CA2105209A1

CA2105209A1 - Bit rate control for hybrid dpcm/dct video codec

Info

Publication number: CA2105209A1
Application number: CA 2105209
Authority: CA
Inventors: Limin Wang
Original assignee: MINISTER OF INDUSTRY SCIENCE AND TECHNOLOGY
Current assignee: MINISTER OF INDUSTRY SCIENCE AND TECHNOLOGY
Priority date: 1993-08-31
Filing date: 1993-08-31
Publication date: 1995-03-01

Abstract

ABSTRACT

A method and apparatus are provided for regulating the bit stream generated by a hybrid DPCM/DCT video codec so that the compressed bit stream at a variable rate can be transmitted over a fixed rate channel. The bit rate regulation is accomplished by appropriately quantizing the DCT
coefficients where the quantization stepsize is obtained by multiplying a fixed weighting matrix by a scaling factor. For each frame, a single quantization scaling factor is determined by iteratively comparing the output bit rate with the target bit rate which is based upon the desirable bit rate, frame code mode and scene changes. The iterative comparison guarantees a quantization scaling factor which results in a minimum accumulated bit rate difference from the target bit rate for each frame.

Description

1 0 ~

BIT RATE CONTROL FOR HYBRID DPCM/~C~ VID~O ÇQ~

The present invention relate~ to the field of electronic tran~mission of digitized video signal~. More particularly, the invention relates to the need to alleviate the channel capacity requirements, by employing a bit compres~ion technique which reduces data rate without losing too much of the subjective quality of the images transmitted.
Generally speaking, hybrid DPCM/DCT i~ the most commonly-used compre~sion technique for digitized video signals having been adopted in many international standards, recommendations and proposals. A hybrid DPCM/DCT video codec generates a compressed bit stream at a variable rate which often needs to be transmitted over a fixed rate channel.
A currently known method for regulating the variable rate bit stream is to use a channel buffer. In order to ensure that the buffer capacity is not exceeded, the buffer occupancy information is fed back to the video encoder and i~ used to adjust the quantization scaling factor. In particular, if the buffer occupancy increa~es, the scaling factor is increa~ed, which results in a decrease in bit rate. Otherwise, the scaling factor is decrea~ed, resulting in an increase in bit rate.
The main difficulty with the buffering technique lies in the implementation complexity. Typical technical i~sues associated with the channel buffering technique include the size of the cha1mel buffer, and the frequency and degree of adjustment of quantization levels. In general, the more often the quantization is adjusted, the smoother the variation of the output bit rate and the smaller the required buffer size.
However, this may result in nonuniform distortion over each frame. For example, two identical blocks in a frame may be assigned two different quantization due to the different buffer occupancy, implying two different quality for the two identical block~. On the other hand, if the adjustment of ~1 0~209 quantization is based upon a longer period of time, a larger buffer size may be required to hold the larger variation of the output bit stream.
The object of the present invention is to provide for a method for regulating the bit stream generated by the hybrid DPCM/DCT video codec. Bit rate regulation is accomplished by appropriately quantizing the DCT coefficients where the quantization stepsize is obtained by multiplying a fixed weighting matrix by a variable scaling factor. The weighting matrix i9 intended to address the human visual characteristics in the DCT domain while the scaling factor controls the actual output bit rate. For each frame, the scaling factor is determined by iteratively comparing the output bit rate with a target bit rate which is based upon the desirable bit rate, frtame code mode, frame complexity and ~cene change.
Therefore, since the weighting matrix is fixed, it is the scaling factor that controls the output bit rate and the quality of the reconstructed images. The smaller the scaling factor, the higher the quality and the higher the bit rate.
The iterative procedure, which is independent of the channel buffer occupancy, guarantees a quantization scaling factor resulting in a bit rate closest to the target bit rate. Since a single scaling factor is used for each frame, a relatively uniform quality over each frtame results.

J ~ ~1 9 BRIEF DES~RIPTI~N OF THE D~RA~IN~S

This invention i~ pointed out with particularity in the appended claims. The above and further advantages of this invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings, in which:
Fig. 1 is a simplified block diagram of an encoder based on hybrid DPCM/DCT video coding.
Fig. 2 is an exemplary mapping characteristic of the quantization scaling factor.

