". . .the higher the resolution of the source file. . .the better the QuteHD performed."
espite the vinyl renaissance, digital downloads dominate the music and, increasingly, hi-fi industries. For the latter, the DAC has moved from a creature of curiosity to an essential piece of kit in an audiophiles musical armory. Sometimes, though, DACs can flatter to deceive. They often gush a wealth of impressive specifications and a raft of supported sound formats, but on many occasions the sonic results prove to be something of a damp squib. Frankly, if a DAC is really going to support hi-res formats such as 24-bit/192kHz PCM or the even rarer DSD, then the basis of that DAC had better be just right. If you want the likes of DSD in your life, then your DAC has got to be both a thorough and thoughtful design.
Which is the perfect preamble for a review of the Chord QuteHD. You have to ask, however, does a selling price of $1795 for a DAC provide the designer with much freedom for thought -- or the buyer with much confidence, for that matter? DSD users are naturally rather demanding. Is the QuteHD a bit too near the "cheap and cheerful" category to be taken seriously? Maybe it was the relaxed smile that played around the lips of Rob Watts, who designed the QuteHD for Chord, that made me hesitate before rushing to judgment.
Talking to me from his lab in Carnarvon, this ex-Deltec man is the center of a flurry of activity. He invented Direct Digital Feedback Amplifier Technology for the chip outfit CSR, the end product of which was seen in the recent range of NAD products, and created the Watts Transient Aligned filter and Pulse Array technology for Chord.
The QuteHD was built for a purpose -- and a personal one at that. "When Im listening at home, I generally use my computer for background music, using the soundcard that offers bad sound quality," said Watts, who, rather than search for a suitable DAC replacement wondered if he could produce an inexpensive example of his own utilizing his own design philosophies and a new Field-Programmable Gate Array (FPGA).
Why not use an off-the-shelf DAC? Something from Wolfson, maybe? BurrBrown, perhaps? The answer, "Because they sound crap," gave me an inkling of Watts single-minded passion for DAC design and construction and made me think that, just maybe, DSD replay might have a chance from a mind so focused.
The key to the QuteHD design is that FPGA. This little chip has been a revelation in DAC design -- and is becoming increasingly popular amongst enlightened designers. The effect on DAC chips is the same effect that punk had on music. It promotes a DIY ethic. It frees you from the corporates and restricted supply and allows anyone to get in on the act. It means that you can get right to the core of digital music and change it to follow your own philosophy, changing the very essence of the digital source instead of having to make do, buying off the shelf and then faffing around, tweaking at the very edges of the music you want to deliver.
The core of a DAC, any DAC, are its filter and noise shaper that accept your 44.1kHz CD-quality signal. The DAC then upsamples that signal by 8, 16 or 32 times to reconstruct the original analog waveform by inserting information in between the digital samples. Doing so moves the sound further away from that harsh-sounding, metallic cliché and more towards a rich, warm output. Part of the reason that some digital music can sound tinny is the timing problem that Watts attempts to tackle using his WTA filter.
"So, lets say that you are sampling at 44.1kHz and you have 22ms in between each sample," said Watts. "The original transient might be in the middle of the 22ms, so you have to reconstruct that." And why would you have to work to such fine tolerances? Because your ear is a delicate instrument, more delicate than many realize, as it can detect delays of 4ms between one ear and the other.
Okay, so timing is important when creating a DAC, but so are taps. Taps? Yes, taps. The number of taps, more fully known as FIR taps, is an indication of the amount of memory required to implement the filter and the amount of filtering the filter can do. "Off-the-shelf DACs contain only a limited number of taps. A filter has a long delay line featuring a bunch of taps along it. If you want to reconstruct the original waveform, you need lots of taps. Theres a theory that says that, if you want to perfectly re-create the original recording, you have to have an infinite tap-length filter. Thats what the math says. I looked at off-the-shelf systems. These DACs were using around 100 taps. I could see that this would create severe problem in timing."
