ffmpeg conversion - keep audio bitrate - batch-file

Question

I'm using ffmpeg to extract the audio from different video formats (flv, mp4) and convert it to mp3.
%~dp0ffmpeg.exe -i %1 -ar 44100 -ac 2 -ab 128k "%~dpn1.mp3"
This works just fine. However, in my input files, the audio bitrate varies, and I want to adjust the output bitrate accordingly. Even by extensive Google searching, I didn't find any hint how to just keep the original bitrate.
What I would need would be something like:
-ab copy
Which, of course, does not work.
Is there anything that will work?
P.S: As you might have figured from the formatting above, I'm using a windows batch file. There would be the hack to use %~dp0ffmpeg.exe -i, get the audio bitrate by grep and insert it in the command line. I just think there has to be an easier and more elegant way.

Answer 1

even though the original thread was looking for an answer without grepping anything, nate's script seems to be the most useful post. but it has some limitations, for example not all outputs give you a bitrate grepped, some turnout to give you just the result "default". here's a little more improved version of it.
#!/bin/env bash
ext=$1
for f in *.${ext}; do
x=${f%.*} ;
x=${x% - YouTube}; # I usually download some song covers from YouTube.
x=$x".mp3";
bit=`ffmpeg -i "${f}" 2>&1 | grep Audio | awk -F", " '{print $5}' | cut -d' ' -f1`
if [ -n "$bit" ]; then
ffmpeg -i "$f" -ab ${bit}k "$x"
else
ffmpeg -i "$f" "$x" # this is if we don't have an output from the grep line above, ffmpeg will keep the original quality, that is 192k for 192k
fi
done

Answer 2

Here is a bash script that will take a file extension and extract audio from any file with that extension, and of course maintain the bitrate. I can't claim the credit of the key piece of the code, as that goes to the gentleman that writes this blog.
#!/bin/bash
ext=$1
for file in *.${ext}; do
tmpfn=${file%.*} ; # get rid of file ext
tmpfn=$tmpfn".mp3"; # add mp3 file ext
# next line gets bitrate of audio from video using ffmpeg
bit=`ffmpeg -i "${file}" 2>&1 | grep Audio | awk -F", " '{print $5}' | cut -d' ' -f1`
# finally, convert to mp3 using proper bitrate
ffmpeg -i "$file" -ab ${bit}k "$tmpfn"
done
Just run it in the directory where you have the files like so:
$bash script.sh flv
where flv is the file extension. Hack it to make it do exactly as you wish or process multiple filetypes to your heart's content.
EDIT: Just a quick note for anyone on Ubuntu/debian/etc. Make sure you install the additional codec package or else it won't work, i.e. you must install ffmpeg and the extra libav codec package or you're gonna have a bad time. This should do the trick:
sudo apt-get ffmpeg libavcodec-extra-53

Answer 3

As LordNeckbeard states, using the same bitrate to encode in different formats isn't necessarily wise. However...
Here is a batch-file solution which captures the input file bitrate and uses that as a parameter for the encoding command line. This approach was hinted at by the original questioner. The mp3 output file is created in the same folder as the input file.
#ECHO OFF
SETLOCAL ENABLEDELAYEDEXPANSION
REM Usage: scriptname.cmd "full-quoted-path-to-input-file"
REM Adjust FFMPEG_PATH variable value to match the path to your FFMPEG binary
SET FFMPEG_PATH=C:\Program Files\ffmpeg-20170807-1bef008-win64-static\bin
SET INPUT_FILE_FULL_PATH=%1
REM Get input file bitrate
FOR /F "tokens=5 delims==," %%i IN ('""%FFMPEG_PATH%\ffmpeg.exe" -i %INPUT_FILE_FULL_PATH% 2>&1 | find "Audio:""') DO (
FOR /F "tokens=1 delims==k" %%j IN ('ECHO %%i') DO (
SET BITRATE=%%j
SET BITRATE=!BITRATE: =!
ECHO Input file bitrate is !BITRATE! kb/s
)
)
REM Encode file using previously captured bitrate
"%FFMPEG_PATH%\ffmpeg.exe" -i %INPUT_FILE_FULL_PATH% -ar 44100 -ac 2 -ab !BITRATE!k "%~dpn1.mp3"
There is scope for tightening the code up, for example, a check to make sure at least one argument was provided, and that the BITRATE is not empty before beginning the encode, but as a rough and ready solution this should do fine.

Answer 4

Current version of ffmpeg (tested 2.1.4) recommends using "-qscale 0" to preserve quality. This worked for me on my mpeg4 video test file.

Answer 5

Try this option: -codec copy or -acodec copy for only audio.
Check this reference http://ffmpeg.org/ffmpeg.html#Stream-specifiers-1

Answer 6

Instead of
-ab copy
try
-sameq
%~dp0ffmpeg.exe -i %1 -sameq "%~dpn1.mp3"