This was a tough example! This dropout is an analogue drop out on 1/4 inch tape. Looking at the Spectrogram above you can see a gap in the audio. There are straight vertical lines to show this but importantly these lines don’t go all the way the top of display, meaning that they don’t go through all the frequencies. This is the first clue that this is not a digitally introduced dropout. Playing the tape back to listen again would be essential here.
Below is a spectrogram showing a dropout created unintentionally during the digitisation process . The line is abrupt and vertical, covering all frequencies. The second image is zoomed in on this same scope:
This type of error is introduced and not in the orignal source. It can also sound like a gap in the audio so it’s important to QC your digitised files to check:
Analogue audio has a smooth waveform curve. When we digitise audio at archival standards (96kHz 24bit) we are trying to best represent that analogue curve. Sometimes there can be a glitch that can be introduced during digitisation, usually due to the performance of the computer. This results in dropout (or interstitial error) on the exact timecode of that glitch, whereby audio is lost in the file. This means that the digitised file is not a good representation of the original analogue recording and requires digitisation again.
Conversely if the loss of audio is in the original analogue source, then the resulting digital file is already an accurate copy and would not need to be redigitised. To confirm this, you should play the source again and double check by listening and looking at the same portion of the tape. Compare to the file you had already made. Does it sound the same? If the heads on your machine are clean then you can confidently assume this error was insource and therefore should be retained with in the preservation file.