A key describing the boost factor allowing to adapt the dynamics of the output to the actual listening requirements. This relies on DRC gain sequences that can be transmitted in the encoded bitstream to be able to reduce the dynamics of the output signal upon request. This factor enables the user to select how much of the gains are applied.
Positive gains (boost) and negative gains (attenuation, see MediaFormat.KeyAacDrcAttenuationFactor) can be controlled separately for a better match to different use-cases.
Typically, attenuation gains are sent for loud signal segments, and boost gains are sent for soft signal segments. If the output is listened to in a noisy environment, for example, the boost factor is used to enable the positive gains, i.e. to amplify soft signal segments beyond the noise floor. But for listening late at night, the attenuation factor is used to enable the negative gains, to prevent loud signal from surprising the listener. In applications which generally need a low dynamic range, both the boost factor and the attenuation factor are used in order to enable all DRC gains.
In order to prevent clipping, it is also recommended to apply the attenuation factors in case of a downmix and/or loudness normalization to high target reference levels.
Both the boost and the attenuation factor parameters are given as integer values between 0 and 127, representing the range of the factor of 0 (i.e. don't apply) to 1 (i.e. fully apply boost/attenuation factors respectively).
This key is only used during decoding.