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A DV(L)-FAQ [e]

DVL-Digest 1373 - Postings:
Index


AUDIO FUNDAMENTALS (was: Speakers for use with FCP)
What DV Cams Are You Using?


AUDIO FUNDAMENTALS (was: Speakers for use with FCP) - "Don Mitchell"


From: bounce-dv-l-5963@dvcentral.org
[mailto:bounce-dv-l-5963@dvcentral.org]On Behalf
Of Danny Grizzle
Sent: Friday, December 20, 2002 2:03 PM
To: DV-L
Subject: [dv-l] AUDIO FUNDAMENTALS (was:
Speakers for use with FCP)
On 12/19/02 12:04 PM, "Don Mitchell"
[runtime@wzrd.com> wrote:
Can you run through the difference between
> monitoring speakers such as the ones you
mention
> and, say, good quality stereo speakers?
The idea behind good audio monitoring is
loudspeakers and listening
conditions which reveal the recorded signal in a
precise way. You are
looking for reproduction accuracy so you can
"see" what you've got. This is
no different than avoiding an inferior display
when evaluating video...
nobody here would consider using a handheld
Casio LCD TV to evaluate
3-chip DV footage, would they?
Good quality stereo speakers might be useful. 20
years ago, there was more
clarity in audio, consumer vs. HiFi. Today,
thanks to CDs, consumer audio is
vastly improved, while HiFi has all but
disappeared. I would characterize
mass-market audio as more or less uniformly
mediocre. The public's rush to
digital CD audio was about convenience, not
quality.
I'm not familiar with the speaker you named, and
don't want to get into a
debate about the merits of various real
loudspeakers, which is entirely
subjective. My only intention is to badmouth low
fidelity computer speakers,
completely inappropriate for critical decision
making.
A quality loudspeaker's job is to reproduce
sound accurately and with no
coloration. The measurement of success is
frequency response. (Human hearing
is from 20 cycles to 20,000 cycles per second.)
The difference between good
speakers and bad speakers is how "flat" the
response curve is -- less
variation is good, because it shows the speaker
is not adding to or taking
away from the sound signal.
HUMAN HEARING
There are several important things to know about
how your ears work.
All things being equal, when comparing two
speaker systems, most people will
believe that louder of the two sounds better. In
fact, the louder speaker
may deliver far less performance. Bottom line:
when evaluating speakers, it
is important to compare at the same volume
level, even if this requires
separate amplifiers and settings.
All speakers should be evaluated "flat", with no
tone controls and no
loudness setting engaged.
In recent years, electronics designers have been
researching and exploiting
the "psychoacoustics" of human hearing. This
goes beyond straight
performance measurements and gets into
perception. This research is closely
related to video compression, both involving
issues of how to eliminate data
without detection. In the case of audio, the
technology of psychoacoustics
is used to either add or subtract. For instance,
MP3 files - compressed
audio for the Internet - are based on lossy data
compression, making files
smaller with little perceived loss in quality.
Principles of psychoacoustics
can also be used to add sound perception, such
as a video producer who wants
to mix audio so that a cheap 2-inch TV speaker
is perceived to make low bass
notes that the laws of physics will not allow
such a device to make, in
reality. Thus, psychoacoutics are useful going &
coming for people who know
what they are doing, but they have no place in a
critical listening
environment. Avoid trick consumer-type stereos
with built-in tricks.
Tone controls. Most people never exposed to HiFi
audio are surprised to
learn that, at the high end, audiophile
equipment provides less controls,
not more. The theory goes like this: anybody
committed to good audio will
own superb speakers that do not color the sound.
Tone controls -- bass and
treble -- are there only to compensate for
inadequate performance of an
inferior loudspeaker or a poor listening room.
Good sound systems do not
have tone controls or equalizers because these
indicate a deficiency in
another part of the system that should be
corrected.
Masking Effect. Means this: when more than one
sound is present, the human
ear will tend to hear the loudest. You can hear
something whispered into
your ear normally, but not standing next to a
jet engine.
SOUND PRODUCTION VS. REPRODUCTION
In the sound world, production & reproduction
are two completely different
animals. For an audiophile, the objective is to
realize the sound created by
an artist, reproduced in the listening room as
if the artist were present.
