Abstract:
According to the exemplary embodiment of the present invention, a method for controlling quantization scales when encoding video signals is disclosed, which comprises: (a) generating a new quantization scale; and (b) comparing the new quantization scale with a prior quantization scale, and if the difference between the new quantization scale and the prior quantization scale exceeds a predetermined range, replace the prior quantization scale with the new quantization scale.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of U.S. patent application Ser. No. 10/904,420, filed on Nov. 9, 2004 and included herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to methods and devices for controlling quantization scales of video signals, and more particularly, to methods and devices for controlling quantization scales of video signals in a video encoding device. 
     2. Description of the Prior Art 
     According to MPEG (moving picture coding experts)  2  standard, an image is compressed by eliminating spatial redundancies by chrominance sampling, discrete cosine transform (DCT) and quantization, and eliminating temporal redundancies due to similarity between frames by motion compensation (MC). 
     Generally, there are chromatic or geometrical similarities between frames. In order to eliminate spatial redundancies, it is required to find out important elements and remove those elements that are less important. According to experiments, the human eye is more sensitive to luminance than chrominance. Thus, the MPEG 2 standard, which symbolizes luminance (a.k.a. luma) with Y and chrominance (a.k.a. chroma) with Cr and Cb, reduces signal volume by decreasing chrominance sampling. The MPEG 2 standard defines three sampling modes—4:2:0, 4:2:2, and 4:4:4—which represent three different chroma sampling frequencies. For instance, 4:2:0 mode means sampling 4 Y blocks (each block 8*8 pixels), 1 Cr block (8*8 pixels) and 1 Cb block (8*8 pixels) from a macro block the size of 16*16 pixels. By reducing the chroma sampling frequency, data compression can be optimized. 
     Video data is actually a continuous series of still frames, which are perceived as a moving picture due to the persistence of images in the vision of human eyes. The frames have a very short time interval, and there is only small difference between neighboring frames. Therefore, the MPEG 2 standard eliminates temporal redundancies due to similarity between frames by motion compensation (MC). The method described above is well known by a person skilled in the art. 
     Please refer to  FIG. 1  showing a conventional video encoder  10 . The video encoder  10  includes a DCT device  12 , a motion estimator and compensator  14 , a quantizer  16 , a variable length encoder (VLE)  18 , and a rate controller  20 . The video encoder  10  utilizes the DCT device  12  and the quantizer  16  to eliminate spatial redundancies and the motion estimator and compensator  14  to eliminate temporal redundancies in order to compress the digital video data. The compressed data is then encoded by the VLE  18  and sent to a system multiplexer (not shown) to output the data in a transport stream or program stream as defined by the MPEG 2 standard. 
     The DCT device  12  performs DCT operation to every block (each block includes 8*8 pixels) sampled from a macro block in order to transform the video data from a spatial domain to a frequency domain. The DCT operation is a completely reversible mathematical operation. A DCT coefficient obtained after transforming the chroma of the block remains as an 8*8 two-dimensional matrix. Generally, there is hardly any intensive change of colors in a frame; thus the DCT coefficient standing for higher spatial frequency in the matrix is small or even 0. Basically, DCT operation does not reduce data volume but instead transforms the data in a format where redundancies can be more easily found. 
     Subsequently, the quantizer  16  quantizes the DCT coefficient to further compress the video data. Quantization is to reduce the description of the bit number of each coefficient; that is, each coefficient is described in a less precise unit. Quantization makes a value close to 0 become 0 and reduces the distribution of coefficients that are not 0 for a better performance on data compression. Quantization is a sort of damaging compression, which means the data quantized is not the same as the original. Therefore, the distortion resulting from compression is dependent on the selection of quantization scale. 
     The rate controller  20  in the video encoder  10  is for adjusting the quantization scale of the quantizer  16  according to a predetermined output bit rate range of the video encoder  10 . The rate controller  20  adjusts the quantization scale for a macro block; that is, every block sampled from a macro block has the same quantization scale. 
     After quantization by the quantizer  16 , the video encoder  10  connects those DCT coefficients in the quantized two-dimensional matrix serially, turning it into a one-dimensional series by way of a specific algorithm, to create the series with the longest length of continuous 0&#39;s in order to optimize the data compression. Subsequently, the VLE  18  compresses the one-dimensional series to output a compressed bit stream, which is called an encoded bit stream. 
