Concerning the correction of possible equalization errors found in
tapes recorded on a machine running off of desired speed, we first
must know the conditions under which that recording machine was
calibrated. Because one first performs a playback response
calibration, before performing the record calibration, it is of
interest to know if the machine was running off-speed when the
playback response curve was set (yes or no). Two different sets of
conditions can follow from this. Yet we seldom (if ever) have such
information about the recorder’s calibration.
> 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?
Yes, because the complementary record and playback equalizations are
rooted in the frequency domain, they are (in a practical sense) blind
to absolute tape speed and drift from nominal (and resulting changes
to recorded wavelengths), provided those speed changes occur equally
in BOTH recording and reproducing (playback).
> What is the point at which an alteration should be made to
> the pre- and/or de-emphasis EQ?
Let's first simplify it by assuming there was no error introduced in
the recorder's record equalization calibration by virtue of someone
having first performed that machine's reproduce alignment with the
machine running badly off speed.
Then, there will be NO such point, IF the playback speed is matched to
the record speed. Under this condition (which should always be the
goal) the problems you are imagining disappear.
Now, in the general case where the playback speed does NOT match the
record speed, you are correct that a playback equalization
(de-emphasis) tracking error will be introduced. As someone else
responded, in most all practical examples experienced, this error is
small enough to be negligible.
> provided one had a way of dialing in the exact speed to make
> the reproducer play back at the (actual) speed of the recorder
You are correct to be emphasizing this, as it is one of the first
things one needs to do in tape playback. Adjusting to known (or
assumed) musical pitch, resolving to known (or assumed) recorded power
line frequency hum, or possible other recorded pilot signal, including
recording bias, are powerful tools for doing this.
Audio Transfer Laboratory