diff options
Diffstat (limited to 'libAACenc/include/aacenc_lib.h')
| -rw-r--r-- | libAACenc/include/aacenc_lib.h | 45 |
1 files changed, 23 insertions, 22 deletions
diff --git a/libAACenc/include/aacenc_lib.h b/libAACenc/include/aacenc_lib.h index 01a9d36..71f7556 100644 --- a/libAACenc/include/aacenc_lib.h +++ b/libAACenc/include/aacenc_lib.h @@ -436,31 +436,32 @@ audio quality. \subsection vbrmode Variable Bitrate Mode -The encoder provides various Variable Bitrate Modes that differ in audio quality -and average overall bitrate. The given values are averages over time, different -encoder settings and strongly depend on the type of audio signal. The VBR -configurations can be adjusted via ::AACENC_BITRATEMODE encoder parameter. +The variable bitrate (VBR) mode coding adapts the bit consumption to the +psychoacoustic requirements of the signal. The encoder ignores the user-defined +bit rate and selects a suitable pre-defined configuration based on the provided +AOT. The VBR mode 1 is tuned for HE-AACv2, for VBR mode 2, HE-AACv1 should be +used. VBR modes 3-5 should be used with Low-Complexity AAC. When encoding +AAC-ELD, the best mode is selected automatically. + +The bitrates given in the table are averages over time and different encoder +settings. They strongly depend on the type of audio signal. The VBR +configurations can be adjusted with the ::AACENC_BITRATEMODE encoder parameter. \verbatim --------------------------------------------- - VBR_MODE | Approx. Bitrate in kbps/channel - | AAC-LC | AAC-LD/AC_ELD -----------+---------------+----------------- - VBR_1 | 32 - 48 | 32 - 56 - VBR_2 | 40 - 56 | 40 - 64 - VBR_3 | 48 - 64 | 48 - 72 - VBR_4 | 64 - 80 | 64 - 88 - VBR_5 | 96 - 120 | 112 - 144 +----------------------------------------------- + VBR_MODE | Approx. Bitrate in kbps for stereo + | AAC-LC | AAC-ELD +----------+---------------+-------------------- + VBR_1 | 32 (HE-AACv2) | 48 + VBR_2 | 72 (HE-AACv1) | 56 + VBR_3 | 112 | 72 + VBR_4 | 148 | 148 + VBR_5 | 228 | 224 -------------------------------------------- \endverbatim -The bitrate ranges apply for individual audio channels. In case of multichannel -configurations the average bitrate might be estimated by multiplying with the -number of effective channels. This corresponds to all audio input channels -exclusively the low frequency channel. At configurations which are making use of -downmix modules the AAC core channels respectively downmix channels shall be -considered. For ::AACENC_AOT which are using SBR, the average bitrate can be -estimated by using the ratio of 0.5 for dualrate SBR and 0.75 for downsampled -SBR configurations. - +Note that these figures are valid for stereo encoding only. VBR modes 2-5 will +yield much lower bit rates when encoding single-channel input. For +configurations which are making use of downmix modules the AAC core channels +respectively downmix channels shall be considered. \subsection encQual Audio Quality Considerations The default encoder configuration is suggested to be used. Encoder tools such as |