On Wed, 5 Jan 2005 11:37:50 +0200, "Iain M Churches"
wrote:


"Don Pearce" wrote in message
...

First the capacitors. Go through many samples with a meter to make
sure they are as far as possible equal in value. Install them in a
switcher box so they can be changed without delay.

OK. The service technician at the studio has promised to select with
a bridge carefully matched samples from those we give him.

Check the whole system to make sure that when a change is made the
output level remains the same.

Yes of course;.) We can see the output levels (sample and hold) on the
console meter bridge.

Also make sure there are no switching
transients that could identify which is being used.

We can guarantee this if we let the console do the switching of two
outputs, but can we guarantee it if we actually switch the capacitors
with 200V across them?

Yes - just arrange a bleed resistor to keep the voltage across the cap
at all times.

Put the whole thing in a separate room to the subjects. Identify the
point at which a change is made with some kind of signal light that
invites the subject to make his choice. Allow the subject to listen
for as long as he needs to make his choice.

The idea was to sit the subject in the studio, (low lights:-) and run the
test from the control room.

Do you think that we should let the subject know that a change had been
made? (it would be simple to rig a cue light for this purpose) I think it
would
be better if the subject were asked to detect the change without any visual
warning of when and if it had taken place.

Let the subject know the start of each trial - whether a change has
been made or not. It is too much of a strain for a subject to try and
tell the exact point - it is quite possible that the music playing at
that instant is not a type which best highlights the change.

At any point in the test, allow the subject to ask to hear either of
the capacitors identified, to verify impressions of difference.

We planned to give the subject a cue button with which he could, on
the fly, send a TC marker when he hears a change. This will alow us to
roll back to the same point to recheck.

The tester should determine the order of switching just before the
test with thirty coin tosses. He should write these down and follow
his list.

Thirty coin tosses? We shall be spending most of our time in the dimly
lit control room on our hands and knees looking for 1 EUR coins:-))

We could use a random numbers generator to pick
from three numbers, and feed these to the desk automation.

OK - do you have a random number generator? Hint - the RND() function
in a PC doesn't produce random numbers.

The order should be a new random set for each subject.
Someone off group has said that every subject should listen to
changes at exactly the same TC and the beauty of our set up
is that we can replicate it exactly for each and every subject.

No - if the changes are the same for every subject, you will fail to
randomize the effects of small changes in musical detail that may feel
like capacitor changes to a subject.

Go for more than 60% - 75% would be more reasonable.
Agreed. Olsen has used 60% in the past, as a "convincing"
percentage.

Do not let any subject meet the proctor at any point.

Agreed.
Thanks
Iain


Pearce Consulting
http://www.pearce.uk.com