Extract audio from mp4

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LXV
Posts: 2
Joined: Thu Apr 29, 2021 8:48 am

Extract audio from mp4

Post by LXV »

INPUT: video file (mp4, about 18 MB), length 227 seconds. Its audio is AAC LC, 128 kbps, stereo, 44100 Hz.

I can drag the mp4 into GoldWave (v6.55, Windows 10), and save the audio as m4a. Here is the funny thing: No matter what length I select to save (1, 17, or the full 227 seconds), the resulting files are too large, each by about 3686400 bytes.

I noticed this when I exported a 17-second selection to both m4a and WAV, and the m4a file was bigger than the WAV file!

However, if I save the mp4 as WAV and then use that as the input, the output m4a file sizes are all correct.
DougDbug
Posts: 2172
Joined: Wed Feb 16, 2005 3:33 pm
Location: Silicon Valley

Re: Extract audio from mp4

Post by DougDbug »

Embedded artwork? Maybe GoldWave (or th4e M4A encoder) is converting a video frame to a still image?

...I don't have an MP4 handy but when I convert an MP3 to M4A the album artwork is retained and when I make a WAV it's lost.
LXV
Posts: 2
Joined: Thu Apr 29, 2021 8:48 am

Re: Extract audio from mp4

Post by LXV »

Thanks, that could be the answer. The difference was 3686473 bytes in 2 of the 3 cases, and 3686402 in the other.

The video is 1280x720 (=921600). 3686473 / 921600 = 4.000079 and 3686402 / 921600 = 4.000002, so 4 bytes per pixel (plus a small header).
Stiiv
Posts: 335
Joined: Sat May 15, 2004 7:29 pm
Location: Fallentown, PA

Re: Extract audio from mp4

Post by Stiiv »

Hi. The method that always works for me is to temporarily change the "mp4" to "mpg", then open it in Goldwave, make my edits/changes, then save it to wav. When you're done, don't forget to change the file suffix back to "mp4".
Stiiv
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