So Watts started experimenting, using his punkified FPGA chip with a longer tap length and algorithms to try to improve the ability of the filter to reconstruct the original timing. Hence, on the QuteHD, you will find 10,000 taps (the older Chord DAC64 had 1000, while a generic, standard, off-the-shelf DAC features 128). The more taps you feature on a DAC, though, the more memory you need, which adds to the expense.
So, if taps are so important, why do the common chips that populate most of the worlds DACs get by with so few? Reportedly, well-known manufacturers dont see the low tap count as being a major problem. "They dont even do listening tests," said Watts. "It doesnt feature on their event horizon. Design is all done mathematically. They make assumptions and then they just do it." According to Watts, technocrat graduates populate these companies, showing a greater interest in mathematics than sound.
The other main component in a DAC is the noise shaper. A noise shaper is like a feedback amplifier, featuring a feedback path that examines and compares the output with the input, flagging an error if a difference occurs. Unlike a standard feedback amp, which is a first-order noise shaper, the QuteHD employs a fifth-order noise shaper that pushes the noise floor to -240dB at 32 bits.
The design of the noise shaper influences how much distortion and noise you will include in the DAC. The oversampling rate of the noise shaper affects this area too: "Off-the-shelf systems tend to run at 6MHz," said Watts. "The problem with that is the amount of feedback that you have available depends on the speed of the output. Running at 6Mhz, you get limited performance. If you run at 60Mhz for a fourth-order noise shaper, then you get around 35dB worth of feedback. I run at 104Mhz."
Oversampling is much misunderstood. The idea, according to Watts, is to increase oversampling as much as possible to re-create the shape of an analog waveform. The more times you oversample, the smoother the waveform becomes. Watts manages to oversample an impressive 2048 times in the QuteHD (a standard high-performance DAC oversamples just 8 times), which produces a smoother waveform with very small steps. A DAC that oversamples just 8 times, for example, creates a waveform with steps that resemble the staircase in your house. The problem with having whopping great steps within your DACs waveform is that the sonics are not as smooth-sounding.
More than that, those large steps cause noise, which can trigger jitter effects. "If you filter that high-frequency noise component, then it becomes less sensitive to jitter. That, in turn, means that noise-floor modulation reduces, which means the sound is smoother and sweeter. The level of noise-floor modulation that you can physically hear is unbelievably low, far lower than you can measure. Thats what Im trying to do when I oversample at silly levels, to strip out the high-frequency noise. Preventing the problems that jitter creates."
Jitter is the result of timing errors in the digital signal. Weve all heard about it and we all recognize that it is the nemesis of digital music. Jitter creates noise-floor modulation that reduces musical detail and instrumental separation while increasing graininess. To tackle jitter in the QuteHD, Watts includes a digital phase locked loop. The incoming sample rate is analyzed over a long time interval. A micro buffer inside the clock accommodates changes, smoothing out and creating a largely jitter-free signal.
hats the essence of QuteHD -- the "bits" you dont see, at any rate. If you took the lid off, however, what would you see? Firstly, you would notice the isochronous/asynchronous USB decoder, which receives its timing instructions from the FPGA. More from Rob Watts: "Its actually less important about whether a USB is isochronous/asynchronous or not; its more important to see how the jitter is being handled in the DAC. I dont care, therefore, if the signal is jittery or not; the end product will sound the same because the DAC will sort it out."
Theres the FPGA itself plus other elements of the nine-piece DAC that sit around it, separated from the DAC chip to reduce noise. There is a lot of filtering and power conditioning going on, so Watts is not too bothered that the external power supply might not exactly be audiophile in nature. In fact, he believes that an improved power supply wouldnt have an effect on the performance of the QuteHD. Watts is a firm believer in internal regulation.
Of course, this sort of attitude will raise the ire of many an anti-distortionist. This guy is admitting that power supplies and, by inference, power cords, dont really matter. Blasphemy! Watts was quick to add that "Power supplies and mains cables can change the sound quality by a large factor, yes, but the reason is down to high-frequency noise. When it leeches into the system it creates intermodulation problems and the noise floor pumps up and down. But if its properly filtered, the mains cable, for example, no longer has an effect."
