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by E. Brad Meyer and David R. Moran
We have received
a number of requests for additional details of this series
of experiments. This supplement was written prior to the
New York AES convention (Columbus Day weekend 2007). The
information is not complete as of this date, and the document
will be expanded in the future and posted here. Copies
may also be had by emailing EbradMeyer@att.net.
We used several systems for the
experiments. The bulk of the trials were done on the first
system listed.
The Principal System
The playback equipment
in this system consisted of an Adcom GTP-450 preamp and
a Carver M1.5t power amplifier. Speaker cables were 8
feet of generic 12-gauge stranded wire; the line-level
connecting cables were garden-variety. Three different
players were used: a Pioneer DV-563A universal player,
a Sony XA777ES SACD model, and a Yamaha DVD-S1500. The
loudspeakers were a pair of Snell C5s. The CD-standard
A/D/A loop was an HHB CDR-850 professional CD recorder.
Level matching to
the required precision of <0.1 dB in each channel was
done with a custom line-level stage made by DB Systems
(P.O. Box 460, Rindge, NH 03461). This device, whose dial
markings were hand-drawn to match its particular level
controls, can be seen in Fig. 2b in the original paper.
Its distortion and noise are well below 0.01% and its
frequency response is flat within 0.02 dB 20-20kHz. As
is evident from the block diagram in the original paper,
this device was in the branch of the circuit containing
the CD-quality A/D/A.
When the subject
was listening to the high-bit audio alone, whether sighted
(with the display showing A) or blind (with the display
showing X), our test system added the following components
in series with the high-bit player’s output: (1)
an ordinary RCA connecting cable 18 inches long, and (2)
a switchbox comprising two RCA connectors, a total of
4 inches of hookup wire, and a reed relay with 0.2-ohm
dc resistance.
None of the subjects,
including the owner of the audiophile-grade System 4 described
below, felt that the addition of these components changed
the sound of the high-bit audio in any way.
Notes
As is evident from
the photograph, the speakers and listening chair were
set up toed in, to focus clean first-arrival sound at
the listening chair. This system has a wide frequency
range, good definition and detail, and a stereo image
with both specificity and depth. Pink noise measured with
temporal averaging was very flat broadband. The large
room was on the basement level of a house in a quiet suburb
of Boston. The background noise level in this room is
lower than that in most urban listening rooms – 19
dBA. This is crucially important if the point of the exercise
is to reveal inner detail and signal quality at low levels.
To maximize important
imaging properties, the speakers are placed out from the
front wall. The wall to the left of the listening area
is only about 8 feet off the center line between the speakers,
where the listening chair sits, so it was treated with
absorptive material for the duration of the tests. The
other side wall did not need treatment, as it is over
12 feet to the right of the center line, and there were
other sources of diffusion and absorption there.
Listeners in this
venue were free to set playback levels as desired, and
did so, up to and including very loud, >100 dBspl.
See the gain note later on.
Several high-bit
players were used for these tests, which made up the majority
of our trials. In the early going we used the Pioneer.
About three months in, we did a trial using the only disc
we came across with an acoustic/electronic noise floor
lower than our CD link (the Hartke disc listed in the
Sources section).. We advanced the gain beyond our nominal
setting by 20 dB and used the player’s A-B repeat
to loop through a short segment containing only room tone
and a couple of extremely quiet musical notes. The noise
of the CD loop was easily audible at the listening chair.
We corroborated this with a double-blind test, conducted
by listening only to X, without the need for comparisons
to the known sources, obtaining results of 9/9 and 6/6
correct.
The first of these
trials was done with the Pioneer player, and the fadeup
of the room tone at the beginning of the Hartke disc revealed
a slight but audible nonlinearity in its left channel
decoder. We did some tests with the Sony, which sounded
clean at any gain setting, and then switched to the Yamaha
DVD-S1500, which was used for the remainder of the tests
at this site.
System 2
We also conducted
a series of tests at a local CD/DVD mastering facility.
I do not currently have a detailed equipment list for
this venue, but the speakers were very large and capable
high-end monitors, approximately 7 feet tall, and the
power amps were sufficient to drive the speakers to very
high levels without audible distortion. Some of the source
material for these trials was a classical production which
was then in process at this establishment. Like all the
others, these trials, which were done under a promise
of anonymity made to those involved, produced no significant
correlations on music at normal levels.
System 3
Another series of
trials took place at a facility at the University of Massachusetts
- Lowell campus, using students in their recording program
as subjects. Their large monitoring room is custom-designed
and has very good acoustics, with a system to match. The
system has a center channel and surrounds, but as in the
other trials we restricted ourselves to the two-channel
versions of our sources, so only the left and right were
working. The equipment list for the two channels is:
Klark Teknik DN-410 custom-modified
2-channel parametric equalizer
Stage Accompany PPA-1200 Dig Control class AB amplifier
w/ crossover card
Stage Accompany ES-20 Class G amplifier w/crossover
card
SLS S1266 3-way monitor (two 12" dynamic drivers,
two 6" dynamic drivers, one 6" ribbon tweeter)
Bag End ELF-1 8-Hz 2-channel low-frequency integrator
Bag End D18E-I dual-18" ELF subwoofer system
This is another
professional monitoring system, installed in a large custom-built
listening room with auditorium-type seating. It was capable
of very high levels with no audible distortion as well
as imaging of a quality not usually found in large spaces
of this kind. We were interested to find that our informal
high-frequency-hearing tests, which we administered to
most of our subjects, indicated that these students had
taken unusually good care of their hearing. Most of them
had an upper limit in our test of 16 to 18 kHz.
