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ARSCLIST  January 2011

ARSCLIST January 2011

Subject:

Re: Vari-Speed - wasTest tones circa 1978

From:

Andrew Hamilton <[log in to unmask]>

Reply-To:

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

Date:

Sat, 29 Jan 2011 14:53:39 -0500

Content-Type:

text/plain

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Thanks Richard and Jerry,

      I have used that EQ translation table (from Choosing &  
Using...) in the past for transferring at 3.75 ips when I only had a  
7.5 ips NAB Cal. tape.   Indeed, from 250-1000 cycles, the NAB curve  
is rather flat.   However, the Cal tape's 2x speed (as compared to  
the target reel,) requires an EQ fudge factor of about -3 dB at 60  
cycles, and -1.5 dB, at 120 cps.   1000 cycles at 2x (500, for the 1x  
target) would be "dead nuts" (=0 dB), but, by 4k cps, there is the  
need to reduce (again) by 0.6 dB (where using the 7.5 ips tape for a  
3.75 ips recording's playback).

      Would it follow that, if a tape had apparently been recorded at  
an amazing 1.5x speed (i.e., 5.6 ips based on a fast-played 3.75 ips  
NAB Cal tape (presumably) at time of recording), those 7.5 ips- 
to-3.75 ips amplitude offsets would need be halved from where they  
are with the 2x (i.e., 7.5 ips) Cal tape, since at 1x, there is to be  
no offset from standard EQ, and 1.5 is half-way between 1 and 2?   If  
so, then for a hypothetical  1.25x-speed "3.75 ips"  NAB tape (i.e.,  
one actually recorded at 4.7 ips), those amplitudes should be offset  
by -0.25 times the 2x (i.e, 7.5 ips) Cal tape's fudge factor...?     
Then, for a 1.1x speed "3.75 ips" NAB tape (i.e., actually recorded  
at 4.1 ips) (again, for sake of discussion), the EQ should be tweaked by
-0.1 times the 2x fudge factor, or -0.3 dB.)

      Fast forward to the ATR Service(s) VS-20.    This vari-speed  
oscillator can drive the ATR-100 with digital re-set-ability at any  
1/100% step above the nominal clock speed, up to +200.00 %.   It can  
also, as you may well know, slow down the same machine to -50.00 %  
nominal clock.    So, one could play a 3.75 ips tape from the 7.5 ips  
setting on the machine, by setting the vari-speed to -50.00%.   He'd  
be wrong, however, not to alter the de-emphasis accordingly.   
Clearly, by then, one may need to use Mr. McKnight's generous cheat- 
sheet so as not to labor under a mistake.

      Perhaps, what you want me to "take away" from this topic of  
discourse is that pitch accuracy is far more significant to sound  
recording than amplitude accuracy, in terms of what is detectable to  
the ear when one vari-speeds for truer pitch on an otherwise properly  
Cal'd reproducer.

      Still, when someone has recorded a tape on a machine that was  
running too fast or too slow, by some margin less than +200.00% or -  
50.00% of a standard (as in EQ) speed, I wonder if there is the  
possibility of untwisting the mis-applied pre-emphasis by de- 
emphasizing at the original (erroneous) record speed - or if, barring  
serendipity, what we get back by vari-speeding a non-standard  
recording, is the right frequencies but always at the wrong  
amplitude, if only by a small amount, since we have strayed, however  
minimally, from the speed-based curve.
      Put simply, does vari-speed repro's effect on de-emphasis add  
any noise (loss of signal) that wasn't there in the first place,  
provided one had a way of dialing in the exact speed to make the  
reproducer play back at the (actual) speed of the recorder?   Does it  
really de-emphasize that which was to however small a degree, not pre- 
emphasized at the standard speed?  Or does it possibly further twist  
the phase and alter the amplitude?   I suppose that the boon of pitch  
correction would outweigh those concerns in almost any application in  
which one would invoke a vari-speed.  (:      But the mastering  
engineer's dictum is "First, do no harm."   So, is it a minor side- 
effect, or actually part of the cure with no inherent insult?



Cheers,
      Andrew



On Jan 29, 2011, at 10:40 AM, Richard L. Hess wrote:

> Hi, Andrew,
>
> Yes there will be changes, but they are not huge and the slope is  
> gentle (less so with AME). Look at www.mrltapes.com and look at the  
> Choosing and Using a test tape document -- there are cross  
> references between standards for applying a calibration tape of one  
> standard to another.
>
> Cheers,
>
> Richard
>
> On 2011-01-29 3:23 AM, Andrew Hamilton wrote:
>> On Jan 28, 2011, at 10:12 PM, Richard L. Hess wrote:
>>
>>> ...One of my clients likes it that we can pitch the tapes for him  
>>> in the analog domain.
>>>
>>
>>
>> Dear Mr. Hess, et al.,
>>
>>      I was wondering how much speed tolerance there is to a given  
>> pre-/de-emphasis EQ, even if a vari-speed oscillator is  
>> available.    Say there is a 1 kHz tone at 990 Hz, and you know  
>> they had intended the EQ to be NAB...   Had the horse already left  
>> the barn (in terms of the emphasis's efficacy) when they were  
>> signaling a 1k sine, but the machine was turning a bit faster than  
>> 15 ips (say, 15.2 ips)  and turned the 1000 cycles per second into  
>> 990 cps when played back at the intended record speed?   Or, by  
>> speeding up the playback machine to 15.2 ips - enough to make the  
>> intended 1k tone actually play 1000 cps - does the vari-sped  
>> playback (at the original record speed) magically de-emphasize the  
>> haunted pre-emphasis?  What is the point at which an alteration  
>> should be made to the pre- and/or de-emphasis EQ?  (Obviously, if  
>> the recorder is turning at 20 ips or higher, a different emphasis  
>> eq has to be set, whence the custom curves - let alone AES and AME.)
>>
>>
>> Thank you,
>>      Andrew
>>
>
> -- 
> 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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