Mathematics needed to convert 78rpm 60Hz recording to 78rpm 50hz Digital conversion

[robwaldron]

My first post, I do hope someone can help me.

I have a US mastered 78rpm recording (60Hz) so the replay speed is governed by the flourescent tube run at 60Hz on the indicator reflectors on the platter.

This recording needs to be played back at the same speed it was recorded at, however it is being played back on 50Hz using the same flourescent indicator light/reflector system.

I am transcrbing the recording to 192kHz 32bit and therefore require a conversion figure as a percentage.

Played back at 50Hz 78rpm the recording is a little slower that it should be and I need to find out by what percentage I should increse the speed so that the 50Hz version runs at exactly the same speed as the Original 60Hz version.

My Maths is not that good so if anyone could help me I would be most grateful
Rob

[Station X]

I assume that you're playing a 78 RPM record on a deck designed to work on a 60Hz supply, but powered from a 50Hz supply?

If so the required speed, as a percentage of the original speed, is 60/50*100% or 120%. An increase of 20%

[bluepilot]

I don't really understand the question. A 78rpm record is recorded at 78rpm regardless of the mains frequency. 78rpm in the US is the same as 78rpm in the UK. The problem seems to be the turntable stroboscope. I assume your turntable only has one set of graduations for 60Hz? Why not just print out a stroboscope for the required speed and strobe light frequency (you can use the room lighting rather than the strobe on the turntable) and play the disc at 78rpm?
Strobe discs
or produce a custom one here:
Strobe generator

[Ted Kendall]

A 50 Hz strobe gives 77.92 rpm, a 60 Hz strobe gives 78.26 rpm, so the difference is of the order of 0.1%. I try to do initial transfers at the correct nominal speed, but often other factors supervene.

[Nickthedentist]

Thanks for that Ted, I always assumed both variants would give you 78.00 rpm.

[Station X]

Originally Posted by bluepilot

I don't really understand the question,

I'm not so sure I do now either. Unless the turntable speed can be varied, any speed correction will need to be made in software.

[Ted Kendall]

The nominal speed for cutting records coalesced on both sides of the pond at 78rpm. Different stroboscopes gave slightly different answers. The difference seldom matters in practice, though. All digital varispeeds are not created equal, either...

[Tractorfan]

Hi,
Why not simply play the disc on a UK spec turntable with 50Hz strobe markings?
All the turntables I've seen in the UK and Europe have both 50 and 60Hz indicators.
Cheers, Pete.

[bluepilot]

Originally Posted by Ted Kendall

A 50 Hz strobe gives 77.92 rpm, a 60 Hz strobe gives 78.26 rpm, so the difference is of the order of 0.1%. I try to do initial transfers at the correct nominal speed, but often other factors supervene.

Maybe that's what OP means. A US disc with nominal speed 78.26rpm will be played at 77.92 rpm on a turntable with a 50Hz strobe. That implies it needs to be speeded up by a factor 78.26/77.92 = 1.00436. If he's recording it at 192kHz then he needs to tell his software that it was sampled at 192.83778kHz and then resample to 192kHz. The difference is negligible for all practical purposes.

[TonyDuell]

It's probably quite difficult to judge the speed of a turntable to that accuracy with a strobe disk (and is the rate of flashing of the strobe lamp accurate enough?)

I would also not be surprised if most (all?) consumer-grade turntables, even quite high-end ones, didn't vary in speed by that sort of amount when playing a record.

['LIVEWIRE?']

I woud also be surprised if anyone could actually hear the difference between 77.92 & 78.26RPM as this is less than1%

[Sideband]

...but surely if the disc was cut at 78RPM then it only has to be played back at the same speed. It's irrelevant if it was recorded in the US or Europe. 78RPM is 78RPM anywhere in the world. It will only matter, as already stated, if a US deck was being played on UK mains where it would run slow. Just use a UK deck set to 78 RPM and it will be correct.

Maybe the OP needs to clarify what he is using.

[Ted Kendall]

Originally Posted by 'LIVEWIRE?'

I woud also be surprised if anyone could actually hear the difference between 77.92 & 78.26RPM as this is less than1%

Well, in some circumstances I can hear discrepancies of this order on familiar material. Many years ago I bought a high speed Revox G36 in very nice condition. I was delighted with it, and put on a familiar tape, a Goon Show as it happens, of good provenance. After a few minutes it dawned on me that something wasn't quite right - it was pace rather than pitch which made me uneasy at first. Investigation showed that the machine was in fact running about 1% slow - it was a conversion by the UK agent, who had fitted an F36 motor with a slightly undersized UK made capstan shaft.

Apologies for earlier brain fade, incidentally - the difference under discussion is of the order of 1%, not 0.1%. This is a sixth of a semitone, and can be significant when remastering, although as I have said other factors frequently come into play.

[bluepilot]

Originally Posted by Sideband

...but surely if the disc was cut at 78RPM then it only has to be played back at the same speed. It's irrelevant if it was recorded in the US or Europe.

Not exactly. In the US 78rpm was actually 78.26rpm, this being the speed of a 3600rpm synchronous motor run from a 60Hz supply, reduced by a 46:1 gear train. In the UK it was 77.92Hz, this being a 3000rpm motor run from a 50Hz supply, reduced by 38.5:1. So if you play a US record on a UK turntable it will be 0.436% too slow. Whether you can hear the difference is another matter but correcting it won't hurt if you feel so inclined.

BTW, the strobe speeds mentioned above are actually 100Hz and 120Hz rather than 50Hz amd 60Hz because the lamps will produce a pulse on both positive and negative half cycles.

[JayBee66]

If you want to copy the recording then save the audio to a computer and use any number of software packages to convert the recording to the correct speed.

[Hartley118]

Originally Posted by bluepilot

Not exactly. In the US 78rpm was actually 78.26rpm, this being the speed of a 3600rpm synchronous motor run from a 60Hz supply, reduced by a 46:1 gear train. In the UK it was 77.92Hz, this being a 3000rpm motor run from a 50Hz supply, reduced by 38.5:1. So if you play a US record on a UK turntable it will be 0.436% too slow. Whether you can hear the difference is another matter but correcting it won't hurt if you feel so inclined.
........

Has anyone ever encountered a turntable with a synchronous motor and a fixed-ratio geared drive?

Martin

[Station X]

I guess they'd be used in a recording machine.

[Brigham]

Cutting machines are a different animal compared to playback turntables.
In the 78 era, they were usually weight-driven, for consistency of speed, gravity being
constant to a high degree of accuracy.
I never realised that 78RPM was a nominal speed, or that the actual speed varied with different regions.
One learns something new...

[Hartley118]

The weight-driven cutting lathes would necessarily have incorporated a governor that could be set to any speed.

AFAIK motor driven cutting lathes initially used a belt drive, so the reduction ratio wasn't limited by available gear tooth ratios. Nowadays, I understand that direct-drive synchronous motors are common.

Martin

[Nickthedentist]

Originally Posted by Hartley118

Has anyone ever encountered a turntable with a synchronous motor and a fixed-ratio geared drive?

Martin

Weren't some of the pre-war electric turntables like this? Could be wrong though.