Bitrate constrained variable bitrate audio encoding

• US 7,895,045 B2
• Filed: 11/02/2009
• Issued: 02/22/2011
• Est. Priority Date: 02/25/2005
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Stewart, William G., Kuo, Shyh-shiaw, Kaura, Hong
• Primary Examiner(s): ALBERTALLI, BRIAN LOUIS
• Application Number:
  US12/610,615
• Publication Number:
  US 20100049532A1
• Time in Patent Office: 477 Days
• Field of Search: None
• US Class Current: 704/501
• CPC Class Codes: G10L 19/035 Scalar quantisation

Abstract

A hybrid audio encoding technique incorporates both ABR, or CBR, and VBR encoding modes. For each audio coding block, after a VBR quantization loop meets the NMR target, a second quantization loop might be called to adaptively control the final bitrate. That is, if the NMR-based quantization loop results in a bitrate that is not within a specified range, then a bitrate-based CBR or ABR quantization loop determines a final bitrate that is within the range and is adaptively determined based on the encoding difficulty of the audio data. Excessive bitrates from use of conventional VBR mode are eliminated, while still providing much more constant perceptual sound quality than use of conventional CBR mode can achieve.

Claims

1. A method for encoding audio, the method comprising:

computing a first bitrate based on a sound quality target for a block of audio data;

determining whether the first bitrate is within a specified range;

if the first bitrate is not within the specified range, then

computing a target bitrate;

based on said target bitrate, computing a final bitrate at which the block of audio data is to be encoded;

wherein the final bitrate is within the specified range; and

encoding the block of audio data using the final bitrate; and

if the first bitrate is within the specified range, then encoding the block of audio data using the first bitrate;

wherein the method is performed by one or more computing devices.

2. The method of claim 1, wherein determining the first bitrate includes adjusting a first quantization step, using a first quantization loop, so that the first bitrate achieves the sound quality target.

3. The method of claim 2, wherein determining the final bitrate includes adjusting a final quantization step, using a second quantization loop, so that the final bitrate is within the specified range.

4. The method of claim 3, wherein the specified range encompasses the target bitrate, the method further comprising:

if the first bitrate is greater than the highest value in the specified range, then determining the final bitrate includes adjusting the final quantization step, using the second quantization loop, so that the final bitrate is the sum of the target bitrate and a specified percentage of the difference between the first bitrate and the target bitrate.

5. The method of claim 3, wherein the specified range encompasses the target bitrate, the method further comprising:

if the first bitrate is less than the lowest value in the specified range, then determining the final bitrate includes adjusting the final quantization step, using the second quantization loop, so that the final bitrate is the difference between the target bitrate and a specified percentage of the difference between the target bitrate and the first bitrate.

6. The method of claim 1, further comprising:

computing the target bitrate based on the first bitrate.

7. The method of claim 1, further comprising:

computing the target bitrate based on a bitrate at which an immediately previous block of audio data was encoded.

8. The method of claim 7, further comprising:

computing the target bitrate based on a ratio of a number of bits used to encode an immediately previous block of audio data and a number of bits available to encode the immediately previous block.

9. The method of claim 1, wherein determining the final bitrate includes determining a final bitrate that violates the sound quality target.

10. The method of claim 1, wherein the sound quality target is a noise-to-masking ratio target.

11. A non-transitory computer-readable medium storing instructions which, when executed by one or more computing devices, cause the one or more computing devices to perform:

computing a first bitrate based on a sound quality target for a block of audio data;

determining whether the first bitrate is within a specified range;

if the first bitrate is not within the specified range, then

computing a target bitrate;

based on said target bitrate, computing a final bitrate at which the block of audio data is to be encoded;

wherein the final bitrate is within the specified range; and

encoding the block of audio data using the final bitrate; and

if the first bitrate is within the specified range, then encoding the block of audio data using the first bitrate.

12. The non-transitory computer-readable medium of claim 11, wherein determining the first bitrate includes adjusting a first quantization step, using a first quantization loop, so that the first bitrate achieves the sound quality target.

13. The non-transitory computer-readable medium of claim 12, wherein determining the final bitrate includes adjusting a final quantization step, using a second quantization loop, so that the final bitrate is within the specified range.

14. The non-transitory computer-readable medium of claim 13, wherein the specified range encompasses the target bitrate, wherein the instructions, when executed by the one or more computing devices, cause the one or more computing devices to further perform:

if the first bitrate is greater than the highest value in the specified range, then the step of determining the final bitrate includes adjusting the final quantization step, using the second quantization loop, so that the final bitrate is the sum of the target bitrate and a specified percentage of the difference between the first bitrate and the target bitrate.

15. The non-transitory computer-readable medium of claim 13, wherein the specified range encompasses the target bitrate, wherein the instructions, when executed by the one or more computing devices, cause the one or more computing devices to further perform:

if the first bitrate is less than the lowest value in the specified range, then determining the final bitrate includes adjusting the final quantization step, using the second quantization loop, so that the final bitrate is the difference between the target bitrate and a specified percentage of the difference between the target bitrate and the first bitrate.

