av subjektivisten » 2007-12-07 00:56
En klassiker är denna:
Digital Challenge: A Report
by Stanley P. Lipshitz (University of Waterloo, Waterloo, Ontario)
(From The BAS Speaker, Vol. 12 No. 11/12. Subscription to the magazine is
the same as membership in the society, and can be had by sending $25 for one
year ($35 outside North America) to The Boston Audio Society, Department C,
Box 211, Boston MA 02126-0002.)
Some readers may be unaware of the background to the "digital test" in which
Ivor Tiefenbrun participated on February 23, 1984 [1-4]. To summarize
briefly: Tiefenbrun has been quite outspoken about the inadequacies of
digital audio recordings and the systems on which they are made, and his
organization, Linn Products Ltd., was instrumental in publishing an analysis
of the Sony consumer PCM-F1 digital audio adaptor [5] outlining their
objections. I, on the other hand, have been using this very system for a
number of years now and have made close to one hundred recordings with it
with superb results and not the "execrable results" reported by Tiefenbrun.
I and my colleague John Vanderkooy have moreover conducted blind listening
comparisons between the PCM-F1's input and its reconstructed output signals,
and had yet to find anyone who can reliably distinguish between them on
musical program material.
I therefore challenged the "anti-digital" community in general, and
Tiefenbrun in particular, to participate in a blind listening test of the PC
M-F1 to give them the opportunity to substantiate their claims of poor
sound. The challenge details were spelled out in [2]. When I learned in
February 1984 of Tiefenbrun's impending visit to Toronto, I reissued this
challenge and was pleased to have him accept.
The test took place in the home of the local Linn distributor, Mr. Michael
Remington, using his all-Linn/Naim system (Linn LP-12 turntable, arm and
cartridge, Naim NAC 32 pre- and NAP 250 power amplifiers and Linn Isobarik
loudspeakers) and his choice of program material (all LP records).
Vanderkooy and Alan Lofft, editor of "Sound Canada" magazine, were also
present. The atmosphere throughout was cordial and more relaxed than I
expected.
The day began with two brief tests of the Tiefenbrun claim that undriven
transducers (digital alarm watches, telephones, headphones, or other
loudspeakers) in the same room audibly degrade the sound quality -- a claim
which forms the rationale behind their "single speaker" demonstration
demand. Firstly, a digital alarm watch with piezoelectric "beeper" was held
about 500 mm behind Tiefenbrun's head while he listened to the loudspeaker
reproduction from his stereo seat on the couch, with the watch either fully
exposed or clasped firmly between the palms of my hands. We were assured
that the latter artifice would muffle any deleterious effects. This was thus
a single-blind test: The testee did not know the covered/uncovered status of
the watch at each trial, but the tester did know. A random series of 20
trials was conducted while Remington cued up the turntable (playing a female
vocalist) on each occasion, as he did throughout the day.
Tiefenbrun's result: 10 correct responses in 20 trials, an outcome which
shows no ability to discriminate between the two situations.
The second test, also single-blind, used a Linn "Kan" loudspeaker as the
undriven transducer. Again the female vocalist was used as source material.
The loudspeaker lay on the thickly-carpeted floor behind the listening
couch. It was placed either on its side (the "uncovered" condition) or on
its face (the "covered" condition) according to a random series of choices.
Ten trials were conducted during which Tiefenbrun achieved a score of 5
correct out of 10. Again, this demonstrates no discrimination ability beyond
what one would expect purely on the basis of chance.
With these preliminaries out of the way (but just for safety all watches and
headphones were left in another room throughout the entire day's
proceedings), we proceeded to the main test of the day -- that of the
audibility of the Sony PCM-F1 digital audio processor in 16-bit mode when
inserted into the audio chain, as proposed in my original challenge.
The PCM-F1's analog-to-digital (D/A) converter output was looped straight
through to the digital-to-analog (D/A) converter input by connecting "video
out" to "video in". The complete encode-decode chain including the two
low-pass anti-aliasing filters as well as the sample-and-hold circuits and
the A/D and D/A converters was thus subject to the test.
In order to remove the F1's polarity reversal it had been internally fitted
with a digital inverter chip (as used in the later non-inverting Sony
PCM-701 model) to interchange the ones and zeroes and hence perform a
polarity correction.
