The Secret Society of Lathe Trolls

tape · Joined: 25 Jan 2007 Posts: 95

A fantastic machine - instead of a pickup it has a photocell, upon which the grooves are projected trough a mirrorconstruction.

http://www.philcorepairbench.com/phono.htm

No record or needle wear!

The 'pick up mechanism' looks quite complicated.
My question is how to make one and attach it to a mordern recordplayer - if possible....

The perspectives of such a device is interesting. Besides playing records you could draw or etch soundwaves on a disc and play them back....

Viewmaster · Joined: 16 Oct 2007 Posts: 12 Location: UK

[quote="tape"]A fantastic machine - instead of a pickup it has a photocell, upon which the grooves are projected trough a mirrorconstruction.

No record or needle wear!
quote]

Not so. A 'jewel' tracks in the grooves so there is wear. See note 7 and NOTE at end of that page where there is reference to this tracking jewel.
Only a modern laser tracking system is non wear like the ELP laser turntable at $10,000 Shocked

Albert.

tape · Joined: 25 Jan 2007 Posts: 95

ok, I dont get the principle behind this then....
Not obvious what the jewel is for.

I thought that the mirror reflected the grooves upon the cell...

Doug 6N · Joined: 21 May 2007 Posts: 68 Location: Washington

cuttercollector · Joined: 11 Jun 2006 Posts: 317 Location: San Jose, CA

Hmm, Mr antique technology here will now do my best to confuse....
It kind of works like an optical sound pickup on a movie projector.

There is a conventional stylus. It tracks the grooves in a normal way.
The idea is there could be less tracking weight needed (though all the components themselves were heaver) because the stylus, instead of moving a coil of wire or twisting a piezo crystal (both in early primitive states at that time) could just cause a mirror to vibrate. This principle is similar to (or is) a type of galvanometer. There is a small light bulb with DC voltage to the filament (AC would cause hum to be in the light source) with a lens to focus the beam and a photo pickup tube with a DC bias voltage applied. This type of tube changes resistance when struck by light, thus the mirror reflecting the beam of light modulated by the stylus/mirror assembly with the vibration of the record groove creates an audio signal as the DC to the tube photocell is modulated by the change in light value. This is sent to a normal audio preamp/amp and then to the speaker in the standard way as with any other phono pickup.

They leveraged available movie projector technology to do this. The only change is that the focused beam of light is shot through the optical sound track along the edge of the film, which has light and dark areas as an analog of the audio signal and then the now audio modulated beam of light is focused on the photo pickup tube cell (primitive photocell) and then amplified in the same way.

Reports were that the performance of the pickup system in the Philco didn't work all that well, was complicated and went out of adjustment pretty easilly.

Typically optical sound track films are not even capable of the same level of fidelity as 78 records allthough both of them and AM radio were about all there was till after WWII when the concept of "high fidelity" became popular.

Doug 6N · Joined: 21 May 2007 Posts: 68 Location: Washington

Hey Cuttercollector:

We should chat sometime. I bet we have a lot in common in our intrests of old technology. Smile

Pretty realistic reproduction can be had from 100 to 6K Is it hi fi? Nope. But not all that bad. The old acdemy motion picture sound track fell into this area with it's pre and post eq. About the same as 78's

Most of us here are obviously still listening to this range of reproduction. Smile

Gives one pause to think about cutting a 78 from a CD. Ah! Technology? LOL!

Doug

cuttercollector · Joined: 11 Jun 2006 Posts: 317 Location: San Jose, CA

You can email me anytime. My email is in my profile.
Yes, I agree with you. I personally will take - oh, say, 40-50Hz on the low end to 6-7Khz on the high end with a reasonably flat to falling response in that area (no big peak up there) and say, 45-50dB between the surface (or other) noise and where it gets to 3-4% distortion, all with decent transient response within the pass band, no artificial audio compression and mid band distortion of less than one percent in general, which could be achieved with proper use of 1940s technology whether it be 78s, optical film tracks or AM radio, over some of the things that pass for "high fidelity" today. Especially mushy low bit rate lossy digital compression with tons of eq and audio compression it doesn't need!
I think the first time I heard a properly played back, well recorded and not worn out 78 played back through a single ended triode amp with a reasonable amount of power into an efficient horn speaker with excelent transient response, my jaw just about dropped because within it's own limitations the sound was very "REAL". Made me scratch my head and think "Wait a minute! What have we lost here along the way?!?"

BTW we started our reply to this thread at the same time. I just type more slowly and am more long winded. : )
I did not read what you had written.