[FFmpeg-trac] #10403(undetermined:closed): Audio sample rate adjustment for simple speed change (lossless) (was: Audio sample rate adjustment for simple speed change)

[FFmpeg]

#10403: Audio sample rate adjustment for simple speed change (lossless)
-------------------------------------+-------------------------------------
             Reporter:  Noitarud     |                    Owner:  (none)
                 Type:  defect       |                   Status:  closed
             Priority:  normal       |                Component:
                                     |  undetermined
              Version:  unspecified  |               Resolution:  invalid
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
Changes (by Noitarud):

 * summary:  Audio sample rate adjustment for simple speed change => Audio
     sample rate adjustment for simple speed change (lossless)


Old description:

> Please correct me if I am wrong: if the audio sample rate is read at a
> different speed, I should get a different audio speed.
>
> I like the idea of the -c copy. But its use does not appear to extend to
> the alteration of playback speed. If I use the command as shown, it will
> copy the content and use original rate, no speed change.
>
> "C:\Program Files\GPAC\ffmpeg" -i INPUT44100.mp3 -c copy -ar 22050
> E:\slow.mp3
>
> I have noticed filter asetrate but don't filters necessitate
> recompression?
>
> Is there anything that can be done in a hex editor to the file to just
> cause the rate change? Just tell MP3 and/or AAC unless you feel like
> telling more.
>
> (Version 2014. No tickets since describing this I could see.)

New description:

 I assume the audio is read at the sample rate speed, so if I somehow get
 the SR changed I should get a different audio playback speed (and this
 would be lossless), but with -c copy and -ar it appears to reencode
 (ignores -ar) and there is no speed change.
 Here is the string I used:
 "C:\Program Files\GPAC\ffmpeg" -i INPUT44100.mp3 -c copy -ar 22050
 E:\slow.mp3

 A little research gives me: sample rate is recorded on each frame, sample
 rates are specific steps, and this may only work per "version" (V1 is
 44.1/48/32k; V2 half of V1) if contents differ, such a conversion step
 (inside V1) may be expedient for some people, but not versitile enough for
 all (proper fractions of speed) even if between V1/2/2.5 were offered.

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Ticket URL: <https://trac.ffmpeg.org/ticket/10403#comment:2>
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