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轉載兩大段有關speaker cable野洋文大家討論下, 呢兩個題目我奇怪左好耐,因為我之前覺得無科學根據, 而家搵到解釋啦~, 大家睇睇認吾認同:
1. Bi-wire 有咩好?
*What good does bi-wiring do?
Some say that any improvement in the sound it makes is strictly due to the decreased total DCR, and this makes the speaker less prone to frequency response variations due to cable resistance. According to this view,simply running the two cables in parallel at both ends will do the samething. In my opinion, this is a very simplistic and incomplete way of looking at the situation.
Once the crossovers have been electrically separated, they present different impedances (loads) tothe power amp within their passbands and outside of their pass bands.The woofer and corresponding low frequency crossover section willpresent a low impedance at low frequencies and a high impedance at HF,while the tweeter section will present a low impedance at high frequencies, and high one at LF's.
With the electrical separation, differing currents will flow within the two cables that make up a bi-wire set. For the separate cable feeding the woofer section, a lot of current will flow at LF's but not much current atHF's, and the tweeter cable will have some current flow at HF, but very little at LF's. A division of labor has occured with bi-wiring, whereby a single cable does not have have to carry the HF currents simultaneuously with the LF current.
Two things happen due to this:
1.The losses in the cable due to "eye-squared-are" losses (currentsquared time the resistance equals voltage drop) are reduced for each frequency band, so that any tendency for the woofer to modulate thetweeter due to current draw is greatly reduced. This form of IM would be in lock-step with the original signal.
2. The magnetic fields due to the HF and LF currents have also been separated out, and any tendency for them to intermodulate and cause sonic artifacts has been greatly reduced. This form of IM would be occuring both at the same time, and in a time delayed form due to mechanical resonances and motor/generator action.
*Can I use the A and B outputs on my receiver to bi-wire?
While this would be very convenient, it can only be done if the outputs are placed in parallel, rather than series. Many receivers and integrated amps that have two speaker outputs will connect them in series when they are both selected at the same time, to protect the amplifier froman excessively low impedance. Check your owners manual to see how the A and B output are wired when they are both selected. A simple test would be to select them both with a speaker only hooked up normally to one ofthem, if the sound goes dead, then they are connected in series, and you will need to connect both of the bi-wire cables to one pair of output terminals.
*Should both cables be the same kind?
This is not absolutely necessary, although it can be argued that certain sensitive speakers need the same exact cable in order to minimize any discontinuities inthe midrange (crossover region). However, as long as both cables are of a high quality, this is much less of a problem than it might seem. An extreme example would be to use heavy gauge zip cord for the woofer,and an exotic high performance cable for the highs. Some audiophiles have used this approach to great effect, and others found the zip cordon the woofer holding them back. It never hurts to try, and see how well a particular pairing will work for you.
*Should both cables be the same length?
This has already been answered above, but many people seem to feel that bi-wiring is even more critical, so the warnings in the answer above about matching length for electrical reasons are perhaps even morerelevant.
*Are the bi-wire cables that are all in one jacket as good as separate cables?
Many cable manufacturer's offer an all-in-one bi-wire cable, primarily for convenience and a tidier wiring arrangement. If you read the explanation of how bi-wiring works, then you would realize that placing the cable's magnetic fields in close proximity within the same cable jacket may compromise the bi-wiring advantage to some degree. Most experienced audiophiles agree, totally physically separated bi-wire cables tend to sound better than those in a common jacket. The degree of compromise is going to depend on the spacing between the bi-wire sections within the common jacket, and how much magnetic coupling and motor/generator resonances are going on inside the common cable jacket.
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2. digital 野點解仲要用靚線?
*How can digital interconnect cables from a CD transport to anoutboard DAC possibly affect the sound? Isn't digital audio just 1sand 0s?
A very common misconception about digital signal
transmission with respect to audio is that if the signal does
not get corrupted to the point of losing or changing the 1's
and 0's, that nothing else can go wrong. If the transmission
system had been designed with cost no object, and by
engineers familiar with all the known foibles and problems
of digital transmission of audio signals, then this might
be subtantially true. No differences could rear their ugly
head.