     Lowering the quantization scale of the quantizer  16  lowers the compression rate, which increases the image quality, but increases the bit rate of the encoded bit stream output by the VLE  18 . In contrast, increasing the quantization scale of the quantizer  16  increases the compression rate, which lowers the image quality, but lowers the bit rate of the encoded bit stream output by the VLE  18 . In the encoded bit stream, each quantization scale applied during encoding is represented by, for example, 7-8 bits. These bits are used for decoding the encoded bit stream. 
     Along with the description above, in the case that the output bit rate of the video encoder  10  is to be kept in a predetermined range, if the quantization scale varies too frequently, there may be a large number of bits used to record quantization scales in the encoded bit stream output by the video encoder  10 . For instance, in the NTSC standard, there are 30 frames per second, and each frame has 1350 macro blocks. In this case, if the output bit rate of the video encoder  10  is 2 Mbps, the share of the output bit rate for each compressed macro block is only 49.3 bits. If the rate controller  20  varies the quantization scale for every macro block, around 14% (7/49.3=0.14) of the output bit rate is used for recording the quantization scale. Such arrangement of the bit rate is not efficient. 
     Therefore in the conventional video encoder  10 , the more frequent the rate controller  20  varies the quantization scale, the more bits are wasted on recording the quantization scale, so the remaining bit rate available for recording video data itself is limited and the image quality is not satisfactory. 
     SUMMARY OF THE INVENTION 
     It is therefore an objective of the invention to provide methods and devices for controlling quantization scales while encoding video signals by limiting the number of changes in the quantization scales to solve the problems mentioned above. 
     According to the exemplary embodiment of the present invention, a method for controlling quantization scales when encoding video signals is disclosed, which comprises: (a) generating a new quantization scale; and (b) comparing the new quantization scale with a prior quantization scale, and if the difference between the new quantization scale and the prior quantization scale exceeds a predetermined range, replace the prior quantization scale with the new quantization scale. 
     According to the exemplary embodiment of the present invention, a video quantizing device for quantizing video signals to generate a quantized matrix is disclosed, which comprises: a rate controller for providing quantization scales; and a quantizer electrically connected to the rate controller for quantizing the video signals according to the quantization scales; wherein the rate controller compares a new quantization scale with the prior quantization scale, and if the difference between the new quantization scale and the prior quantization scale exceeds a predetermined range, the rate controller provides the new quantization scale to the quantizer to replace the prior quantization scale. 
     According to the exemplary embodiment of the present invention, a method for encoding digital video signals including a plurality of macro blocks is disclosed, which comprises: (a) encoding a current macro block using a first quantization scale; and (b) determining whether to encode a next macro block using a second quantization scale instead of the first quantization scale according to a difference between the first quantization scale and the second quantization scale. 
     According to the exemplary embodiment of the present invention, an apparatus for encoding digital video signals including a plurality of macro blocks is disclosed, which comprises: a rate controller for providing a quantization scale; and a quantizer coupled for receiving the quantization scale to perform quantization to respective one of the macro blocks; wherein when the new value is significantly different from a prior value, the quantization scale provided by the quantizer is replaced with the new value. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a conventional video encoder. 
         FIG. 2  illustrates a video encoder capable of controlling the number of changes in the quantization scales according to one embodiment of the present invention. 
         FIG. 3  depicts a flowchart illustrating the operation of controlling the quantization scale of a video signal according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Please refer to  FIG. 2  showing a video encoder  100  capable of controlling the number of changes in the quantization scales. Some components in the video encoder  100  could be similar to those in the conventional video encoder  10 . These common components have the same numbering in this embodiment. The video encoder  100  uses a DCT device  12  and a quantizer  16  to eliminate spatial redundancies and a motion estimator and compensator  14  to eliminate temporal redundancies in order to compress video data. The compressed data can then be encoded by a VLE  18  to be output in form of an encoded bit stream. 
     The quantizer  16  quantizes a block according to a quantization scale determined by a rate controller  120 . However in the present invention, the rate controller  120  can also determine whether to replace the original quantization scale with a new quantization scale according to the number of changes in the quantization scales during encoding. 
