Abstract:
A transcoding method for inserting a visual element into an encoded bit stream with minimal decoding of the encoded bit stream. The transcoding method includes the steps of receiving an encoded bit stream containing a picture, partially decoding a copy of the bit stream, determining if a segment of the picture in the bit stream is affected by the insertion of the visual element, re-encoding the segment if the segment is affected by the insertion of the visual element, and outputting the re-encoded bit stream. The encoded and re-encoded bit streams may be MPEG-encoded bit streams. The re-encoded bit stream may be output using a bit rate control process that randomly selects an initial processing point within the re-encoded bit stream, requantizes the re-encoded bit stream using a modified quantization table, and/or utilizes a slice-level stopping criteria for selectively starting or ending bit rate control.

Description:
GOVERNMENT LICENSE 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Government Contract No. 70NANB5H1174 awarded by NIST (National Institute of Standards &amp; Technology). 
    
    
     FIELD OF THE INVENTION 
     The invention generally relates to a process for modifying coded bit streams. More particularly, the invention is directed to a process for inserting a visual element into a moving pictures group-type 2 (“MPEG 2”) encoded video bit stream. 
     BACKGROUND OF THE INVENTION 
     Traditionally, broadcast stations insert visual elements (e.g., logos, station identifications, video sequences, and the like) into video signals broadcast to the end viewer. Before the advent of compressed video stream transmission, the insertion of a logo into a broadcast was a relatively straight forward procedure of superimposing or substituting the logo image over a selected portion of a video frame. However, with the advent of compressed video stream transmission (e.g., MPEG-coded video streams) the insertion of logos into video broadcast has become complicated due to temporal dependencies in the coded video stream. In particular, each frame in the video stream no longer contains all the information necessary to reconstruct the video image. Rather, only a few frames (e.g., I frames) in the video stream contain all the information that is needed to re-construct a video image. Other “predictive” frames (e.g., P frames and B frames) reference the portions of the complete frames (e.g., I frames) that are necessary for the reconstruction of the predictive frames, as discussed in further detail below. 
     One method of inserting a logo into an MPEG-coded video stream is to decode the video stream, insert the logo in the pixel domain, and re-encode the video stream. However, the process of decoding and re-encoding the entire video stream has several drawbacks. One drawback is that the decoding and re-encoding process results in a loss in image quality since errors are introduced into the video stream. These errors are due, in part, to the non-reversible nature of some of the coding techniques (e.g., quantization and inverse quantization). Another drawback is that decoding and re-encoding the entire video stream requires a significant amount of computation. 
     In addition to the drawbacks discussed above, there is an additional difficulty encountered when inserting a logo into an MPEG-encoded video stream. The insertion of the logo may affect the bit count of the video stream. In particular, the insertion of the logo may cause the bit count of the video stream to increase or decrease and this, in turn, may affect Video Buffer Verifier (VBV) compliance. More specifically, the input buffer of a decoder, under the MPEG standard, has a fixed buffer size and handles transitions between a constant channel bit rate, i.e., the rate at which the video is displayed, and a fluctuating MPEG coded bit rate, i.e., the rate at which the compressed video is passed. Preferably, data flows through the buffer so underflow and overflow conditions are avoided. However, altering the bit count of the MPEG-encoded video stream may cause underflow or overflow conditions to occur in the input buffer of a downstream decoder. 
     The present invention is directed to overcoming the drawbacks and difficulties discussed above. 
     SUMMARY OF THE INVENTION 
     Briefly stated, a transcoding method for inserting a visual element into an encoded bit stream with minimal decoding of the encoded bit stream is provided. The transcoding method includes the steps of receiving an encoded bit stream containing a picture, partially decoding a copy of the bit stream, determining if a segment of the picture in the bit stream is affected by the insertion of the visual element, re-encoding the segment if the segment is affected by the insertion of the visual element, and outputting the re-encoded bit stream. The encoded and re-encoded bit streams may be MPEG-encoded bit streams. The re-encoded bit stream may be output using a bit rate control process that randomly selects an initial processing point within the re-encoded bit stream, requantizes the re-encoded bit stream using a modified quantization table, and/or utilizes a slice-level stopping criteria for selectively starting or ending bit rate control. 
