SIGNAL CLIPPING PROTECTION USING PRE-EXISTING AUDIO GAIN METADATA

• US 20110208528A1
• Filed: 10/26/2009
• Published: 08/25/2011
• Est. Priority Date: 10/29/2008
• Status: Active Grant

• Current Assignee: Dolby International AB (Dolby Laboratories Incorporated)
• Original Assignee: Dolby International AB (Dolby Laboratories Incorporated)
• Inventors: Groeschel, Alexander, Schildbach, Wolfgang A.
• Granted Patent: US 8,892,450 B2
• Time in Patent Office: Days
• US Class Current: 704/500
• CPC Class Codes: G10L 19/008 Multichannel audio signal coding or decoding using interchannel correlation to reduce redundancy, e.g. joint-stereo, intensity-coding or matrixing
• CPC Class Codes: G10L 19/173 Transcoding, i.e. converting between two coded representations avoiding cascaded coding-decoding

Abstract

The application describes a method and an apparatus to prevent clipping of an audio signal when protection against signal clipping by received audio metadata is not guaranteed. The method may be used to prevent clipping for the case of downmixing a multichannel signal to a stereo audio signal. According to the method, it is determined whether first gain values (4) based on received audio metadata are sufficient for protection against clipping of the audio signal. The audio metadata is embedded in a first audio stream (1). In case a first gain value (4) is not sufficient for protection, the respective first gain value (4) is replaced with a gain value sufficient for protection against clipping of the audio signal. Preferably, in case no metadata related to dynamic range control is present in the first audio stream (1), the method may add gain values sufficient for protection against signal clipping.

Claims

1. A method of providing protection against signal clipping of an audio signal derived from digital audio data, the method comprising:

determining whether first gain values based on received audio metadata are sufficient for protection against clipping of the audio signal, the received audio metadata embedded in a first digital audio stream; and

in case a first gain value is not sufficient, replacing the respective first gain value to with a gain value sufficient for protection against clipping of the audio signal.

2. The method of claim 1, wherein the step of determining comprises the steps of:

computing second gain values based on the digital audio data, the second gain values sufficient for clipping protection of the audio signal; and

comparing

the first gain values based on the received audio metadata and

the computed second gain values.

3. The method of claim 2, wherein the step of computing second gain values comprises:

determining maximum allowable gain values.

4. The method of any of claims 2-3, wherein in dependency on the comparing step, gain values are selected from the first gain values and the computed second gain values, where replacing with a gain value is performed by selecting a second computed gain value.

5. The method of claim 4, wherein the minimum of a pair of first and second gain values is selected.

6. The method of any of the preceding claims, wherein the method is performed in the course of transcoding

the first audio stream coded in a first audio coding format into

a second audio stream coded in a second audio coding format different from the first audio coding format, the second audio stream comprising audio metadata having the replaced gain values sufficient for protection against clipping of the audio signal or having gain values derived therefrom.

7. The method of any of the preceding claims, wherein the audio signal is a downmixed audio signal and the method provides protection against signal clipping of the downmixed signal.

8. The method of any of the preceding claims, wherein the step of determining whether first gain values are sufficient for protection comprises the step of:

downmixing the digital audio data according to at least a first downmixing scheme.

9. The method of claim 8, wherein the step of determining whether first gain values are sufficient for protection comprises the step of:

computing peak values, wherein a peak value is computed by determining the maximum of the absolute values of at least two audio signals at a time, the at least two audio signals selected from the following group of;

one or more audio signals after downmixing according to the first downmixing scheme,

one or more audio signals before downmixing, and

one or more audio signals after downmixing according to a second downmixing scheme.

10. The method of any of the preceding claims, wherein the step of determining whether first gain values are sufficient for protection comprises the step of:

determining the maximum of a plurality of consecutive signal values derived from the digital audio data.

11. The method of claim 10, wherein the step of determining whether first gain values are sufficient for protection comprises the step of:

computing peak values, wherein a peak value is computed by determining the maximum of the absolute values of at least two audio signals at a time, the at least two audio signals selected from the following group of;

one or more audio signals after downmixing according to a first downmixing scheme,

one or more audio signals before downmixing, and

one or more audio signals after downmixing according to a second downmixing scheme, and

wherein the plurality of consecutive signal values correspond to consecutive peak values or consecutive filtered peak values.

