Umschalten zwischen bitratenreduzierten Signalen

摘要

Decoders A and B decode MPEG-2 bitstreams A and B. A switch (S1) switches from decoded stream A to decoded stream B to achieve a splice. The resulting bitstream is re-encoded in an encoder 4. A new transitional GOP is defined beginning at the splice. The new GOP is defined by picture type decision rules which may have the effect of changing the GOP compared to the GOPs of streams A and B. The new GOP provides an initial prediction of the position in stream B where the occupancy value of stream C should coincide with that of B. A target for the new number of bits in the new GOP is calculated as the sum of the normal allocation of bits for the new GOP plus the difference between the occupancy value of stream C at the splice and a prediction of the occupancy of stream B at the predicted position. The occupancy value of stream C is controlled in accordance with the target so that it tends towards the occupancy value of stream B at the predicted position. The target is updated regularly throughout the GOP on I and P frames. After the first update at the beginning of the transitional GOP, the target changes at the updates nominally by an amount VBV_diff which is the difference between occupancy of bit stream B on the current I or P frame and the occupancy at the next I or P frame. If VBV-diff is negative, the amount of change is limited to a limit value. The maximum value of the limit may be exceeded only once and the limit is then set to zero. The excess of VBV_diff over the limit is a value reduction_carry. Reduction _carry is monitored, and if at the end of the transitional GOP it exceeds a threshold (e.g. zero), another transitional GOP is started. If VBV_diff is negative but does not exceed the limit, the target is reduced by VBV_diff and the limit is reduced by a proportion of VBV_diff. If VBV_diff is positive, then VBV_diff is averaged out over the target values for the remaining I and P frames in the GOP. <IMAGE>