How do the ashes-level audiophiles create a portable DAC and ear extension?
Chord Electronics Hugo, a battery-powered portable DAC and ear extension for hardcore audiophiles, uses a Xilinx Spartan-6 FPGA (using a 1/2 unit) to implement a 32-bit, 26K tap The WTA interpolation filter creates a ladder to music lovers for music lovers. As the Audiostream.com web page commented, "The longer the tap length, the closer it is to the perfection of math..."
The Audiostream web page also pointed out that the low-power Spartan-6 FPGA reduced the size of the Chord Hugo to a palm size of 20x100x132mm and reduced the weight to only 0.4kg, while a full charge lasted for 10 hours.
Hugo is small, but still can accommodate multiple digital input interfaces, including Toslink fiber optic terminal, coaxial SPDIF interface, driverless USB interface (16bit/48khz), and high-definition (HD) USB interface (32bit/384KHZ and DSD128). There is also a wireless Bluetooth access function. The analog output includes two 3.5 mm headphone jacks, a 6.35 mm headphone jack (1/4 inch), and a pair of RCA stereo jacks.
Chris Martens, who specializes in complete audio detection reports, quoted John Franks, CEO and senior engineer at Chord, as saying: “Without Spartan-6 FPGAs, it’s hard to make Hugo DACs both high-end and lightweight, even desktops. It's also hard to do (because it consumes a lot of power and is very bulky). If you are not a team player, the 26K tapped audio filter is a bit scary (most audio DACs can implement 150 tap filters and the largest commercial-grade DACs can only provide 256-tap filters, see Chord's web page for information on DAC technology), but the pursuit of the ultimate tap length is to produce a cleaner sound, so you can attract more high-end music enthusiasts.
What is the principle behind the 26k tap WTA audio filter? First, WTA is short for (Watts Instant Alignment), and this "Watts" refers to Robert Watts, a well-known DAC design expert. Watts originally developed this FPGA-based filter technology from a 2002 Chord product called DAC64. According to Watts's sound selection magazine published in the UK in June 2014 (reproduced from the Chord electronic official website) "As early as the 80s, when I was an electronic major, I realized that the interpolation filter has been Used up to now, it has its inherent limitations and has very serious timing problems. This is based on sampling theory, which clearly shows that perfectly replicating a signal with a limited sampling bandwidth requires an infinite length filter. Using traditional filtering (about 100 taps), there will be serious timing problems. I also know that by studying hearing, the ear or brain can handle the 4us timing, but the CD can only be accurate to 22us, this missing timing can be used by the interpolation filter reconstruction"
Here's more about the WTA interpolation filter: Chord's DAC technology page
“It explains why the higher the sample rate, the better the sound quality. It is well known that 96KHZ (DVD-Audio) recordings sound better than 44.1KHZ (CD) recordings. Most people think this is because the existing ultrasound information can be heard. Even though the best human hearing is limited to 20khz, what is unknown is that the recording quality of 768khz is better than 384khz, and the sampling area with limited sound quality is in MHZ"
An important reason why the 768khz recording is not good is that when the information exceeds 200khz, not only the instruments, microphones, amplifiers and speakers do not work in this frequency band, and humans cannot hear this sound. So suppose there is no extra bandwidth (which is important), why do sounds with high sample rates sound better?
The answer is that although we can't hear the ultrasound, we can clearly "hear" this "transient time." We have known for a long time that the human ear and brain can distinguish the sounds of the ear to the different stages between the microsecond sequences. Since the transient can be detected in the microsecond level, the recording system only needs to complete the timing change within 1 microsecond, which is done with a sampling rate of 1 MHz.
However, by using a digital oscilloscope, the 44.1khz sampling can accurately complete transients. Digital filtering can increase resolution to some extent without requiring a higher sampling rate.
To do this, the filter requires an infinitely long tap. Currently, all reassembled filters have relatively short taps, and the longest commercial device is only 256 taps. Short transients and corresponding filtering algorithms will produce erroneous transient times. These errors can be clearly heard."
Watts and Chord Electronics have developed 64-bit, 1024-tap filters using Xilinx Virtex FPGAs more than a decade ago, and the technology continues to improve across a range of products. To design this lightweight Hugo DAC and ear extension, the company later ported the technology to the Spartan-6 FPGA. The low power consumption and high thermal efficiency of the Spartan-6 FPGAs make this lightweight battery-powered Hugo. Although Spartan-6 is considered to be a low-end FPGA from Xilinx, it is striking that the number of taps of the filter is directly increased from 1024 to 26K, and the sampling rate is also significantly improved. The second Spartan-6 chip implements a high-performance input interface in Hugo.
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