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DETAI~ED DESCRIPTION

Referring first to Fig.1, there i9 shown a block diagram of an encoder based on hybrid DPCM/DCT video coding arranged in accordance with the present invention.
A video signal on line 1 is first input and i8 then partitioned, by a Discrete Cosine Transform (DCT) 2 used to transform spatial domain signals into frequency domain signals, into macroblocks consisting of one luminance block of 16 x 16 pixel~, which is further divided into four 4 x 4 blocks, and two chrominance blocks of 8 x 8 pixels.
Block matching motion estimation 3 is performed on the 16 x 16 luminance block for each macroblock where a closest macroblock is determined from the reconstructed previous frame 4 within a specific tracking range of up to ~ 15 x + 15 pixels. The criterion used to determine the "closest"
macroblock is the absolute difference, given by, ABS(V~h) = ~ I Xm til~ m-i ti~VI j+h) I tl) where xm (i,j) is the luminance pixel value in the macroblock in the current frame, and ~m-l ti+v, j+h) is the corresponding luminance pixel in the macroblock that is shifted v pixel vertically and h pixels horizontally in the previous frame.
The vector, tv*, h*), which yields the minimum absolute difference, ABS tv*, h*) = min V,h ABS tv,h) t2) is called the motion vector. The macroblo~k in the previous frame specified by the motion vector, tv*, h*), is taken as the prediction for the macroblock in the current frame.
~ Generally speaking, due to the temporal correlation between successive frames during continuous scenes, the current macroblock can be fairly well represented by its prediction.
However, when there is a major change in picture content due to uncovered background, fast movement or scene change little temporal correlation is left to be exploited, implying 2 ~ 9 that a poor prediction for the current block results. In such situations, the macroblock should be coded in intra-frame mode, instead of the inter-frame mode. The decision of inter/intra frame coding for each macroblock is based on energy comparison i.e. if the energy of the prediction error macroblock, a2 difference block, is greater than that of the original macroblock a2 original block, 1.5 o2 difference block > o2 original block, (3) intra-frame code mode is selected; otherwise, inter-frame code mode is used. The criterion of the above equation favours the intra-frame code mode by a factor of 1.5 because there usually exists a higher degree of spatial correlation in the original blocks than in the difference blocks with a similar energy level. The higher spatial correlation of the original macroblock can be well addressed by the following DCT in the coding loop.
The difference between the original block and its prediction in inter-frame mode (or the original macroblock in intra-frame mode) undergoes the two dimensional DCT of 8 x 8. -The DCT coefficients are then quantized.
The quantization 5 step sizes, ql~, i, j = 0, 1, 2, 3, 4, 5, 6, 7, are obtained by multiplying a weighting matrix of i, j = 0, l, 2, 3, 4, 5, 6, 7, by a scaling factor, Q, i.e. -25qV-[ ~ ~ int~

(4) where [a] int is the truncation of (a + 0.5). The weighting ~-matrix is designed to address the human visual characteristic in the DCT domain. For each frame, a single quantization scaling factor is issued for all the blocks, the reason being . ~ :

21~S2~9 that is to maintain a relatively uniform distortion or quality over each frame.
The quantized DCT coefficients are scanned along a zigzag path and the resulting sequence usually contains long strings of zeros especially toward the end of the sequence. The sequence is entropy encoded where each run of consecutive zeros and the following non-zero coefficient value is coded as an event.
To synchronize the encoder and decoder, the reference frame for motion compensation 6 should be identical for both encoder and decoder. The inverse operation of quantization 7 and DCT 8 coefficients, generates the reconstructed frame 9. That reconstructed frame is then stored in the frame buffer and use as the reference for the next frame.
A quantization scaling factor is determined for each frame for a given target bit rate. The quantization scaling factor selection 10 is an extra sub-loop attached to the hybrid DPCM/DCT codec which involves only quantization 5 and variable length coding (VLC) 11. The iteration starts with the quantization scaling factor used for the previous frame.
The output bit rate generated by variable length coding is compared with the target bit rate. If the output bit rate 12 is higher ~han the target bit rate, the quantization scaling factor is increased; otherwise it is decreased. The new quantization scaling factor is then applied to the DCT
coefficients, which results in a new compressed bit stream.
In general, the new compres~ed bit rate should be closer to the target bit rate. The quantization and comparison process continues until the quantization scaling factor re~ulting in a bit rate closest to the target bit rate is found.
Note that due to the continuity of natural video signals, the quantization scaling factor used for the previous frame is often fairly good for the current frame. This implies that the number of iteration~ required for determining the quantization scaling factor may be very small.