     Please refer to  FIG. 3  showing a flowchart  200  of the video encoder  100  controlling the quantization scale of a video signal as follows:
         Step  202 : Start.   Step  204 : Set up parameters required for rate controller  120 &#39;s operation, such as timeframe P for calculating the number of changes in the quantization scales, initial value of the number of changes Chg_Q, upper limit TH of the number of changes in the quantization scales, and threshold QTH of the difference between successive quantization scales.   Step  206 : The rate controller  120  generates a new quantization scale Q new  corresponding to the next macro block.   Step  208 : The rate controller  120  determines if the new quantization scale Q new  is equal to the original quantization scale Q old . If yes, go to Step  216 ; otherwise, go to Step  210 .   Step  210 : Calculate a difference between the new quantization scale Q new  and the original quantization scale Q old  and determine if it is equal to or larger than the threshold QTH. If yes, go to Step  214 , and if no, go to Step  212 .   Step  212 : Determine if the number of changes in the quantization scales Chg_Q is equal to or larger than the upper limit TH. If yes, go to Step  216 ; otherwise, go to Step  214 .   Step  214 : The rate controller  120  passes the new quantization scale Q new  to the quantizer  16  to replace the original quantization scale Q old  with the new quantization scale. Go to Step  218 .   Step  216 : The rate controller  120  does not replace the original quantization scale Q old  with the new quantization scale Q new . The quantizer  16  utilizes the original quantization scale Q old  for quantization.   Step  218 : The rate controller  120  refreshes the number of changes in the quantization scales Chg_Q.   Step  220 : Determine if any other macro blocks need to be processed. If yes, go to Step  206 , otherwise, go to Step  222 .   Step  222 : End.       

     In the following description, the quantizer  16  is assumed to quantize a macro block L according to a quantization scale Q old . 
     In Step  204 , the video encoder  100  sets up the parameters required for rate controller  120 &#39;s operation. For instance, set up a timeframe P for calculating the number of changes in the quantization scales for twenty macro blocks, which means calculating the number of changes in the quantization scales every time period of twenty macro blocks. Set up the upper limit TH of the number of changes in the quantization scales as ten, which means the number of changes in quantization scales can be up to ten times in a time period of twenty macro blocks. Set up the initial value of the number of changes in the quantization scales Chg_Q as zero, which means the rate controller  120  calculates the number of changes in the quantization scales starting from zero. Finally, set up the threshold QTH of the difference between successive quantization scales. 
     In Step  206 , the rate controller  120  generates a new quantization scale Q new  corresponding to the next macro block M to be quantized, which is next to the macro block L. 
     In Step  208 , the rate controller  120  compares the new quantization scale Q new  with the original quantization scale Q old  to see whether they are equal to each other. If they are equal to each other, Step  216  is then performed. In Step  216 , the rate controller  120  notifies the quantizer  16  to keep the original quantization scale Q old  instead of passing the new quantization scale Q new , which is the same as the original quantization scale Q old , to the quantizer  16 . Thus, the quantizer  16  quantizes the macro block M according to the original quantization scale Q old . If the new quantization scale Q new  does not equal to the original quantization scale Q old , go to Step  210  to find the difference between the new quantization scale Q new  and the original quantization scale Q old . 
     In Step  210 , the rate controller  120  checks whether the difference between the new quantization scale Q new  and the original quantization scale Q old  is larger than the threshold QTH; that is, to see if |Q new −Q old |≧QTH. |Q new −Q old |≧QTH means that the difference between the new quantization scale Q new  and the original quantization scale Q old  which corresponds respectively to the macro block L and the macro block M, is larger than the threshold QTH. The quantization scale calculated by the rate controller  120  represents the complicacy of the macro blocks. The quantization scale difference between the macro block L and the macro block M being large generally indicates strong preference to the new quantization scale Q new . Therefore, if the difference between the new quantization scale Q new  and the original quantization scale Q old  is larger than the threshold QTH, the rate controller  120  passes the new quantization scale Q new  to the quantizer  16  for the next quantization without any limitations from the number of changes in the quantization scales Chq_Q. In Step  210 , if the quantization scale difference is not larger than the threshold, i.e. |Q new −Q old |&lt;QTH, go to Step  212  to determine whether or not to replace the original quantization scale Q old  with the new quantization scale Q new . 
