The answer is not much. Speed changes may be detectable by pitch, or
frequency, changes but not amplitude resulting from equalization.
The NAB curve rolls off from 20 db of boost at 20 Hz to 20 dB cut at 20 kHz,
crossing 0 dB at 1 kHz. The curve is not perfectly linear, but it averages
at about 4.6 dB per octave. Therefore a 1% frequency shift would cause an
amplitude change of about 0.07 dB or 0.8% (assuming I did the math right),
not enough to be detectable by the human ear.
Media Sciences, Inc.
> -----Original Message-----
> From: Association for Recorded Sound Discussion List
> [mailto:[log in to unmask]] On Behalf Of Andrew Hamilton
> Sent: Saturday, January 29, 2011 3:23 AM
> To: [log in to unmask]
> Subject: Re: [ARSCLIST] Vari-Speed - wasTest tones circa 1978
> 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,