12. The method of any of claims 10-11,

wherein the method is performed in the course of transcoding

the first audio stream coded in a first audio coding format into

a second audio stream coded in a second audio coding format different from the first audio coding format, the second audio stream comprising audio metadata having the replaced gain values sufficient for protection against clipping of the audio signal or having gain values derived thereform, and

wherein

the second audio stream is organized in data segments, and

the maximum of a plurality of signal values associated with a segment of the second audio stream is determined

13. The method of any of claims 10-12, wherein

a maximum signal value is divided by the determined maximum.

14. The method of any of claims 10-12, wherein

the determined maximum is inverted.

15. The method of any of the preceding claims,

wherein the method is performed in the course of transcoding

the first audio stream coded in a first audio coding format into

a second audio stream coded in a second audio coding format different from the first audio coding format, the second audio stream comprising audio metadata having the replaced gain values sufficient for protection against clipping of the audio signal or having gain values derived therefrom, and

wherein

the first audio stream is organized in data segments, at least one gain value being received per data segment of the first audio stream,

the second audio stream is organized in data segments, and

the method further comprises the step of:

resampling gain values of the first audio stream.

16. The method of any of the preceding claims,

wherein the method is performed in the course of transcoding

the first audio stream coded in a first audio coding format into

a second audio stream coded in a second audio coding format different from the first audio coding format, the second audio stream comprising audio metadata having the replaced gain values sufficient for protection against clipping of the audio signal or having gain values derived therefrom, and

wherein

the first audio stream is organized in data segments, at least one gain value being received per data segment of the first audio stream,

the second audio stream is organized in data segments,

the method further comprises the step of:

determining the minimum of a plurality of consecutive gain values of the first audio stream.

17. The method of claim 16, wherein the plurality of consecutive gain values have each an impact area, and the impact areas of these gain values overlap with the impact area of a gain value in the second stream.

18. The method of any of the preceding claims, wherein in case no metadata related to dynamic range control is present in the first audio stream, gain values sufficient for protection against clipping of the audio signal are added.

19. The method of claim 18,

wherein the method is performed in the course of transcoding

the first audio stream coded in a first audio coding format into

a second audio stream coded in a second audio coding format different from the first audio coding format, the second audio stream comprising audio metadata having the replaced gain values sufficient for protection against clipping of the audio signal or having gain values derived thereform, and

wherein in case no metadata related to dynamic range control is present in the first audio stream, gain values sufficient for protection against clipping of the audio signal are added in the second audio stream.

20. The method of any of claims 18-19, wherein the added gain values are limited to a maximum gain of 1.

21. The method of claim 20, the method comprising the step of computing second gain values based on the digital audio data, the second gain values sufficient for clipping protection of the audio signal, wherein

in case a respective computed second gain value has a gain below 1, the added gain value conesponds to the computed second gain value;

and

in case a respective computed second gain value has a gain above 1, the added gain value conesponds to a gain of 1.

22. The method of any of claims 2-21, wherein a smoothing filter is used for generating the second gain values.

30. The method of any of claims 1-22, wherein the method is performed in the course of transcoding

the first audio stream coded in a first audio coding format into

a second audio stream coded in a second audio coding format different from the first audio coding format, the second audio stream comprising audio metadata having the replaced gain values sufficient for protection against clipping of the audio signal or having gain values derived therefrom, and

wherein

the first audio coding format is AAC or HE-AAC, and

the second audio coding format is Dolby Digital.

31. The method of claim 30, wherein the first audio stream is part of a DVB video/audio stream.

32. The method of claim 9, wherein the method is performed in the course of transcoding

the first audio stream coded in a first audio coding format into

a second audio stream coded in a second audio coding format different from the first audio coding format, the second audio stream comprising audio metadata having the replaced gain values sufficient for protection against clipping of the audio signal or having gain values derived therefrom, and

wherein

the second audio stream is organized in data blocks,

the audio metadata embedded in the first audio stream comprises metadata indicating a loudness of the audio content, and

second gain values are computed based on the digital audio data, the second gain values sufficient for clipping protection of the audio signal, the computing of the second gain values comprising:

determining the maximum value of a plurality of peak values for a data block of the second audio stream; and

level adjusting the maximum value in dependency on the metadata indicating a loudness of the audio content, and

first gain values based on the received audio metadata and the computed second gain values are compared.

