Vinyl lathe

[Kitwn]

https://thevinylfactory.com/features...r-own-records/

Personally I think the guys in that video are barking mad! But plenty of people think that of me so no problem. The most important thing to understand about the whole process of making gramophone records is that the only thing it has in common with anything anybody usually talks about on this forum is the the use of the word 'lathe'.

The following is a memory dredge of stuff I learned a long, long time ago. Only the seriously geeky nerds should bother reading this.

Mono recordings make a constant depth track that is modulated from side to side to record the music.The master disc is cut into shelac or something similar layered onto an aluminium substrate. In the old days of 78 recordings the radial inward speed which sets the track spacing was constant and had to accommodate the highest possible amplitude of signal. In later 45 and 33 rpm systems the recording engineer had control of the track spacing and could reduce it for quieter passages, thus increasing the recording time available on a side. This was part of the art being a disc cutting engineer.

The master disc was then used to create positive mold masters (can't remember what they were made out of if I ever knew) which would be used to stamp the discs. Since the master was only ever used to make molds it would last a long time and could be used to replace worn out stamps many times over.

Before the advent of tape recording you could go to your local music shop and record your own 78 rpm disc. The earliest recording my sister and I have of our mother singing is one of those. You can see an example of such a machine being used in the film 'The King's Speech'.

When stereo came along the need was to have a single groove carrying two channels of audio and it's geometry had to be mono-compatible for existing players so the recording head has two electromagnetic solenoids driving the cutting tool along paths at 45 degrees to the disc surface, 90 degrees to each other, one for the left audio channel (L) one for right (R).

There's a link below to a video that explains this idea using model aeroplane servos to move a model head.

This produces an end result where the sideways deflection of the track is derived form the Left + right (L+R) signal amplitude divided by two and the vertical movement is left minus right (L-R) divided by two, the difference signal which depends on the pan position of the source. A mono player only recovers the sideways component, L+R for a satisfactory compatible output but the stereo cartridge has two transducers at 45 degrees like the cutting head.



Off to the side for a moment, FM radio uses a similar idea to add stereo transmission to an existing mono system by transmitting (L+R)/2 voltage as the standard mono 'sum' signal and adding the (L-R)/2 'difference' signal as a subcarrier. Mono radios ignore the subcarrier but a stereo receiver recovers L and R by adding and subtracting the difference. (L+R)/2 + (L-R)/2 = L. (L+R)/2 - (L-R)/2 = R.

Clear as mud?

Kit

https://www.youtube.com/watch?v=s62GfJEr6PI