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Positive Feedback ISSUE 67
Cables - Do We Really Need Them? How Future Audio Systems Will Eliminate Cables
One of the things I hate when I buy a new audio component is all the time, effort and expense it takes to find the right cables to get the best sound out of my new purchase. All right, I could just be content using the same old cables I used before, but I've learned from experience that doing so almost always leaves some sound inside the box. I'm enough of an audio freak that I want to get the most I can out of what I buy, within the limits of affordability. As a reviewer I am confronted with this problem all the time. Am I really hearing the review sample in the best possible light given the cables on hand? I usually try to swap cables a few times, but I only have a limited set on hand or that I can borrow.
I've been thinking about this quite a bit lately and I've concluded there is something fundamentally wrong with how we currently put our audio systems together. This, in turn led to musings about the way audio systems might look in the not-so-distant future. In the 1970's when Robert Fulton, Bruce Brisson and other pioneers first demonstrated that cables could affect, if not always improve, the sound of the audio systems of the day it led to the founding of a huge new sub-industry within the audio universe, and a very profitable one. I know more than one audiophile who has more money tied up in cables than in electronics. Companies like Monster Cable and Audioquest have managed to trickle down the fascination with cables into mid-fi areas and home theater systems at all price and quality levels.
PFO Editor-In-Chief David Robinson and others have written in these pages about system synergy. This has to do with fundamental things, like matching amplifier output power with speaker efficiency and impedance specifications, and more subtle things like mating an amplifier with a warm tonal balance to a speaker with dryer high end to yield a more neutral overall balance. This is actually quite complicated and whereas many combinations of preamps, amps, speakers, etc. sound pretty good, it is almost serendipitous when a particular group of separates really "gel". When one throws cables into this discussion it greatly adds to the complexity of getting it right.
I believe the industry took a wrong turn when this cable business first came up. Instead of establishing a new component category of "cables" (I'm lumping interconnects, speaker cables and power cables together right now) what the audio industry should have done collectively is say: "Aha! Cables change the way my electronics, source components and speakers sound. Therefore, I need to do something to eliminate this source of variability." In the electronics field outside audio it is standard practice to standardize interfaces between components so that everyone can use cables made to the same specification that have a negligible effect on operation of the system. Somehow this didn't happen in audio. The Audio Engineering Society (AES) maintains a library of around a thousand documents describing standards and measurement methods for interfaces between professional and consumer audio subsystems. Unfortunately these standards fall short of making sure the interfaces support the best possible sound. On the other hand most of the powers that be in the AES think audiophiles who hear differences between cables are nuts.
Signal cables (as distinct from power cables) affect the sound of our systems in many ways. My way of looking at it is that no such cable is really making your system sound better than it is. Cables help unlock the potential sound quality or fidelity inherent in your components. Given that you need cables to connect two components together, a "poorly" designed and built one will degrade the sound more than a "well" executed one. A poorly designed cable for one system might work quite well with a different system and degrade the sound less. It really depends on how the designers of the individual components in your system took cables into account in their designs and what assumptions they made about the cables and the electrical interfaces between components. For example, most preamp designers assume the power amp presents a moderately high impedance load (say 50 kohms or more). They would prefer this load to be purely resistive. However, all power amps don't have a purely resistive input impedance (almost none do), there is a reactive component. Moreover, that complex (i.e. combined resistive and reactive) load interacts with the electrical characteristics of some unknown length of interconnect that has its own resistance, capacitance and inductance. This is all going to muck with the sound to some degree that is almost impossible to predict, especially with so many different types of cables to deal with. Of course, there are some sound engineering techniques to minimize the effects of these interface variations but the evidence of 40+ years of audio system and cable design I think shows that the residual effects are quite audible. Maybe we would have been better off all these years if everyone used the same old pink zip cord to connect our amplifiers to our speakers. Then, with this as a standard part of the interface, amplifier and speaker designers might have been able to adapt and come up with a way to eliminate the deleterious effects of the cord. Who knows? It's too late now.