     In one advantageous embodiment of the invention, a transcoding method for inserting a visual element into an encoded bit stream representing a series of image frames is provided. The transcoding method includes receiving the encoded bit stream, partially decoding the encoded bit stream, inserting the visual element into the partially decoded bit stream, re-encoding the partially decoded bit stream after the visual element is inserted, and outputting the re-encoded bit stream. 
     In another advantageous embodiment of the invention, a transcoding method for inserting a visual element into an insertion region of an encoded bit stream representing a series of image frames is provided. The transcoding includes receiving a first encoded bit stream, decoding a copy of the first encoded bit stream to determine a position of a segment of an image frame, comparing the insertion region to the position of the segment to determine if the insertion of the visual element would affect the segment, re-encoding the segment if the insertion of the visual element affects the segment, and outputting a second encoded bit stream, the second encoded bit stream being a combination of the first encoded bit stream and the re-encoded segment. 
     In a further advantageous embodiment of the invention, a transcoding method for inserting a visual element into an insertion region of an encoded bit stream is provided. The transcoding method includes receiving a first encoded bit stream representing an image, decoding a copy of the first encoded bit stream to detect characteristics of respective segments of the image, comparing the characteristics of the segments to the insertion region to determine if the insertion of the visual element into the image would affect the segments, re-encoding the affected segments, and outputting a second encoded bit stream such that the second encoded bit stream contains the same number of bits as the first encoded bit stream, the second encoded bit stream being a combination of the first encoded bit stream and the re-encoded segments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned advantages of the invention, as well as additional advantages thereof, will be more fully understood as a result of a detailed description of the preferred embodiment when taken in conjunction with the accompanying drawings in which: 
     FIG. 1 illustrates an exemplary image and logo of the present invention; 
     FIG. 2 is illustrates pixel macroblock slices, macroblocks, and blocks in the FIG. 1 image; 
     FIG. 3 is a block diagram illustrating a logo insertion process of the present invention; 
     FIG. 4 is a is a flow diagram illustrating a by-pass process of the present invention; 
     FIG. 5 is a flowchart illustrating a frame determination process of the present invention; 
     FIG. 6 is a flowchart illustrating a logo insertion process for an I frame of the present invention; 
     FIG. 7 is a flowchart illustrating a logo insertion process for a P frame of the present invention; 
     FIG. 8 is a flowchart illustrating a logo insertion process for a B frame of the present invention; and 
     FIGS. 9,  10  and  11  are flowcharts illustrating a rate control process of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The characteristics and advantages of the present invention will become more apparent from the following description, given by way of example. 
     Referring now to FIG. 1, an exemplary image  10  and logo  12  are shown. Image  10  may be contained in a field picture or a frame picture having an I, P, or B structure. For clarity and ease of reference, only frame pictures will be discussed below. An insertion region  14  in image  10  corresponds to the position where logo  12  is to be inserted. As briefly mentioned above, a frame having an I structure is similar to a frame coded as a still image. As such, an I frame contains all the necessary image data needed for the reconstruction of the image contained within it. A frame having a P structure is known as a predicted frame and requires, for reconstruction, the image data contained in the most recently reconstructed I or P frame. A frame having a B structure is known as a bidirectional frame and is predicted from the closest two I and/or P frames, one in the past and one in the future. Therefore, a B frame requires the image data contained in two other frames (I and/or P) for reconstruction. 
     Referring now to FIG. 2, image  10  is encoded as an I, P, or B frame and passed in an encoded bit stream as a series of macroblock slices  16 . Each macroblock slice  16  contains luminance and chroma data for a horizontal group of pixel macroblocks  18 . Each pixel macroblock  18  has a size of 16×16 pixels (not shown) and is made up of four blocks  20  of luminance and two blocks of chrominance (one Cr and one Cb) covering the same area. Thus a transcoder implementing the method of the present invention receives an encoded video bit stream as a series of macroblock slices  16 . Although an exemplary transcoder is not shown, the implementation of the present invention in a conventional MPEG transcoder is considered within the ability of one skilled in the art. 
