As a turntable rotates at a constant RPM, scratches change in pitch (if there is such a thing) but it will always "pop" once per rotation. My guess would be so

As a turntable rotates at a constant RPM, scratches change in pitch (if there is such a thing) but it will always "pop" once per rotation. My guess would be some filtering (to remove the actual music) and some kind of autocorrelation algorithm to detect some periodic patterns with a period matching the expected rotation speed of a record within a few percent (33.3 RPM, it doesn't talk about 45/78).