I should add a note here that many signal cables shape the tone of a system by boosting or attenuating some frequencies. This can indeed make a system sound better. But wouldn't it be better to use a tone control to do this rather than a cable? At least a tone control has an adjustable and reproducible frequency curve that can be used for many different things. It amazes me that some audiophiles will manipulate the tonality of their systems to taste by trying dozens of different cables yet hate the idea of tone controls. Of course it goes without saying that any tone controls inserted into the signal path need to be executed with subtlety and using superior components.
Let me focus on a just a few headaches brought on by cables. First, given that our various audio components are separated by some distance and connected by cables, then those cables are exposed to an external environment. There are variations in temperature, vibration and exposure to many electromagnetic fields from all the broadcast signals around us, and inadvertent fields from various electrical sources in the home like lights, motors and noisy switches. There is also static electricity arising from materials in the home and the bodies of us audiophiles and our families and pets. Most important there are the fields emanating from other cables in our audio systems. Now, much has been written about ways to minimize some of this inconvenient but unavoidable environmental stuff. Cables can be shielded to some degree of effectiveness and static charge can be bled off and minimized. We are all supposed to keep our signal cables separated from power cables and/or cross them at right angles (how many of you can really do that when all those cords are crowded behind your preamp?). Despite all these known methods it is almost impossible to keep it all out. So, what are these external perturbations doing to the sound? Nothing good.
Next, I want to touch on the effect of impedance mismatch and reflections. Most engineers would tell you that audio signal cables do not need to be matched to the source and load impedance of the electronics at either end as is done in interconnecting microwave circuits. This is considered to be true because of the low frequency of audio signals relative to their speed of propagation within the cable and due to the short lengths of cables used in typical audio systems. (This is not true for digital cables. Since the digital signals being carried today often have content at frequencies of several megahertz their characteristic impedance and terminations need to be handled more carefully. Some cable people actually pay attention to this). However, I have grown skeptical of this typical assumption based on my listening experience. I believe our ears can hear some things that are difficult to measure but can be explained by analogy to signal behavior in microwave systems. First, impedance mismatches in microwave systems cause reflections that result in signals traveling both ways in cables. The amplitude and speed of propagation of the reflections is frequency dependent and varies with the load impedance if that impedance varies (e.g. in some loudspeakers). The reflected signals interact with the forward propagating desired signal in a complex way, distorting time, phase and amplitude. Even though such reflections in audio cables are tiny at best it is possible the changes in imaging, depth and even some tonal variations noted when changing cables is due to such reflections. Certainly we have come to learn that reflections happen in digital signal cables where they produce jitter, an artifact that our ears are very sensitive to.
Further, I mentioned above that an external electromagnetic environment bombards cables lying about in our systems. This consists of frequencies far above the audio range and into the microwave range, and these signals can enter the cable and propagate. Even if the audio signal reflections are negligible, the high frequency signals that sneak their way into the cables will bounce back and forth within the cable and all this junk can interact with the audio signals producing spurious signals in the audio band. Again the effect would be small and almost impossible to measure. Further these high frequency signals that can enter our systems via the cables can leak into the electronic circuits and affect their operation in other complex ways.
Additionally, I have observed that in microwave circuits when mismatches occur the effects tend to be very sensitive to vibration. If one shakes the cables the reflections are amplified producing even stronger (and wider bandwidth) amplitude and phase modulations. I've not found any scientific study of this for audio but it is well known in microwave systems. I think if our audio systems are sensitive to vibration, and many audiophiles would agree they are, then they will be more sensitive if long cables are hanging off each component. It does little good to vibration isolate the electronics while ignoring vibration isolation for the cables.
So I've mentioned several problems caused by the mere presence of signal cables in our audio systems today. I'm not saying it's impossible to solve these problems, but it is difficult, and doing so while producing a cable that is compatible with all the diverse sources, amps and speakers out there seems like a tall order.
I've reached two conclusions from thinking about these things. The first, at the risk of alienating several friends who are in the cable business, is that the best cable is no cable at all (sorry folks, still love ya!). Well, it's going to be difficult to put together an audio system without cables if we think along the same lines we do now. This leads to my second conclusion: future audio systems need to be designed as tight, integrated systems from the source through the loudspeaker instead of as a collection of individual components, and this integrated system must minimize cables wherever possible. I hope I have not made too big of a leap for you so let me try to explain a bit more.