2 1 ~ ~ 2 ~ 9 The input experience is segmented into groups of N
frames. For each gxoup, the first frame i9 coded in intra-frame mode and the other~ are in inter-frame mode. In order to achieve a similar image quality, intra-frame coding usually requires more bits than inter-frame coding because intra-frame does not take advantage of the temporal correlation. To address the difference in bit rate required for the two types of frames, two target bit rates are given as follows: -J BintrA target .~ Bd~sirable ¦ Binte~_target~ +N-lBdeg (5) '~:~
where Bde81rabl0 is the desirable bit rate in bits per group and a i5 the ratio of Bintrafr~ over Binterfr~e- A logic~l bit allocation over the frames within each group is given by J Bk ( ) tllrg~t ~ Bi~trA targ~t ¦ B~(i) target ~ Binter_ta~get~ 2, N-1 -(6) whçre Bk(i) iS the target bit rate assigned to the ith frame ~
of group k. -I When a scene change occurs, it is necessary to code the change in intra-frame mode as it has little correlation with the previous frameO Consequently, a group containing scene changes is allocated more bits because more than one frame in the group is assigned the intra-frame target bit rate. This results in peaks in the output bit rate and in order to avoid the po~sible build up in the channel buffer, the extra intra-~3~V3 frame target bit rates assigned to the scene change frameshave to be balanced. The adopted method is to code the fir~t frames of the following groups in inter-frame mode, in~tead of intra~rame mode, provided they are not scene changes, until all the extra assignments of intra-frame target bit rate are balanced.
Let SC be the scene change index indicating the number of extra assignments of intra-frame target bit rate up to group k. The procedure for assigning either intra or inter-frame target bit rate to the first frame of group k + 1 canbe generalized as follows:
1. If SC = 0, implying that all extra assignments of intra-frame target bit rate have already been balanced, the first frame of group k ~ 1 is assigned the intra-frame target bit rate and coded in intra-frame mode.
2. If SC ~ 0, implying that all extra assignments of intra-frame target bit rate need to be balanced, the first frame of group k + 1 is assigned the inter-frame target bit rate and coded in inter-frame mode provided it is not a scene change, the scene change index is then decreased by 1, SC = SC - 1 (7) as one extra assignment of the intra-frame target bit rate has been absorbed. However, if the first frame is also a scene change, it is assigned the intra-frame bit rate and coded in intra-frame mode. The scene change index therefore remains unchanged.
~he scene change index is then updated by i SC = SC + n ~8) where n is the number of scene changes (except the first fra~.e) occurring in group k + 1.
A good choice of a is important for obtaining good quality images as a controls the bit allocation over intra-and inter~frames, as indicated in equation (5). It is desirable to have a bit allocation so that equal distortion -:

21~S209 g or quality for both intra- and inter-frames result~. However, the numbers of bits required for intra- and inter-frames for a given quality vary from scene to scene. To follow this variation, a is updated on a group-by-group basis according to the frame complexity measure. Here, the frame complexity measure is defined as the product of the bit rate generated by encoding the frame and the quantization scaling factor used. Specifically, let ak1 be the a value for group k-1, the a value for each group k is given by Bintr~xQintra sc= O
ak ~ Binte~XQinter ak l, SC~O

(9) where Qintra and Bintra are, respectively, the scaling factor and the bit rate for the most recent intra-frame before group k, and Qlnter and Bint~r are the average scaling factor and bit rate 15 for the following inter-frames up to group k. - - -There are a few remark~ on equation (9~. First of all, if the scaling factors for intra- and inter-frames are -identical, i.e. Qintr~ = Qint~r/ the a value will be equal to the ratio of the intra-frame bit rate and the average inter-frame bit rates, i.e., :
Bintra :
a Binter ( 10 ) .
'~' - '~ -Since the fluctuation of the output bit rate can be kept within a narrow margin, the a value will change only slightly.