     Referring now to FIG. 3, in conjunction with FIG. 2, an overall logo insertion process  22  of the present invention is shown. Logo insertion process  22  includes a logo insertion pass  24  and a rate control pass  26 . As discussed in further detail below, logo insertion pass  24  includes a decoding step  28  and a re-encoding step  30 . Generally, decoding step  28  includes partially decoding an incoming encoded video stream and determining if a segment (e.g., macroblock slice  16 , macroblock  18 , or a block  20 ) of the video stream overlaps or is otherwise affected by the insertion of logo  12 . If the segment is not affected by the insertion of logo  12 , the encoded segment by-passes decoding step  28  and re-encoding step  30  and is stored in a temporary buffer. However, if the segment (e.g., macroblock slice  17 ) is affected by the insertion of logo  12 , the affected portion of the segment, during decoding step  28 , is decoded up to the pixel domain and logo  12  is combined with, or otherwise assimilated into, the affected portion of the decoded segment. Afterwards, during re-encoding step  30 , the segment, now containing logo  12  or logo related data (e.g., motion vector information), is re-encoded and stored in the temporary buffer. 
     Next, during rate control pass  26 , the encoded and re-encoded segments of the video stream are output from the temporary buffer such that the re-encoded segments are inserted into their original positions in the encoded video stream. During a decoding step  32 , the segments of the encoded video bit stream are decoded up to the DCT domain. In the DCT domain the DCT coefficients of the segments are requantized using different quantization scalars, as discussed in further detail below. After requantization, the decoded segments, during a re-encoding step  34 , are re-encoded and stored in a temporary buffer. The re-encoded segments are then output via a selective rate control that is enabled or disable based on characteristics of the requantized segments, as discussed in further detail below. 
     Although logo insertion pass  24  is illustrated as being distinct from rate control pass  26 , one skilled in the art will readily devise that logo insertion pass  24  may be integrated with the rate control pass to further reduce the computational cost of the overall logo insertion process  22 . However, logo insertion pass  24  and rate control pass  26  are illustrated as distinct passes for clarity, ease of reference, and to facilitate an understanding of the present invention. 
     Referring now to FIG. 4, in conjunction with FIG. 2, a partially decoding process  38  of the insertion method of the present invention is illustrated. Data in an incoming bit stream  40  is passed through a transcoder at the highest level possible. For example, an encoded macroblock slice  16  that does not overlap, or is not otherwise affected by, insertion region  14  of logo  12  is passed through the transcoder in the macroblock slice domain  46  and stored as an encoded macroblock slice. If a portion of a macroblock slice (e.g., macroblock slice  17 ) is found to overlap, or to be otherwise affected by, the logo&#39;s insertion region  14 , the macroblock slice is decoded into the macroblock domain  48  and any macroblocks  18  that don&#39;t overlap the logo&#39;s insertion region  14  are passed through the transcoder in the macroblock domain  48  and stored as encoded macroblocks. Furthermore, in the macroblock domain  48  any motion vectors affected by the logo&#39;s insertion region  14  are processed, as discussed in further detail below. This by-passing process continues up to pixel domain  44  where the pixels in the overlapping portion of the macroblock slice are combined with logo  12 . Afterwards, the pixels are re-encoded down to macroblock slice domain  46  such that encoded and re-encoded blocks are combined into re-encoded macroblocks, encoded and re-encoded macroblocks are combined into re-encoded macroblock slices, and encoded and re-encoded macroblock slices are passed in an outgoing bit stream  42 . Thus, partially decoding process  38  of the present invention facilitates the insertion of logo  12  into an MPEG bit stream  40  without fully decoding the entire bit stream and, as a result, reduces the complexity, cost, time, and re-coding loss traditionally encountered during logo insertion. 