If our goal is to eliminate most if not all of the cables we use today then the entire audio system will have to be built into the speaker boxes, thus creating what is called an active speaker. An active speaker is a speaker with amplifiers built inside and fed by either a line level analog input or a digital input. Some companies have already been doing this for years. In professional audio and PA applications active speakers are ubiquitous. This is largely for user convenience and efficiency (lots fewer cables to deal with when one has to set up and break down a system daily or more often). Active speakers are also becoming popular in home theater applications where integrating the loudspeaker and amplifier simplifies some of the installation challenges for home theater. Virtually every subwoofer for home theater, and high-end audio for that matter, already contains a built-in amplifier. High-end audio companies such as Vandersteen and others use powered subwoofers within their speaker boxes. Probably the best known high-end company that actively powers all the drivers in their speakers is Meridian, which has been selling fully active speakers for at least 20 years, but I see more and more companies trying this out at recent audio shows.
The approach being used by Meridian and by many of the other companies working with active loudspeakers is to drive the speakers with a digital signal. Despite the opinion of many audiophiles that digital is not as good as analog, an opinion that I agree with by the way, I believe that the future path to eliminate the most damaging cables in our systems is a digital path. I'm encouraged by the recent progress in high-resolution digital audio and in particular in DSD and double DSD audio. If these high-resolution formats continue to advance then I think there is a way to get rid of the worst of our cable woes.
The figure is a notional block diagram for a type of integrated digital active speaker I am talking about. This is not the only way to do things but I think the easiest to understand. I assume a classic 3-way speaker with no particular driver technology as a model in the figure, but in fact I believe an active digital speaker architecture will lead to new speaker configurations as well. For now in the figure, each speaker driver has its own amplifier that is tailored to the power requirements, impedance characteristics and bandwidth of the driver. Integrated with each amplifier is a digital-to-analog converter (DAC) and a digital signal processor (DSP), which is likely a special purpose computer. The DSP implements the crossover at each driver in the form of a digital filter instead of an analog one. The DSP can also perform other functions such as frequency equalization (i.e. tone control) and time and phase alignment among others. It is important that the amplifier in particular be integrated as tightly as possible with the driver if not installed inside of it. This is one difference between my proposed architecture and the approach used by Meridian and others. This implies, of course, that the amplifier cannot be some big-ass tube amp (although a real small tube amp may not be out of the question). In fact I am looking forward to a new class of modular amplifiers that are very small yet pack a punch.
In the figure, music in the form of DSD digital files stream into the speaker box into a function called the Control and Timing Computer. This computer makes sure only left channel stuff goes to the left channel and only right channel stuff goes to the right. It also ensures that all of the timing between the multiple speaker drivers in the box remains rigidly synchronized (including right to left channel timing, or more channels if this gets applied to a multi-channel system). This box in the diagram also acknowledges that there will still be some interconnect in the system. I haven't figured out a way to get rid of everything (although someday maybe we will – I don't think wireless is the answer). I believe fixing the digital interconnects is easier than fixing the analog ones though some may disagree. Clearly the combination of audio and computer engineers have mucked up the process of moving digital audio files around but this problem has been solved in other fields (like sonar which has similar problems and also involves frequencies in the audio range) so I believe it can be solved in audio as well.
So, I have probably alienated at least 70% of the general audiophile population out there by advocating not only active speakers but going all digital to boot. Some will say my thesis must be wrong since the Meridian active speakers don't sound as good as numerous other systems out there using cables, even ones that are cheaper than the comparably sized Meridian designs. Well, some Meridian speakers do sound pretty decent to me but I admit I like my own system better even though it sports all sorts of big fat cables. Remember when you listen to Meridian speakers you are not just hearing the active and digital parts, but also the specific set of drivers and box design that reflect the tastes of the designer. I can't prove it, but I feel strongly that if you were to pull the electronics out of the Meridian speaker systems and then hook everything back together with the best interconnects and speaker cables money can buy, the result would sound worse than their integrated active speaker system. Meridian hasn't come up with a sound that interests a large number of audiophiles. In researching this article I ran across a blog discussion on the Absolute Sound web site where someone asked why the Meridian systems haven't caught on better. To paraphrase the answer from editor Robert Harley, it is because audiophiles like to tinker with the parts of their system in order to tailor the sound to their taste, and Meridian didn't allow for this (much).
