Berkeley Audio Design Alpha USB Interface - Son et image

Berkeley Audio Design Alpha USB Interface - Son et image

Here are just a few examples: the CD format (itscreators would be astounded at the advances injust, say, digital filtering); the RCA plug and jack(compare the connections on 1970s equipment totoday’s Cardas or WBT RCAs), and the CompactCassette (Nakamichi 1000, anyone?).A more recent, and perfect, example is theUniversal Serial Bus, or USB interface. Designedto connect computer peripherals, USB was neverintended to be a high-resolution digital-audiointerface. But the rapid growth of computerbasedmusic systems has, once again, foistedupon the high end a standard that requiresexceptional re-engineering to meet the demandsof high-quality music reproduction. Because ofthe contributions of high-end designers, today’sbest USB interfaces are light years beyond thebasic implementations.In this overall drive toward a good-soundingUSB interface, one company stands out forpushing the envelope—Berkeley Audio Design.The company that brought us the amazing AlphaDAC has turned its considerable engineeringchops and uncompromising work ethic towardsolving the USB interface problem. The AlphaUSB reviewed here—only the second productfrom Berkeley—has been nearly two years in themaking, largely, I surmise, because Berkeley isrun by engineering-driven perfectionists who keptdiscovering during the design process better andbetter techniques and implementations. Berkeleyis the kind of company that would repeatedlydelay a product launch until it had wrung outevery last bit of performance.WHY DO WE NEED A USB CONVERTER?Before looking at the Alpha USB in detail andconsidering its sound quality, let’s review theoptions for getting music out of a computer-basedserver and into a digital-to-analog converter(DAC). For now, we’ll ignore the turnkey systemssuch as Sooloos and the Olive O6HD (reviewedin this issue) to focus on do-it-yourself serversbased on a personal computer.The first option is to install a soundcard that hasbuilt-in DACs in your PC. You simply connect thesoundcard’s analog outputs to your preamplifierand you’re in business. The compromises ofthis approach are fairly obvious—the inside of acomputer is not the best place to perform digitalto-analogconversion. The second option is touse a soundcard’s S/PDIF or AES/EBU digitaloutput for connection to an external DAC. In ourprevious issue (May/June) Karl Schuster surveyedand reported on eight such sound cards. Thisapproach requires opening your PC to installthe card, and configuring software. Moreover,building a PC server that is “bit transparent”(one that doesn’t change the ones and zerorepresenting the music) is easier said than done.The third method, which is by far the mostpopular, is to simply run a USB cable from thecomputer to a DAC equipped with a USB input.Although simple in practice, the USB interfaceaudibly degrades the signal passing through it,even in the better implementations. As noted, USBwas never designed for audio; it is a “packetizeddata” format in which data are split up intodiscrete chunks, wrapped up with informationabout those chunks, transmitted, and then putback together at the receiving end. This is insharp contrast with the continuous bitstream ofdigital audio formats such as S/PDIF. Moreover,until recently USB has been limited to a maximumsampling frequency of 96kHz. And let’s not forgetthat many of us have older DACs that still soundgood but lack a USB input. It is for these reasonsthat my own server, which I use exclusively toplay high-resolution music, is a PC fitted with aLynx AES16 card that outputs its digital signal asAES/EBU on an XLR plug. (I also use a MeridianSooloos to access my CD music library.)The solution to these myriad problems is anoutboard box that takes USB from the computerand outputs S/PDIF—if this can be accomplished

