Constant-amplitude vs. constant-velocity, in layman's terms

  1. Greeting Physics Forums,

    I run a tiny online magazine and am trying to explain to my readers the difference between ceramic and magnetic cartridges. In particular, I'm trying to explain the how each type equalizes the audio signal in accordance to the RIAA curve.

    I posted a question on HydrogenAudio.org and one helpful member explained ceramic cartridges are "constant-amplitude" devices whereas both magnetic cartridges and the record mastering cutting head are "constant-velocity" devices. Since the amplitude of the RIAA-equalized record is (very roughly) flat, it is not necessary to apply the equalization to the ceramic cartridge signal.

    Here's the full response:

    I get the gist of his explanation but not enough to explain it succinctly to my readers. So, I thought I'd get back to the basics and learn the difference between constant-amplitude vs. constant-velocity here at PF.

    I appreciate any guidance you can offer.

    - Jeff
     
  2. jcsd
  3. The magnetic pickup is based on Faraday's Law of Induction, which produces a voltage proportional the motion of a pickup coil in a magnetic field. There are two types; moving coil and moving magnet. The motion of the coil in a magnetic field, or vice-versa, produces the output voltage. Because the motion is a velocity, and hence equal to the time derivative of position, the output voltage is proportional to frequency times displacement.
    The ceramic cartridge is based on the piezoelectric effect, in which the displacement of the pickup produces pressure on a piezoelectric crystal that produces a voltage.
    There is a discussion of both magnetic and piezoelectric pickups at
    http://en.wikipedia.org/wiki/Phonograph#Direct_vs_belt_drive
     
    Last edited: Apr 28, 2009
  4. Thanks for your reply Bob.

    I should have been clearer from the start: What I'm really hoping you'll answer is, "Define constant-amplitude and constant-velocity and compare the two. Optionally, describe how an RIAA equalized audio signal on the record is of seemingly little consequence to a ceramic cartridge’s ability to produce a roughly flatly equalized signal."

    Responses on Hydrogen Audio, like the one quoted in the original post, quickly get into science I don't understand.

    That's why I'm coming to PF- to fill in the gaps in basic physics- so I can confidently say why a ceramic cartridge approximates RIAA equalization because the amplitude is reasonably constant even though mastering LPs involves attuning the low frequencies and raising the highs.

    If my question is too elementary for the EE forum, please point me to an introductory forum.
     
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