From: Patent Tactics, George Brock-Nannestad
if the mains frequency is at the wrong speed, then a motor running off that
will run at the wrong speed. If you use a stroboscope light off the same
supply, you will think things are o.k., unless the speed would be off even
with the correct mains frequency. This is all assuming you use a synchronous
or hysteresis-synchronous motor. In the 1950s one of the broadcast stations
in Germany had a special crystal controlled mains supply for their recording
and reproducing equipment to avoid such problems. In the 1930s already,
German portable film recording equipment had a stabilised frequency and
voltage power supply for cameras to avoid the variable mains frequency. BBC
had record labels for instantaneous discs where you could mark the mains
frequency at recording.
With electronic speed control all of this was moot, and certainly Lyrec had a
crystal control for their speeds in the Frida model. The worst were the tape
recorders where you could accidentally record with the variable speed pot
engaged. This was the reason that I made my added-on speed pot for my UHER
4000 go via a cable to a specially fitted jack, so that it could only ever be
on if you plugged it in.
> Hi Bob:
> Question on that -- if the lights are blinking at the wrong frequency and
> the tape motor is thus
> running a bit fast or slow, why wouldn't that register on a calibrated
> strobe wheel? Won't lights
> that aren't 60hz make the strobe pattern move just the same as wheel speed
> that's not right? Sorry
> if it's a dumb question!
> -- Tom Fine
> ----- Original Message -----
> From: "Bob Olhsson" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Friday, January 28, 2011 5:20 PM
> Subject: Re: [ARSCLIST] Test tones circa 1978
> > -----Original Message-----
> > From Tom Fine: "...Shai -- how about a stroboscope wheel in the tape
> > path?..."
> > I was told when I was working in motion picture post production that
> > unfortunately power line frequencies are not a reliable source of speed
> > calibration because they are adjusted to maintain the long term accuracy
> > electric clocks.
> > A flutter tape is really the only solution.
> > Bob Olhsson Audio Mastery, Nashville TN
> > Mastering, Audio for Picture, Mix Evaluation and Quality Control
> > Over 40 years making people sound better than they ever imagined!
> > 615.562.4346 http://www.bobolhsson.com http://audiomastery.com