The PCM-F1 was inserted into the signal path between the preamplifier and
the power amplifier by means of an A/B/X switchbox of our design (see
[6,7]). The setup is shown in Fig. 1. When the switchbox was in the "A"
position the PCM-F1 was inserted into the signal path, while the "B"
position represented the "straight-through" configuration. The "X" position
was a randomly selected choice made by the switchbox. For each trial "X" was
chosen by the box to be either "A" or "B", but its identity was unknown to
any of the participants, thus making the experiment double blind. The
subject could at his leisure compare any of all of "A", "B" and "X" with
each other. What he had to do was identify whether "X" was "A" or "B". Once
his decision was made, the switchbox was interrogated to discover the true
identity of "X", and then "X" was re-randomized in preparation for the next
trial.
The gains of the "A" and "B" paths were matched in both left and right
channels to within 0.05 dB at 1 kHz using the PCM-F1's gain controls. This
was done by measuring across the amplifier output terminals. The match was
then confirmed to be within +/- 0.25 dB across the whole audio band. The
PCM-F1's "peak hold" feature was used to keep a record of the peak signal
levels passing through it during the test, especially in view of the
relatively high sensitivity of the Naim power amplifier (<1 Vrms at
clipping) and the relatively low listening levels chosen by the
participants. More about this shortly.
After an acclimatization period, a set of 10 trials was conducted in an
unhurried fashion before breaking for lunch, after which a further set of 10
trials was conducted. Tiefenbrun's score for the series was 11 correct
decisions out of 20, a result which shows no statistically significant
ability to discriminate between "A" and "B" any more accurately than would
be expected on the basis of random guessing.
At this point I thought that I could reliably distinguish between the "A"
and "B" paths on the basis of the *slight* noise level increase which
occurred when the PCM-F1 was inserted into the chain, and which was
marginally audible due to the high gain of the Naim MAP 250 power amplifier
combined with the low peak signal levels through the F1, which the peak-hold
meters showed to have risen no higher than -20 dB. (0 dB is the digital clip
point, and these peak levels were somewhat unfair to the digital processor
since 20 dB of its signal-to-noise ratio was being thrown away.) [In other
words, for this segment of the test the F1 was in effect a 13-bit
processor. -- Ed.]
I expressed my desire to try the test, and Remington went to cue up the
record again, but I requested to be allowed to undertake the test with no
signal passing though the system. Before realizing the import of what he was
saying, Vanderkooy interjected: "Ah! You're going to listen to the sound of
the relays." Yes, there is indeed a slight audible difference between the
acoustic "click" made when the "A" and "B" relays pull in. This is due to
the unavoidable differences in the mounting positions of the relay on the
A/B/X box chassis and, although slight, it can be heard if one listens for
it. I replied that I was going to listen to the difference in background
hiss, and the subsequent series of blind trials showed conclusively that the
two signal paths could be reliably distinguished on this basis alone.
What conclusion can we draw from this? Tiefenbrun's random results show that
he had not been aware of either the background noise difference -- masked as
it most likely was during his trials by the record surface noise -- or the
relay click difference. The null result of this first PCM-F1 listening test
is thus valid in spite of the potential for non-blindness due to the
subliminal noise differences. I had been aware of the hiss difference from
the outset from listening while the stylus was off the record, but had
decided to proceed with the test in the knowledge that if the test produced
a null result, the hiss difference could not have affected the outcome -- a
decision vindicated by the actual data.
The "secret" about the slight relay noise difference was, however, now out
in the open, and if listened for could from then on influence (albeit
subconsciously) the blindness of any succeeding tests made using the
switchbox. Nevertheless we decided to proceed with a second series of PCM-F1
listening tests, this time with the digital system inserted into the
preamplifier's tape monitor loop. Once again levels were carefully equalized
and frequency responses checked to be very closely matched between "A" and
"B". This insertion point precedes the preamplifier's volume control, where
the signal levels are significantly higher, thus more fully exercising the
F1's full dynamic range while at the same time rendering its output noise
level inaudible at the replay volume setting then being used. This
configuration removed the hiss difference as a potential factor.
A set of 23 double-blind trials was conducted listening to music through the
system, with Tiefenbrun voting for the identity of "X" at each trial. It was
then that Vanderkooy pointed out that, although the relays in the A/B/X box
were switching normally during this series, no tiny electrical clicks were
audible from the loudspeakers during their operation, as normally would be
the case. Investigation revealed that the preamplifier's "source/tape"
monitor switch had inadvertently been left in the "source" position, and as
Fig. 2 reveals, the A/B/X box and digital system were thus not being
inserted into the chain at all during this run!
