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Old 4th Dec 2019, 12:13 pm   #21
'LIVEWIRE?'
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Default Re: Mathematics needed to converst 78rpm 60Hz recording to 78rpm 50hz Digital convers

I still maintain that very few people would notice that a piece of music was 0.436% slow (or fast for that matter), and, in any case isn't the playing speed of most turntables likely to be something like + or - 1 or 1.5% of nominal, simply because there is an allowed tolerance from nominal in the specification of all turntables, whether the humble BSR or Garrard, etc., or even the most sophisticated belt or direct drive Hi-Fi Deck? I was more familiar with cassette players whilst working full time in the trade, particularly the in-car ones, and, IIRC, all had an allowable tolerance of, at best = or - 1& of the nominal 1.875 i.p.s. (4.76 cm.p.s ) playing speed.
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Old 4th Dec 2019, 12:31 pm   #22
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Default Re: Mathematics needed to converst 78rpm 60Hz recording to 78rpm 50hz Digital convers

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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.
Certainly pre-war 78 turntables normally had a geared drive, but also incorporated a speed governor so that the gear ratio wasn't wholly responsible for the turntable speed. The excellent Garrard 201 used a direct drive motor, but also had a governor for variable speed.

Martin
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Old 4th Dec 2019, 12:43 pm   #23
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Default Re: Mathematics needed to converst 78rpm 60Hz recording to 78rpm 50hz Digital convers

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I still maintain that very few people would notice that a piece of music was 0.436% slow (or fast for that matter), and, in any case isn't the playing speed of most turntables likely to be something like + or - 1 or 1.5% of nominal, simply because there is an allowed tolerance from nominal in the specification of all turntables, whether the humble BSR or Garrard, etc., or even the most sophisticated belt or direct drive Hi-Fi Deck? I was more familiar with cassette players whilst working full time in the trade, particularly the in-car ones, and, IIRC, all had an allowable tolerance of, at best = or - 1& of the nominal 1.875 i.p.s. (4.76 cm.p.s ) playing speed.
I guess that's true, though I can't claim an understanding of the 'perfect pitch' that some musicians seem to possess.

Certainly, we sometimes see variations in mains frequency as much as the 0.4% you quote. Such a change will directly impact on the speed of the direct-drive synchronous motors used in the top-of-the-line Neumann disc cutting lathes. Unless these are nowadays electronically driven?

Martin
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Old 4th Dec 2019, 2:32 pm   #24
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Default Re: Mathematics needed to converst 78rpm 60Hz recording to 78rpm 50hz Digital convers

As a kid, we had an HMV/Colombia turntable. This was gear driven with a worm on the motor shaft.

It wasn't a synchronous motor, it had to be an induction motor because there was a small lever and brake to slur the speed down in an adjustable way. Of course, if the OP is setting whatever turntable by a mains driven strobe, then mains frequency gets in on the act.

Mains frequency also wanders.

Maybe for such critical work, a precise quartz crystal timed strobe is needed. There was a thread on such not long ago.

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Old 4th Dec 2019, 7:03 pm   #25
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Default Re: Mathematics needed to converst 78rpm 60Hz recording to 78rpm 50hz Digital convers

Films are normally at 24 frames per second and speeded up by a whopping 4% for TV, anyone notice?
 
Old 4th Dec 2019, 7:26 pm   #26
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Default Re: Mathematics needed to converst 78rpm 60Hz recording to 78rpm 50hz Digital convers

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Films are normally at 24 frames per second and speeded up by a whopping 4% for TV, anyone notice?
Is that what they do? I thought there was a system where they repeated a frame every now and then to keep the speed right. Maybe that's only for NTSC. However there are certainly a lot of old films where it is quite obvious that they have been speeded up. Keystone cops for example. I recently watched Metropolis and everyone seemed to move unnaturally fast.
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Old 4th Dec 2019, 8:45 pm   #27
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Default Re: Mathematics needed to convert 78rpm 60Hz recording to 78rpm 50hz Digital conversi

The technology certainly exists to convert 24 to 25 fps whilst retaining pitch, but I think most films are still run at 25fps and transmitted as such. Most of the audience don't notice anyway, although some soundtrack collectors took trouble to modify machines to record slightly fast.
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Old 4th Dec 2019, 9:32 pm   #28
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Default Re: Mathematics needed to convert 78rpm 60Hz recording to 78rpm 50hz Digital conversi

The question of gear-driven turntables and the derivation of standard rotational speeds from feasible gear ratios came up in this earlier thread: Question on Recording speeds, https://www.vintage-radio.net/forum/...d.php?t=159711.

The RCA 70-series transcription turntable was quoted as an example with synchronous motor and gear drive, and more comment on that turntable had been provided in this thread: Why both 33 and 45 ? https://www.vintage-radio.net/forum/...d.php?t=151267 (towards the end)


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Old 4th Dec 2019, 9:40 pm   #29
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Default Re: Mathematics needed to convert 78rpm 60Hz recording to 78rpm 50hz Digital conversi

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Maybe that's only for NTSC
60Hz TV uses the 3:2 pulldown technique (https://en.wikipedia.org/wiki/Three-two_pull_down) you can see this on some US video-recorded film back converted to 25 frame per second and shown here (50Hz land). They look a bit juddery on pans etc.. Usually on cheap programs, couldn't be bothered to either find the original film or it was lost/thrown in the bin once transferred.

You can just about notice that a 24 frame/s film on US TV is a bit odd, much less than the Film up to 60Hz back to 25 conversion above.
 
