Apparatus and methods for transcoder-based adaptive quantization

A method for forming a re-encoded bitstream portion, the method comprising:

• (a) receiving a compressed bitstream portion having a first quantization parameter, Q1;
• (b) initiating decoding of said compressed bitstream portion to produce said first quantization parameter, Q1;
• (c) receiving a second quantization parameter, Q2;
• (d) determining that Q2 is less than Q1;
• (e) determining that a signal derived from Q1 is substantially equal to Q2 when the quantization error signal is below a predetermined threshold;
• (f) copying said compressed bitstream portion to produce a re-encoded bitstream portion;
• and
• (g) reconstructing said bitstream portion to form a reconstructed bitstream portion.

A computer readable storage medium storing program code for causing a processing system to perform the steps of:

• receiving a decoded quantization parameter, Q1, of a bitstream portion;
• receiving a re-encode quantization parameter, Q2, of the bitstream portion;
• comparing Q1 and Q2;
• and
• selecting Q2 as the optimal re-encode quantization parameter if Q2 is greater than Q1, and otherwise selecting Q1 as the optimal re-encode quantization parameter.

A transcoder comprising:

• means for receiving a decoded quantization parameter, Q1, of a bitstream portion;
• means for receiving a re-encode quantization parameter, Q2, of the bitstream portion:
• means for comparing Q1 and Q2 and
• means for selecting Q2 as the optimal re-encode quantization parameter if Q2 is greater than Q1, and otherwise selecting Q1 as the optimal re-encode quantization parameter.

A method for forming a re-encoded bitstream portion, comprising:

• (a) receiving a compressed bitstream portion having a first quantization parameter, Q1;
• (b) initiating decoding of said compressed bitstream portion to produce said first quantization parameter, Q1, and an inverse quantization output;
• (c) initiating re-encoding of said decoded bitstream portion to form a second quantization parameter, Q2, and a quantization input;
• (d) determining that Q2 is less than Q1;
• (e) determining that said bitstream portion corresponds to a B-picture;
• (f) copying said compressed bitstream portion to produce the re-encoded bitstream portion;
• and
• (g) reconstructing said bitstream portion to form a reconstructed bitstream portion and storing said reconstructed bitstream portion in a frame buffer.

A method for disposing of spare bits resulting from transcoding, comprising:

• receiving a decoded quantization parameter, Q1, of a bitstream portion;
• receiving a re-encode quantization parameter, Q2, of the bitstream portion;
• determining that Q1 is greater than Q2;
• selecting Q1 as an replacement re-encode quantization parameter;
• and
• passing spare bits resulting from said replacement to another bitstream portion.

A method for re-encoding a first bitstream to a second bitstream, the method comprising:

• receiving a decoded quantization parameter, Q1, of a first bitstream portion;
• receiving a re-encode quantization parameter, Q2, of the first bitstream portion;
• comparing Q1 and Q2;
• selecting Q2 as the optimal re-encode quantization parameter if Q2 is greater than Q1, and otherwise selecting Q1 as the optimal re-encode quantization parameter;
• and
• using the selected re-encode quantization parameter to produce a second bitstream at a different bitrate from the first bitstream.

A method for re-encoding an MPEG-compliant bitstream having macroblocks and I, P and B pictures, wherein the method uses hardware including a decoder and an encoder, wherein the bitstream is initially encoded with a first quantization parameter, Q1, wherein it is desired to re-encode the bitstream at a second quatization parameter, Q2, the method comprising:

• selecting a macroblock in the bitstream;
• determining whether one of the following three conditions is true regarding the selected macroblock
• (a) that the selected macroblock is coded in intra-frame mode in an I, P or B picture using complete or partial re-encode transcoding;
• (b) that the selected macroblock is coded in inter-frame mode in a P or B picture with a same prediction macroblock at both the decoder and encoder, and that cascade-based transcoding is used;
• (c) that the selected macroblock is coded in inter-frame mode in a P or B picture with a zero corresponding quantization error compensation derived from a discrete-cosine transformation calculation, and simplified transcoding is used
• then, if one of the conditions in the determining step is true, performing the following steps:
• selecting Q1 over Q2 for the selected macroblock;
• copying compressed bits from the selected macroblock directly into a re-encoded bitstream;
• if cascaded-based transcoding is used then storing at least a portion of the selected macroblock for later access by the decoder or encoder;
• else if simplified transcoding is used and the selected macroblock has zero quantization error then copying the selected macroblock into the current frame buffer.

A method for re-encoding an MPEG-compliant bitstream having macroblocks and I, P and B pictures, wherein the method uses hardware including a decoder and an encoder, wherein the bitstream is initially encoded with a first quantization parameter, Q1, wherein it is desired to re-encode the bitstream at a second quatization parameter, Q2, the method comprising:

• selecting a macroblock in the bitstream;
• first determining whether a target number of bits assigned for a macroblock during transcoding is not less than a source number of bits used for the same macroblock in a source precompressed bitstream and, if so, proceeding to the second determining step, below;
• second determining whether one of the following three conditions is true regarding the selected macroblock
• (a) that the selected macroblock is coded in intra-frame mode in an I, P or B picture using cascaded-based or simplified transcoding;
• (b) that the selected macroblock is coded in inter-frame mode in a P or B picture with a same prediction macroblock at both the decoder and encoder, and that cascade-based transcoding is used;
• (c) that the selected macroblock is coded in inter-frame mode in a P or B picture with a zero corresponding quantization error compensation derived from a discrete-cosine transformation calculation, and simplified transcoding is used;
• then, if one of the conditions in the second determining step is true, performing the following steps:
• copying compressed bits from the selected macroblock directly into a re-encoded bitstream;
• if cascaded-based transcoding is used then storing at least a portion of the selected macroblock for later access by the decoder or encoder;
• else if simplified transcoding is used and the selected macroblock has zero quantization error then copying the selected macroblock into the current frame buffer.