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

[Knowzy]

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 http://en.wikipedia.org/wiki/RIAA_curve" .

I posted a http://www.hydrogenaudio.org/forums/index.php?showtopic=71491" 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:

Magnetic cartridges are constant-velocity devices: the signal is proportional to the velocity of the stylus. Since high frequencies cause the stylus to "wiggle" faster, you naturally get an increasing velocity (and hence signal level) as frequency rises. The cutting head used to make the master is also a constant-velocity device, so a magnetic cartridge "matches" the characteristics of the cutting head. The RIAA EQ is applied during cutting in order to restrict excessive groove excursion (bass cut) and to overcome surface noise (treble boost). When you play a record that was cut with a constant-velocity cutter using a constant-velocity cartridge, you therefore need to apply inverse EQ to correct the frequency response.

In contrast, ceramic cartridges are constant-amplitude devices. In other words, the signal level is proportional to the amplitude of the side-to-side motion of the stylus. There is no natural tendency for the signal level to increase at higher frequencies (because the amplitude doesn't get bigger), and hence RIAA EQ should not be applied. Of course, the frequency response characteristic you get out of an unequalised ceramic is only approximately OK-ish. But since its intrinsic sound quality is so low that's an academic point.

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

 

[Bob S]

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

 

[Knowzy]

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.