WittNet

Create a spectrogram (image)

The following command creates a WAV file using Alsa (Device 2.0). In this case, Sox is additionally piped, which is completely unnecessary here, however, Sox could still add various filters to the resulting test.wav change:

arecord -D hw:2,0 -r 32000 -f S16_LE -c 1 -t wav | sox -t wav -c 1 -L -b 16 -r 32000 - test.wav

Using the file test.wav that has just been created, a spectrogram is now generated:

sox test.wav -n spectrogram -o image.png

The file image.png then looks something like this (in this case a simple whistle)

Similar can also be created using avconv:

avconv -i test.avi -lavfi showspectrumpic=s=hd480:legend=0,format=yuv420p out.png

Create a spectrogram (Video)

The following instruction creates a spectrogram from a video test.avi. This is saved as a video under the name out.avi.

avconv -i test.avi -filter_complex "[0:a]showspectrum=s=854x480:mode=combined:slide=scroll:saturation=0.2:scale=log,format=yuv420p[v]" -map "[v]" -map 0:a -b:v 700k -b:a 360k out.mp4

Instead of avconv, ffmpeg could also have been used. The Debian version does not currently support all parameters here.

Alternatives

A spectrogram of a video can also be created with mplayer or with the mplayer/sox combination.

First a WAV file is created using MPlayer:

mplayer test.mp4 -ao pcm:file=/dev/stdout -vo null > test.wav

This is followed by the creation of the spectrogram:

sox test.wav -n spectrogram -o test.png

The graphics created in this way can be compared directly. With some (apparently identical) WAV files this can be difficult.

For this reason, the display of a spectrogram diff is recommended.

First a WAV diff is created:

sox -m -v 1 source1.wav -v -1 source2.wav diff.wav

Then the spectrogram diff is created:

sox diff.wav -n spectrogram -o diff.png

Individual files can also be transferred using netcat. To do this, the following instructions must be used on the transmitter and receiver:

On the receiver:

nc -l -p 8888 -w 5 > ziel < /dev/null

At the sender:

nc servername 8888 < quelle

Explanation: The recipient starts a server service based on the first parts, which reacts to requests on TCP port 8888. With the instruction > ziel the content of received data is written to a file name ziel.

The file transfer is started on the transmitter by entering the second instruction. Here a connection with the server servername via port 8888 is initiated and the file quelle is transferred.

The file is transmitted unencrypted and uncompressed. Using tools like gzip and openssl this could be implemented transparently.

There are several approaches here. The order of the images is problematic. The easiest way would be by:

cat *.jpg | ffmpeg -f image2pipe -r 25 -vcodec mjpeg -i - test.mp4

A warning is issued here because the pipe is aborted and ffmpeg does not seem to be able to recognize this.

Another method would be via image names which contain a sequential counter in the file name. If this is not the case, the file names can be changed as follows:

#!/bin/bash

COUNTER=1
for i in *.jpg; do
        NEW_FILENAME=$(printf "%04d.jpg" "$COUNTER")
        mv -i -- "$i" "$NEW_FILENAME"
        let COUNTER=COUNTER+1
done

The video can be created using:

ffmpeg -start_number 1 -i %04d.jpg -vcodec mpeg4 test.avi

In this case, the 4 fixed numerical digits on the left are filled in with 0. If the number is higher, the mask %04d must be changed accordingly.