System 4
Another set of trials
was performed during the evening at another suburban location
– a custom-built listening room with good acoustics
(with the help of an assortment of professional absorbers
and diffusers), very low background noise, and equipment
that we trusted would pass muster with most audiophiles:
Denon 2900 Universal Player with
full PartsConnexion mods
Conrad-Johnson 17 LS line stage preamp
Sim Audio Moon 7 monoblock power amplifiers
Quad ESL 989 electrostatic speakers
Muse Model 18 subwoofer, 24 dB/octave crossover @ 50
Hz
Nordost SPM interconnects and speaker cable
A few of these trials
were conducted with some of our own classical source material.
The majority were run using the owner’s preferred
recordings, which were mostly small-ensemble jazz groups
with generous amounts of high-frequency detail in the
percussion and reeds. These are listed along with the
other sources below.
We did not do any
trials at elevated gains here. The owner of this system
has moved and is building another dedicated listening
room at his new house. We plan to conduct high-gain tests
at his new facility to determine if the threshold of detection
is lower there than for our main system, though this is
likely to be dependent solely on the background levels,
which are or will be very low in both places.
A note on system gain
The overall system
gain is an essential factor in these experiments, or in
any attempts to duplicate the work. Our standard system
gain was calibrated using an octave of pink noise recorded
at -16 dBFS, which produced a wideband SPL of 85 dB at
the listening chair.
This signal is available for download
here:
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If you
click on the link it will simply open
into your default media player.
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download .mp3 files, Right Click
and select "Save
Target As..."
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File 1: 01_ABX_Level_set.mp3
( 787k )
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| This MP3
file is the level setting signal mentioned in
the paper on the audibility of a CD-quality
loop inserted into a high-bit audio stream,
by E. Brad Meyer and David Moran, published
in the September, 2007 issue of the Journal
of the Audio Engineering Society. The reference
system gain mentioned in the paper can be duplicated
by playing this signal, a one-octave band of
pink noise centered on 1 kHz, so that the level
at your listening chair is 85 dB SPL |
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One of the authors,
using a short repeated section of room tone on the Hartke
disc mentioned above, obtained a positive result (15/15)
at a gain of only 10 dB above our standard level. This
setting produced sound levels clearly higher than those
at the site, as the peak levels for this small vocal/percussion
ensemble would have been 111 dB SPL on the loudest part
of the disc. Many of our trials using classical and jazz
material took place at higher than the standard gain setting,
at the request of those subjects who wanted to listen
for more details in softer passages; none were conducted
at lower than the nominal setting.
The Hartke disc
is not, we understand, the only source that can be used
for this test, but we tried other discs at high gain without
any detectable differences emerging. The vast majority
of productions have a minimum noise level that swamps
the residual noise in the CD link, and no differences
in the quality of that noise, or of reverberant tails,
could be heard.
The two variables
that determine whether differences would emerge, in our
experience, were the source material (of which very little
is quiet enough) and the system gain. As mentioned, the
test signal we used for our reference gain is available
at www.bostonaudiosociety.org or at the author’s
email address listed above.
The music
While this list is not complete,
most of the tests were done using these discs.
Patricia Barber – Nightclub
(Mobile Fidelity UDSACD 2004)
Chesky: Various -- An Introduction
to SACD (SACD204)
Chesky: Various -- Super Audio Collection
& Professional Test Disc (CHDVD 171)
Stephen Hartke: Tituli/Cathedral
in the Thrashing Rain; Hilliard Ensemble/Crockett (ECM
New Series 1861, cat. no. 476 1155, SACD)
Bach Concertos: Perahia et al; Sony
SACD
Mozart Piano Concertos: Perahia,
Sony SACD
Kimber Kable: Purity, an Inspirational
Collection SACD T Minus 5 Vocal Band, no cat. #
Tony Overwater: Op SACD (Turtle
Records TRSA 0008)
McCoy Tyner Illuminati SACD (Telarc
63599)
Pink Floyd, Dark Side of the Moon
SACD (Capitol/EMI 82136)
Steely Dan, Gaucho, Geffen SACD
Alan Parsons, I, Robot DVD-A (Chesky
CHDD 2003)
BSO, Saint-Saens, Organ Symphony
SACD (RCA 82876-61387-2 RE1)
Carlos Heredia, Gypsy Flamenco SACD
(Chesky SACD266)
Shakespeare in Song, Phoenix Bach
Choir, Bruffy, SACD (Chandos CHSA 5031)
Livingston Taylor, Ink SACD (Chesky
SACD253)
The Persuasions, The Persuasions
Sing the Beatles, SACD (Chesky SACD244)
Steely Dan, Two Against Nature,
DVD-A (24,96) Giant Records 9 24719-9
McCoy Tyner with Stanley Clark and
Al Foster, Telarc SACD 3488
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