Mr.T wrote:
wrote in message
ups.com...
The fundamental arm/stylus resonance is more like 8-12 Hz, not
15 kHz. Playing it at half speed then converting to normal would move
for example, into the 16-24 Hz region.

Yes, but since you are only copying to a computer, then it may not matter
all that much. Any artefacts below 30Hz can be filtered out, (usually
nothing below that on the record), and you needn't use monitor speakers, or
even stay in the room, so physical vibration induced problems will not be so
much of an issue.
However it would require the TT/cartridge system to have a relatively flat
response to 15 or 20 Hz, (to get 30 or 40Hz) which is not so easy to achieve
IME.

The mechanical system consisting of the tone arm
effective mass and the stylus compliance forms a
second order mechanical resonant system. As such,
that means it's a second-order high-pass filter with
the cutoff frequency corresponding to the resonant
frequency.

Pplaying an LP at half speed,a s one example, means
that ALL the information is shifted down one octave.
But that mechanical high-pass filter remains the same.
Thus, the effect, once the half-speed play is compensated
for, is to have that high-pass filter move up an octave.

That means that, under the somewhat optimistic
assumption that the resonance is damped enough
to give a Butterworth high-pass at, oh, 12 Hz, playing
at half speed and compensating makes it a high-pass
at 24 Hz.

In fact, the vast majority of turntable systems I examined
over the years were seriously UNDERdamped, with effective
Q's in the realm of 2-5, which meant a pretty sizeable peak
(in the range of +6 to +14 dB) at resonance (12 Hz). Now,
move that peak to 24 Hz, and we begin to see the problem.

Now, for sure, the response is minimum-phase, and
can be completely compensated for by a complementary
equalizer, but there are several issues:

1. How many people know, with reasonablt certainty,
precisely what the resonant frequency of the arm/
cartridge system REALLY is, and what is REALLY
the system Q at resonance? (hint: almost none)

2. Regardless of whether it is equalizable (it is), what
you have done by shifting all the audio down by low-
speed playing is that you have now placed it in the
realm of that (likely) under-damped resonance. Now
you face the problem that you have significantly MORE
signal to stimulate that resonance and, being under-
damped, increase the likelihood of potential mis-
tracking problems, rather than decreasing it.

I also wonder just what benefits would be expected,
since a good system can play all the treble available
on any record at normal speed, and the biggest
problem in many cases is in the bass region.
Might be better to increase the playback speed instead.

But you trade one set of problems for another.

Rather, IF people would take the time and effort (and it's
NOT easy) to make sure the arm cartidge resonance is
both at the right frequency AND has a Q in the realm of
about 0.6 to 0.8, then things will be fine.

The problem is that the normal practice of applying some
indiscriminant amount of damping goo DOES NOT WORK.
The ONLY way to do it is to apply the right amount of the
right jind of damping, and the ONLY way to do THAT is
to MEASURE the result with appropriate test equipment.

I have, in fact, done that and when properly done, results
in even rather "ordinary" LP playback equipment being
able to track damned near anything and, by the way, also
reduces isolation problems (an underdamped arm/cartridge
system is more prone to isolation issues, because you
have more gain at the resonant frequency, partically defeating
the low-pass filtering effect of the turntable suspension).

And, for those of you out there eager to jump in an tell
me that their tone arm IS properly damped and all, please,
spare me the waste of time. Of the many hundreds of turntables
examined, ranging to the most esoteric, I never saw ONE
that was even close to the proper Q.