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Q How does a speaker produce harmonic distortion?
A It
is a result of mass and velocity (inertia). 
Remember
that mass is different than weight. A speaker cone in space might
be weightless, but it would still have mass. When the electrical
signal (music) passes through the voice coil of the speaker, an
electromagnet is created, and the force of the magnetic field
in the voice coil repelled or attracted to the magnetic field
of the permanent magnet causes the cone to move forward or backward.
Initially, the movement lags behind the signal because it begins
from a stopped condition, but once the cone is moving back and
forth with a constant signal (never really the case with music,
but it would be so for a sinewave), the cone is moving back towards
the resting position by itself rather than being pulled from the
signal alone because the cone's mounting is elastic.
Within a certain range, the movement of the cone matches the signal to a satisfactory degree, but once we turn the volume up really high, the cone reaches its limit of travel and mass with velocity causes it to continue travelling beyond what the electrical signal would dictate. The diagram on the left shows this, and the red waveform illustrates that the physical movement of the cone in these extreme conditions is like a square wave, and that translates to harmonic distortion, i.e, harmonics, or multiples, of the basic waveform.
Even at low volume, mass and velocity result
in harmonic distortion, because all cones have mass. This is why
cones are designed to be strong, but low in mass. At any one frequency,
the velocity increases as the volume increases, also increasing
the inertia, so with increasing volume, the harmonic distortion
increases too.
Q Last
night I believe I heard the best reproduced music I have ever
experienced. I played the Chesky Super Audio DVD (96 kHz/24 bit)
on my system consisting of a Marantz DVD-810 player, Proceed AVP
preamp/processor, ATI 5 channel power amp, and Cambridge Soundworks
Tower II speakers. The sound was extraordinarily clear, rich,
and involving. The question I have is that although the AVP is
capable of processing a 96 kHz/24 bit audio bitstream, I have
read that DVD players downsample 96 kHz to 48 kHz before outputting
the bitstream. Is this right? And does it matter considering the
frequency response of speakers and human hearing? I have also
heard that the next generation DVD player (4th generation?) will
not have 96 kHz/24 bit DAC because of the new DVD audio standard.
What's the story here?
A Even
the early DVD players could handle the 96/24 discs, but they truncated
the least significant bits (LSBs) to 96/20. The latest players
apparently process all 24 bits, although how accurately, we won't
know until detailed testing is done. Because of copyright issues
(again), most of the current players will decode 96/24 inside
the player and output two-channel analog audio to your preamplifier,
but if you use the digital output of the DVD player, it has been
downconverted to 48 kHz. There may be a couple of players that
will output 96 kHz bitstreams, but I don't know which ones. Of
course, you would need a 96/24 DAC to decode this bitstream. Most
of the literature you see on 96/24 implies that the benefit of
96 kHz is an increased frequency response to 40 kHz. Although
the frequency response does increase, it is really the 10 kHz
to 20 kHz region that benefits from the increased sampling rate.
Even though Nyquist's formula predicts that it requires only two
samples to recreate a waveform (44.1 kHz provides two samples
at 20 kHz), it just does not work out this way in practice, and
at 10 kHz, the sound starts to fall apart in spite of having four
samples. So, by going to 96 kHz, there will be nine samples at
10 kHz, and four samples at 20 kHz, giving the DAC a much better
set of information to use in recreating the waveform. I had heard
rumors of future DVD players not having digital outputs because
of fears about pirating, but so far, they are just rumors.
Q With
all the confusion around the audio "standards" for DVD,
I decided to spend my upgrade money on speakers. Dynaudio 80s
to be exact. Now that my back end is handled, I've returned to
trying to decide about the front end. I hate to invest in electronics
(preamp processors especially) that may be outdated in a year
or so. I would like to improve my sound now that I have speakers
that can benefit from higher end gear. I currently have the H/K
AVR75. I had intended to mate this with a 5-channel amp, but after
listening to what a good integrated two channel like the Classe
150 could do for my stereo listening, I'm now very torn. There
just doesn't seem to be a non-megabuck solution for an upgrade
that would improve both HT and Stereo sound. Or is there?