For lovers of DSD, you will be pleased to know that Watts has been very careful to treat it with kid gloves. Chord supports the current protocol for DSD over USB. The use of the FPGA also allows for future upgrades. In the QuteHD, DSD is dealt with directly, so there is no PCM conversion involved and no "processing" inducing data and timing losses. Hence, DSD is maintained as a pure stream, filtered and then upsampled from there.
"What you dont want to do is convert it to 88.2kHz because then you have the problems inherent in DSD -- the particular noise and distortion issues it has -- plus the problems of PCM, that has timing issues. Hence, I treat the DSD natively and it gets upsampled 2048 times without any decimation."
The USB port supplies the principle DSD interface, but if the S/PDIF signal has a DSD header on it, an 8-bit header, the DAC will treat it as DSD data. Only FPGA-type DACs handle DSD in this way. Interestingly, while Watts is a fan of DSD, he doesnt see it as the ultimate format currently available. His eye has been taken by 24-bit/384kHz PCM. Actually, the QuteHD supports 384kHz through the S/PDIF jack, with a possible USB upgrade on the way.
"The 24-bit/384kHz format is the ultimate, it is the future," said Watts. "It is far superior to DSD. In comparison, DSD lacks resolution and detail." Which is all well and good, but 384kHz recordings are few and far between. This will, however, surely be down to time, as will assimilating the much larger audio files that accompany them.
Measuring 6 1/4"L x 2 3/4"W x 1 5/8"H and weighing in at just under one pound, the QuteHD features one USB port, coax and TosLink inputs, along with a pair of RCA outputs and a 12V wall-wart power supply. The aluminum chassis also features a quartz oscillator that is clamped directly to it to improve RF isolation. A window illuminates when the DAC plays a particular sample rate. So, for example, when a 44.1kHz Red Book signal is being played, the circular window emits a red light. Similarly, an orange light emerges when playing a 48kHz, yellow for 88.2kHz, green for 96kHz, light blue at 176.4kHz, dark blue at 192kHz and purple for DSD.
Listening to the silence
he QuteHD was created for audiophiles who want a quality listening experience from their computers, so lets address that issue first. My basic digital-file replay system currently includes an Arcam rDAC, priced at around $400. Considering that most computer-based setups are less than audiophile approved in nature, residing on desktops as they are, this system presents a nearfield listening experience. Theres little in the way of a hi-fi room setup, isolation is minimal and the listening position is constantly on the move. Is it worth spending over and above $400 for a QuteHD?
I started the listening with a WAV file of Billy Joels "My Life" [Columbia PC 35609], ripped via EAC onto a Quad Core PC. The Chord DAC raised an immediate eyebrow as its clarity -- perhaps the result of lower distortion -- shone through. Percussion, meanwhile, reveled in its new authority. Drum strikes not only had weight, but each became a discussion in itself, carrying breadth, timbre and tonal realism.
Moving to the upper midrange, both vocal performance and piano featured air and space. Periods of extra reverb were noticeable via the Chord, while the instrumental separation relaxed into a broad and expansive soundstage. The previously muffled acoustic guitar was now happily strumming in the center of the stereo image.
Turning to the 24-bit/96kHz version of "Let Us Garlands Bring" from Come Away Death by Marianne Beate Kielland & Sergej Osadchuck (FLAC [2L 064-FLAC96]), the entire vocal performance was lifted above and beyond the piano accompaniment. Previously, the Arcam rDAC had merged the two, providing a coincident melange. Having moved from this position, the vocal was able to exhibit itself as a fully rounded entity. Vibrato showed far more control, while Kielland exhibited more power potential during her crescendos. The piano accompaniment was dynamic in nature, rolling under and around the vocal and providing an ebb and flow that complemented the mood and the structure of the arrangement.
I paused here before moving on, to push something sorbothane-flavored underneath the QuteHD. Uncontrollable upper mids and a floppy bass sounded distinctly like external distortions that the sorbothane grasped and eliminated, adding a measure of control and focus to those wayward frequencies.
