-02-13 1988-02-11 Codeur

Country Status (2)

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GB (1) GB8703354D0 (fr)
WO (1) WO1988006390A1 (fr)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1991011057A1 (fr) * 1990-01-12 1991-07-25 Telenokia Oy Procede et appareil de codage predictif
WO1991011883A1 (fr) * 1990-01-31 1991-08-08 Telenokia Oy Procede et appareil de codage predictifs
EP0588419A2 (fr) * 1992-09-14 1994-03-23 Philips Patentverwaltung GmbH Codeur adaptif DPCM

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE2454232A1 (de) * 1974-11-15 1976-05-20 Bosch Gmbh Robert System zur codierung von videosignalen nach dem sogenannten differenz-signalverfahren
US4051530A (en) * 1974-10-21 1977-09-27 Nippon Telegraph And Telephone Public Corporation Frame-to-frame coding system
US4093962A (en) * 1976-12-01 1978-06-06 Nippon Electric Co., Ltd. Adaptive predictive encoder
US4200886A (en) * 1977-09-10 1980-04-29 Licentia Patent-Verwaltungs-G.M.B.H. Method for transmitting video signals with the aid of DPC modulation and controlled quantizer
US4562468A (en) * 1982-05-14 1985-12-31 Nec Corporation Adaptive predictive coding apparatus of television signal

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4051530A (en) * 1974-10-21 1977-09-27 Nippon Telegraph And Telephone Public Corporation Frame-to-frame coding system
DE2454232A1 (de) * 1974-11-15 1976-05-20 Bosch Gmbh Robert System zur codierung von videosignalen nach dem sogenannten differenz-signalverfahren
US4093962A (en) * 1976-12-01 1978-06-06 Nippon Electric Co., Ltd. Adaptive predictive encoder
US4200886A (en) * 1977-09-10 1980-04-29 Licentia Patent-Verwaltungs-G.M.B.H. Method for transmitting video signals with the aid of DPC modulation and controlled quantizer
US4562468A (en) * 1982-05-14 1985-12-31 Nec Corporation Adaptive predictive coding apparatus of television signal

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1991011057A1 (fr) * 1990-01-12 1991-07-25 Telenokia Oy Procede et appareil de codage predictif
WO1991011883A1 (fr) * 1990-01-31 1991-08-08 Telenokia Oy Procede et appareil de codage predictifs
EP0588419A2 (fr) * 1992-09-14 1994-03-23 Philips Patentverwaltung GmbH Codeur adaptif DPCM
EP0588419A3 (fr) * 1992-09-14 1994-08-31 Philips Patentverwaltung

Also Published As

Publication number Publication date
GB8703354D0 (en) 1987-03-18

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