"Jim Lesurf" wrote in message
...
In article , Alan Murphy
wrote:
"Don Pearce" wrote in message
...
On Thu, 16 Sep 2004 08:48:58 +0000 (UTC), "Alan Murphy"
wrote:

Because the DAC is much quieter and I can identify it every time :-)


What do you mean by quieter - less background noise or less volume?

Both, but I was just trying to make a point really, Don, about the
difficulty of establishing proper procedures when testing sensory
descrimination. In the visual field, with which I am familiar, very
slight alterations in test procedure, such as seperating contiguous
samples by a few mm or so can decrease discrimination of colour
difference by an order of magnitude.

I can see the above as being a justification for equalising the output
levels in order to remove a controllable variable - assuming that
the concern is to see if differences of some other kind can be
perceived.

Assuming that your belief is that you can hear differences which
do *not* come from sound level inequalities alone. Please see below as
I'd like to clarify your point of view on this....

Presenting the samples, in series, in A/B fashion, further greatly
decreases discrimination depending on the time interval between
viewings.

The above seems to me to be a generalised assertion - but I am not sure
that it can be shown to always be reliable. I'd presume it would depend
upon the circumstances and the manner of any 'difference' which the
subjects are being tested for perception.

The differences are still there of course but are masked by
the method of testing.

To say "masked" here seems to me to be presupposing that any
difference was both due to some effect *other* than the level
inequality and was then indeed 'masked' rather than being 'removed'
by equalising the levels. Again, I'd like to clarify this below...

Resort to instrumentation is not helpful in judging differences below
about 5 - 10 jnd's, depending on position in colour space, due to the
acuity of the visual system. I suspect the same holds true for auditory
differences.

My interest here is not in what you "suspect", but in trying to clarify
your views and their implications. To do this I'd like to post a series
of
questions and I'd be interested in your answers. Apologies if what
follows
is not clearly expressed...

Firstly, am I correct in understanding that you are saying that you would
be willing to do a comparision test if there were no attempt to correct
for
measureble level differences, but would not be willing to take such a
test
if the levels were equalised beforehand?

If the answer to the above is essentially "Yes", the next question is: Is
this because you are confident you can hear a difference when the levels
are different, but not when they have been equalised?

If the answer to the above is essentially "Yes", the next question is: Is
it your belief that the anticipated inability to perceive a difference is
somehow "masked" (using your term) by level equalisation?

If the answer to the above is essentially "Yes, the next question is:
What
test do you suggest that can be carried out to distinguish between the
hypothesis that the failure to discriminate when the levels are equalised
is due to "masking" rather than the anternative hypothesis that the only
audible distinction was simply due to a difference in level? i.e. that
the
difference percieved was solely due to a level difference.

If you cannot suggest a performable test that could falsify one
hypothesis
and support the other, how can you regard the hypothesis that the level
equalisation "masks" rather than simple "removes" the difference as being
scientifically or academically supportable?

So far as I am concerned, the above questions assume you could arrange
the
protocol to chose lengths and pattern of sampling "A"/"B"/"X" as you
would
feel most reasonable. So - for example - if you'd personally prefer each
"A", etc, to be half an hour, or twenty seconds, this can be assumed to
be
as you prefer.

As a separate point, I would also be curious about your view on how
significant a difference may be if it becomes un-detectable when the
levels
are equalised and each player was giving the desired loudness which you
might prefer (without knowing which player was in use). As someone who
used
to design equipment I can appreciate there are times when even tiny
effects
may be worth persuing. However when in normal use, given all the other
uncontrolled variables of domestic listening, it does seem to me that
many
of the 'differences' people debate may be of doubtful concern when simply
listening to and enjoying music.

Slainte,

Jim

--
Electronics
http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

I think it might be clearer, Jim, if I just outline my
views on the subject and hope that you find this acceptable.

I agree that, for the results to be meaningful in AB
testing, levels should be equalised and regret that my
devious attempts to wind up Stewart were misinterpreted.

I do feel however that AB testing is possibly not a suitable
test for revealing differences close to 1 jnd and is accurate
to perhaps 5 jnd. My reasoning is that this is analogous
to the case in the visual field where it is possible to view
samples both simultaneously and serially but where
discrimination is greatly reduced in the case of serial viewing.
As you probably know, the specification of small colour
differences is of significant commercial importance and
international standards and an extensive body of research,
both published and unpublished, exists. I published an early
computer study on equal visual spacing as long ago as 1977,
("A two-dimensional colour diagram based on the sensitivity
functions of cone vision". JOCCA, 1977, 60, 307-310).

As for a test to determine whether AB testing is sufficiently sensitive
to distinguish small audio differences I would propose the following:
It should be possible to determine minimum audible differences
of 1 jnd over a discrete range of frequencies on a test setup,
say from 1000 to 15000Hz at 2000 Hz intervals.
A set of digital AB samples to Red Book CD
standard at normal listening levels would then be prepared,
one of which would be a pure tone at each of the
frequencies and the other would be the same tone corrupted
at alternate values by positive then negative random increments
of digital noise varying from 0 to 5 jnd. My prediction is that
an AB test on these samples would not be able to distinguish
differences of less than 5 jnd. Over to you.

BTW Europe won the Ryder Cup by a record margin but I am
sure that in St Andrews you already knew that :-)

Alan.