Spectrograms are images formed through audio-spectrographic analysis. With respect to 77 Days, thus far, the technique has been applied to the YouTube videos of both Pronunciation Book and renunciationbook.
Audio-spectrographic analysis was first applied to PB by analysing the silences at the end of every countdown video starting from 77, with a new layer added every day a new countdown video (with accompanying silence at the end for analysis) has released.
The 154x154 spectrogram produced, referred to as the spectrogram, appears to resemble wartime recruitment posters such as Lord Kitchener's 1914 "your country needs you", as well as later imitations such as Uncle Sam's 1917 "I want you!".
According to the ARG, the figure seems to be Jackie Dalton.
Apparent noise increased up to the line from Day 55 and from then upwards the image is much clearer. This could be a hidden message in the image, or something made to look like a paused magnetic video tape, or a sync issue, like those experienced when a TV screen is recorded on camera.
However, another explanation is that the creator of the videos saw how the script works and adjusted the subsequent videos to be decoded more accurately by removing the noise. The script shades a pixel based on the RMS of the samples in that pixel's frame. This means that a frame with more noise (i.e., greater magnitude) will get a darker pixel. Whereas the pulse either exists in that frame or it doesn't, so there should be no shades of gray. This theory is backed up by the evidence in timestamp analysis of the videos after day 54.
The spectrogram in the picture on the right was obtained and updated by running this python script.
Though the user renunciationbook only has one video, it was also analysed using this software, though initially the resultant spectrogram was circular in format.