Unfortunately, the systems we ended up with DO NOT remain
unaffected by such things as jitter, where the transistion
from a 1 to a 0 is modulated with respect to time. There are
many ways that jitter can affect the final digtial to analog
conversion at the DAC. Jitter on the transmitted signal can
bleed or feed through the input reciever, and affect the DAC.
How? Current drain on the power supplies due to the changing
signal content and the varying demands made on the power
supply to the logic chips and the DAC. Modulate the power
supply rails, and the DAC will convert at slightly different
times.This is due to the fact that a logical one or zero is detected by thesignal swinging through a regoin from around zero volts to around 5volts. The digital logic chips detect the change at a specificPERCENTAGE of that total voltage swing.
HOWEVER the power supply gets modulated, it will
affect the DAC. One version of this has been popularly
refered to as LIM or Logic Induced Modulation by the
audiophile press. See:
"Time Distortions Within Digital Audio Equipment Due to Integrated Circuit Logic Induced Modulation Products"
AES Preprint Number: 3105 Convention: 91 1991-10
Authors: Edmund Meitner & Robert Gendron
Many of the logic chips in a digital audio system behave
very poorly with respect to dumping garbage onto the rails
and even worse, onto the ground reference point. Even as I
post, logic manufacturers such as TI are advertising the
benefits of their latest generation of logic chips that
reduce ground bounce. The circuitry itself generates it's
own interference, and this can be modulated by almost
anything that also affects the power supply or ground.
The amount of jitter that it takes to affect the analog
output of the signal used to be thought of as fairly high,
somewhere on the order of 1,000 to 500 pS worth. Now, the
engineers on the cutting edge claim that in order for jitter
to be inaudible and not affect the sound of the signal, it
may have to be as low as 10 to 20 pS. That's for 16 bit
digital audio. That's a very tiny amount of jitter, and
easily below what most all current equipment is capable of.
Computer systems never convert the 1's and 0's to time
sensitive analog data, they only need to recover the 1's
or 0's, any timing accuracy only has to preserve the bits,
not how accurately they arrive or are delivered. So in this
regard, computer systems ARE completely different than
digital audio systems.
Look into digital audio more thouroughly, and realize that
the implementations are not perfect or ideal, and are
sensitive to outside influences. Just because they could
have been and should have been done better or more nearly
perfect does not mean they were! People are not hearing
things, they are experiencing the result of products designed
to a cost point that perform the way they do in a real
world because of design limitations imposed by the consumer
market price conciousness all the mid-fi companies live and
die by.
With digital cables, there are three things that are paramount:
proper impedance, proper cable termination, and wide bandwidth.
It may be that a particular cable more nearly matches a systems
actual impedance. The other factor, proper termination includes,
but is not limited to, the actual electrical termination inside
the components, as well as the connector on the end of the
cable. If the connector is NOT a perfect 75 ohm, 110 ohm, or
whatever, it will cause minor reflections in the cable, which
makes our old friend JITTER raise it's ugly head again.
The third factor, bandwidth, is only an issue because both the
AES/EBU and the SP/DIF interface formats were designed before
Sony/Phillips knew all there was to know about digital problems,
and they require PERFECT unlimited bandwidth cables in order for
the transimission systems to be free of jitter. The more you limit
the bandwidth, the more jitter. This is a known engineering
fact, and an AES paper was given about this very subject not
too long ago.
"Is the AES/EBU/SPDIF Digital Audio Interface Flawed?"
AES Preprint Number:3360 Convention: 93 1992-10,
Authors: Chris Dunn & Malcolm O. J. Hawksford
The effective data rate of SP/DIF is about 3 Mhz, and the
design of the transmitters and recievers is abysmal. Maybe
if everything else was done right, then cables, etc. wouldn't
matter. So much was done wrong or cost cut till it screwed
up that they do come into the picture. |
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發表於 2012-4-23 11:07
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