We will utilise Gimp, Enscribe, Sonic Visualiser, MPlayer and Audacity to insert and view a watermark in audio.

We will create two figures using the Gimp with 11px of width and 11px of height in PNG format. We will put a white background and a black letter X in the first file( BlackX_WhiteBackground.png ) and black background and a white letter X in the second file( WhiteX_BlackBackgroud.png ), as the figures below:

-> BlackX_WhiteBackground.png

-> WhiteX_BlackBackgroud.png

We will use the Enscribe after the creation of figures, so that through the fast transformations of Fourier generate the watermark in the output files of this program.

These commands below will generate the files with watermark concerning with two previous figures, the parameter -transform-size=0 is utilized for that the watermark have a shorter duration.

root@computer4en6# enscribe -transform-size=0 BlackX_WhiteBackground.png BlackX_WhiteBackground.wav

root@computer4en6# enscribe -transform-size=0 WhiteX_BlackBackgroud.png WhiteX_BlackBackgroud.wav

We will use specially the Sonic Visualizer to view the watermark, that allow see the spectrogram. In the figure below, the menu for add the spectrogram of the WhiteX_BlackBackgroud.wav file.

In the figure below, the spectrogram generated through the WhiteX_BlackBackgroud.wav file.

Utilising again the Sonic Visualizer, but now to add the spectrogram of the  BlackX_WhiteBackground.wav file.

In the figure below, the spectrogram generated through the BlackX_WhiteBackground.wav file.

Both files of the PNG type have the same dimensions and quantity of bytes, and were also generated the WAV files with equals quantity of bytes, as we shall see in the command below.

root@computer4en6# du -b *.png *.wav
685     BlackX_WhiteBackground.png
685     WhiteX_BlackBackgroud.png
524332  BlackX_WhiteBackground.wav
524332  WhiteX_BlackBackgroud.wav

However, we can evidence visually that the spectrograms generated previously are different, despite of the equalities in the dimensions and file sizes.

We will choose the WhiteX_BlackBackgroud.wav file, because that generates a noise of shorter duration in an audio player program, such as Mplayer.

root@computer4en6 # mplayer WhiteX_BlackBackgroud.wav

For leave inaudible the watermark , must be open the WhiteX_BlackBackgroud.wav file in the Audacity and reduce the gain in decibels to -36, as show in the figure below:

We can export the resulting file to a format such as OGG and even then the watermark will continue.

In the figure below, the exportation of the WAV file to OGG with maximum quality.

The previous technique eliminates the noise in the watermark, due to the reduction of the gain in decibels, which was done in Audacity. We will percieve this when execute the WhiteX_BlackBackgroud.ogg file in Mplayer.

root@computer4en6 # mplayer WhiteX_BlackBackgroud.ogg

We can utilise the Sonic Visualizer in the WhiteX_BlackBackgroud.ogg file and verify if even encoded in OGG the watermark is maintained, as shown below: