Bit rate automatic gear

1 . A method for controlling a video stream bit rate while encoding a video stream having pictures, the method comprising the step of using a quantiser_scale_code of the general form:

quantiser — scale — code= ROUND( A+B ( D +log C (MIN{ VAR[luma — 0 ]; . . . ,VAR[luma — n ]})))

with

A being an adjustable gear shift parameter; and

B being an adjustable gas pedal parameter; and

[luma_n] being a nth luminance block of a macroblock of said picture with n beiing equal or larger than 1,

D being a constant depending on the picture type,

and the quantiser_scale_code being saturated.

2 . The method of claim 1 wherein C=2.

3 . The method of claim 1 wherein n=3.

4 . The method of claim 2 wherein said quantiser_scale_code is modified to the form:

quantiser_scale_code=ROUND(A+B′*([I_GEAR or P_GEAR or B_GEAR]+log 2 ([MIN[VAR[luma — 0], . . . ,VAR[luma_n]])))

with I_GEAR, P_GEAR and B_GEAR being numbers dependent on the type of said picture.

5 . The method of claim 1 wherein said gas pedal parameter for a current picture is adjusted by:

(a) increasing said gas pedal parameter if a measured actual size of a part of said current picture that is already encoded is larger than its target size; and

(b) decreasing said gas pedal parameter if said measured actual size of a part of said picture that is already encoded is smaller than its target size.

6 . The method of claim 5 wherein said target size of said current picture is calculated by:

Calculating a difference by subtracting a macroblock counter from a current macroblock counter; and

Calculating a first sum by adding an average macroblock size to an extension value of the average macroblock size; and

Multiplying said first difference by said first sum and a picture type factor and by an IPB_factor.

7 . The method of claim 1 wherein during the encoding process each repeated field eliminates a redundency of a half of the average picture size; and

each time a new picture is encoded, a notification is created whether or not the last field was repeated; and

if the notification is true then a residual register (RESIDUAL) is reduced by the size of the half of the average picture size.

8 . The method of claim 1 , wherein a half picture counter counts the fields in the current GOP; and

when a new GOP begins then

a maximum threshold size of the previous GOP is calculated by multiplying a size of half of an average picture with a maximum bit rate factor of said GOP and said half picture counter; and

a minimum threshold size of said previous GOP is calculated by multyplying said size of half of an average picture with a minimum bit rate factor of said GOP and said half picture counter.

9 . The method of claim 8 wherein said gear shift parameter (A) is changed from GOP to GOP by the steps of:

if said actual size of the previous GOP is larger than said maximum threshold size then said gear shift parameter is increased; and

if said actual size of said previous GOP is smaller than said minimum threshold size then said gear shift parameter is decreased.

10 . The method of claim 1 wherein a size of at least one of an I-picture, a P-picture, and a B-picture are monitored.

11 . The method of claim 1 wherein said gas pedal parameter is saturated between a minimum value and a maximum value.

12 . The method of claim 9 wherein for every macroblock,

a minimum saturation counter of consecutive times that said gas pedal parameter is saturated to its minimum value is counted; and

a maximum saturation counter of consecutive times that said gas pedal parameter is saturated to its maximum value is counted; and

when said gas pedal parameter holds a value between its minimum value and its maximum value then said minimum saturation counter and said maximum saturation counter are reset to zero; and

whereby every picture in which said minimum saturation counter is larger than a counter limit then said gear shift parameter is decreased and minimum saturation counter is reset to zero; and

whereby when said maximum saturation counter is larger than the counter limit then said gear shift parameter is increased and said maximum saturation counter is reset to zero.

13 . The method of claim 1 wherein

for every GOP, a target GOP size is calculated by:

multiplying said current macroblock counter with said average macroblock size and reducing this from the actual GOP size; and

adding said current residual value, which is the new residual value from the previous iteration or which is zero at the beginning, and using the result as a new residual value; wherein

said new residual value is used to derive said extension value of the average macroblock size for the next GOP by:

summing up all of said residual values that have been processed; and

adding a previous integrator value to said new residual value that has been processed.

14 . The method of claim 13 wherein

the step of summing up said residual values includes an arithmetic right shift of 19 and summing them up into an integrator value; and

the step of adding includes right shifting the new residual value by 17.

15 . The method of claim 1 wherein at least one out of the following conditions holds:

(i) the final quantiser_scale_code of each macroblock is saturated.

(ii) the gear shift parameter is saturated between a minimum value and a maximum value.

(iii) the residual value is not allowed to overflow.

(iv) said integrator value and said extension value are saturated.

(v) said gas pedal parameter is saturated between its minimum value and its maximum value.

16 . A method for encoding a video stream using the method of claim 1 .

17 . The method of claim 16 wherein the encoding is of the MPEG-2-type.

18 . An encoding system using the method of claim 1 .

19 . An integrated circuitry implementing the method of claim 1 .

20 . A computer program able to execute the method of claim 1.