Thus, no tone controls, etc., because use of
these would be to introduce
changes in the sound, thus not respect the
artist's wishes for how the
performance was to be rendered.
Sound production, on the other hand, has no
rules. It is not important if a
guitar amplifier loudspeaker is flat or not, so
long as it produces a sound
that pleases the artist. Extreme example: Jimi
Hendrix deliberately used
amplifier feedback and distortion, overdriving
electronics, etc. Cool --
these became part of the instrument he was
playing, and thus the job of the
audiophile's megadollar sound system is to
reproduce Hendrix's distortion
and feedback perfectly, exactly how he played
it.
Fidelity is not a primary consideration for
sound reinforcement, and public
address (PA) type equipment should never be used
for critical listening. In
fact, speaker designs for sound reinforcement
frequently "creatively
degrade" fidelity for specific purposes. For
instance, intelligibility of
spoken words is not enhanced by full fidelity.
That's why both telephones &
AM radio have a particular sound quality, to
enhance intelligible speech,
and to conserve bandwidth (telephones only).
Similar design optimizations
are made in churches and large halls. This may
be hard to discern since
there are so many ignorant amateur
implementations, but top professional
installations are tuned, and not necessarily for
perfect accuracy, but
towards a specific set of objectives in a
particular venue.
BASIC LOUDSPEAKER DESIGN
There are different types of speaker design. Two
primary approaches are
acoustic suspension and bass reflex. With
acoustic suspension, speaker
drivers are enclosed by a sealed box. Bass
Reflex has a ported box, with an
opening allowing air to move to & from the
inside of the speaker enclosure.
Acoustic suspension speakers are easier to
design for accuracy, but require
a lot of amplifier power. Bass reflex are louder
with smaller amplifiers,
but much more difficult to design for flat
frequency response.
There are many other loudspeaker designs, some
more exotic than others. None
of which you are too likely to see. Horns are
sometimes used for tweeter
elements, but become too big at low frequencies.
Interestingly, Bose
Acoustic Wave, if you ask me, holds a lot in
common with Klipsch folded horn
designs. True horn designs are excellent, but
not commercially feasible to
package and ship since they require a speaker
something like 20 or 30 feet
long. There are a lot of exotic speaker
technologies on the market, too much
to get into here.
One design variation that you *are* likely to
see anymore is various
cross-over networks and subwoofers. JBL was the
first to popularize this
design in a pricy HiFi speaker called the L212
(if I remember correctly).
Basically, an electronic crossover network
strips low frequencies from the
signal sent to stereo speakers (or a surround
system), and routes low
frequencies to a dedicated sub-woofer. Bass
sounds are not directional, so 1
speaker is plenty, no need for stereo imaging.
Mackie is using sub-woofers
with their studio monitors. I like (but can't
afford) Bob Carver's Sunfire
cubes, a really innovative design.
PHASE CANCELLATION
Sound is a vibration in the air, molecules being
jostled about, to and fro,
push and pull. The "push" portion of the
vibration is a miniscule amount of
compression, like a microscopic high pressure
front. The "pull" portion of
the vibration is called rarefaction, like a
microscopic low pressure front.
If push and pull were to occur at the same point
in time, they would net out
to zero, no different than summing +1 and -1.
The major problems with
speaker installation -- listening room and
speaker position -- is due to the
fact that reflecting surfaces such as walls make
sound waves bounce and
rebound. Once sound is arriving at any given
spot via two different routes
of different lengths, it is inevitable that
phase cancellation begins to
occur. Factor into this that various sound
frequencies have various
wavelengths, and you will see that phase
cancellation is not uniform, with
dead silence at particular listening positions,
but rather frequency
dependent.
Nearfield monitoring (below) addresses this
problem in a direct manner based
on the human ear's masking effect. Problem is,
nearfield monitoring is
highly tuned for a specific listening sweetspot,
a single human head. Anyone
else, see acoustic treatments, below.
NEARFIELD MONITORING
The terms "active" and "nearfield" monitors are
often used in the same
breath, but they are not related.
Active loudspeakers contain built-in power
amplifiers, and have become very
popular in recent years. They are not superior
