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Comment by sambeau | Hacker News Reader

My first thought was that it is listening to the scratches rather than the music, but I guess they speed up as the record gets played and the arm moves inwards. So, my second guess is that it's listening to the pitch of the notes being played. Of course, old albums (especially punk albums) are probably tuned to whatever worked on the day—maybe a tuning fork, possibly a piano, maybe just to one of the instruments.

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Umm, no, scratches meet the needle once per revolution, regardless of the needle's position. So that would definitely be a possible method. But not all records have scratches, or they could be at an angle, which would give the wrong estimate. Also, actual sounds on the record could have scratch-like qualities.

>scratches meet the needle once per revolution, regardless of the needle's position

I need to go away and think about that :D

The needle doesn't pass at a constant "ground speed" everywhere on the record. On the outside the needle "travels faster" and in theory you have more detail. Near the middle the needle travels over the record surface slower.

That's one of the reasons you'd put your best song on side A, song 1 and your 2nd best song on side B, song 1 - you got the most detail on the outside track. I remember seeing an interview with Led Zeppelin quite a few years back where they were claiming they knew they'd have a smash hit with Stairway to Heaven. Which was funny, because I remembered then seeing an interview several years before where they said it was a track they never thought was going to go anywhere.

Which is it? It's track position - 4th track on the A side, i.e. the last track, tells you how important they thought it was. Which is to say, it was a "throwaway" track that made it big. It happens.

The other odd thing about that album is "When the Levee Breaks" is considered to be the 2nd-most popular song off of the album and it too was the last track on the B side.

What this tells me is the album's producer, one Jimmy Page, was out of touch with what Led Zeppelin fans liked most. Based off the sound of the next album, Houses of the Holy, I'd say Mr. Page got the message loud and clear.

I’m not sure about this. Thriller (by Michael Jackson) is one of the biggest pop songs of all time. It was also the fourth track on the A side. Bohemian Rhapsody (another one of the best selling singles of all time) was the fourth track on the B side.

Those are two examples off the top of my head. If you give me a bit of time to put my kid to bed, I’m sure that I’ll find many more.

What you've just learned is that pop producers are not in tune with what's good. It's a hard lesson to learn.

Not really. I found data that shows that released singles aren’t necessarily placed first on a side.

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).

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