Bitrate constrained variable bitrate audio encoding

• US 8,442,838 B2
• Filed: 02/22/2011
• Issued: 05/14/2013
• Est. Priority Date: 02/25/2005
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Kuo, Shyh-Shiaw, Kaura, Hong, Stewart, William G.
• Primary Examiner(s): ALBERTALLI, BRIAN LOUIS
• Application Number:
  US13/031,963
• Publication Number:
  US 20110145004A1
• Time in Patent Office: 812 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:

receiving a series of blocks of audio data to encode;

encoding a particular block of audio data in the series at a first bitrate;

prior to encoding any block that follows the particular block in the series, determining whether the first bitrate is within a range, determining a target bitrate based on a bitrate at which a previous block of audio data in the series was encoded, in response to determining that the first bitrate is not within the range, determining a second bitrate within the range based on the target bitrate, and encoding the particular block of audio data at the second bitrate that is within the range;

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

2. The method of claim 1, wherein encoding the particular block of audio data at the first bitrate includes adjusting a first quantization step, using a first quantization loop, so that the first bitrate achieves a sound quality target.

3. The method of claim 2, wherein the second bitrate violates the sound quality target.

4. The method of claim 2, wherein the sound quality target is a noise-to-masking ratio target.

5. The method of claim 1, wherein encoding the particular block of audio data at the second bitrate includes adjusting a second quantization step, using a second quantization loop, so that the second bitrate is within the range.

6. The method of claim 1, wherein the range has a highest value and a lowest value, the method further comprising:

if the first bitrate is greater than the highest value in the range, then the second bitrate is the sum of the target bitrate and a first percentage of the difference between the first bitrate and the target bitrate;

if the first bitrate is less than the lowest value in the range, then the second bitrate is the difference between the target bitrate and a second percentage of the difference between the target bitrate and the first bitrate.

7. The method of claim 6, wherein the first percentage and the second percentage are different.

8. The method of claim 1, wherein the particular block of audio data has an immediately previous block of audio data in the series;

and

wherein the previous block is the immediately previously block.

9. The method of claim 1, further comprising:

encoding the particular block of audio data at the second bitrate based on a ratio of a number of bits used to encode the previous block and a number of bits available to encode the previous block.

10. A non-transitory computer-readable medium having processor-executable instructions recorded thereon for encoding audio, the instructions, when executed by one or more processors, cause performance of a method comprising:

receiving a series of blocks of audio data to encode;

encoding a particular block of audio data in the series at a first bitrate;

prior to encoding any block that follows the particular block in the series, determining whether the first bitrate is within a range, determining a target bitrate based on a bitrate at which a previous block of audio data in the series was encoded, in response to determining that the first bitrate is not within the range, determining a second bitrate within the range based on the target bitrate, and encoding the particular block of audio data at the second bitrate that is within the range.

11. The medium of claim 10, wherein the instructions for encoding the particular block of audio data at the first bitrate include instructions for adjusting a first quantization step, using a first quantization loop, so that the first bitrate achieves a sound quality target.

12. The medium of claim 11, wherein the second bitrate violates the sound quality target.

13. The medium of claim 11, wherein the sound quality target is a noise-to-masking ratio target.

14. The medium of claim 10, wherein the instructions for encoding the particular block of audio data at the second bitrate include instructions for adjusting a second quantization step, using a second quantization loop, so that the second bitrate is within the range.

15. The medium of claim 10, wherein the range has a highest value and a lowest value, the instructions further comprising instructions for:

if the first bitrate is greater than the highest value in the range, then the second bitrate is the sum of the target bitrate and a first percentage of the difference between the first bitrate and the target bitrate;

if the first bitrate is less than the lowest value in the range, then the second bitrate is the difference between the target bitrate and a second percentage of the difference between the target bitrate and the first bitrate.

16. The medium of claim 15, wherein the first percentage and the second percentage are different.

17. The medium of claim 10, wherein the particular block of audio data has an immediately previous block of audio data in the series;

and

wherein the previous block is the immediately previously block.

18. The medium of claim 10, further comprising instructions for:

encoding the particular block of audio data at the second bitrate based on a ratio of a number of bits used to encode the previous block and a number of bits available to encode the immediately previous block.

19. A computing device comprising an audio encoder, the audio encoder comprising logic for:

receiving a series of blocks of audio data to encode;

encoding a particular block of audio data in the series at a first bitrate;

prior to encoding any block that follows the particular block in the series, determining whether the first bitrate is within a range, determining a target bitrate based on a bitrate at which a previous block of audio data in the series was encoded, in response to determining that the first bitrate is not within the range, determining a second bitrate within the range based on the target bitrate, and encoding the particular block of audio data at the second bitrate that is within the range.

20. The device of claim 19, wherein the logic for encoding the particular block of audio data at the first bitrate includes logic for adjusting a first quantization step, using a first quantization loop, so that the first bitrate achieves a sound quality target.

21. The device of claim 20, wherein the second bitrate violates the sound quality target.

22. The device of claim 20, wherein the sound quality target is a noise-to-masking ratio target.

23. The device of claim 19, wherein the logic for encoding the particular block of audio data at the second bitrate includes logic for adjusting a second quantization step, using a second quantization loop, so that the second bitrate is within the range.

24. The device of claim 19, wherein the range has a highest value and a lowest value, the audio encoder further comprising logic for:

if the first bitrate is greater than the highest value in the range, then the second bitrate is the sum of the target bitrate and a first percentage of the difference between the first bitrate and the target bitrate;

if the first bitrate is less than the lowest value in the range, then the second bitrate is the difference between the target bitrate and a second percentage of the difference between the target bitrate and the first bitrate.

25. The device of claim 24, wherein the first percentage and the second percentage are different.

26. The device of claim 19, wherein the particular block of audio data has an immediately previous block of audio data in the series;

and

wherein the previous block is the immediately previously block.

27. The device of claim 19, the audio encoder further comprising logic for:

encoding the particular block of audio data at the second bitrate based on a ratio of a number of bits used to encode the previous block and a number of bits available to encode the previous block.