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Re: [ARSCLIST] Non-RIAA preamp
From: Patent Tactics, George Brock-Nannestad
Hi Eric (no confusion this time)
> Hi George,
> I was hoping you'd chime in!
> Thanks for the language and conceptual precision. See my further questions
> On September 13, 2006 George Brock-Nannestad wrote:
> > ----- the compression you perceive is probably due to the fact that when
> > reproducing at half speed, you still use a corner frequency at the
> > frequencies mentioned rather than 150 Hz or 125 Hz. Or that is what you
> > eventually have to do, if you have used the electromagnetic pickup with
> > constant-velocity characteristic without any de-emphasis.
> When doing half-speed transfers I do these sans EQ (ie. FLAT).
----- it is probably good to do any declicking at this stage as well.
> (double the speed in the DAW). And then apply EQ as the very last step.
> Time to do some listening tests. I haven't half-speed transferred anything in
> long while, but noticed the compression when doing this on a client's system.
> Perhaps their system response was not as linear as it needed to be for the
> initial flat transfer.
> However, I'm still a bit confused. Again, assuming a flat transfer...
> At half-speed (transfer), wouldn't the dynamic range (the difference
> between the loudest and the quietest sounds be halved as well?
----- in absolute terms, as subtraction of two amplitude values, you are
correct. However, we measure dynamic range as proportions and logarithmically
at that, and call it decibels. And the decibels come out the same, although
perhaps the noise floor (because the absolute signal level out of the
transducer is halved) is slightly higher.
And if so,
> repitching will not affect the dynamic range, only the frequencies, right?
----- I think that if we make formal maths out of it, it will be clear.
Anyway, given that you are using a velocity pickup, its output at 10 Hz
(which would have been 20 Hz in real time for the recording) is only 1/100 of
that at 1000 Hz (calibration value of the pickup), i.e. 40 dB down (7½ bits
less to describe the waveform). That is why you need a bass lift in the EQ.
And incidentally, that is also why you need a disco single for dance floors:
in order to get punchy bass, you need to amplify strongly at the bass end,
and you risk positive feedback via the record surface as bass receiver and
the pickup as the transducer. If the record has coarser grooves and hence
more absolute bass velocity in the grooves already, you do not need so much
amplification, and positive feedback is avoided. Obviously with digital,
where the player only reads a table of loudspeaker cone positions at any one
instant, there is no feedback risk at all.