Back to digital and 24-bit/192kHz via the slightly odd pairing of Sigmund Groven and Iver Kleive ("Gloria" from HarmOrgan, FLAC [2L 077-FLAC192]), who offer the rather fruity combination of harmonica and a grand, cathedral-like organ. The introductory harmonica sequence at the beginning of the track was, via the QuteHD, atmospheric. Again, lower distortion gave this instrument a truly atmospheric, rather haunting manner. Sonic elements drifted into the darkness of the presentation, providing an almost plaintive cry from this simple little apparatus. Importantly, with the magnificence of the organ, which built into an Everest-like cacophony of competing upper-mid crescendos, there was control and focus that kept each melodic sweep firmly in check, enabling a precise separation of each tonal aspect, giving more expression and emotive force.
So now it was time to bring out the big guns and go DSD. I prepared the software-based Audirvana Plus as a player on my iMac and MacBook and pushed through the US-based singer-songwriters, Keith Greeninger and Dayan Kai's Looking for a Home (DSD [Blue Coast BCRSA1012]). This piece of Americana features slices of blues, country and folk and contains nothing but a pair of vocals, an acoustic and a steel guitar. What was startling about the DSD rendition was how, even on this less-than-ideal system, the music sounded so relaxed and yet under control. At no time did the upper mids resort to shouting, which could happen with the other files, even those of a high-res nature. Sometimes, this effect can initiate an unconscious tension; you know that something like that may happen, so you prepare for it. Instead, on this DSD track, the upper midrange had a calming effect that set me immediately at ease. The guitars were almost clinical in their instrumental separation but only spatially, not in their tonal quality, which was warm, reassuring and secure in the soundstage. Possibly the most remarkable aspect of the performance was the vocal presentation, which enabled a contoured stereo image, enhancing the realism of the vocal by examining, in fine detail, the resonance of the vocals themselves.
I then connected the QuteHD to my reference system and the transport of my Denon B-475 CD player via an Audiomeca digital cable. Spinning Helen Merrills quirky rendition of "Angel Eyes," from the Helen Merrill/John Lewis CD [Verve 846 011-2], the vocal presentation was clear and quite sparse. Merrill used the range of her delivery to manipulate the silence around her, giving the QuteHD a chance to display its dynamic range. The sole instrumental accompaniment, the piano, was crisp, light and sparkling, providing a playlful contrast with the vocal performance, the slightly forward upper mids giving it a lively essence. That said, Merrills delivery was smoky and yet precise: a sort of nightclub arrangement with the deliberate and careful placing of the lyric. The upper mids were at the heart of that precision while the lower frequencies, what there where on this track, were nicely characterful and textured.
Moving to rock and Gentle Giants Giant CD [Vertigo 6360 020], the QuteHD rocked with both passion and vigor. The upper mids were fast and crisp, and vocals danced over the lyric like Nureyev on point. The lead vocal of Derek Shulman was packed with a throaty grit and a rasping attack, which the Chord tracked with aplomb. Percussion had power and punch, while a favorable instrumental separation and low distortion never allowed the percussive components to bleed into each other. The soundstage was broad and clear in its arrangement, while the stereo image was rock solid with a 3D element that produced a sense of layered depth.
initially saw the QuteHD as a potential companion to a desktop setup, offering no more than touch of class to standard computer-based digital fare, but the more I used it, the more I understood that it can easily be utilized as an all-rounder, fitting within a serious hi-fi setup. It showed a level of performance that not only covered all digital bases (with a degree of future proofing), but it also fit well within a mid-price system, handling not only digital files but also physical media with the aid of a suitable transport.
Offering flexibility in terms of connection but also in the file formats it handles, the QuteHD is a compact, well-designed DAC that represents value for the money. Easy to use, the Chord QuteHD is an ideal fit into a digital system, the higher the resolution of the source file, in fact, the better the QuteHD performed. Great for budget, second, office or small-room systems, it also promises quality performance from a specialist DSD rig.
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