What we need is more people trying the active approach in order to push the performance envelope higher. Because the few active designs out there are complicated, expensive and not selling all that well it discourages others from trying. I hope this article stimulates a dialog that will encourage others to try this out. However, I don't believe the state of the audio art is ready yet to come up with a world-class active speaker system, but it's getting close. Someone will figure it out soon. On the other hand I believe it is possible to produce a very good audio system today with active speakers that is more affordable than a cable-rich alternative. The savings in cables and cabinetry offsets the cost of more electronics. And I believe that more and more systems will come to market. For example, I read in recent coverage of the Munich High-End Show that Avangarde Audio in Germany, known for their high-efficiency horns, has come out with an active digital speaker much like the one I describe in this article.
What are some of the enabling technologies required for this approach to reach audio nirvana?
The most significant need may be in power amplifiers. Although it's not out of the question to install a good tube amplifier inside a loudspeaker I think that approach is unlikely to be practical over the long run. Packaging several linear class A or class AB solid-state amplifiers within the loudspeakers may be easier but the best ones are still pretty large. I believe we need small, efficient and powerful amplifiers that can be tightly integrated with loudspeaker drivers. I continue to follow the development of digital amplifiers with interest for this reason. I have reviewed a couple of class D designs for PFO and listened to several more at shows and some homes. They are not bad, not good enough but getting better. I haven't yet explored the new class of direct digital amplifiers that have received some good magazine reviews recently, but I hope to do so in the coming year (see, for example, Jim Merod's review of the NAD M2 in PFO Issue 66). Digital amplifiers are already used quite frequently in quality subwoofers or in woofer sections of some large floor-standers so the low frequency part of the architecture may already be near a solution. As for the other amps in the system the digital active loudspeaker approach offers some opportunities to make the job of improving digital amplifiers easier. For example, output power requirements are probably lower (compared to the standalone monoblock commonly used today) because the amplifier can be designed to a fixed set of input/output matching loads, elimination of cable, connector and crossover losses, and a narrower bandwidth per amplifier (assuming the loudspeaker design is a multi-way design). The digital signal processing (more about that below) and other components in front of the amplifier can also be put to work to mitigate gain and bandwidth issues for typical amplifiers. All of this is under control of the loudspeaker systems designer. I do know there are some amazing new solid-state device technologies emerging from research laboratories that may readily fit the needs for this type of audio system.
A second area of concern, that may sound less significant to you but really isn't, is packaging and integration. Installing active electronics inside the loudspeaker comes with its own set of new problems that need to be solved. Among them are thermal issues, electromagnetic interference (not only emanating from the digital electronics but also from the strong varying magnetic fields that come with some loudspeaker drivers), and vibration and its effect on electronic devices. Plus, installing the amplifiers directly on or within the speaker drivers, as I think is desirable, will require some new thinking and may result in new speaker driver configurations. There are many other basic system integration issues for this type of design that will require loudspeaker companies to take an overall systems engineering approach in order to resolve.
Next, for the digital active loudspeaker to work well I believe some new thinking will be necessary in digital crossover design. One advantage of this approach is that it opens up a vast new array of alternative filter designs that are not practical with analog components. On the other hand, I don't think there has been much research in how different digital crossover filters sound. Over the last decade or two audio designers have experimented with various filter designs for the outputs of our DACs. This provides a useful foundation of knowledge, but the application to loudspeaker crossovers is different. Once one settles on a particular filter and its parameters there are issues in how to implement that filter within a specific computing architecture. There is often more than one way to perform the math in a digital computer and there are questions over mathematical precision, processing speed, latency and so on. Many engineers would argue that, past the equation stage of the design, most of the rest of the problem is trivial, but I have respect for the tremendous sensitivity of the human ear and its ability to hear the smallest of sonic differences. This leads me to believe all these minor things might matter.
