Convert a folder of LibreOffice .ODT files to .DOCX files

I don’t spend much time in traditional “word processors”, but when I do, it is usually in LibreOffice. Then I prefer to save the files in the native .ODT format. But it happens that I need to send a bunch of files to someone that prefers .DOCX files. Instead of manually converting all the files, here is a short one-liner that does the trick using the magical pandoc, the go-to tool for converting text documents.

for i in *.odt; do name=`echo $i | cut -d'.' -f1`; pandoc "$i" -o "${name}.docx"; done

Paste it into a terminal window opened in the directory of choice and watch the magic!

Add fade-in and fade-out programmatically with FFmpeg

There is always a need to add fade-in and fade-out to audio tracks. Here is a way of doing it for a bunch of video files. It may come in handy with the audio normalization script I have shown previously. That script is based on continuously normalizing the audio, which may result in some noise in the beginning and end (because there is little/no sound in those parts, hence they are normalized more).

It is easy to add a fade-in to the beginning of a file using FFmpeg’s afade function. From the documentation, you can do a 15-second fade-in like this:


And a 25-second fade-out like this:


Unfortunately, the latter requires that you specify when to start the fade-out. That doesn’t work well in general, and particularly not for batch processing.

A neat trick

Searching for solutions, I found a neat trick that solved the problem. First, you create the normal fade-in. Then you make the fade-out by reversing the audio stream, applying a fade-in, and then reversing again. The whole thing looks like this:

ffmpeg -i input.mp4 -c:v copy -af "afade=d=5, areverse, afade=d=5, areverse" output.mp4

A hack, but it works like a charm! And you don’t need to re-encode the video (hence the -c:v copy message above).

Putting it together

If you want to run this on a folder of files and run a normalization in the same go (so you avoid recompressing more than once), then you can use this bash script:


shopt -s nullglob
for i in *.mp4 *.MP4 *.mov *.MOV *.flv *.webm *.m4v; do 
   name=`echo $i | cut -d'.' -f1`; 
   ffmpeg -i "$i" -c:v copy -af "loudnorm=I=-16:LRA=11:TP=-1.5,afade=d=5, areverse, afade=d=5, areverse" "${name}_norm.mp4"; 

Save, run, and watch the magic!

Removing audio hum using a highpass filter in FFmpeg

Today, I recorded Sound Action 194 – Rolling Dice as part of my year-long sound action project.

The idea has been to do as little processing as possible to the recordings. That is because I want to capture sounds and actions as naturally as possible. The recorded files will also serve as source material for both scientific and artistic explorations later. For that reason, I only trim the recordings non-destructively using FFmpeg.

Recording the dice example, however, I noticed an unfortunate low-frequency hum in the original recording:

The original recording has an unfortunate low-frequency hum.

I like the rest of the recording, so I thought it would be a pity to skip publishing this sound action only because of the hum. So I decided to break my rule of not processing the sound and apply a simple highpass filter to remove the noise.

Fortunately, FFmpeg, as always, comes to the rescue. It has myriad audio filters that can be combined in various ways. I only needed to add a highpass filter, which can be accomplished using this one-liner:

ffmpeg -i input.mp4 -c:v copy -af highpass=400 output.mp4

Here I use the -c:v copy to copy the video stream directly. This avoids re-compressing the file and saves time. Then I use the -af highpass=400 function to add the highpass filter to the audio stream with a frequency of 400 Hz. This is relatively high but works well for this example.

The recording with highpass-filtered audio.

Adding a filter means that the audio stream needs to be re-compressed. So it breaks with the original (conceptual and technical) idea. However, the result sounds more like how I experienced it. I didn’t notice the hum while recording, and this project is focused on foreground sounds, not the background. However, this example is relevant for my upcoming project, AMBIENT, in which I will focus on the background sound of various in-door environments.

Export images from a PDF file

I have previously written about how to export each of the pages of a PDF file as an image. That works well for, for example, presentation slides that should go on a web page. But sometimes there is a need to export only the images within a page. This can be achieved with a small command line tool called pdfimages.

One way of using it is:

pdfimages -p -png file.pdf image

This will export all images in file.pdf and label them with something like image-001-010.png, where the first number refers to the page and the second is a count of images.

Sometimes I just take a screenshot if I want to grab something from a PDF. But this is a more robust method if you want to grab several different images from a PDF file.

Create timelapse video from images with FFmpeg

I take a lot of timelapse shots with a GoPro camera. Usually, I do this with the camera’s photo setting instead of the video setting. That is because I find it easier to delete unwanted pictures from the series that way. It also simplifies selecting individual photos when I want that. But then I need a way to create a timelapse video from the photos easily.

Here is an FFmpeg one-liner that does the job:

ffmpeg -r 10 -pattern_type glob -i "*.JPG" -s 1920x1440 -vcodec libx264 output.mp4

To break down the different parameters a little:

  • -r 10″: the framerate (fps)
  • “-pattern_type glob”: to allow for selecting all JPGs using “*.JPG”
  • “-s 1920×1440”: downscales the images to a pseudo-like HD format
  • “-vcodec libx264”: force to use this codec