Demuxing from MP4 - Unexplained Quality Loss

[bqi]

I've been using Pazera Audio Extractor's option to extract original audio stream.

On the original MP4 video, MediaPlayerClassic's properties will reveal a bit rate of (for example):

Audio: AAC 44100Hz stereo 151kbps

Then after demuxing, the resulting AAC file shows up in MPC's properties as [Audio: AAC 44100Hz stereo 5kbps], and Windows Explorer's built in bit rate display shows the same file as 70kbps.

I'm confused at this, espescially since some files seem to demux perfectly from MP4 to AAC with the before/after detected bit rates nearly identical (e.g. 192kbps --> 196kbps, etc).

[netmask56]

What settings did you use in Pazera Audio Extractor? You do know you can alter the settings and profile.

[bqi]

What settings did you use in Pazera Audio Extractor? You do know you can alter the settings and profile.

I'm using "Try Extract Original Audio Stream" which greys out many of the options including bit-rate and sampling freq.

I haven't added any other specs from the default (e.g. no file size limit and "audio track to extract" set to auto).

[hello_hello]

AAC is a raw format. I think some of the information needs to be stored at the container level and read by the player to be accurate, whether the container be MP4, M4A, MKV or MKA etc. MPC-HC is probably scanning the initial part of the AAC audio stream and taking a guess. If the initial part contains a lot of silence and the bitrate is variable, then it might display a much lower bitrate than what it really is. It's nothing to worry about. You'll probably find the duration MPC-HC displays is also a huge guess.

I just extracted the AAC audio from an MP4 and opened it with MPC-HC. The reported bitrate was wrong and the duration was way off. For multi-channel audio it's likely to be more wrong. If I load an AAC file into foobar2000 and it's variable bitrate, foobar2000 will report the bitrate as the audio is playing, but it doesn't even try to guess the average bitrate or duration.
Thinking about it..... MPC-HC uses MediaInfo to report on a file's properties so you'll probably find if you open an AAC stream with MediaInfo it'll be doing the same guessing.

Here's what the audio in an MP4 looks like as reported by foobar2000.



Here's what foobar2000 reports for the extracted AAC (I used My MP4Box GUI to extract it).



Here's what foobar2000 reports for the same audio after it's been muxed into an MKA using MKVMergeGUI.



Here's MPC-HC/MediaInfo taking a guess when opening the extracted audio. Note the duration is also wrong.



And MPC-HC reporting again, after it's been muxed into an MKA.



Okay.... the last one is a bad example. MediaInfo doesn't report the birate for individual AAC streams in MKV/MKA files. Therefore, neither does MPC-HC. I don't know why that is. Someone else might. It's getting the duration correct again though, and if I switch to the MediaInfo tab in MPC-HC's File/Properties menu, the "overall bitrate" is reported as being 96.2Kbps.
Are you editing the AAC audio? If not, you should put it into another container, or put it into a container after editing.

Edit. I just noticed that after muxing the AAC audio (14.6GB) into an MKA, the file size of the MKA was slightly smaller than the original (14.4GB). I'd have expected it to be the other way around. Once again I don't think it's anything to worry about, but I don't know why that is.
As I quick experiment I extracted the AAC from the MKA with MKVCleaver and it went back to it's original size (14.6GB), so obviously nothing was lost. I'll have to research that one myself later.... unless someone else knows.

[El Heggunte]

The file extension "AAC" usually means *an AAC stream with ADTS headers*.

And the the container overhead of ADTS is always greater than the overhead of Matroska and MP4.

http://en.wikipedia.org/wiki/Advanced_Audio_Coding#Container_formats

[bqi]

Good info here. I'm still seeing some inconsistency with Pazera - e.g. the portable version extracted some streams fine, while the install version came up with errors on the same files when using the exact same settings.

In any case, while we're on the topic, does anyone have a better rec than Pazera for batch extractions of original streams (mostly AAC) from (predominantly) MP4s? Free is nice, especially since I don't need anything too complicated, but I'm willing to pay if it's worth it.

[hello_hello]

My Mp4Box GUI seems to happily batch extract from MP4s.
Well I switched to it's Demux tab, opened two MP4s at the same time, selected the audio streams in each, hit extract and it extracted both. I don't know if there's a limit to how many files you can open simultaneously.
MKVCleaver will batch extract streams form MKVs.

[bqi]

My Mp4Box GUI seems to happily batch extract from MP4s.
Well I switched to it's Demux tab, opened two MP4s at the same time, selected the audio streams in each, hit extract and it extracted both. I don't know if there's a limit to how many files you can open simultaneously.
MKVCleaver will batch extract streams form MKVs.

If demuxing multiple mp4 files with MP4box, is there some kind of "select all" option I'm not seeing? or does one have to individually check/select every audio file?

[hello_hello]

I think you'd need to uncheck each video stream individually if you just want to extract the audio (they're both checked by default if memory serves me correctly).