16. The non-transitory computer-readable medium of claim 11, wherein the instructions, when executed by the one or more computing devices, cause the one or more computing devices to further perform:

computing the target bitrate based on the first bitrate.

17. The non-transitory computer-readable medium of claim 11, wherein the instructions, when executed by the one or more computing devices, cause the one or more computing devices to further perform:

computing the target bitrate based on a bitrate at which an immediately previous block of audio data was encoded.

18. The non-transitory computer-readable medium of claim 17, wherein the instructions, when executed by the one or more computing devices, cause the one or more computing devices to further perform:

computing the target bitrate based on a ratio of a number of bits used to encode an immediately previous block of audio data and a number of bits available to encode the immediately previous block.

19. The non-transitory computer-readable medium of claim 11, wherein determining the final bitrate includes determining a final bitrate that violates the sound quality target.

20. The non-transitory computer-readable medium of claim 11, wherein the sound quality target is a noise-to-masking ratio target.

21. A computer system comprising:

an audio encoder, the audio encoder comprising logic for;

computing a first bitrate based on a sound quality target for a block of audio data;

determining whether the first bitrate is within a specified range;

if the first bitrate is not within the specified range, then

computing a target bitrate;

based on said target bitrate, computing a final bitrate at which the block of audio data is to be encoded;

wherein the final bitrate is within the specified range; and

encoding the block of audio data using the final bitrate; and

if the first bitrate is within the specified range, then encoding the block of audio data using the first bitrate.

22. The computer system of claim 21, wherein the logic for determining the first bitrate includes logic for adjusting a first quantization step, using a first quantization loop, so that the first bitrate achieves the sound quality target.

23. The computer system of claim 22, wherein the logic for determining the final bitrate includes logic for adjusting a final quantization step, using a second quantization loop, so that the final bitrate is within the specified range.

24. The computer system of claim 23, wherein the specified range encompasses the target bitrate, the audio encoder further comprising logic for:

if the first bitrate is greater than the highest value in the specified range, then determining the final bitrate includes adjusting the final quantization step, using the second quantization loop, so that the final bitrate is the sum of the target bitrate and a specified percentage of the difference between the first bitrate and the target bitrate.

25. The computer system of claim 23, wherein the specified range encompasses the target bitrate, the audio encoder further comprising logic for:

if the first bitrate is less than the lowest value in the specified range, then determining the final bitrate includes adjusting the final quantization step, using the second quantization loop, so that the final bitrate is the difference between the target bitrate and a specified percentage of the difference between the target bitrate and the first bitrate.

26. The computer system of claim 21, further comprising logic for:

computing the target bitrate based on the first bitrate.

27. The computer system of claim 21, further comprising logic for:

computing the target bitrate based on a bitrate at which an immediately previous block of audio data was encoded.

28. The computer system of claim 27, further comprising logic for:

computing the target bitrate based on a ratio of a number of bits used to encode an immediately previous block of audio data and a number of bits available to encode the immediately previous block.

29. The computer system of claim 21, wherein the logic for determining the final bitrate includes logic for determining a final bitrate that violates the sound quality target.

30. The computer system of claim 21, wherein the sound quality target is a noise-to-masking ratio target.

31. A method for encoding audio, the method comprising:

executing a first quantization loop to determine a first number of bits for use in encoding a block of audio data, wherein the first number of bits satisfies a sound quality target for the block;

determining whether the first number of bits is within a first specified range of bits, wherein the specified range is based at least in part on a number of bits available for encoding the block;

if the first number of bits is not within the first specified range, then

computing a target number of bits for use in encoding the block;

executing a second quantization loop, based on the target number of bits, to determine a final number of bits for use in encoding the block; and

encoding the block of audio data using the final number of bits; and

if the first bitrate is within the specified range, then encoding the block of audio data using said first number of bits;

wherein the method is performed by one or more computing devices.

32. A non-transitory computer-readable medium storing instructions which, when executed by one or more computing devices, cause the one or more computing devices to perform:

executing a first quantization loop to determine a first number of bits for use in encoding a block of audio data, wherein the first number of bits satisfies a sound quality target for the block;

determining whether the first number of bits is within a first specified range of bits, wherein the specified range is based at least in part on a number of bits available for encoding the block;

if the first number of bits is not within the first specified range, then

computing a target number of bits for use in encoding the block;

executing a second quantization loop, based on the target number of bits, to determine a final number of bits for use in encoding the block; and

encoding the block of audio data using the final number of bits; and

if the first bitrate is within the specified range, then encoding the block of audio data using said first number of bits.

33. A computer system comprising:

an audio encoder, the audio encoder comprising logic for;

executing a first quantization loop to determine a first number of bits for use in encoding a block of audio data, wherein the first number of bits satisfies a sound quality target for the block;

determining whether the first number of bits is within a first specified range of bits, wherein the specified range is based at least in part on a number of bits available for encoding the block;

if the first number of bits is not within the first specified range, then

computing a target number of bits for use in encoding the block;

executing a second quantization loop, based on the target number of bits, to determine a final number of bits for use in encoding the block; and

encoding the block of audio data using the final number of bits; and

if the first bitrate is within the specified range, then encoding the block of audio data using said first number of bits.