     Referring now to FIG. 5, a general overview of the visual element insertion process  50  is illustrated. As illustrated, at step  52 , the transcoder examines an incoming frame to determine if the frame is an I frame. If so, the transcoder, at step  54 , carries out the process illustrated in FIG.  6 . If not, the transcoder, at step  56 , determines if the incoming frame is a P frame. If the incoming frame is a P frame, the transcoder, at step  58 , carries out the process illustrated in FIG.  7 . If the incoming frame is not a P frame, the transcoder, at step  60 , determines that the incoming frame is a B frame and carries out the process illustrated in FIG.  8 . Frame type may be determined by examining picture or frame header data accompanying respective coded frames. As discussed in further detail below, after the visual element has been inserted into the incoming frame, the transcoder, at step  62 , repeats the process illustrated in FIG. 5 to determine the frame type of the next incoming frame. This process continues until the insertion of the visual element into the incoming bit stream is completed. 
     Referring now to FIG. 6, an I frame insertion process  70  is shown. First, at step  72 , the transcoder stores and copies the encoded bits of a macroblock slice of the incoming I-frame. Next, at step  74 , the transcoder decodes the copy of the encoded bits of the macroblock slice and stores two copies of the decoded slice in memory. The first copy is stored in a “Ref (slice_pos)” block of memory. The decoded macroblock slices stored in the “Ref (slice_pos)” block of memory may be utilized for decoding macroblocks in later frames as is done in a standard MPEG decoder. The second copy is stored in a “Ref_w_logo (slice_pos)” block of memory and may be utilized for re-encoding macroblocks in later frames that are affected by the logo insertion, as discussed in further detail below. At step  76 , the transcoder determines whether the decoded macroblock slice overlaps the logo&#39;s insertion region  14 . If not, at step  78 , the transcoder passes the encoded bits of the macroblock slice that were previously stored. If so, at step  80 , the transcoder combines the overlapping portions of logo  12  and the macroblock slice (e.g., macroblock slice  17 ), as discussed above, and overwrites the previous content of the “Ref_w_logo (slice_pos)” block of memory with this new data. It should be noted that the combining step may represent, but is not limited to, a replacement or mixing operation. It should be further noted that logo  12  may advantageously be in encoded form (e.g., an encoded macroblock) if the combining step is a replacement operation since replacing the macroblock slice with macroblock portions of logo  12  in the macroblock domain requires less computation than replacing the macroblock slice with pixel portions of logo  12  in the pixel domain. To further reduce unnecessary computations the logo insertion region  14  may be sized to align with upper and lower boundaries of overlapping macroblock slices to ensure that the entire logo replacement operation occurs in the macroblock domain. At step  82  the transcoder re-encodes the combination of logo  12  and the overlapping macroblock slice and, at step  84 , passes the re-encoded combination, as discussed above. DC quantization values undergo DPCM (Differential Pulse Code Modulation) across a macroblock slice. Thus, a macroblock to the right of a re-encoded macroblock (i.e., the macroblock combined and re-encoded with a portion of logo  12 ) must also be re-encoded to account for the change in the DC quantization value of the macroblock containing the logo. Next, at step  86 , the transcoder determines whether the current macroblock slice is the last macroblock slice in the I frame. If not, at step  88 , the transcoder returns to step  72  to store and copy the next encoded macroblock slice of the I frame. If so, at step  90 , the transcoder returns to the process illustrated in FIG.  5  and begins to examine the next frame. 
     Referring now to FIG. 7, a P frame insertion process  92  is shown. First, at step  94 , the transcoder stores and copies the encoded bits of a macroblock slice of the incoming P frame. Next, at step  96 , the transcoder decodes the copy of the encoded bits of the macroblock slice and stores two copies of the decoded slice in memory. The first copy is stored in a “Ref (slice_pos)” block of memory. The decoded macroblock slices stored in the “Ref (slice_pos)” block of memory may be utilized for decoding macroblocks in later frames as is done in a standard MPEG decoder. Although completely decoding P frame segments is disclosed, it is considered within the scope of the present invention to use other techniques for re-encoding later frames that are affected by the insertion of logo  12 . For example, the transcoder may store encoded P frames and decode selected parts of the P frame on an as needed basis. The second copy is stored in a “Ref_w_logo (slice_pos)” block of memory and may be utilized for re-encoding macroblocks in later frames that are affected by logo insertion, as discussed in further detail below. 
