Multi-mode audio codec and CELP coding adapted therefore
In an embodiment, bitstream elements of sub-frames are encoded differentially to a global gain value so that a change of the global gain value results in an adjustment of an output level of the decoded representation of the audio content. Concurrently, the differential coding saves bits. Even further, the differential coding enables the lowering of the burden of globally adjusting the gain of an encoded bitstream. In another embodiment, a global gain control across CELP coded frames and transform coded frames is achieved by co-controlling the gain of the codebook excitation of the CELP codec, along with a level of the transform or inverse transform of the transform coded frames. In another embodiment, the gain value determination in CELP coding is performed in the weighted domain of the excitation signal.
1. A multi-mode audio decoder for providing a decoded representation of audio content on the basis of an encoded bitstream, a multi-mode audio decoder comprising a memory; and a processor configured to
decode a global gain value per frame of the encoded bitstream, wherein a first subset of the frames being coded in a first coding mode and a second subset of the frames being coded in a second coding mode, with each frame of the second subset being composed of more than one sub-frames,
decode, per sub-frame of at least a subset of the sub-frames of the second subset of frames, a corresponding bitstream element differentially to the global gain value of the respective frame, and
complete decoding the bitstream using the global gain value and the corresponding bitstream element in decoding the sub-frames of the at least subset of the sub-frames of the second subset of frames and the global gain value in decoding the first subset of frames,
wherein the multi-mode audio decoder is configured such that a change of the global gain value of the frames within the encoded bitstream results in an adjustment of an output level of the decoded representation of the audio content.
2. The multi-mode audio decoder according to claim 1 , wherein the first coding mode is a frequency domain coding mode, and the second coding mode is a linear prediction coding mode.
3. The multi-mode audio decoder according to claim 2 , wherein the multi-mode audio decoder is configured to, in completing the decoding of the encoded bitstream, decode the sub-frames of the at least subset of the sub-frames of the second subset of frames by using transformed excitation linear prediction decoding, and decode a disjoined subset of the sub-frames of the second subset of the frames by use of CELP.
4. The multi-mode audio decoder according to claim 1 , wherein the multi-mode audio decoder is configured to decode, per frame of the second subset of the frames, a further bitstream element revealing a decomposition of the respective frame into one or more sub-frames.
5. The multi-mode audio decoder according to claim 1 , wherein the frames of the second subset are of equal length, and the at least subset of the sub-frames of the second subset of frames exhibit a varying sample length selected from the group comprising 256, 512 and 1024 samples, and a disjoined subset of the sub-frames exhibit a sample length of 256 samples.
6. The multi-mode audio decoder according to claim 1 , wherein the multi-mode audio decoder is configured to decode the global gain value on fixed number of bits and the bitstream element on a variable number of bits, the number depending on a sample length of the respective sub-frame.
7. The multi-mode audio decoder according to claim 1 , wherein the multi-mode audio decoder is configured to decode the global gain value on fixed number of bits and to decode the bitstream element on fixed number of bits.
8. AN SBR decoder comprising a core decoder for decoding core coder portion of a bitstream to acquire a core band signal according to claim 1 , the SBR decoder configured to decode envelope energies for a spectral band to be replicated, from an SBR portion of the bitstream, and scaling the envelope energies according to an energy of the core band signal.
9. A multi-mode audio encoder comprising a memory; and a processor configured to encode an audio content into an encoded bitstream with encoding a first subset of frames in a first coding mode and a second subset of frames in a second coding mode, wherein the second subset of frames is respectively composed of one or more sub-frames, wherein the multi-mode audio encoder is configured to determine and encode a global gain value per frame, and determine and encode, per sub-frames of at least a subset of the sub-frames of the second subset, a corresponding bitstream element differentially to the global gain value of the respective frame, wherein the multi-mode audio encoder is configured such that a change of the global gain value of the frames within the encoded bitstream results in an adjustment of an output level of a decoded representation of the audio content at the decoding side.
10. A multi-mode audio decoding method for providing a decoded representation of audio content on the basis of an encoded bitstream, the method comprising
decoding a global gain value per frame of the encoded bitstream, wherein a first subset of the frames being coded in a first coding mode and a second subset of the frames being coded in a second coding mode, with each frame of the second subset being composed of more than one sub-frames,
decoding, per sub-frame of at least a subset of the sub-frames of the second subset of frames, a corresponding bitstream element differentially to the global gain value of the respective frame, and
completing decoding the bitstream using the global gain value and the corresponding bitstream element in decoding the sub-frames of the at least subset of the sub-frames of the second subset of frames and the global gain value in decoding the first subset of frames,
wherein the multi-mode audio decoding method is performed such that a change of the global gain value of the frames within the encoded bitstream results in an adjustment of an output level of the decoded representation of the audio content.
11. A multi-mode audio encoding method comprising encoding an audio content into an encoded bitstream with encoding a first subset of frames in a first coding mode and a second subset of frames in a second coding mode, wherein the second subset of frames is respectively composed of one or more sub-frames, wherein the multi-mode audio encoding method further comprises determining and encoding a global gain value per frame, and determine and encode, per sub-frames of at least a subset of the subframes of the second subset, a corresponding bitstream element differentially to the global gain value of the respective frame, wherein the multi-mode audio encoding method is performed such that a change of the global gain value of the frames within the encoded bitstream results in an adjustment of an output level of a decoded representation of the audio content at the decoding side.
12. A non-transitory computer readable medium comprising a program code for performing, when running on a computer, a method according to claim 10 or claim 11 .
13. The multi-mode audio decoder according to claim 1 , wherein the multi-mode audio decoder is configured to decode the global gain value from a fixed number of bits which is equal for frames of the first and second subsets of frames, respectively.
14. The multi-mode audio decoding method according to claim 10 , wherein the step of decoding the global gain value per frame of the encoded bitstream is performed such that the global gain value is decoded on a fixed number of bits which is equal for frames of the first and second subsets, respectively.
15. The multi-mode audio encoder according to claim 9 , wherein the multi-mode audio encoder is configured to determine and encode the global gain value per frame such that the global gain value is encoded on a fixed number of bits which is equal for frames of the first and second subsets, respectively.
16. The multi-mode audio encoding method according to claim 11 , wherein the determining and encoding of the global gain value per frame is performed such that the global gain value is encoded on a fixed number of bits which is equal for frames of the first and second subsets, respectively.