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Vinyl records, RPMs, and velocity. How does it work?

  1. Jan 17, 2012 #1
    Hey all, I'm relatively new here, so I hope I'm posting in the right place. Please forgive me if not. This seems like a simple question to me, but my dad vehemently disagrees.

    Setup:
    Let's say we have a vinyl record spinning at a standard 33RPM's. The stylus begins at the outside edge of a standard record and moves slowly towards the center over the duration of play time. Laws of motion dictate that, under a constant angular velocity, a point at the outer edge of said record must inherently move at a faster speed than a point near the center.

    My question is this:
    Given that the record maintains a constant 33RPMs, an angular frequency of about 3.45 rad/s, and the radius between the stylus and center of the record grows gradually smaller, do the shapes/size of the groove not also have to change to keep the record from sounding as though it is playing slower?

    My logic is that because the radius grows smaller, so too does the velocity of the groove as it moves under the stylus needle. Because the stylus remains fixed, this means that the groove moves past the stylus faster at the edge, and slower near the center, and thus would produce an altered sound unless the grooves/player were designed to compensate for this.

    My father says this is not so. I am only 24, so I have admittedly less experience than he does with record players, and perhaps I'm misunderstanding their function. He says the radius at which the stylus sits doesn't matter because the record still spins at 33RPMs.

    I used a theoretical hundred mile wide record to challenge this theory, given that a 33RPM record would pass under the stylus at hundreds of miles an hour near the edge, and he maintains that the record would play the just same.

    Another question I posited which he was unable to answer is what would happen if we then took one ring of the groove from the center, and stretched it out along the outer edge. I maintain that the inner ring which previously passed the stylus in exactly one revolution, would pass in only a fraction of a revolution when placed at the edge, thereby playing faster.

    Honestly, his theory seems silly to me, but mine is apparently equally silly to him. I feel like Physics I taught me all I need to know about this problem, but I am unsure if there is something I am simply overlooking here or what. Unfortunately, I have been unable to locate any definitive answer about this using Google, and just don't know enough about records to challenge someone who used them excessively in their heyday.

    So, Physics Forum, which is it? Does a record player accommodate for this difference in velocity through design (grooves, stylus arm, etc.) or is the groove cut the same at any point with the radius of stylus location having no effect, and making me a complete fool who doesn't understand how rotation works?

    If I am right, anybody have any other ways of trying to impart this to him?

    Thanks, and sorry again if I posted in the wrong place.
     
  2. jcsd
  3. Jan 17, 2012 #2

    AlephZero

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    If you recorded a tone with constant frequency, then the linear length of the "waves" in the grooves would be longer at the outside than at the inside. There would be the same number of waves around each complete revolution of the groove.

    The amplitude (loudness) of the sound depends on the sideways motion of the stylus (actually, both sideways and vertical motion for a stereo record) and the scale for that is the same over the whole record. So the "width" of a groove (i.e. the radial distance between the grooves) is constant.

    The "design" to make the records this way was quite simple. The "master recording" was cut with a stylus onto a blank disk in real time, i.e. using the reverse process to playing the disk, so the correct size changes just happened automatically. Then a mould was made from the master recording and used to make identical copies.

    FWIW, CDs and DVDs are recorded with the disk moving at a constant linear speed, and the rotation speed does change while the disk is playing. Because the data is recorded digitaally, it is easy for the player to "measure" how fast it is playing the disk, and so the rotation speed of a CD is actuallly controlled by the disk that is being played. That is the different from a vinyl record, where the constant RPM is controlled by the record deck, not thne record itself.
     
  4. Jan 17, 2012 #3
    Thank you so much for your reply, Aleph! It was quite clear and exactly the explanation I was looking for.

    I knew whatever markings in the groove responsible for sound had to be different along the record, but I didn't ponder how simple it was until reading your reply, and I'm slightly shamed it wasn't intuitive to me. You've definitely cleared up my question. Because of my limited vinyl playing knowledge, I could only use classical mechanics to explain my point of view to my father, but it just wasn't coming across right. Following the vinyl back to the manufacturing process seems like a good example to help explain this in another way.

    The whole situation arose when, on TV, there was something similar to this and I pondered out loud how, if at all, it would control its speed to coordinate with the record. My father replied by explaining why that wouldn't be an issue, and the argument was born.

    Anyhow, thanks for clearing this up for me, and also for adding in the bit about disks, I did not know that. I need to be less shy about getting on here and asking questions, this place is great!
     
  5. Jan 18, 2012 #4

    Borek

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    I have a feeling this can be slightly misleading (although could be my English is failing me). Rotation speed of CD is controlled by the player using information present on the disk - so it is not that CD has any control over how fast it is rotated, it is about as passive as the vinyl in the old gramophone. That being said there is no doubt its role in keeping its rotation speed correct is much larger than in the case of vinyl disc.
     
  6. Jan 18, 2012 #5
    You are absolutely right in thinking that something must exist to compensate for the (critically) different linear speeds on the outside and inside of the record. As AlphZero pointed out, the shapes of the grooves are in fact different to compensate, although this difference is fortunately inherent to the recording process.

    I also didn't know how they adjusted for this. I would assume that the rotation speed of the record changes, but I guess not. Good to know!

    I guess as a consequence though, records don't have as much capacity as they could, and also the quality is different (probably imperceptibly) when on the inside/ outside of the record?
     
  7. Jan 18, 2012 #6

    Andrew Mason

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    Apparently there was a vinyl disc player that moved at constant linear speed: the Audograph which came out in 1945. The record spindle moved toward the stylus instead of the stylus moving inward and as it moved the rotation speed increased. It was a little too complicated, apparently and never got much market share.

    AM
     
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