|
Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
|
Thread Tools |
4th Dec 2019, 12:13 pm | #21 |
Rest in Peace
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,306
|
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.
|
4th Dec 2019, 12:31 pm | #22 | |
Nonode
Join Date: Aug 2015
Location: Cambridge, Cambs. UK.
Posts: 2,196
|
Re: Mathematics needed to converst 78rpm 60Hz recording to 78rpm 50hz Digital convers
Quote:
Martin
__________________
BVWS Member |
|
4th Dec 2019, 12:43 pm | #23 | |
Nonode
Join Date: Aug 2015
Location: Cambridge, Cambs. UK.
Posts: 2,196
|
Re: Mathematics needed to converst 78rpm 60Hz recording to 78rpm 50hz Digital convers
Quote:
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
__________________
BVWS Member |
|
4th Dec 2019, 2:32 pm | #24 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,799
|
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. David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
4th Dec 2019, 7:03 pm | #25 |
Guest
Posts: n/a
|
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?
|
4th Dec 2019, 7:26 pm | #26 |
Heptode
Join Date: Feb 2010
Location: Duffort, Gers, France
Posts: 714
|
Re: Mathematics needed to converst 78rpm 60Hz recording to 78rpm 50hz Digital convers
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.
__________________
Stuart The golden age is always yesterday - Asa Briggs |
4th Dec 2019, 8:45 pm | #27 |
Dekatron
Join Date: Jul 2008
Location: Kington, Herefordshire, UK.
Posts: 3,657
|
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.
|
4th Dec 2019, 9:32 pm | #28 |
Nonode
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
|
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) Cheers, |
4th Dec 2019, 9:40 pm | #29 | |
Guest
Posts: n/a
|
Re: Mathematics needed to convert 78rpm 60Hz recording to 78rpm 50hz Digital conversi
Quote:
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. |
|
5th Dec 2019, 9:42 am | #30 |
Pentode
Join Date: Jan 2013
Location: Powell River, British Columbia, Canada.
Posts: 217
|
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.
__________________
Steve Dow VE7ASO |
5th Dec 2019, 11:06 am | #31 | |
Heptode
Join Date: May 2009
Location: Birmingham, West Midlands, UK.
Posts: 708
|
Re: Mathematics needed to converst 78rpm 60Hz recording to 78rpm 50hz Digital convers
Quote:
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. |
|
5th Dec 2019, 12:35 pm | #32 |
Hexode
Join Date: Nov 2003
Location: Ossett, West Yorkshire, UK.
Posts: 482
|
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. |
5th Dec 2019, 1:13 pm | #33 | |
Nonode
Join Date: Aug 2015
Location: Cambridge, Cambs. UK.
Posts: 2,196
|
Re: Mathematics needed to convert 78rpm 60Hz recording to 78rpm 50hz Digital conversi
Quote:
Martin
__________________
BVWS Member |
|
5th Dec 2019, 1:37 pm | #34 |
Nonode
Join Date: Jan 2009
Location: Bristol, UK.
Posts: 2,059
|
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 |
5th Dec 2019, 2:18 pm | #35 | |
Octode
Join Date: May 2009
Location: Stevenage, Herts. UK.
Posts: 1,515
|
Re: Mathematics needed to converst 78rpm 60Hz recording to 78rpm 50hz Digital convers
Quote:
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. |
|
5th Dec 2019, 2:37 pm | #36 |
Administrator
Join Date: Dec 2002
Location: Cardiff
Posts: 9,060
|
Re: Mathematics needed to convert 78rpm 60Hz recording to 78rpm 50hz Digital conversi
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.
|
5th Dec 2019, 3:49 pm | #37 |
Dekatron
Join Date: Jul 2008
Location: Kington, Herefordshire, UK.
Posts: 3,657
|
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...
|