without compromising sound quality. AlthoughUSB converters are widely available, nonecould be considered an all-out assault onthe state-of-the-art. Rather, they are largelyutilitarian in purpose.Which is where the Berkeley Alpha USBcomes in. Berkeley’s goal with the AlphaUSB was not just to create the best-soundingUSB interface, but to completely eliminatethe problems of USB and build a state-ofthe-artsolution for getting music out of acomputer and into a DAC. Concomitantly,Berkeley wanted to create a product thatallowed anyone, not just those with technicalexpertise, to realize state-of-the-art computerbasedaudio performance. When Berkeley’sMichael Ritter told me about the Alpha USB,he invited me to compare its sound with that ofthe AES/EBU output of the Lynx AES16 cardin my fan-less, drive-less PC server, a setupthat many considered the state-of-the-art incomputer audio (see my review of this systemin Issue 189). The PC with the Lynx card startsoff with the considerable advantage of neverconverting the audio data to the USB format.If the Alpha USB did sound better than myPC (with both driving the same DAC), it wouldnot only represent a breakthrough in soundquality, but make it much easier for non-geekymusic lovers to enjoy the benefits of computersourcedaudio.FUNCTIONAL AND TECHNICALDESCRIPTIONThe Alpha USB is housed in an unusual chassisthat’s significantly larger than that of mostUSB converters, but smaller than full-sizedcomponents. As with the Alpha DAC, the AlphaUSB features a no-cosmetic-frills chassis.The front panel’s only indicator is a singleLED that lights up amber when powered on,and then switches to green when connectedto an active USB source (the computer). Therear panel is also minimalist, with AES/EBUand S/PDIF outputs (the latter on a BNC jack)selectable via a small toggle switch. The AlphaUSB lacks an RCA output because the RCA’scharacteristic impedance is 50 ohms, not the75-ohm standard for S/PDIF. A BNC interface,which has a characteristic impedance of 75ohms, is vastly superior to RCA for carryingdigital audio. AC power is via an IEC jack.There is no power switch; the unit, whichdraws 4.5W in standby mode, is designedto be left continuously powered. Macintoshcomputers running Snow Leopard or later willautomatically talk to the Alpha USB. Windowsusers will need to install driver software, whichis included on a CD.The Alpha USB is the epitome of “form followsfunction.” The unusual chassis size and shapewere chosen for sonic performance. In broadterms, the Alpha USB isolates the “dirty” USBcircuitry from the “clean” digital-audio output,and provides a high-precision clock for thataudio output. The USB input jack is mountedon a plastic insert rather than directly in thechassis to prevent capacitive coupling of noisebetween the USB input and the digital-audiooutput. This isolation between the two “halves”of the Alpha USB is improved by powering thecircuitry associated with the USB input fromthe computer via the USB bus, and poweringthe clocks and digital-audio output driversfrom a clean, linear power supply built into theAlpha USB. Berkeley extends this isolationconcept by recommending that the computerbe powered from one AC outlet and the AlphaUSB, DAC and analog components fromanother, and that the USB cable between thecomputer and Alpha USB be routed away fromthe Alpha USB’s chassis and any analog cablesor components. It goes without saying that theAlpha USB’s USB interface is “asynchronous,”meaning that the Alpha USB’s output clock isnot locked to the computer’s clock. That is, thetiming precision of the Alpha’s S/PDIF outputis not affected by the computer. A conventional “adaptive”USB interface, in which the computer serves as the masterclock, is simply a non-starter for critical applications.The digital audio signal is clocked with one of twoprecision oscillators, one for the 44.1kHz family ofsampling frequencies (44.1kHz, 88.2kHz, and 176.4kHz)

and the other for the 48kHz family (48kHz, 96kHz,192kHz). To give you an idea of how meticulouslydesigned and executed the Alpha USB is,consider that each oscillator is measured on a$90,000 instrument that creates a spectragraphof the oscillator’s phase noise. Only the bestmeasuring parts go into the Alpha USB; therest are rejected. This measurement processcosts many times what the actual part costs,but Berkeley discovered that this performanceparameter was crucial to sound quality.The Alpha USB’s designer, Michael “Pflash”Pflaumer, comes from a multidisciplinarybackground that includes writing the DSP codethat made HDCD possible, analog design, and RFdesign. When the first microprocessor becameavailable, Pflash wrote his own operating systemfor it and built a computer around it. It’s rare for anengineer who can write DSP code to also have anintimate understanding of how electromagneticfields behave. It’s even rarer when that engineeris also a musically sensitive listener who uses hisears to guide product development.SYSTEM CONTEXT AND SET-UP NOTESBerkeley sent to me a bare-bones iMac ($1199)loaded with music, including most of theReference Recordings HRx 176.4kHz/24-bittitles (which I also have on my PC server). Theyalso shipped a Straightwire Info-Link AES/EBUcable and a Straightwire USB-Link USB cable(both 1.5 meters). The iMac ran iTunes along withthe latest version of Pure Music, a $129 piece ofsoftware that improves sound quality (see StevenStone’s review of Pure Music in Issue 209).Berkeley recommends Pure Music, and includeswith the Alpha USB a $25 discount coupon forthe software. The complete digital front end ofan iMac, Alpha USB, Alpha DAC, Pure Music,Straightwire USB and Straightwire AES/EBUcosts about $8300.The iMac was unbelievably easy to set up anduse; my Windows PC was a different story. To usethe Alpha USB with a Windows machine, you mustinstall the supplied driver. This will let the PC andAlpha USB talk to each other but will not allow youto realize the system’s full sonic potential. You mustalso install an ASIO driver to avoid data corruption.For Windows XP users, the available ASIO driverswill degrade the sound. Machines runningWindows 7 can use a more sophisticated WASAPIdriver that reportedly delivers performance asgood as that possible from a Macintosh. Installingthe ASIO driver on my XP machine was a hassle.Moreover, switching between the Lynx card andthe USB output required going into a couple ofmenu layers in MediaMonkey to manually changea setting.Considering my experience with both an iMacand a Windows PC, I can unequivocally saythat the Macintosh is vastly superior (and I’veused PCs for all other computing since the late1990s). The Mac is far more elegant, easier toset up, better sounding, and doesn’t require thatyou jump through hoops to realize its optimumperformance. Even if you own a PC that you arethinking of using as a server, I encourage you tospend the $1199 for an iMac—you’ll be glad youdid.LISTENINGI began by comparing the sound of the iMac/Alpha USB to my PC-based server equippedwith the Lynx AES16 card. The bitstreams fromeach computer driving the Berkeley Alpha DACwere identical. How do I know this? Every HDCDrecording carries a flag in the least significant bitthat identifies the recording as HDCD-encoded.An LED on the Berkeley Alpha DAC’s front-panelilluminates when this flag is detected. If the datawere corrupted, that LED would not illuminate.In all my tests, the HDCD LED remained lit whenplaying any Reference Recordings title (CD orhigh-res file). It follows that the bitstreams wereidentical for non-HDCD recordings, as well.Listening to 176.4kHz/24-bit ReferenceRecordings HRx files through the ConstellationAudio electronics ($65k Altair preamplifier and$140k Hercules power amplifiers) driving Sonusfaber’s $200,000 flagship loudspeaker is ascritical a situation as one could devise. Theseexquisite recordings contain so much fineinformation, dense spatial cues, micro-transientdetail, and rich timbral colors that any degradationis instantly identifiable. The playback system istruly of reference quality.Comparing the PC-based server with the Lynxcard to the iMac and Alpha USB (by changingwhich AES/EBU cable was connected to theAlpha DAC) revealed that the PC setup I’dthought was the state-of-the-art was actually astep down from what was possible. Simply put,the Alpha USB took the system to another level ofresolution and musicality.Although the PC server sounds phenomenallygreat, the iMac/Alpha USB was better in virtuallyevery sonic criterion and inferior in none. Firstwas the sense of space and the naturalness ofthe staging. The iMac/Alpha USB had a slightlyless forward perspective along with much greatersoundstage depth. Although the front of the stagewas a little set back, instruments in the back ofthe hall sounded much more distant. The senseof bloom around individual instrumental outlineswas more realistic and palpable. The Alpha USBmade the PC/Lynx sound slightly congested andhomogenized by comparison.Within this sense of expansive space, I couldmore easily hear fine recorded detail, particularlyinstruments toward the back of the hall. Switchingover to the Alpha USB system was like sharpeningthe focus on a camera; very-low-level detail thathad been just a bit indistinct or smeared snappedinto vivid clarity. I thought I had heard the HRxrecordings in their full glory with my PC server,but I was astonished to discover another level ofresolution and clarity.The treble through the Alpha USB was smootherand, paradoxically, slightly more prominent. Thepresentation wasn’t brighter, just more aliveand vivid. The treble had greater texture andincreased density of information, yet was morefinely filigreed and delicate. The top octaves alsohad greater smoothness and ease, particularlyon high-level, high-frequency transients such asthe upper octaves of fff piano passages.The sense of hearing more information waspartially the result of the Alpha USB’s superiorrendering of transient detail. Listen to a Latinpercussion instrument such as the güiro; theAlpha USB better resolved the instrument’s

