AES conference London: High Resolution perception, paper about listening test Options

[Kees de Visser]

Unfortunately I wasn't able to visit the june 2007 AES Conference in London about High Resolution Audio.
The paper/presentation about a high-res audio listening test seems interesting. I'm wondering if anyone on HA happens to have been there and can share some information.

preview of the paper session:

Monday, June 25 11:00 – 12:30
Paper Session 2 — Perception

2-1 Which of the Two Digital Audio Systems Meets Best with the Analog System?— Wieslaw Woszczyk,1 Jan Engel,2 John Usher,1 Ronald Aarts,3 Derk Reefman3
1McGill University, Montreal, Quebec, Canada
2Centre for Quantitative Methods CQM BV
3Philips Research, Eindhoven, The Netherlands

In this listening test, two digital audio systems (B and C), and one analog system (A) were tested by 10 test persons who listened to a surround sound scene “live” (without recording). The main question to be answered was: “Which of the two digital systems meets best with the analog system?” Both digital versions had 24-bit dynamic resolution but differed in sampling rate with which the analog signal was sampled. One version © was sampled with a CD rate of 44.1 kHz, the other (B) 8 times faster. There were also two test conditions, where in one condition there was a bandwidth cut off at 20 kHz instead of the 100 kHz that was possible with special 100 kHz microphones and added super-tweeters. For each subject, the experiment was replicated six times, in each of the two conditions. The outcome of each experiment was a 0 or 1, where the 1 means that the, technically best, digital system B has been chosen as meeting the analog quality. The paper describes the test and the outcome.

Without having read the paper, it's not clear to me whether the test was double-blind or not. Apparently it was not possible to replay sources, since the audio source was "live". How reliable would a test like this be ?

[cabbagerat]

This paper is certainly worth reading for anybody interested in the subject.

There were also two test conditions, where in one condition there was a bandwidth cut off at 20 kHz instead of the 100 kHz that was possible with special 100kHz microphones and added super-tweeters.

This is extremely interesting. The authors write about tests done at NHK to identify the audibility of ultrasonics (above 21kHz), and seem to have chosen to concentrate only on the low frequencies:

(1) Is there a possible benefit of high-resolution audio at lower frequencies without the necessity for the reproduction of supersonic components?

Their second goal is even more interesting:

Is a noticeable benefit of high-resolution revealed in surround sound listening?

The results are extremely interesting. Some extracts:

This means that at the 100 kHz bandwidth the low sampling system Y is more often than X judged to be most like the analog system A.

This means that with the cut-off at 20 kHz bandwidth the high sampling system X is more often than Y judged to be most like the analog system A.

In the conclusions, they hypothesize that the wide bandwidth system sounds less transparent because of what they call "noise-like artifacts" in the high frequencies (above 20kHz), which are not passed in the lower sampling rate version of the wide bandwidth system. I am not sure how they support this hypothesis, given that the analog reference system described also passed the ultrasonics.

They also claim that "Listeners judge high sampling conversion as sounding more like the analog reference when listening to standard audio bandwidth." If this effect can be reproduced in other papers using similar equipment, it suggests that the practical limitations in sampled data systems need to be addressed through oversampling (that is, sampling significantly above the Nyquist limit).

This post has been edited by cabbagerat: Dec 11 2007, 10:32

[Kees de Visser]

In the conclusions, they hypothesize that the wide bandwidth system sounds less transparent because of what they call "noise-like artifacts" in the high frequencies (above 20kHz), which are not passed in the lower sampling rate version of the wide bandwidth system. I am not sure how they support this hypothesis, given that the analog reference system described also passed the ultrasonics.

It's not very clear in the paper, but IMO it's possible that ultrasonic artifacts are introduced, added and/or modified in the AD/DA process.
It's also important to know that only two sample rates were used: 44.1 and 352.8 kHz, derived from a 128xFs 5 bit delta-sigma modulator. It would have been interesting to know if popular standards like 96 and 192 kHz would behave differently. I can imagine however that time and budget were limited.
Note also that only one brand of AD/DA converter has been used (Digital Audio Denmark). AFAIK it hasn't been tested if results remain identical with other brands.

Kees de Visser

This post has been edited by Kees de Visser: Dec 10 2007, 11:38