mp4: volume + or -

diff --git a/mp4.md b/mp4.md
index 070ec0ef479eb2b5a75619aeae0d796f7f5699aa..e044b88eefe5d2b6d81a9d1b1081e621eeb188dd 100644 (file)
--- a/mp4.md
+++ b/mp4.md
@@ -18,6 +18,7 @@ Table of contents:
 * Conversion to projo
 * FFMPEG flags
 * Slow down audio
+* Volume up and down
 
 ## Cut a movie part
 
@@ -264,10 +265,12 @@ Output arguments:
 It is possible to slow down the audio of a video while style keeping the same pitch, with a complex filter.
 
 You need to use a correlated number for the speed and the pitch:
+
 * speed (tempo) = X
 * pitch = 1 / X
 
 For instance, to slow down to 75% of the original speed, you would use:
+
 * 0.75 for the tempo
 * 1.33 for the pitch
 
@@ -278,3 +281,9 @@ For instance, to slow down to 75% of the original speed, you would use:
         OUTPUT.EXT
 ```
 
+# Volume up and down
+
+You can increase or decrease the volume with a filter (here, 150%):
+
+* -filter:a "volume=1.5"
+