     At step  98  the transcoder determines whether the position of the second copy of the decoded macroblock slice overlaps the logo&#39;s insertion region  14 . If so, at step  100 , the transcoder combines the overlapping portions of logo  12  and the macroblock slice (e.g., macroblock slice  17 ) and overwrites the previous content of the “Ref_w_logo (slice_pos)” block of memory with this new data. Afterwards, at step  102 , the transcoder re-encodes the combination of logo  12  and the overlapping macroblock slice using a previously encoded anchor frame (e.g., I-frame or P-frame) that contains the logo and, at step  104 , passes the re-encoded combination, as discussed above. As previously noted, the combination may represent, but is not limited to, a replacement or mixing operation. In an alternative approach, the transcoder may set a motion vector, associated with the overlapping macroblock, to zero to avoid the computational cost of the motion vector search process. In either case, it should be noted that the motion vector and DC quantization values are DPCM across a macroblock slice. Therefore, if a motion vector and/or macroblock is re-encoded the next unskipped motion vector and/or macroblock must be differentially encoded to account for the re-encoding of the motion vector and/or macroblock. 
     If the transcoder determines that no overlap exists the transcoder, at step  106 , determines whether there are any motion vectors in the macroblock slice that point to the logo&#39;s insertion region  14 . If not, at step  108 , the transcoder passes the encoded macroblock slice that was previously stored. If so, at step  110 , the transcoder preferably re-encodes the affected macroblocks using the reference frames that were previously saved in the “Ref w/ logo” block of memory. As discussed above, the previously stored macroblocks are macroblocks of earlier I or P frames that were stored and modified by logo insertion. In an alternative procedure, the transcoder may set motion vectors pointing to the logo insertion region  14  to zero. This alternative procedure would reduce the computations otherwise needed for re-encoding the affected macroblocks. Furthermore, motion vector extrapolation techniques may also be utilized to reduce computation costs. One exemplary motion vector extrapolation technique is disclosed in applicant&#39;s U.S. patent application Ser. No. 026,140, filed Feb. 20, 1998, entitled “Motion Vector Extrapolation For Transcoding Video Sequences” and hereby incorporated herein by reference. In any case, it should be noted that the motion vector and DC quantization values are DPCM across a macroblock slice. Therefore, if a motion vector and/or macroblock is re-encoded the next unskipped motion vector and/or macroblock must be differentially encoded to account for the re-encoding of the motion vector and/or macroblock. At step  104  the transcoder passes the re-encoded macroblock slice. 
     After the stored macroblock slice or re-encoded macroblock slice is passed, the transcoder determines, at step  112 , whether the current macroblock slice is the last macroblock slice in the P frame. If not, at step  114 , the transcoder returns to step  94  to store and copy the next encoded macroblock slice in the P frame. If so, at step  116 , the transcoder returns to the process of FIG.  5  and begins the examination of the next frame. 
     Referring now to FIG. 8, a B frame insertion process  118  is shown. Initially, at step  120 , the transcoder stores and copies the encoded macroblock slice of the incoming B frame. Next, at step  122 , the transcoder determines if insertion region  14  of logo  12  is within the f-code range of the macroblock slice. The f-code range is contained in the picture header. If not, at step  124 , the transcoder passes the stored macroblock slice and determines, at step  126 , whether the current macroblock slice is the last macroblock slice in the B frame. If the current macroblock slice is not the last macroblock slice in the B frame then, at step  128 , the transcoder returns to step  120  to store and copy the next encoded macroblock slice in the B frame. If the current macroblock slice is the last macroblock slice in the B frame then, at step  130 , the transcoder returns to the process illustrated in FIG.  5  and begins the examination of the next frame. 
     If insertion region  14  of logo  12  is within the f-code range the transcoder, at step  132 , determines if the macroblock slice overlaps insertion region  14  of logo  12 . If so, at step  134 , the transcoder decodes the macroblock slice and, at step  136 , combines the overlapping portions of the macroblock slice (e.g., macroblock slice  17 ) and logo  12 . Afterwards, at step  138 , the transcoder re-encodes the combination of logo  12  and the overlapping macroblock slice using previously encoded anchor frames (e.g., I-frames and/or P-frames) that contain logo  12  and, at step  140 , passes the re-encoded combination. As previously noted, the combination may represent, but is not limited to, a replacement or mixing operation. In an alternative approach, the transcoder may set the motion vectors associated with each overlapping macroblock to zero to avoid the computational cost of the motion vector search process. In either case, as discussed above, the next unskipped motion vector and/or macroblock must be differentially encoded. 