dynamic envelope to create a greater impressionof hearing the instrument itself rather than a recreationof it. Percussion seemed to “pop” fromthe soundstage with greater life. Micro-transientswere also noticeably superior; listen to brusheson cymbals, to triangles, and to tambourine. Bybetter resolving this low-level transient detail,the Alpha USB made the presentation morelifelike and musically vivid. I could better hear themechanisms by which the sounds were created,which is always a good sign.Finally, the Alpha USB had a greater senseof ease on high-level peaks, particularly duringdense and complex passages. The music gotlouder more gracefully, with smoother texturesand less homogenization of images. Thepresentation remained more coherent duringthe loudest orchestral passages, contributingto the overall sense of ease and involvement Iexperienced.Because the Alpha DAC was receiving thesame bitstream from both music servers, the onlydifference was in the timing precision—jitter.Some of these differences could beattributable to the different computer platforms,so I compared the sound of the Lynx card in myPC to the PC’s USB output through the AlphaUSB. The Alpha USB sounded better overall,although the disparity was not as great as whencomparing the PC/Lynx to the iMac/Alpha USB.The specific sonic characteristics were the same,but the magnitude of the difference was reduced,suggesting that the Macintosh platform has asonic advantage over the PC. That differencemight be erased by a PC running Windows 7and a better WASAPI driver, but I was unable tohear that configuration. Even if a Windows 7 PCcan sound as good as the Macintosh, the Appleplatform is much more pleasant to use.CONCLUSIONThe Berkeley Audio Design Alpha USB is abreakthrough product that not only overcomesthe limitations of the USB interface, but providesa state-of-the-art method of getting audio out ofa computer. Moreover, the Alpha USB makes thisreference-quality performance available to nontechnicalmusic lovers who have a Macintoshand a DAC.Though the Alpha USB’s $1695 price isconsiderably more than that of other USBconverters, the Alpha is a bargain when youconsider that it provides a simple, foolproofpath for creating a state-of-the-art music server.Moreover, the entire digital front end of the iMac,Alpha USB, Alpha DAC, Pure Music software,and Straightwire digital interconnects costsabout $8500. That’s not chump change by anymeasure, but it’s eminently reasonable for amusic server and a DAC that deliver this level ofperformance. I listened to this digital front end asa source for electronics and loudspeakers thattogether cost more than $400k, yet never felt thatthe digital source was the weak link in the chain.In fact, I had the opposite reaction: This sourceallowed me to hear these ultra-exotic electronicsand loudspeakers at their best.One day computer-based music systems willbe simple to set up, foolproof, ubiquitous, anduncompromised in sound quality. The BerkeleyAlpha USB represents a giant leap forward inrealizing this goal.

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