Stylus Rake Angle

Surely the groove depth changes with signal? A right channel signal for example must cut into and out of the lacquer at 45 degrees. I can see that a heated cutting stylus will help the process, but then does that affect the final profile as the material cools?

I used to hear about this thing called rebound associated with lacquers but yet to experience it. Yes, the groove wall is cut at a 45 degree angle, so if you were to cut an out-of-phase bass signal, the groove would indeed modulate up and down. Of course, the needle would not be able to track it, and such a signal fortunately does not exist in nature. If we encounter studio productions where out-of-phase information is a problem, we have a simple passive processor that takes care of it. So groove depth can be considered a constant.

Another way to look at this is if the cartridge body rises higher, that is a change in groove depth, if the needle simply traces a signal, the groove depth has not changed.

Atmasphere, Does azimuth vary from cutter to cutter and lathe to lathe? If so it would not surprise me.

Csontos, you said:

You are mistaken. VTA has nothing to do with the angle of the cantilever. The angle of the stylus relative to the cantilever can vary wildly from one cart to another and therefore result in huge differences in cantilever/surface angles. VTA is set by the cutting stylus itself. Therefore there doesn't need to be a reference to VTA adjustment at all. It's done with as soon as the laquer is cut. The pertinent function to play back is SRA, the thing that follows. The fact that the cantilever angle changes with SRA adjustment is incidental.

You need to do your homework and think more on this.

Vertical Tracking Angle is a cartridge property. The clue is in the word "tracking".
The fact that it may not be the same as the equivalent angle used when the record was cut, as you mention but fail to realise its relevance, is what can lead to distortion in playback.

Atmasphere, you said:

Yes, the groove wall is cut at a 45 degree angle, so if you were to cut an out-of-phase bass signal, the groove would indeed modulate up and down. Of course, the needle would not be able to track it, and such a signal fortunately does not exist in nature. If we encounter studio productions where out-of-phase information is a problem, we have a simple passive processor that takes care of it. So groove depth can be considered a constant.

I wasn't talking about out of phase signals, but left or right signals. I can see that a mono signal varying only in amplitude, side to side, will maintain a constant groove depth which doesn't vary, but stereo?

I'm not trying to make a point but just trying to understand the process and its implications for playback, as coming from a mechanical engineering perspective, I am familiar with lathes and cutting tools, and the issues involved in machining different materials. That's why I'm interested in knowing more.

.

Sarcher30, it might, but if so it would be very very slight! If you look at any lathe you will see at once that they are built to be quite sturdy and machined to exact tolerances. Thus the cutterhead mount is exactly perpendicular to the platter surface; one of the better examples of how excellent machining really looks, such that the azimuth error cannot be measured. So in essence, I am saying 'no, azimuth does not vary from lathe to lathe'.

I wasn't talking about out of phase signals, but left or right signals. I can see that a mono signal varying only in amplitude, side to side, will maintain a constant groove depth which doesn't vary, but stereo?

I imagine my response was not entirely clear so I will try to restate in a less ambiguous way. There is no variation in groove depth due to signal. There is only signal modulation that varies, whether stereo or mono. It sounds to me like you are confusing the two- and I can see why.

Look at it this way- groove depth is something you can set on the cutterhead. Take it as a definition unique to the technology that 'groove depth' means how deep the groove is with no signal. The signal is a function of the electronics. Think of modulation in an radio signal and its the same idea- if you want to reproduce the signal, the groove depth is a constant, just as the frequency of the radio station is a constant. If you want to look at the groove as being variable depth according to the signal, you won't be able to reproduce it.

If the cutterhead were to rise and fall with respect to the lacquer surface, then we would see a varying groove depth. In theory, during playback the cartridge body is held in constant locus by the mass of the arm so that the cartridge can only respond by the motion of the needle in the groove, not anything else.

John, Thanks for the treatise. I agree with your definitions of the terms, but I cannot see the point of your statement:

"Now while the stylus might be set at 92 degrees, the cantilever may not be at the same angle as the the cutter."

The cartridge is a transducer whose optimal function depends upon a fixed spatial relationship between the magnets and coils, as one or the other vibrates. Once the LP is cut, why should the cantilever give a damn about the cutter angle? The stylus "cares" about that in terms of SRA. So I would think that you set VTA so as to obtain proper or optimal SRA, first of all to assure proper energy transfer between groove and stylus, and then it is VTF that mainly assures the proper spatial relations between the transducing parts of the cartridge. VTA is just a convenient surrogate for SRA. Probably this has become a discussion about semantics.