A There
are a couple of different possibilities. One is to use the Classe
amp for the front left/right channels and a different amp for
the center/rear, such as the Adcom 5503. That way, you get the
Classe sound for stereo, yet have good overall sound for surround,
without breaking your bank account. Another possibility is to
get an efficient, powerful five channel amp like the Sunfire Cinema
Grand. It gives 200 watts/ch, but because it uses a switching
power supply, it is not too expensive. We are using this amp to
review a set of Von Schweikert speakers, and it sounds incredibly
good.
Q I
currently own Parasound separates for my home theater. The preamp
does an outstanding job decoding Dolby Pro Logic and acting as
a straight stereo preamp, but is not Dolby Digital (DD) ready
(no set of 5.1 input jacks). I would like to add Dolby Digital
to my setup without changing my preamp. Any decoder that I buy
will need to have a good bypass mode that will not degrade the
signal path. The Marantz DP-870 is the only product that I have
located that I can connect to my non-DD setup. What is your opinion
of this product, and do you know of any similar products? Also,
if I buy a DVD player prior to buying a DD decoder, can I connect
the DVD player to my existing equipment and watch movies with
Dolby Pro Logic?
A Even
pass-through circuitry has to use jacks and switches, and as such,
they will degrade the signal. There is no way around that if you
want to use such circuits. The way I would do this is as follows:
Use a set of Y connectors to split the output of your current
audio source, such as a CD player. One set of outputs goes to
the CD input on your current preamplifier (Parasound). The other
set goes to the CD input on your new surround sound processor
with DD decoding capability. The Parasound preamplifier audio
outputs go to your current two-channel power amplifier which drive
your front left/right speakers. The center and rear left/right
analog outputs of the Parasound preamplifier go to the analog
inputs on the DD processor (for pass through), and the center
and rear left/right analog outputs go to your center and rear
left/right power amplifiers. The front left/right analog outputs
of the DD processor go to an auxiliary set of analog inputs on
your Parasound preamplifier. When you get your DVD player, connect
the digital output to the digital input on your DD processor (in
the meantime, you could connect the two-channel analog outputs
from the DVD player to your Parasound and it will give you Pro
Logic). Now, when playing CDs, you will get front left/right stereo
without having any pass through degradation. When using Pro Logic
or DD, the pass through will be limited to the center and rear
left/right.
Q My
center speaker has a prominent chestiness to it, and I'm sure
it is bass loading. My question is why is it common in center
speakers? My center speaker is an Infinity Kappa Video II and
my brother's is a B&W CC6. I have heard good reviews about
the CC6 and yet it still sounds chesty to me. I know part of the
reason is the contact/proximity to the TV. But why at 100 Hz to
150 Hz region? Does it have something to do with center speakers
having a low frequency cut-off at about 80 Hz? Is this the same
as bass reinforcement when you place main speakers or a sub close
to a wall? Or does it also have something to do with the small
size of a center speaker?
A Speaker
designers try to build their products with a flat frequency response
because they don't know where you are going to place them. Although
most of us put center channel speakers on top of the TV, not everyone
does this. The bass loading is caused by reinforcement of the
120 Hz part of the spectrum when sound comes out of the speaker,
reflects off the face of the TV screen, and comes toward the listening
position. That is the region of the audio spectrum where the waveforms
add on top of each other for the speaker - TV - listener triangle
(there are other regions too, but they don't seem to be so noticeable).
Some center channel speakers have a switch that puts a notch filter
into the circuit to reduce output in the 120 Hz region. You can
try aiming the speaker slightly upward, putting it on a shelf
above the TV, putting it underneath the TV, or using an equalizer.