Beyond the digital crossover issue, the overall use of digital signal processing in a digital active loudspeaker has tremendous potential to manipulate the sound and to address many audio problems, but not without attention from talented audiophiles and music lovers. There are at least a couple of areas where new work is required. First, digital signal processing within a loudspeaker can be used to mitigate shortcomings in the speaker drivers themselves, such as flattening the frequency response. It can also be used to adjust time and phase alignment. But there is more than one way to do this and there is a need for designers with very good ears to experiment and find the best solutions. Further, DSP can perform additional functions, such as room correction. This is already the norm in active subwoofers and should be extended to other parts of the frequency spectrum, but there is a danger that doing so might disturb other sonic factors. Finally, the architecture shown in the figure has multiple digital signal processors for the most general case. Keeping all the processors working together in a coherent way may seem like a simple synchronization issue, but it isn't that simple. Once again a good systems approach is required to make all elements of the digital active loudspeaker work together to achieve the best possible sound.
So, some invention is required but all the technology pieces are close or already here. I am especially encouraged about the activity in DSD downloads and in DSD-capable DACS. DSD and double DSD is slowly but surely becoming the audiophile standard for playing digital music files. Moreover, several DSD-capable DACs have already come on the market in the $1000 - $2000 price range that outperform, in terms of sound quality, many conventional PCM DACs that cost thousands more. It is just a matter of time before some of these DSD DACs are integrated with high-end amplifiers. There are already a few home theater receivers available that can play back DSD files over 7 or more 100W+ audio channels for less than $2000. They may not be up to audiophile standards but they show the way.
In order for the digital active loudspeaker approach to take hold, high-end audio companies will have to consolidate and become more vertically oriented including disciplines in analog and digital electronics, loudspeakers, power supplies, digital signal processing, mechanical engineering and software. A very strong systems engineering component will also be necessary as all these diverse parts will have to work together optimally. We are already seeing this happen. Aside from Meridian, one can cite Harmon International that has electronic companies like Mark Levinson under its umbrella as well as speaker companies like Revel and JBL. More recently, the Italian organization Fine Sounds SpA has consolidated Audio Research, McIntosh, Wadia, Sonus Faber and Sumiko. Also, take a look at TAD Laboratories, MBL and other European companies that work the entire spectrum of audio gear (with the notable exception of cables I believe). It goes without saying that many Asian audio companies already fit the bill.
Audiophiles like to tinker and tweak. One of the knocks on the Meridian products mentioned earlier is that they don't allow for enough tinkering although I am sure the Meridian engineers would disagree with this. What I see happening is that the digital active architecture places many, many things under software control. So, a key to acceptance by audiophiles will be an easy to use "app" and control interface that allows the tweakers among us to move things around without getting crazy. At the same time the "app" for the systems needs to be simple enough for others who want to just set it and forget it. I believe this is very, very important.
I haven't talked about power cords specifically in this article. They too can have an effect on how a system sounds, as many of us know. Aside from cutting down the number of power cords required, I believe the digital active loudspeaker topology should make the whole business of power distribution in audio systems easier. It should be addressed as part of the overall audio system design. For example, power conditioning should be done inside the speaker and not outside, and all the various power supplies in the speaker should be designed to a coherent set of requirements allocated among all the elements of the system in a balanced fashion. I believe the power supply area of the system will offer options to manage the cost of these cable-less new loudspeakers.
In summary, let me be clear that I am not criticizing the cable industry. There may be a tiny percentage among them who are charlatans and a few others who are well meaning but don't have a clue. But, the vast majority of folks who work on cables are great engineers and designers who truly love music. The fact that our systems do sound better with the right cables despite the chaotic panoply of equipment and ill-defined interfaces of current audio systems is testimony to their greatness. My story instead is that this journey we are on with so many cable variables does not have a good ending or any ending at all. To completely unlock the magic that our systems are capable of and to elevate the level of fidelity and perhaps bring down the cost we need to take a different road; one that seeks to minimize cable interfaces and to maximize the flow of music between equipment stages. I see this as getting back to the roots of our hobby embodied in the Edison phonograph of 1877. What can be better than a system that goes directly from the phonograph needle to the loudspeaker with not a single cable in sight?