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ARSCLIST  January 2011

ARSCLIST January 2011

Subject:

Re: Test tones circa 1978

From:

"Richard L. Hess" <[log in to unmask]>

Reply-To:

Association for Recorded Sound Discussion List <[log in to unmask]>

Date:

Fri, 28 Jan 2011 19:56:15 -0500

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (111 lines)

Hi, Bob and Tom,

The guarantee of x number of cycles per year (which was the old thing to 
keep the clocks on time) is pretty much a thing of the past now that we 
have the huge grids of interconnected generating stations. They are 
generally run now on GPS-derived frequency standards and, since the 
1950s, the frequency standard of the power lines in the U.S. and Canada 
(where they are grid-connected) has improved greatly.

If you look here, you will see that there are essentially three isolated 
(by DC interconnects) grids in the U.S. and Canada (Alaska, Hawaii, and 
remote portions of Canada, including Newfoundland excluded as they are 
not on the grid, yet). There is the Eastern Interconnect, the Western 
Interconnect, and the ERCOT Interconnect for Texas.

> A generator that drops 2 Hz below 60 Hz will
> rapidly build up enough heat in its bearings
> to destroy itself.

> In the Eastern Interconnect, a
> 30-mHz drop in frequency reduces power
> delivered by 1 GW
That's 0.03 Hz or 0.05%

http://www.aip.org/tip/INPHFA/vol-9/iss-5/p8.html (see map at top)

The 0.03 Hz variation would not be tolerated for long, but that 
corresponds to 1.8 seconds per hour of error, well within the timing 
tolerances of most tape machines.

Another interesting paper showing some limiting conditions
http://gridwise.pnl.gov/docs/pnnlsa44073.pdf

And, for more than you ever wanted to know:
http://www.scribd.com/doc/20275676/Robust-Power-System-Frequency-Control

Anyway, long-term (most frequency disturbances are short term as loads 
change), the strobe is not a bad idea at all. You can use a frequency 
counter to measure the line frequency, but its calibration may be no 
better than the AC line. My used, old Fluke in the studio is doing 
pretty well, but there is ambiguity at the fourth digit on my word clock 
bus. Most of the time it reads 44.10 or 96.00, but sometimes the last 
digit flicks.

Better than the strobe is a tachometer which is what is built into 
modern machines. All of the machines I use have a tachometer on a 
large-diameter roller. That includes the Studer A80, A810, and A807 and 
the Sony APR-5000 and APR-16. You can use external crystal oscillators 
that are accurate and stable as a reference.

As with any measurement, we have to make certain we're not using rubber 
rulers. The larger the grid, the more likely it is to be stable. I would 
suspect the Eastern North America grid is more stable than the Texas one.

No matter how you measure it: strobe, flutter tape, or tachometer, you 
are at the mercy of your measurement timebase. Your local frequency 
counter is probably better than you need for analog tape with any of 
these inputs. Slippage in the tach (electronic or strobe) and stretch, 
wrap issues, and slippage in the flutter test tape all introduce a level 
of uncertainty.

Since most of the above mentioned machines are crystal controlled, 
counting down from the microprocessor clock frequency, the only machine 
type I check regularly is the A80 which uses the analog discriminator on 
the capstan servo (same basic circuit as the A77, but with refinements). 
With that, I measure the tach frequency of the capstan motor and make 
sure it's 1600 Hz.

Cheers,

Richard



On 2011-01-28 6:09 PM, Tom Fine wrote:
> 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 of
>> electric clocks.
>>
>> A flutter tape is really the only solution.
>>
>

-- 
Richard L. Hess                   email: [log in to unmask]
Aurora, Ontario, Canada           (905) 713 6733     1-877-TAPE-FIX
http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.

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