ffmpeg 降低聲音品質來縮小 mp3 檔案大小

 

目的縮小MP3檔案大小

• 雙聲道改單聲道

• 範例為音訊位元速率(bitrate) 從 160 kb/s 降為 64 kb/s

指令語法

ffmpeg -i <160k stereo filename>.mp3 -ac 1 -b:a 64k <output 64k mono filename>.mp3

-ac <channels> 設定音訊頻道數(set number of audio channels)

• 1 表示單聲道(mono)

• 2 表示雙聲道(stereo)

-b:a <bitrate> 音訊位元速率(bitrate)

小結

檔案大小縮小約為原來的 39.88%

~$ python
Python 3.9.6 (default, Jun 29 2021, 04:45:03)
..
>>> round(80973 / 203040 * 100,2)
39.88

執行指令範例

$ ffmpeg -i 1_setup_160k_stereo.mp3 -ac 1 -b:a 64k 1_setup_64k_mono.mp3
...
$ ffmpeg -i 1_setup_160k_stereo.mp3
Input #0, mp3, from '1_setup_160k_stereo.mp3':
Duration: 00:00:10.12, start: 0.069063, bitrate: 160 kb/s
  Stream #0:0: Audio: mp3, 16000 Hz, stereo, fltp, 160 kb/s
  Metadata:
    encoder         : LAME3.98r
​
$ ffmpeg -i 1_setup_64k_mono.mp3
Input #0, mp3, from 1_setup_64k_mono.mp3':
Metadata:
  encoder         : Lavf58.45.100
Duration: 00:00:10.08, start: 0.069063, bitrate: 64 kb/s
  Stream #0:0: Audio: mp3, 16000 Hz, mono, fltp, 64 kb/s
​
​
$ ls -la 1_setup*
-rw-r--r--@ 1 tester staff  203040 Jul 24 18:38 1_setup_160k_stereo.mp3
-rw-r--r--@ 1 tester staff   80973 Jul 24 18:43 1_setup_64k_mono.mp3

延伸閱讀

• 使用 ffmpeg 指令合併的多個MP4檔案

Dev Tips - command line snippets

 

Command - equal to ldd

objdump -p <path/to/program> | grep -i needed

Command - while loop with date and sleep

while [ 1 ] ; do date +"%M:%S" ; sleep 1 ; done

Command - cp

merge one folder to another folder

if file exists in dummy_tmp, it will replace by file in origin_tmp

$ cp -RT origin_tmp/ dummy_tmp/
$ diff -ur dummy_tmp origin_tmp

Bash Tip - date 日期指令轉換 unix epoch time 範例

 

Command - date

Example 1 date - convert unix epoch timestamp to date string

$ date  --date='@1617694020'
Tue 06 Apr 2021 07:27:00 AM UTC

Example 2 date - convert unix timestamp to custom formatted date string

$ date +"%Y-%m-%d %H:%m" --date='@1617694020'
2021-04-06 07:04
$ date  --date='@1617694020.904'
Tue 06 Apr 2021 07:27:00 AM UTC

Example 3 date -- MacOS Mojave convert timestamp to date string

## MacOS Mojave not support --date
$ date  --date='@1617694020.904'
date: illegal option -- -
usage: date [-jnRu] [-d dst] [-r seconds] [-t west] [-v[+|-]val[ymwdHMS]] ...
          [-f fmt date | [[[mm]dd]HH]MM[[cc]yy][.ss]] [+format]
## use -r to instead
$ date -r 1625043610 +"%Y%m%d %H:%M:%S"
20210630 17:00:10