At 12:18 PM 4/6/02 -0500, you wrote:
>This is exactly the sort of problem I was describing. I'll have to see
>what happens with the UA 30. I know it gives nothing that's audible, but
>the difference test is much more sensitive.
>
>This area is one of the problems of digital audio. Unlike the popular
>belief digital audio does not guarantee that your signal will be
>unchanged. I happen to believe that we got rid of more problems (that
>were in analog recording methods) than we gained from digital. Making
>digital worth it.
>
>Walt
Excellent points, Walt, and can certainly be argued either way. The one
thing that digital has always screwed up royally is arrival times!
compare this crude signal:
- - - - - - /= = = =\_______
sampled at these points:
| | | | | | |
and then "restored to analog" as:
_ _ _ _
- - - - - - =/ \ - - -______
the point here, is the arriving times are shifted ahead and back at random,
as it all depends on the exact sampling time, and not on the actual sound
arrival times.
Much of the animal acoustic research since the 50's has repeatedly
discovered that arrival times meant everything to many biological
analytical systems. Especially, bats, whose published nerve analyses of
echoes is repleat with references to sloped-loudness-form-matching. But
that is another story (D.Griffin, N. Suga).
In 1965, Dwight Wayne Batteau at Tufts showed to my satisfaction (on me)
that the human ear, although limited to upper sine wave detection around 20
kHz (ah, to be young again) actually analyzes sharp pulses using much
shorter arrival times than the 20 - 20k range would ever predict (down to
25 microseconds).
Specifically, he used a human head model, with pinnae molded to exactly fit
a single person. He then recorded, using near-video quality techniques,
from two tiny eXpenSive mics embedded inside the artificial head. Using
rich sound sources, such a jingling a ring of keys, he moved the sound
source toward and away from, and all around, up and down, the head
location, while recording and keeping a video record of his actual planar
coordinates of the key locations in time. If you think your recording
system is better than mine, record some jingling keys and listen to the
result. Don't worry, they all sound that bad!
The person would later listen on headphones in another room, while the
recordings were played back, binaurally. Not only could the person point
nearly exactly to the direction in which the key jingling had come, they
could pinpoint the distance as well, with some accuracy. This, as far as I
know, has never been explained to my satisfaction by hearing theory, before
or since.
Wayne told me (now here comes the interesting part): He could impose or
bypass line filters on the headphone output circuit.
If people "can only hear to 20 kHz" (the PREMISE that all later digital
sound HiFi sampling is based on) then he should do fine imposing a sharp
filter in the headphone line of 25 kHz. Right? NO. NOT. The position
sensing went away, as I remember his explaining it, when the line filter
came below about 80 kHz, and was very, very crude when he got into the 20
kHz range.
He also manipulated the pinnae shape, (it didn't hurt, it was only a rubber
model) and it changed the recordings he made adequately to also ruin much
of the subject's position sensing. He interpreted this as meaning: the
very short reflections on the major and minor features of each person's
pinnae contribute echo arrival information that allows near 3 dimensional
direction decoding.
But, alas, all this is gone, even at the famous new 24 bit 96 kHz
"standard". Why? because the filters that are imposed after the digital
signal is turned back into voltage and before it is turned into sound
pressure, can at best represent a sample every 11 microsec. The echoes
Batteau was talking about, created by pinnal features with dimensions of 2
to 8 mm, create sound echo delays on the order of 5 to 10
microseconds*. At 96 kHz sampling rate, these are all lost in the filter
ringing.
But, luckily for me, I can no longer hear any of this. Good luck youngsters!
my very best,
Marty Michener
MIST Software Associates
75 Hannah Drive, Hollis, NH 03049
coming soon : EnjoyBirds bird identification software.
* these calculations all come out of the fact that a wave of 30kHz, going
about 1000 ft per second in air, will have a wavelength of 1 cm, and an
inter-peak interval of about 33 microseconds. So 5 mm travel or bounce
distance, becomes an interval of 16 microseconds, etc. These are worst
case calculations, the analytic system having to do much better than this,
about 1/10 of a cycle, to come up with beam-forming information
analysis. But, after all, that it has been doing for millions of years,
before engineers came along to measure and mis-understand it all.
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