33. The method of claim 32, wherein the metadata indicating a loudness of the audio content is program reference level metadata.

34. The method of any of claim 32 or 33, wherein

the first audio stream comprises gain metadata for a first mode and different gain metadata for a second mode, wherein the second mode allows higher dynamic range compression than the first mode;

second gain values for the first mode are computed based on level adjusted maximum values, the second gain values for the first mode sufficient for clipping protection in the first mode;

gain values based on the received audio metadata for the first mode and the computed second gain values for the first mode are compared;

second gain values for the second mode are computed by amplifying level adjusted maximum values by 11 dB, the second gain values for the second mode sufficient for clipping protection in the second mode;

gain values based on the received audio metadata for the second mode and the computed second gain values for the second mode are compared.

35. The method of claim 9, wherein the method is performed in the course of transcoding

the first audio stream coded in a first audio coding format into

a second audio stream coded in a second audio coding format different from the first audio coding format, the second audio stream comprising audio metadata having the replaced gain values sufficient for protection against clipping of the audio signal or having gain values derived therefrom, and

wherein

the second audio stream is organized in data blocks;

the first audio stream comprises gain metadata for a first mode and different gain metadata for a second mode, wherein the second mode allows higher dynamic range compression than the first mode;

second gain values for the first mode are computed based on maximum values, wherein a maximum value is the maximum value of a plurality of peak values for a data block of the second audio stream, and wherein the second gain values for the first mode are sufficient for clipping protection in the first mode;

gain values based on the received audio metadata for the first mode and the computed second gain values for the first mode are compared;

second gain values for the second mode are computed by amplifying by 11 dB the maximum values or values being dependent thereon, the second gain values for the second mode sufficient for clipping protection in the second mode; and

gain values based on the received audio metadata for the second mode and the computed second gain values for the second mode are compared.

36. The method of any of claim 34 or 35, wherein the second gain values for the second mode are computed by downsampling from a block rate to a frame rate.

37. The method of claim 36, wherein the downsampling is performed by determining the minimum of computed gain values for a total number of 6 consecutive blocks.

23. An apparatus for providing protection against signal clipping of an audio signal derived from digital audio data, comprising:

determining means for determining whether first gain values based on received audio metadata are sufficient for protection against clipping of the audio signal, the received audio metadata embedded in a first digital audio stream; and

replacing means for replacing a first gain value with a gain value sufficient for protection against clipping of the audio signal in case the first gain value is not sufficient for protection.

24. The apparatus of claim 23, wherein the determining means comprise:

computing means for computing second gain values based on the digital audio data, the second gain values sufficient for clipping protection of the audio signal; and

comparing means for comparing

the first gain values based on the received audio metadata and

the computed second gain values.

25. The apparatus of any of claims 23-24, wherein the apparatus is part of a transcoder, the transcoder configured to transcode

the first audio stream coded in a first audio coding format into

a second audio stream coded in a second audio coding format different from the first audio coding format, the second audio stream comprising audio metadata having the replaced gain values sufficient for protection against clipping of the audio signal or having gain values derived therefrom.

26. The apparatus of any of claims 23-25, wherein the audio signal is a downmixed audio signal and the apparatus provides protection against signal clipping of the downmixed signal.

27. A transcoder configured to transcode

a first audio stream coded in a first audio coding format into

a second audio stream coded in a second audio coding format, the transcoder comprising the apparatus of any of claims 23-26.

28. The transcoder of claim 27, wherein the first audio stream is a digital broadcast signal.

29. A method of providing protection against signal clipping of an audio signal derived from digital audio data, wherein the method is performed in the course of transcoding

a first audio stream coded in a first audio coding format into

a second audio stream coded in a second audio coding format different from the first audio coding format, and

wherein in case no metadata related to dynamic range control is present in the first audio stream, gain values sufficient for protection against clipping of the audio signal are added in the second audio stream.