Old 5th Dec 2019, 9:42 am   #30
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Default Re: Mathematics needed to convert 78rpm 60Hz recording to 78rpm 50hz Digital conversi

50 and 60 Hz power supply frequencies are not absolute. They vary
simply because electricity is moved around the grid, simply by shifting
the phase. Utility companies aimed at giving the correct number of
minutes in a day.

Power systems can export exact timing. However recent political
quarrelling in Europe has played old harry with people's clocks that
depended on synchronous motors.

As to ever encountering a synchronous motor turntable with a gear
driven motor, I believe I have about 60 years ago.

I was working at ETCO (Electronic Tube Company) in Montreal,
and they had purchased a slew of obsolete turntables from the
Canadian Broadcasting Corporation. I think people bought them
for the motor.

A reluctance synchronous motor was very commonly used
in Teletype machines and has plenty of torque and no
need for a DC excited rotor.

They were gigantic machines, with an electric motor in the bottom
about the size of a dishwasher. And I had to move them about.

If you want to know RPM of turntable, use an optical tachometer.

Digitize at will but know the eccentricity of the hole in the particular
record copied will make the recording truly unique.

Below from A.E.S. Disk Recording anthology, 1953-1980.
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Old 5th Dec 2019, 11:06 am   #31
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Default Re: Mathematics needed to converst 78rpm 60Hz recording to 78rpm 50hz Digital convers

Quote:
Originally Posted by bluepilot View Post
Quote:
Originally Posted by merlinmaxwell View Post
Films are normally at 24 frames per second and speeded up by a whopping 4% for TV, anyone notice?
Is that what they do? I thought there was a system where they repeated a frame every now and then to keep the speed right. Maybe that's only for NTSC. However there are certainly a lot of old films where it is quite obvious that they have been speeded up. Keystone cops for example. I recently watched Metropolis and everyone seemed to move unnaturally fast.
The old tube film scanners could only run at 25fps, hence movies made at 24fps were running slightly faster and old silent hand-cranked films would always look speeded up.....not relevant today they can progressively scan each frame rather than worry about getting the 2 fields right. Today they can run a film at effectively any speed they like so if a TV station finds it is running short of time they can almost unnoticeably speed it up to fill the slot they have got.They tend to do this on long credit sequences at the end of the film.

In the past in US they had a different problem, trying to match 24fps of the film to the 29fps odd television rate, basically they repeated frames to fill in the gap,which was noticeable on say a pan shot as a slight judder...

Last edited by cheerfulcharlie; 5th Dec 2019 at 11:21 am.
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Old 5th Dec 2019, 12:35 pm   #32
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Default Re: Mathematics needed to convert 78rpm 60Hz recording to 78rpm 50hz Digital conversi

I was going to suggest the Garrard 301 or 401 turntable as a possible solution until I saw the current asking price, eeeeeek!

I sold my 401 about 35 years ago for £18 now they are worth close on £1000 and the 301 three times that.

They both used reluctance synchronous motors with an eddy current brake to vary the speed + / - about 10%. The brake was a piece of aluminium that moved between two pole pieces.
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Old 5th Dec 2019, 1:13 pm   #33
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Default Re: Mathematics needed to convert 78rpm 60Hz recording to 78rpm 50hz Digital conversi

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I was going to suggest the Garrard 301 or 401 turntable as a possible solution until I saw the current asking price, eeeeeek!

I sold my 401 about 35 years ago for £18 now they are worth close on £1000 and the 301 three times that.

They both used reluctance synchronous motors with an eddy current brake to vary the speed + / - about 10%. The brake was a piece of aluminium that moved between two pole pieces.
Not synchronous methinks, otherwise the brake couldn't slow them down.

Martin
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Old 5th Dec 2019, 1:37 pm   #34
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Default Re: Mathematics needed to convert 78rpm 60Hz recording to 78rpm 50hz Digital conversi

For a 50Hz strobe, a platter with 500 'dots' would give exactly 78RPM as every 13th dot would be highlighted !

dc
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Old 5th Dec 2019, 2:18 pm   #35
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Default Re: Mathematics needed to converst 78rpm 60Hz recording to 78rpm 50hz Digital convers

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Originally Posted by bluepilot View Post

Is that what they do? I thought there was a system where they repeated a frame every now and then to keep the speed right. Maybe that's only for NTSC. However there are certainly a lot of old films where it is quite obvious that they have been speeded up. Keystone cops for example. I recently watched Metropolis and everyone seemed to move unnaturally fast.
Silent movies were not shot at 24FPS, that standard was only established with the coming of sound. The original 'standard' was 16FPS, but cameras were hand cranked and speeds slowly crept up allegedly reaching about 20FPS before the coming of sound.

If a silent is just shown at 24/25FPS everything is unnaturally sped up. Back in the day there was a technique called stretch printing which repeated frames to make up the difference. I'm sure these days much better digital techniques are available.
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Old 5th Dec 2019, 2:37 pm   #36
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Default Re: Mathematics needed to convert 78rpm 60Hz recording to 78rpm 50hz Digital conversi

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Originally Posted by radiotechnician View Post
... but know the eccentricity of the hole in the particular record copied will make the recording truly unique.
And if the hole is larger than the spindle, which it will be to some extent especially on autochangers, every time it is played could be slightly unique too. Probably so minor so to be unnoticeable, but unique nonetheless.
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Old 5th Dec 2019, 3:49 pm   #37
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Default Re: Mathematics needed to convert 78rpm 60Hz recording to 78rpm 50hz Digital conversi

Tell me the old, old story - centring discs has long been one of life's minor miseries...
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