Audibility of a CD-Standard A/D/A Loop Inserted into
High-Resolution Audio Playback

Sources, Venues, and Equipment

 

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:

If you click on the link it will simply open into your default media player.

To download .mp3 files, Right Click and select  "Save Target As..."

File 1:  01_ABX_Level_set.mp3  ( 787k )

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

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

There are other downloadable .mp3 tracks and information here: www.bostonaudiosociety.org/media
 
 

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This page updated 11/6/07