     In Step  212 , the rate controller  120  checks whether the number of changes in the quantization scales Chg_Q equals or is larger than the upper limit TH of 10. If the number of changes in the quantization scales Chg_Q is ten or larger than ten, the rate controller  120  notifies the quantizer  16  to maintain the original quantization scale Q old  instead of passing the new quantization scale Q new  to the quantizer  16 . Thus, the quantizer  16  quantizes the macro block M according to the original quantization scale Q old . Accordingly, in the encoded stream output by the VLE  18 , the quantization scale does not vary during the quantization of the macro block M, and therefore additional bits (for example, 7 to 8 bits) for recording the new quantization scale Q new  are saved. 
     If the number of changes in the quantization scales Chg_Q is less than ten, the rate controller  120  performs Step  214  to pass the new quantization scale Q new  to the quantizer  16  so the quantizer  16  quantizes the macro block M according to the new quantization scale Q new . Thus in the encoded stream output by the VLE  18 , the new quantization scale Q new  used for quantizing the macro block M is recorded with 7-8 bits. 
     In Step  218 , regardless of whether the rate controller  120  replaces the original quantization scale Q old  with the new quantization scale Q new  or not, the rate controller  120  refreshes the number of changes in the quantization scales Chg_Q. It is noted that the timeframe P for calculating the number of changes in the quantization scales was set up as twenty macro blocks in Step  204 ; thus when refreshing the number of changes in the quantization scales Chg_Q, the rate controller  120  will remove the 20 th  macro block previous to the macro block M (i.e. the farthest previous macro block from the current macro block M being processed) and add the macro block M into calculation, where it will sum up the number of changes in the quantization scales of the macro block M and its previous nineteen macro blocks, which serves as the refreshed Chg_Q. 
     Step  220  is to determine if any other macro blocks need to be quantized after the macro block M. If the macro block M is the last macro block, end the flow. If there are still other macro blocks need to be quantized, the rate controller  120  generates a quantization scale corresponding to the next macro block N before going back to Step  206  and repeating the steps described above. 
     The sequence for performing Step  204  and Step  206  is not limiting. Step  204  could be performed prior or after Step  206 . Moreover, in this embodiment it is assumed that the timeframe P for calculating the number of changes in the quantization scales is twenty macro blocks, the upper limit TH of the number of changes in the quantization scales is ten, the initial value of the number of changes in the quantization scales Chg_Q is zero, and the threshold of the difference in the quantization scale range is QTH. These parameters are only for illustrative purpose and should not be taken as a limitation to the present invention and could be adjusted as required. For instance, the timeframe P for calculating the number of changes in the quantization scales could be other specific period of time; the upper limit TH of the number of changes in the quantization scales could be any number larger than 0, and could be either integer or non-integer number; the initial value of the number of changes in the quantization scales Chg_Q could be zero or any other numbers; and the threshold of the difference in the quantization scale range can be a specific value or a specific percentage. It is also possible to write those required parameters into a firmware controlling the rate controller  120  in advance. 
     In the above embodiment, the difference between the new quantization scale Q new  and the original quantization scale Q old  is checked, and the historical changes in the quantization scales Chg_Q is also evaluated to determine whether or not to replace the original quantization scale Q old  with the new quantization scale Q new . In another embodiment, Step  210  can be omitted; that is, when the new quantization scale Q new  does not equal to the original quantization scale Q old , only historical changes in the quantization scales Chg_Q are evaluated to determine whether or not to replace the original quantization scale Q old  with the new quantization scale Q new . Other combinations of the quantization difference and/or the historical changes in the quantization scales are also feasible according to the spirit of the invention. 
     In contrast to the related art, the rate controller  120  according to the present invention can limit the number of changes in the quantization scales during encoding digital video data in order to reduce the bit volume for recording the quantization scale in the encoded bit stream. Therefore more bits are available for recording the digital video data itself, and better image quality can be achieved. 
     The above description relates to the video encoder  100 . The video encoder  100  could be realized in an integrated semiconductor chip. The encoder  100  could also be realized in a computer system, and the DCT device  12 , the motion estimator and compensator  14 , the quantizer  16 , the VLE  18  and the rate controller  120  could be respectively programs or an integrated encoding program. In this case, the flowchart  200  shown in  FIG. 3  is the algorithm of the encoding program with a program code edited by any program language. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.