[HELP] encoded audio info is not what expect

[ssdde]

try to encode a MKV file(X264+PCM), to a MP4(X264+AAC), encode ASS embed into video,

the original MKV audio info is:

Audio
ID                            : 2
Format                        : PCM
Format settings                : Little / Signed
Codec ID                      : A_PCM/INT/LIT
Duration                      : 44 min 47 s
Bit rate mode                  : Constant
Bit rate                      : 1 536 kb/s
Channel(s)                    : 2 channels
Sampling rate                  : 48.0 kHz
Frame rate                    : 2.000 FPS (24000 SPF)
Bit depth                      : 16 bits
Stream size                    : 492 MiB (3%)
Language                      : Chinese



and i set AAC bitrate in Hybrid is 3072K



 
but encoded file audio bitrate is 439K/S, max bitrate is 520K/S, is not what i set 3072K, is much lower than 3072K, why?

Audio
ID                            : 2
Format                        : AAC LC
Format/Info                    : Advanced Audio Codec Low Complexity
Codec ID                      : mp4a-40-2
Duration                      : 44 min 47 s
Bit rate mode                  : Variable
Bit rate                      : 439 kb/s
Maximum bit rate              : 520 kb/s
Channel(s)                    : 2 channels
Channel layout                : L R
Sampling rate                  : 48.0 kHz
Frame rate                    : 46.875 FPS (1024 SPF)
Compression mode              : Lossy
Stream size                    : 140 MiB (1%)
Language                      : Chinese
Encoded date                  : UTC 2021-02-27 17:38:07
Tagged date                    : UTC 2021-02-27 17:38:08

[Selur]

Because 3072 is was too high for stereo audio that does not make any sense and aac is saturated,...
If the uncompressed content has a bit rate of "1536" how do you expect compressed content to have double the size?
-> that does not make sense at all,...

You know that "Hybrid is intended for advanced users", right?

Cu Selur

[ssdde]

so no matter what value is set, for AAC, the stereo audio maxium bitrate is 439 kb/s, right?

[Selur]

Wrong.

Do you know anything about aac?

It's considered "transparent" for the ITU for stereo at a bit rate of 128kbit/s.
see: https://en.wikipedia.org/wiki/Advanced_Audio_Coding
Still the maximum achievable bit rate for stereo content can be much higher, the maximum heavily depends on the complexity of the audio, but it's definitely limited by the bit rate of the uncompressed data.

Cu Selur

[ssdde]

can i understand like this: for stereo content, no matter how complexity of the audio is, no matter what the uncompressed data bitrate is,

i can set the highest bitrate 3072K directly to make sure the highest value for AAC, what kind of result is actually it can.

 
eg: the uncompressed PCM data bitrate is 1536K, i want encode to AAC, i directly set to the maximum value 3072K in hybrid, to make sure can get the best quality for AAC, and the encode result is 439K, in spite of only 439K, but 439K is the highest quality for this file, is that right?


is like that CRF18 is lossless, but i want to make sure the highest quality, so set CRF0 directly, it's acceptable no matter how big size the encode file is, as long as the hard disk has enough space.

[Selur]

about the audio: yes, for the aac encoder you choose with the configuration you chose it setting it to it's max bit rate will cause it to archive the max possible quality under those restrictions. Different aac encoders (Hybrid supports aac through ffmpeg, fdk aac, nero aac, fhg aac and qaac) produce different quality, so using one of those encoder and causing it to preserve most of the info it can might still be less than one of the other encoders, so assuming it's the best the AAC format can deliver is probably wrong.

about video: I no nobody who would call CRF 18 transparent or even near lossless. CRF 18 is a good value for detail preservation for common content to get good results. CRF 0 is lossless.

Cu Selur

[ssdde]

i heard of nero aac, it's the best, how to set nero aac to encode? thanks.

[Selur]

NeroAACEnc.exe needs to be placed inside the Hybrid/32bit-folder for Hybrid to detect it on start-up.
Sadly due to license restrictions I can't ship NeroAACEnc with Hybrid, also Nero itself doesn't offer it anymore on their website, but you can still get it over at https://www.videohelp.com/software/Nero-AAC-Codec
Once you copied the NeroAACEnc.exe into the Hybrid/32bit-folder you can select it next to qaac, faac, fdkc, fhg.
see: [INFO] *hidden* Hybrid options,... under "Additional aac encoders".

Which encoder you select is up to you, personally I prefer qaac over nero, I don't care that much about audio quality.

Cu Selur

[ssdde]

OK, thank you very much