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ARSCLIST  January 2008

ARSCLIST January 2008

Subject:

ABX Testing and perceptual coding tests...

From:

"Richard L. Hess" <[log in to unmask]>

Reply-To:

Association for Recorded Sound Discussion List <[log in to unmask]>

Date:

Mon, 28 Jan 2008 15:02:00 -0500

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text/plain

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Hello,

I have a few comments on the recent thread about ABX testing and 
perceptual coding.

I am not a statistician, but had some assistance from a statistician 
in performing the tests...and this was also several years ago so I 
may have forgotten the details.

This was a video ABX test, not an audio one, but it was designed 
after many of the audio tests that were done with some enhancements. 
I am under NDA about the client/project.

Basically, we obtained excellent quality original, uncompressed (D5) 
video clips for the tests. These clips were run through the 
equipment/system suspected of the degrading. We had multiple clips. 
The same clips were randomized and were run a number of times before 
moving on to the next clip. We had a box with a switcher A/B/X and 
two vote buttons: X is A, X is B.

We ran up to about 15 tries (votes) on each clip, although there were 
some statistical "tricks" that could shorten the test on each clip 
(like 5 out of 6 correct or something like that). The clips would 
keep looping and the operator had control over the ABX switch. The 
loops were short (10-30 seconds). After each vote, X was randomized 
(only the computer knew). I forget if A was always the unprocessed or 
if that were randomized, too.

We had two systems so we had two sets of processed clips. The clips 
were played back from an uncompressed DOREMI Labs digital server or 
two (as I said it was a while ago and the details are becoming a blur).

We did show that one system was more visible than the other.

We also showed that some people who said it "was easy" ended up 
getting about 50% (chance) right.

We found that "golden eyes" didn't necessarily do better than clerks, 
although most of the people who did see the differences were people 
with significant video shading experience.

One comment I received was "I'd never give the viewers the ABX box, 
it's too critical--it allows too much training".

Since I wasn't a statistician, I was booed out of a room 
(essentially) when trying to describe the technique to another group 
that was considering some tests when I couldn't answer their 
statistical questions. My clients and I were confident that the 
statistical basis of the tests were sound and if any testing would 
have caught perceptual artifacts, this would have.

Our test sessions were long, and the test subject was given a break 
and food during their session and I think some were allowed to come 
back another day to finish if they wished.

The test sessions were one at a time for as long as it took. I think 
we saw some fatigue factors, but since we were doing multiple tests 
on multiple clips of the same system (and I'm pretty sure we 
randmoized which system we started with but kept the same system 
together so the training would not be lost.

============

I think I've told this story before, but I was presenting to an 
archivist's workshop and someone asked about the quality of MP3s, so 
I took out my Palm and played the MP3 version of a song I had in the 
original quality demo. It sounded awful. I said I think there was 
something wrong with the test.

I then took the song in MP3 and 44.1/16 WAV format and inter-cut it 
on line boundaries of the song so one line would be the MP3 and the 
next line would be the WAV. For future workshops, I've always played 
this cut-up version rather than the different players. I had the 
organizer of the first workshop send an email to all the participants 
that the demo was really about the poor analog circuitry in the Palm 
rather than the MP3 process.

While the MP3/WAV differences (I think it was a 256 or 320 kb/s MP3) 
are slightly noticeable by some people some of the time on some 
systems, they are very, very close. Oddly, I hear a lack of 
distinction in the bass as well as a loss of detail in the highs in 
the MP3 version, but it's very subtle.

Cheers,

Richard

Richard L. Hess                   email: [log in to unmask]
Aurora, Ontario, Canada       (905) 713 6733     1-877-TAPE-FIX
Detailed contact information: http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes. 

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