Because the "A" and "B" positions were completely identical due to this
error, an unbiased decision for "X" should have produced a 50:50 split of
"A" and "B" votes. Interestingly, it turned out that this was not the case,
Tiefenbrun's votes being 14 for "A" and 9 for "B". Could something have been
influencing his voting, or did he just prefer the letter "A"? A comparison
of the actual "X" choices showed that during this series Tiefenbrun voted
incorrectly in 16 out of 23 trials, although the sound from the loudspeakers
never changed. [This proportion of incorrect guesses would occur in a truly
random situation less than 5% of the time. -- Ed.] What does this mean? We
cannot say for certain, but the most logical explanation is that he was
(perhaps subconsciously) voting on the basis of the relays' acoustic click
difference which we had now revealed. If so, his remembrance of the sound of
the A and B clicks was inverted. Be this as it may, the error we had made by
leaving the tape monitor switch in the "source' position turned out to
provide an interesting sidelight on the question of personal bias. It also
illustrates one of the potential pitfalls of high-resolution blind testing,
and the danger of jumping to conclusions before very carefully checking the
test setup.
With the tape monitor switch now correctly set, a rather rapid series of 37
trials took place with Tiefenbrun voting. The results: 10 correct decisions
out of 37, a result far worse than would have been expected by chance alone.
What was influencing the voting? Was it the relay sound? We do not know, but
this seems likely. It should be remembered that using the A/B/X box, a
direct comparison between "A" or "B" or "X" is always available, so a series
of trials in which there is a tendency as above to produce a consistent
reverse identification seems to implicate some extraneous factor unrelated
to, but correlated with, the loudspeaker sound. Tiefenbrun did very little
switching during this series, leaving the A/B/X box set to the "X" position
most of the time. It does indeed seem possible that the identifications were
being made on the basis of the relay sound incorrectly remembered.
To wrap up the day's work, we conducted a final series of tests to assess
whether the relay contacts in the A/B/X box could have been electrically
affecting the sound in any audible way. For this single-blind test the A/B/X
box was powered up and left permanently in one position, and the relay
contacts inserted into the signal path by switching the preamplifier's
monitor switch to the "tape" position. The switching sequence (relay
contacts "in" or "out") was determined by a random number sequence. Thirty
trials were conducted, interrupted three times to provide Tiefenbrun with a
reference comparison of the "direct" sound versus the relay contacts. His
score in identifying the presence of the relay contacts was 12 correct out
of 30 trials. This of course also shows no statistically significant ability
to identify correctly when the relay contacts were in circuit.
In summary, then, no evidence was provided by Tiefenbrun during this series
of tests that indicates ability to identify reliably:
(a) the presence of an undriven transducer in the room,
(b) the presence of the Sony PCM-F1 digital processor in the audio chain, or
(c) the presence of the relay contacts of the A/B/X switchbox in the
circuit.
The tests were conducted in an amicable rather than confrontational
atmosphere, and the parties departed feeling that the day's work had been
worthwhile. Further carefully-conducted blind tests will be necessary if
these conclusions are felt to be in error.
I would like to acknowledge the friendly cooperation and assistance of both
Michael Remington and John Vanderkooy in carrying out these experiments, and
would like to take this opportunity to express to Ivor Tiefenbrun, the
guinea pig of the day's experiments, our genuine admiration for having the
sincerity and guts to put his professed beliefs on the line.
References
[1] S.P. Lipshitz, "Views", "HFN/RR", Aug. 1984, p.15
[2] S.P. Lipshitz, "Views", "HFN/RR", Sept. 1983, p.19
[3] I.S. Tiefenbrun, "Views", "HFN/RR", Jan. 1984, pp.19,21.
[4] I.S. Tiefenbrun, "Views", "HFN/RR", June 1984, pp.13,15.
[5] A. Orlowski, "Digital Sound: The View from Scotland", "HFN/RR", June
1983, pp.34-37.
[6] S.P. Lipshitz and J. Vanderkooy, "The Great Debate: Subjective
Evaluation", "J. Audio Eng. Soc.", Vol 29, pp.482-491 (1981 July/Aug.).
[7] D. Clark, "High-Resolution Subjective Testing Using a Double-Blind
Comparator", "J. Audio Eng. Soc.", Vol 30, pp.330-338 (1982 May).
Ino i32s*(med ES filter), pY-4, cr80s, Nad 208, Rotel 1090, Pioneer 668, Denon 2106.
