On Wed, 17 Oct 2018 14:30:04 +0100, "J. P. Gilliver (John)"
wrote:

I sometimes want to extract just the audio from a video clip (e. g. so I
can play it on my SatNav/GPS [I have an old one that doubles as an audio
player; I gather modern ones don't do that!], or just because the video
is static or irrelevant).

I would very much prefer to extract the _original_ audio stream:
although _I_ am unlikely to hear the difference, the purist in me feels
that any transcoding _in theory_ will degrade the material. I find very
few "extract audio from video" seem even to offer this option - and
where they do, they don't make it _clear_ that they do. [I want a GUI
one - I'm too set in my ways to mess with command lines. Sorry, ffmpeg
fans.] So many of them offer a choice of data rates, sample rates, and
mono/stereo - which to me seems obvious is doing some transcoding.

I currently use the Pazera utility
(http://www.pazera-software.com/produ...o-extractor/): may not be
the best, but (a) handles lots of video formats, (b) has a
clearly-identified "Try extract original audio stream" option. It's
clearly genuine, as when selected, it does it in the blink of an eye, so
no processing is being done. (It can also extract to raw .wav, though
obviously that will still contain any artefacts of the original
encoding.)

However, whenever (I think) I do it on .mp4 videos (certainly the ones
from YouTube), it always produces .m4a files. I had begun to think this
was a quirk of the Pazera software, but I recently did it on a video
file I'd got from somewhere other than YouTube (can't remember if it was
.mp4), and it produced a .mp3 file.

So is .m4a an intrinsic part of .mp4?

No, M4a is a container format created by Apple to hold AAC audio.
YouTube recommends uploading AAC audio in MP4 containers. When
those containers have an AAC component, most audio extractors can
either output the raw AAC as a file or "repackage" it into an M4a
container. The latter is very desirable as far more devices can
play M4a than raw AAC, and M4a supports a full range of metatags,
which AAC does not.

When you play an M4a file, you are in fact playing the original
AAC audio. In other words, no transcoding was done, only
"repackaging". That means that there is no degradation.

I would very much prefer to extract the _original_ audio stream

I'm with you on this but it's worth noting that youTube itself
often re-encodes video submissions. Thus the audio that we hear
on youTube may have a lower bitrate than it did in the video as
submitted. That means that the quality may be less even though
the format is the same as the original.