Q In
an essay by John Busenitz titled "Digital Audio: What does
this term really mean?", written in February, 1998, the values
for bandwidth and signal to noise ratio are given for CDs. I was
just curious what the values are for vinyl records and cassette
tapes. In what ways do CDs provide better quality sound than vinyl,
and in what ways does vinyl outperform CD?
A The
bandwidth of CDs and other digital formats goes into the range
of radio frequencies (RF). LPs (vinyl) and cassettes are analog
and stay within the audio band (conventional analog audio cassettes,
not DAT), so the bandwidth of vinyl and cassettes is 20 Hz - 20
kHz. The S/N ratio for both LPs and cassettes is not very good
compared to digital, because LPs have lots of surface noise, and
cassettes have tape hiss. Many people feel that analog sources
sound better than CDs. This is probably due to habit as much as
anything else, but also, 44.1 kHz sampling is not as good as was
originally thought. That is why we are going to 96 kHz, and possibly
even to 192 kHz. Once we have affordable high sampling rate DAT,
and recordable DVD at high sampling rates, the LP and analog cassette
will probably just be collector's items.
Q I
actually have two LD players (one at one point broke and I needed
a spare to replace it while the other was fixed). I now have the
Pioneer CLD-D504 and the CLD-D505. Both have the same RCA output
jacks. I just bought the Yamaha DSP-A1, and DTS sort of came as
a "bonus" on it (all my LD's are obviously of the DD
variety). What confuses me is that the DTS website says that any
"digital" output should work. Yet it SEEMS as if the
MSB Technology website implies that you need optical outs to play
back DTS laserdiscs. If I can play DTS LDs, that'll be great,
but I really don't want to spend the amount that "MSB Technology"
charges to add optical outputs onto the LD player (I get the feeling
that in two years you won't be able to find Laserdiscs...they'll
all be DVDs). I'd rather save up my money and eventually upgrade
my DVD player and get one that can play DTS DVD discs. By the
way, thanks for replying and your site is THE place I go for reviews/articles
on home theater (part of the reason I ended up with the DSP-A1).
A On
laserdiscs, the DTS bitstream is recorded in place of the regular
two-channel digital audio, so as long as your laserdisc player
has a digital output jack, whether it is an RCA coax or Toslink
optical, you will have the DTS bitstream to go to your DSP-A1.
Connect the AC-3 RF output jack on the player to the AC-3 RF input
jack on the DSP-A1 for playing DD laserdiscs. So, you don't need
to add anything to your laserdisc player. The nice thing about
the DSP-A1 is that it will recognize the type of digital bitstream
coming in, so if it is a two channel PCM conventional CD stream,
the DSP-A1 will play it as such, and if it is the DTS bitstream,
the DSP-A1 will play it. All you need to do is connect the digital
output jack on your laserdisc players to digital input jacks on
the DSP-A1. There are lots of DTS laserdiscs available, and also
plenty of DTS CDs. Your laserdisc player can play all of these
with the DSP-A1, but your CD player can also play DTS CDs if you
connect the digital output jack on the CD player to one of the
digital input jacks on the DSP-A1.
Q My
question concerns the following hypothetical (but common) situation:
I have an amplifier, preamplifier, and musical source that all
offer flat response from 20 - 20,000 Hz. The amplifier is connected
to speakers that have response down to 60 Hz. If there is a 40
Hz tone in the music, will the amplifier reproduce this tone (in
full force, which could strain the amp because of the bass energy
demands) even though the speakers do not respond to the frequency?
Or, instead, will the amplifier not be strained by the 40 Hz note
because it is not "called upon" by the speakers (since
they don't respond to it)?
A The
amplifier would send the 40 Hz signal to the speakers, and the
current would pass through the crossover network and voice coil,
but the cone would not move very much air. So, the amplifier would
be using energy that is getting wasted, more or less. This is
why some people like to use an "active" crossover between
the preamplifier and power amplifier, that would reduce the signal
below, say, 40 Hz. This way, no signal goes to the power amplifier
that the speakers don't deliver enough air for an audible response.