     If, at step  132 , the transcoder determines that the macroblock slice does not overlap insertion region  14  of logo  12 , the transcoder, at step  142 , acquires the motion vectors for each macroblock in the macroblock slice. Next, at step  144 , the transcoder determines if there are any motion vectors that point to insertion region  14  of logo  12 . If not, at step  124 , the transcoder passes the encoded macroblock slice that was previously stored and, at step  126 , determines if the current macroblock slice is the last macroblock slice in the B frame. If there are motion vectors pointing to the insertion region the transcoder, at step  146 , decodes the relevant portion of the macroblock slice (i.e., the macroblocks having the relevant motion vectors) and, at step  138 , preferably re-encodes the affected reference frames that were previously stored in the “Ref w/ logo” block of memory. As discussed above, the previously stored macroblocks are macroblocks of earlier I or P frames that were stored and modified by the logo insertion. In an alternative procedure, the transcoder may examine the motion vectors (forward and backward) of each macroblock to determine if a motion vector that doesn&#39;t point to insertion region  14  exists. If such a motion vector is detected, the transcoder may discard all other motion vectors for the macroblock. In another alternative procedure, the transcoder may set the motion vectors of each affected macroblock to zero. Both alternative procedures would reduce the computations otherwise needed for re-encoding the affected macroblocks. In any case, as discussed above, the next unskipped motion vector and/or macroblock must be differentially encoded. Afterwards, at step  140 , the transcoder passes the re-encoded macroblock slice and, at step  126 , determines whether the current macroblock slice is the last macroblock slice in the B frame. 
     Referring now to FIGS. 9,  10 , and  11 , the rate control pass  26  of the present invention is shown in further detail. As discussed above, the affected segments of an incoming encoded bit stream are partially decoded and re-encoded for logo insertion prior to rate control pass  26 . More specifically, the affected region for I frames is that region containing slices that overlap the logo insertion region  14 . For P and B frames, the affected region is the region encompassing slices which contain macroblocks that overlap, or have motion vectors pointing to, the logo insertion region  14 . As a result, the outgoing bit stream may not have the same bit count as the original incoming bit stream. However, maintaining the same bit count for the input and output bit streams is desirable for a number of reasons. One reason is that if the same bit count is maintained for each frame (e.g., I, P, or B frame), the transport stream header can be re-used for the outgoing bit stream since data in the header (e.g., DTS data, PTS data, etc. . . ) will not change. Another reason is that the VBV compliance of an incoming bit stream will be maintained for an outgoing bit stream if the bit count for the frames of both bit streams is the same. Thus, rate control pass  26  permits the re-use of the transport stream header and maintains the VBV compliance of the video bit stream. However, it should be noted that rate control pass  26  is an exemplary rate control process of the present invention and that other rate control processes may be used to facilitate transport stream header re-use and/or VBV compliance without departing from the spirit and scope of the present invention, as defined in the appended claims. 
     Turning now to FIG. 9, the transcoder initially, at step  150 , computes the bit count of a frame in the original input bit stream, i.e., the bit count of the frame in the video bit stream prior to the insertion of logo  12 . Next, at step  152 , the transcoder computes the bit count of a corresponding frame in the output bit stream affected by the insertion of logo  12  as well as the bit counts of the macroblock slices of the corresponding frame. At step  154  the transcoder calculates the difference (delta) between the bit count of the frame in the output bit stream and the input bit stream. Delta represents the number of bits that the output bit stream must be reduced by or increased by to ensure that the number of bits of the frame (post logo insertion) in the output bit stream is equal to the number of bits of the frame (pre logo insertion) in the input bit stream. At step  156  the transcoder determines if delta is negative. If delta is negative the number of bits in the input bit stream is greater than the number of bits in the output bit stream and the transcoder, at step  158 , inserts dummy bits into the output bit stream to ensure that the output bit stream is equal to the input bit stream. Afterwards, at step  160 , the transcoder outputs macroblock slices of the re-encoded frame. 