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Secondly, if the actual bit rates are equal to the target bit rates, i.e., J Bintra ~ Bintr~tar~et ~ Bint~r ~ ~ ~nt~_ ta~get ( 1 1 ) the a value for group k is determined by Cl~k ~ r~k 1 Qintra Qin ter (12) That is, a will be corrected by the ratio of intra- and inter-frame scaling factors. For example, if Qintr4 ~ Qinter~ a will be increased. This is quite reasonable because Qlntra > Qint~r means more quantization distortion may be introduced for the intra-frame using the bit allocation controlled by ak1. An increase in a will allocate more bits to the intra-frame and will reduce the intra-frame scaling factor for the new groups.
Thirdly, if SC ~ O, a will remain unchanged because the following new groups need to balance the extra as~ignments of intra-frame target bit rates caused by scene changes.
Referring now to Fig. 2, there is shown an exemplary mapping characteristic of the quantization scaling factor.
The output bit rate, Bo~ is obtained by using the scaling factor, QO (the scaling factor used for the previous frame).
The output bit rate, Bo~ is seen to be less than the target bit rate, Bt4rget, or the bit rate difference, ~0 = Bo -Btarget < The guantization scaling factor is therefore reduced by one adjusting unit, ~q. The new quantization scaling factor, Q1, = Q~ - ~q, is now applied to the DC~
coefficients, generating a new output bit rate Bl. The new output bit rate, B1, is seen to be closer to the target bit - `
210~2~9 ~11-rate, but still less than the target bit rate, Bt~rg~t~ The quantization scaling factor needs to be further decreased by . The process continues until the bit rate and scaling factor curve, B1 (Q1), crosses the straight line, B = Bt~rg~t ;
that is, the current and previous bit rate difference have the opposite signs. Now, one of the last two quantization scaling factors, Q2 and Q3, which result in the two closest bit rates -;~
to the target bit rate, will be selected depending upon their impact on the bit rate regulation. That is, one resulting in a smaller overall accumulated difference in bit rate is selected.
The procedure for determining the quantization scaling factor for the next frame k + 1 can be summarized by the following algorithmic form: -lS 1. Set the iteration index, i = 0, and the bit rate difference, At1 = 0. Take the quantization scaling .
factor used for the previous frame a~ the initial one for frame k + 1, Q1-2. Apply the quantization scaling factor, Qi,to the DCT
coefficients, resulting in the output bit rate, B1. -~ .

3. Calculate the difference between the output bit rate, B1, and the target bit rate, Btarg~t ~i = Bi ~ Bt~lrget ( 13 ) -If the bit rate difference, ~i, has a different sign from the previous bit rate difference, ~ gO to step 5; otherwise continue. -4. If ~1 > 0, increase Qi by one adjusting unit of ~9~ Qi =
Qi + ~q, otherwise decrease Qi by ~q, Qi = Qi - ~q- Let i = i + 1. Go back to Step 2.

5- Ifl ~ Btotal (k) + ~1 I c I Btotal (k~ + ~ , select Q1 as the final quantization scaling factor; otherwise, Qil. - .
The finally-selected quantization scaling factor is ~ ~.
applied to the DCT coefficients, which results in the actual -output bit rate, .

r soutput (k+1) = ¦ B1, for Q1 Bil, for Qi1 (14) and the bit rate difference, +l) ~ Boutput(k+l)~Btarget ~(Bi l-Btal~t~ for Pi l (15) The ~otal accumulated difference in bit rate up to frame k + 1 is therefore updated by Btotal ( k+ 1 ) = 1~ BtOtal ( k ) + ~ B ( k+ 1 ) . ( 16 )

Claims

1. A bit rate control apparatus comprising:
means for setting a constant weighting matrix;
means for iteratively determining a scaling factor which is independent of a channel buffer occupancy;
means for quantizing an input digital signal, where a quantization step size is obtained by the factor of -(i) the weighting matrix, by (ii) the scaling factor; and means coupled to the quantizing means for controlling an output bit rate.

2. A bit rate controller as recited in Claim 1, in which means for comparing the resulting bit rate and target bit rate are provided.

3. A bit rate controller as recited in Claim 1, in which further means for macroblock intra-frame and inter-frame coding are provided.

4. A bit rate controller as recited in claim 3, in which means for adaptively allocating the target bit rate for each frame based upon the desirable bit rate, frame code mode, frame complexity and scene change are provided.

5. A bit rate controller as recited in Claim 3, in which further means for balancing out the appropriately assigned bit rates of the frames are provided.

6. A method of controlling an output bit rate generated by a video codec, comprising the steps of:
setting a constant weighting matrix;
determining an appropriate scaling factor; and establishing a quantization stepsize at which a compressed bit stream can be transmitted over a fixed rate channel.

7. A method as recited in Claim 6, said establishing step comprising multiplying said weighting matrix by said scaling factor to obtain said quantization stepsize.

8. A method as recited in Claim 6, said establishing step including the sub steps of:
setting an appropriate target bit rate; and comparing a resulting bit rate with said target bit rate.