System and method for non-destructively normalizing loudness of audio signals within portable devices
Many portable playback devices cannot decode and playback encoded audio content having wide bandwidth and wide dynamic range with consistent loudness and intelligibility unless the encoded audio content has been prepared specially for these devices. This problem can be overcome by including with the encoded content some metadata that specifies a suitable dynamic range compression profile by either absolute values or differential values relative to another known compression profile. A playback device may also adaptively apply gain and limiting to the playback audio. Implementations in encoders, in transcoders and in decoders are disclosed.
1 . A method comprising:
receiving, by a decoding device, encoded audio information and metadata associated with an audio signal, the metadata including a measure of a loudness of the encoded audio signal, one or more parameter values specifying dynamic range compression (DRC) according to a first profile associated with a first reference reproduction level, and one or more parameter values specifying DRC according to a second profile associated with a range of reference reproduction levels;
selecting the one or more parameter values specifying DRC according to the second profile when a playback reference reproduction level is within the range of reference reproduction levels associated with the second profile, wherein the playback reference reproduction level is controlled through an interface to the decoding device;
applying, by the decoding device, a decoding process to the encoded audio information to obtain subband signals representing spectral content of the audio signal;
modifying, by the decoding device, the subband signals using the one or more DRC parameter values specifying DRC according to the second profile to obtain modified subband signals with changed dynamic range characteristics; and
applying, by the decoding device, a synthesis filter bank to the modified subband signals to obtain a decoded audio signal,
wherein amplitudes of the decoded audio signal are adjusted in response to the measure of the loudness of the encoded audio signal and the playback reference reproduction level.
2 . The method of claim 1 , wherein the first reference reproduction level is −31 dB FS or −20 dB FS .
3 . The method of claim 1 , wherein the range of reference reproduction levels is between −14 dB FS S and −8 dB FS .
4 . A computer program product including a non-transitory storage medium storing instructions for performing the method of 1.
5 . An apparatus comprising:
at least a processor;
memory coupled to at least the processor and configured to store instructions, which when executed by the processor, cause the processor to perform operations comprising:
receiving encoded audio information and metadata associated with an audio signal, the metadata including a measure of a loudness of the encoded audio signal, one or more parameter values specifying dynamic range compression (DRC) according to a first profile associated with a first reference reproduction level, and one or more parameter values specifying DRC according to a second profile associated with a range of reference reproduction levels;
selecting the one or more parameter values specifying DRC according to the second profile when a playback reference reproduction level is within the range of reference reproduction levels associated with the second profile, wherein the playback reference reproduction level is controlled through an interface to the apparatus;
applying a decoding process to the encoded audio information to obtain subband signals representing spectral content of the audio signal;
modifying, by the processor, the subband signals using the one or more DRC parameter values specifying DRC according to the second profile to obtain modified subband signals with changed dynamic range characteristics; and
applying a synthesis filter bank to the modified subband signals to obtain the decoded audio signal,
wherein amplitudes of the decoded audio signal are adjusted in response to the measure of the loudness of the encoded audio signal and the playback reference reproduction level.
6 . The apparatus of claim 5 , wherein the first reference reproduction level is −31 dBFS or −20 dBFS.
7 . The apparatus of claim 4 , wherein the range of reference reproduction levels is between −14 dBFS and −8 dBFS.