     If delta is not negative the transcoder, at step  162 , determines if delta is zero. When delta is zero the number of bits in the input bit stream is equal to the number of bits in the output bit stream and the transcoder, at step  164 , outputs macroblock slices without using rate control. 
     Turning now to FIG. 10, if delta is positive the transcoder, at step  166 , randomly selects a starting slice position for bit rate control. When delta is positive the number of bits in the output bit stream is greater than the number of bits in the input bit stream and the transcoder must reduce the number of bits in the output bit stream. By randomly selecting a starting slice position, the transcoder minimizes perceptual problems (e.g., cyclic patterns or breathing errors) that are generated when the same initial slice location is constantly distorted. However, it is considered within the scope of the present invention to process the macroblock slices in a fixed or sequential manner. Next, at step  168 , the transcoder retrieves the selected slice from the memory buffer and decodes the selected slice up to the DCT domain. Afterwards, the transcoder, at step  170 , adjusts the quantization scales of each macroblock in the decoded slice and, at step  172 , requantizes the DCT data of the macroblocks in the decoded slice. The adjustment involves dividing the DCT coefficients by a predetermined integer and multiplying the quantization scale by the same predetermined integer. The predetermined integer may be a fixed integer that remains constant during the processing of the macroblock slices in the current frame. Alternatively, the predetermined integer may be a variable integer that increases in magnitude as the number of processed slices for the current frame increases. Increasing the magnitude of the predetermined integer on a slice by slice basis reduces the computational complexity and processing time for controlling the bit rate of each frame since the number of macroblock slices that are processed, via rate control, is reduced. One exemplary adjustment, using a fixed integer, is halving the DCT coefficients and multiplying the quantization scales by a factor of two. Halving the DCT coefficients generally reduces the bit count since smaller coefficient values tend to require shorter codes. To restore the coefficients to their approximate value the quantization scales are multiplied by a factor of two prior to re-quantization. At step  174  the transcoder re-encodes the requantized DCT data into a video bit stream and, at step  176 , outputs the bit stream. 
     Turning now to FIG. 11, rate control pass  26  continues with the transcoder, at step  178 , maintaining a record of the bit count of an outgoing slice and calculating the difference between the bit count of the original slice (prior to re-quantization) and the current slice (post re-quantization) at a first predetermined position (e.g., midway through the slice). Next, at step  180 , the transcoder determines if the calculated difference is greater than or equal to the delta value previously calculated. If so, the transcoder, at step  182 , ends the rate control for the current frame and passes the rest of the frame using the original quantization values. The rate control is ended because the number of bits output in the current frame (i.e., post log insertion) is less than or equal to the number of bits in the original frame (i.e., pre logo insertion). It should be noted that dummy bits may be inserted into the output bit stream as needed. If the calculated difference is less than the delta value previously calculated, the transcoder, at step  184 , continues to process the slice via rate control. Next, at step  186 , the transcoder calculates, at a second predetermined position (e.g., at the end of the slice), the difference between the bit count of the original slice (prior to re-quantization) and the current slice (post re-quantization). At step  188  the transcoder determines if the calculated difference is greater than or equal to the delta value previously calculated. If so, the transcoder, at step  190 , ends the rate control for the current frame and passes the rest of the frame using the original quantization values. The rate control is ended because the number of bits output in the current frame (post log insertion) is less than or equal to the number of bits in the original frame (pre logo insertion). Again, it should be noted that dummy bits may be inserted into the output bit stream as needed. If the calculated difference is less than the delta value previously calculated, the transcoder returns to step  168  and begins to process the next slice via rate control. 
     A distortion may be introduced when a macroblock is requantized. This distortion will propagate to any other macroblock block using the requantized macroblock for motion compensation. This distortion is not significant, but for some applications compensating for the distortion may be desired. To compensate, subsequent macroblocks pointing to the requantized region will have to be re-encoded, in a manner similar to the one previously described for logo insertion. 
     While the present invention has been described with reference to the preferred embodiments, it is apparent that that various changes may be made in the embodiments without departing from the spirit and the scope of the invention, as defined by the appended claims.