b'How geophysics ruined musicFeatureFigure 3.The I am T-Pain microphone (not to be confused with I am in pain).in 1987 with China in your hand (rather bizarrely, a song about the book Frankenstein). Readers, like ourselves, who seemed to have somehow missed this rich period of musical history, will be perhaps surprised to learn that T-Pain had a number one album in 20073 that sold over 800 000 copies in the US and has received two Grammy Awards4T-Pain used Auto-Tune to create a sound that was so distinctive it became eponymous and led to the launch of the I am T-Pain microphone (Figure 3), perhaps the worlds most ubiquitous geophysical instrument (pun intended). Despite being listed at number seven on the worst Christmas toys list in 2001 with the claim your kid can get the illusion that he can sing, even if he sounds like a cat held over a bathtub when he belts out the national anthem5 (number one was the long since forgotten Moxie Teenz doll), the microphone was surprisingly successful although has since been rendered obsolete by the release of similar mobile phone apps6. Figure 4.The first page of the Auto-Tune patent granted to Andy Hildebrand in 1999.The Auto-Tune algorithm process is shown in Figure 5. Figure 5a shows the original To geophysicists the correction of pitch seems relativelysignal with each period indicated by the red lines. Figure 5b straightforward. Pitch is just frequency, so we just need tois a display familiar to geophysicists, a spectrogram, in the measure the frequency using the FFT and then increase/ calculation of the spectrogram the time windows are equal decrease it as appropriate. As described in the Auto-Tunein size whereas the sampling of the frequency of each period patent (US 5973252, Figure 4), however, such a simplistic(the black points), is affected by the window size. Figure 5c method is unsuitable as (a) better estimations of the pitch areshows the signal after the periods have been equalised (i.e. obtained using longer windows but then applying a single- the pitch is now the same), note how some of the higher value pitch correction to a large time window is undesirablefrequency detail in the segments that have been shortened and (b) the use of windows, and in particular their overlapshas been removed.introduces distortions. An alternate, time domain methodA similar approach can be used to change the tempo of music, involves dividing a waveform into single periods and thenbut to avoid changing the pitch of the sound the original signal expanding/contracting the waveform within the window sois windowed and then the windows summed together with that the pitch (the period) is corrected. Although effective,overlaps sufficient to increase the tempo. Another issue with this method similarly results in unnatural sounds due tothese types of methods is that they do not work in real-time, the distortion of the amplitude spectrum. An example of thedata must be recorded and then processed. What was required, therefore, was a system capable of calculating and correcting 3https://www.billboard.com/music/t-pain/chart-history/top-album-sales pitch in real-time, which is what Andy Hildebrand created.4https://www.grammy.com/grammys/artists/t-pain/69985https://www.csmonitor.com/Business/2011/1206/Seven-worst-Christmas- The Auto-Tune algorithm is based on the autocorrelation. toys-for-kids/I-am-T-Pain-Mic Hildebrand noted that for a periodic signal, such as the sine wave 6https://www.rollingstone.com/music/music-news/i-am-t-pain-Auto-Tune- in Figure 6a, the autocorrelation at lags equal to multiples of and-recording-app-comes-to-iphone-104445/ the period (in this case 1 s) is equal to the autocorrelation at t=0. APRIL 2022 PREVIEW 44'