About Vertical Movement

[J. Perry]

Hey Guys, I am wanting to talk about vertical movement of the stylus during cutting.

Here's what I know:

- some out-of-phase audio, especially under 500hz, will cause vertical movement of the cutting stylus resulting in a thinner (less deep) region of the groove. if the depth change is extreme enough, playback can be affected.

Now, it's simple prescription to deal with this: elliptical EQ (summing L and R to mono) is applied as needed to lessen any vertical movement. Or, if lateral movement is not extreme, and if the groove pitch (distance between groves) is wide enough, allowing a deep groove (usually around 3 thousandths or more), elliptical EQ can sometimes be avoided. But all that's besides the point.

I'm just kinda wanting some insight into the actual physics behind vertical movement. Why, exactly, does it happen from a scientific standpoint? As I said, I know it's a result of out-of-phase audio, especially in the lower and upper bass frequencies. Could be that, if cutting rock music etc, a drum kit was recorded and not checked for phase cancellation during tracking. Or an artist/band/engineer applied a phase effect on guitars or bass guitar. Or could be from any number of other stereo effects applied during mixing. Just can't seem to wrap my heard around the actual cause. Any discussion or reference to articles, online or otherwise is totally appreciated.

First Post, btw!

[opcode66]

Ping Pong delay on just about anything with bass and you'll have out of phase audio. There are a number of ways to get there sonically in the studio. Having several room mics setup and recording simultaneously could also generate out of phase audio due to different reflections at different points in the room. Out of phase bass cut at high volume makes little ramps that the playback stylus will ride an jump righr off the record. Even at 3 - 4 mils groove width.

[J. Perry]

Ping Pong delay on just about anything with bass and you'll have out of phase audio. There are a number of ways to get there sonically in the studio. Having several room mics setup and recording simultaneously could also generate out of phase audio due to different reflections at different points in the room. Out of phase bass cut at high volume makes little ramps that the playback stylus will ride an jump righr off the record. Even at 3 - 4 mils groove width.

Indeed, good sir. You familiar with any of the science behind it? Like, i know out-of-phase material is having this result, but it's the 'why' I'm having trouble with.

[opcode66]

Well, bass grooves look like elongated sine waves when viewed under a microscope. In phase bass looks like an S. Out of phase bass looks like an elongated 8. The elongated 8 shape at the point where the out of phase material comes together is the ramp I was refering to earlier. That is the launching point for the playback stylus/tonearm.

[mossboss]

Try this simple answer The science is somewhere out there but does it really matter ??
On a 45/45 stereo cutting head this is what takes place in simple terms
An in phase signal will give you a lateral movement an out of phase will give you a vertical movement
A manipulation of both of those conditions will cut you a stereo track on a lacquer
(keep in mind there is no vertical only lateral movement when you cut a mono track)
A straight out of phase signal will present only a vertical movement on the head since there is no lateral movement therefore there is a loss of the widh of the track that is needed for safe play back
Thats should be enough for any one out there in an every day environment cutting or pressing records
You want a bit more reading not quite all of the the science but very good explanation of the whole thing here is a link for you

http://www.sickoftalk.com/whyvinyl.html
Cheers

[markrob]

Hi,

Its just the application of vector addition. Here is a way to visualize this.

First, assume the same signal has been applied to both driver coils such that a positive voltage results in outward movment of the driver. For this example assume we are looking at a positive DC voltage applied to the coils

On a piece of paper, draw an arrow that points downward at a 45 degree angle from left to right. This represents the movement of the cutter head due to the left channel driver only. The movement is from top to bottom and left to right on the paper. From the end of this arrow (lower right), draw an arrow that moves the same way. Remember, both drivers are seeing the same signal. So it will also move outward. Make sure this arrow is the same length as the first (this represents the amplitude of the signal). In this case, from top to botton and right to left at 45 degrees. This represents the movement of the stylus due to the right channel driver. The total net movement is from the start of the first arrow to the end of the second. Notice that the net movemnt is only in the top to bottom direction. This is vertical movment.

Now go back to the start of the second arrow amd draw it from the end of the first, but pointing up and to the right at 45 degress. This will occur if the right driver polarity is reveresed with the same signal present. Now the net movement is lateral.

On a 45/45 cutterhead, one of the two drive coils is actually wired with the opposite polarity with respect to the other. If it were not, in phase or mono signals (L+R) would results in vertical motion (the first example above).

So from the above, given a head wired as described, L+R infomation results in lateral motion, while L-R results in vertical motion. L+R is infomation that is in phase (A badly used term. Polarity is the correct term.). L-R information indicates the signals are identical, but one is out of phase/polarity with respect to each other. Also note that you can also have a problem with heavy bass that is panned hard left or right as it will result in a combination both lateral and vertical movement (only the left driver moving with the right stationary).

Hope this clears it up for you.

Mark

[TotalSonic]

You want a bit more reading not quite all of the the science but very good explanation of the whole thing here is a link for you

http://www.sickoftalk.com/whyvinyl.html
Cheers

That link is absolutely 100% WRONG about how PCM digital audio actually works!!! That graph is NOT what the audio output looks like from a PCM source!
I'd suggest reading Ken Pohlmann's "Principles of Digital Audio" to clear up what are some common misunderstandings about digital audio being propagated by websites such as the one above -
http://www.amazon.com/Principles-Digital-Audio-Ken-Pohlmann/dp/0071348190

Regarding the original posters question - as some animations are worth lots of words indeed - this site should probably help clear some of this up for you ->

http://www.vinylrecorder.com/stereo.html

Best regards,
Steve Berson