MOV is a video format developed by Apple but compatible with most software and players. MOV uses the MPEG-4 codec for compression making the conversion between MOV to MP4 very simple, as seen below:

$ ffmpeg -i input.mov output.mp4 

Tip: MOV files are usually larger in size but a higher quality video in comparison to MP4.But the MOV format does run the risk of compatibility issues due to it being maintained for Apple products.

Tip: Instagram and Twitter support MOV video uploads.

There are times when the audio on a video needs to be replaced. Here filters are not even needed to replace audio but the use of ​-map becomes important to understand:

$ ffmpeg -i input.mp4 -i input.mp3 -map 0:v -map 1:a -shortest output.mp4

Here the video from the first input and the audio from the second input is mapped to the output.

Challenge:​ Using the same concepts in the above example, if two inputs were videos, how would you replace the first input video with the second input video but keep the first input’s audio?

Although this next command isn’t exactly a filter, it’s still useful to know how to remove or mute, audio in an MP4.

Below is the quickest way to remove audio from any form of video using the ​-an​ option:

$ ffmpeg -i input.mp4 -an -vcodec copy output.mp4

Tip:​ ​-an​ indicates no audio output.

Documentation https://ffmpeg.org/ffmpeg-all.html#Examples

A low-pass filter is an audio filter that cuts off frequencies that are higher than the desired cutoff frequency. This filter is popular in music production as it can be used to soften audio or remove undesired noise. 

With FFmpeg the filter lowpass is used by setting the desired cutoff frequency in Hz. In this example, any frequency above 3200Hz is cut off:

 $ ffmpeg -i input.mp3 -af "lowpass=f=3200" output.mp3

The low-pass filter is great for getting closer to the bass of the audio. For the opposite effect, a high-pass filter is used which can be found in question 36, “How to Add a High-Pass Filter to an Audio Track?”.

Flitevox or Flitelib is an open source small run time speech engine. Pass it text and create an audio file with a robot saying it. Really cool and useful for some projects.

Flitelib is not a native filter available in FFmpeg build nor inside the source. Although the documentation states that –enable-libflite is required for config, installing flitelib is required before installing FFmpeg. If you do try to enable the filter you’ll get this error:

$ ./configure --disable-indevs --enable-libflite --enable-cross-compile
ERROR: libflite not found

If you think configure made a mistake, make sure you are using the latest
version from Git.  If the latest version fails, report the problem to the
ffmpeg-user@ffmpeg.org mailing list or IRC #ffmpeg on irc.freenode.net.
Include the log file "ffbuild/config.log" produced by configure as this will help
solve the problem.

In this post, I’ll show you how to install flite and get it working with FFmpeg

First download flitevox from source and install

$ git clone https://github.com/festvox/flite.git
$ cd flite/
$ ./configure
$ make
$ sudo make install

If you’re running linux this installation works perfectly. If you’re running MacOS (if on Monterey or newer, look at the bottom of this tutorial for new instructions), you’ll get this error:

$ sudo make install
Password:
Installing
mkdir -p /usr/local/bin
mkdir -p /usr/local/lib
mkdir -p /usr/local/include/flite
/usr/bin/install -c -m 644 include/*.h /usr/local/include/flite
/usr/bin/install -c -m 755 ../bin/flite_time /usr/local/bin
cp -pd ../build/x86_64-darwin19.0.0/lib/libflite_cmu_us_kal.a ../build/x86_64-darwin19.0.0/lib/libflite_cmu_time_awb.a ../build/x86_64-darwin19.0.0/lib/libflite_cmu_us_kal16.a ../build/x86_64-darwin19.0.0/lib/libflite_cmu_us_awb.a ../build/x86_64-darwin19.0.0/lib/libflite_cmu_us_rms.a ../build/x86_64-darwin19.0.0/lib/libflite_cmu_us_slt.a ../build/x86_64-darwin19.0.0/lib/libflite_usenglish.a ../build/x86_64-darwin19.0.0/lib/libflite_cmu_indic_lang.a ../build/x86_64-darwin19.0.0/lib/libflite_cmu_grapheme_lang.a ../build/x86_64-darwin19.0.0/lib/libflite_cmulex.a ../build/x86_64-darwin19.0.0/lib/libflite_cmu_indic_lex.a ../build/x86_64-darwin19.0.0/lib/libflite_cmu_grapheme_lex.a ../build/x86_64-darwin19.0.0/lib/libflite.a /usr/local/lib
cp: illegal option -- d
usage: cp [-R [-H | -L | -P]] [-fi | -n] [-apvXc] source_file target_file
       cp [-R [-H | -L | -P]] [-fi | -n] [-apvXc] source_file ... target_directory
make[1]: *** [install] Error 64
make: *** [install] Error 2

That’s because MacOS uses different “cp” variables then linux. I found this Stack Overflow answer with the solution: https://stackoverflow.com/a/29075638/525576 but here’s the steps to fix it.

In the folder “flite/main” you’ll need to edit the Makefile with the MacOS version of the command:

$ cd main/
$ vim Makefile

Replace the following:

#       The libraries: static and shared (if built)
        cp -pd $(flite_LIBS_deps) $(INSTALLLIBDIR)
ifdef SHFLAGS
        cp -pd $(SHAREDLIBS) $(VERSIONSHAREDLIBS) $(INSTALLLIBDIR)
endif

to (-pd to -pR):

#       The libraries: static and shared (if built)
        cp -pR $(flite_LIBS_deps) $(INSTALLLIBDIR)
ifdef SHFLAGS
        cp -pR $(SHAREDLIBS) $(VERSIONSHAREDLIBS) $(INSTALLLIBDIR)
endif

How we can try installing flite again:

$ sudo make install

flite should not show any errors and the installation should be complete.

Now back in ffmpeg source filter:

$ ./configure --enable-libflite --enable-cross-compile
$ make install

Installation will complete. To test if flite is working open a new terminal and type:

$ ffplay -f lavfi -i flite=text='Hello World!'

Hello work will speak!

---

Update Install on macOS Monterey

I recently installed this on macOS Monterey, and it seems the installation is a lot easier than before. Here’s the history of my install:

git clone https://github.com/festvox/flite.git
cd flite/
./configure
make
sudo make install
git clone https://github.com/FFmpeg/FFmpeg.git
cd FFmpeg/
./configure --enable-libflite --enable-cross-compile
make install

Then run this to try it out

ffplay -f lavfi -i flite=text='Hello World!'

For more information on how to use the flite filter in ffmpeg, check out this tutorial, How to generate text to speech in FFmpeg.