to non-amplified loudspeakers
in any way, other than perhaps the convenience
factor of requiring less
wiring and fewer devices to clutter your desktop
or equipment rack.
Nearfield is a design strategy in how you
position loudspeakers in your
listening room. It relies on a basic fact of
physics (hope I get it right):
at twice the distance from a sound source,
volume will be 1/4th as loud. By
locating monitors very close to your listening
position, away from walls &
hard surfaces, sound enters directly into your
ears. Reflected sound, the
reverberation present in all rooms, is vastly
diminished as a factor in
coloration of what you hear, because distance
from reflecting surfaces is a
much greater distance, resulting in much more
volume fall-off, with the
consequence that your ear's masking effect no
longer hears the lower volume
reverberant sound.
In essence, nearfield monitoring splits the
difference between headphones
feeding sound direct into your ear, and
conventional loudspeaker placement
where the reverberant qualities of the room
combined with specific speaker
position introduce excessive coloration
(frequency dependant phase
cancellation) into the audio.
The thinking behind most active monitors such as
the Mackie HR824 and
Behringer Truth is that they will be used in a
nearfield application.
Absolute volume levels are not critical, but
location in relation to your
listening position is.
ACOUSTIC TREATMENTS
We've all seen photos of acoustic foam and other
sound treatments in
recording studios and listening rooms. How do
these work?
See various Auralex websites for details:
www.auralex.com/>
Typical problem for video producers: nearfield
works great for critical
listening, but not as well for playback to
groups of people -- like clients,
for instance. Either that, or the video producer
simply does not have the
space to position speakers away from walls
appropriate for nearfield
monitoring. Situations like these are where
acoustic treatments can benefit
a studio.
Here's a short, simple, & practical way to
determine basic acoustic
treatment of a given room. It will be perfectly
clear to anyone who has ever
banked a shoot on a pool table.
With your loudspeakers in final position and
also your listening positions
defined, take a mirror and hold it flat to the
wall. Anyplace on the wall
that you can see the speakers in the mirror from
a listening position needs
to have acoustic treatment applied to that
portion of the wall. This deadens
direct reflective sound from reaching your ears
along two paths, one direct
one reflected, producing inevitable phase
cancellation.
In addition to wall treatments, you will
probably also want to treat corners
(vertical -- wall-to-wall -- & horizontal
wall-to-ceiling). This traps bass
frequencies. Remember, bass is non-directional,
so line of sight is not a
factor at low frequencies.
===========================Read Jay Rose book for details and insight on
audio, not just studio, but
production issues, too. BTW - Jay Rose had a
recent column thoroughly
trashing the idea of using computer monitors
such as the clear plastic stick
speakers for video production, although he
admired them for industrial
design and general utility. Some people expouse
the theory that you should
do everything on a "typical end user TV".
Besides the fact that there is no
such animal, I think this is generally a sloppy
compromise that tends to
make a fool out of the producer. The Clint
Eastwood line, "A man has got to
know his limitations" is more important for
producers than it is for the
general populations. It is inexcusable for
anyone claming to do
professional-level video work to be clueless
about audio, yet we see this
constantly.
Sorry so long, and errors. I'm out of time, so
no editing or fact checking.
Danny Grizzle
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What DV Cams Are You Using? - "Perry Mitchell"


From: Bill P
Here we go again. Have you actually tried a 250 in the
electronic 16:9 mode, or are you just repeating what
you have read or seen from compressed still pics? I
find a very small dropoff in quality, with the footage
still very acceptable. It's better in 4:3, but the
16:9 degradation is so small that most people probably
wouldn't notice it.
--- Enrico Quaglia
>
> Remember the DSR250 doesn't support true widescreen
> like the DSR500. What
> you're getting is cropped 4:3 with an unacceptable
> compromise in terms of
> vertical resolution loss. Don't use it unless you
> really need to....




(diese posts stammen von der DV-L Mailingliste - THX to Adam Wilt and Perry Mitchell :-)


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