Listening to the first of the NTS sessions by Autechre, I noticed something about the  track ‘32a_reflected’. Parts of it sounds like it’s played backwards. On playing the track twice in a row, you can hear that it start how it ends, but in reverse. The spectrogram below shows the last 15 seconds of the track on the left and the first 15 seconds on the right. Clearly they mirror each other.

So ‘32a_reflected’ is a musical palindrome. Here’s a spectrogram of the whole thing:

But it turns out to be a bit more complicated than that. Each of the stereo channels are reflected versions of each other. The left channel is the same as the right channel played backwards, and vice versa. This spectrogram of the first and last 15s shows the stereo channels separately (left channel at the top, right channel at the bottom):

This means that if you play the whole track in reverse, it sounds the same but with the stereo channels switched round. The structure can also be seen in the waveform:

But it’s not exactly symmetrical – the second half is a bit louder (around 1dB on average). You can see this in the two images above: the left-hand portion of the spectrogram is slightly brighter, whilst the right-hand portion of the waveform is slightly wider.

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