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Design Notes 1: iFi Pro iDSD

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Publisher’s note: Every product comes with an Owner’s Manual to familiarize the customer with its operations. These documents are often written in a dry style and to the point. But once in a while, we come across vast sections in Manual that reads like a smooth presentation of ideas and juicy description of brilliant technologies, and if we’re lucky, even a story on the challenges faced by the engineers and designers in bringing the concept to fruition. ‘Design Notes” is a new series created to feature the informative and well written documentary inside those Manuals to the readership, to offer knowledge and viewpoints that we believe will benefit and even entertain us. All materials are reprinted by permission.

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iFi Pro iDSD Streamer/DAC/Head Amp/Preamp

Design Notes from the AMR/iFi Research & Development department

The Pro iDSD is an all-out assault on the state-of-the-art and its ground breaking digital engine will be used across the next generation of iFi products.

1)DAC section
The Pro iDSD uses a quad ‘core’ of iFi audio’s bit-perfect DSD & DXD DAC by Burr-Brown in a custom ‘interleaved’ configuration. A total of eight pairs (four for each channel) of differential signals are used and mixed.

While the Pro iDSD does include an FPGA for digital filter duties, the FPGAs themselves do not perform d/a conversion. There are always external d/a sections, with a limited number of elements. In the Pro iDSD we use four interleaved 64-element converters, to give a total of 256 element DAC per channel, manufactured to the highest level of precision. Discrete DACs added can neither offer those elements’ numbers nor the accuracy required.

All signals to the DACs are re-clocked with the low-jitter Global Master Timing® derived master clock from the AMR DP-777.

2) Digital Processing

Bitperfect or Upsampled
All digital processing is bit-perfect, without employing ASRC or up conversion unless digital filters embedded the Crysopeia FPGA remastering engine are explicitly selected. Divergent digital filters (including bit-perfect mode without digital filtering) are selectable and operate in synchronous mode.

The Pro iDSD uses the second generation XMOS XU216 X-Core 200 Series 16-core processor with a maximum of 2,000 MIPS (two billion instructions per second) calculation power in dual-issue mode as USB interface.

The X-Core 200 handles decoding of signals from all Inputs, be it USB (up to 768kHz/DSD512), AES/EBU or S/PDIF (up to 192kHz/24Bit) and WiFi/network/mass-storage (up to 192kHz/32Bit).

Hi-res Network Audio
In addition to traditional inputs, the Pro iDSD integrates a WiFi/network playback system, with built-in Spotify and Tidal (and other streaming services), wide protocol support including Airplay and USB memory/HDD/SD card playback to make a full standalone streamer/network audio player with 32 bit/192kHz & DSD64 support.

The following playback options are available with the Pro iDSD:
•    Airplay network audio playback from iPhone, iPad and Mac computers
•    DLNA network audio playback from smartphones, tablets and Windows/Linux computers
•    Playback from a hard disk drive (HDD), USB memory or SDHC memory card
•    Playback from Network Attached Storage (NAS)
•    Streaming playback includes Spotify, TIDAL, Napster, QQ Music and others
•    The Pro iDSD is a flagship, hence it has MQA decoding on-board for the maximum quality of playback, be it on MQA music les or streaming.

Zero Jitter Memory Buffer for all inputs
Data for all inputs is sent to a large ‘dynamic’ memory buffer, where it is de-jittered to eliminate any transmission of source jitter to the DAC output. The memory buffer data is then re-clocked by the low-jitter Global Master Timing® clock, which also drives the X-Core 200 & FPGA engine.

Studio Grade DSD1024 Remastering
While the X-Core 200 is optimal for USB audio, AES/EBU/S/PDIF, MQA, DSD etc. decoding, it is not the best platform for DSP, digital lters and PCM-to-DSD conversion.

The other half of the digital processing is carried out by the Crysopeia FPGA engine. It handles digital filtering and PCM-to-DSD remastering up to DSD1024. We believe that FPGA excels in upsampling and digital filtering duties.

On-board hardware upsampling allows us to not only overcome the current DSD512 USB limitation, but also to implement multiple filters optimized for specific time-domain behavior related to both stages; digital and analogue. This level of optimization is simply not possible in generic software upconversion (as found in i.e. Foobar 2000).

There are five filters available:

‘Bit-Perfect’ – No digital filtering is applied, one tap
‘Bit-Perfect+’ – No digital filtering is applied, one tap, SINC roll-off is corrected
‘Gibbs Transient Optimised’ – Minimum filtering, no pre-ringing, minimum post ringing, 32 taps
‘Apodising’ – Modest filtering, no pre-ringing, modest post ringing, 128 taps
‘Transient Aligned’ – Max filtering, max pre-ringing, maximum post-ringing, 16,384 taps

Full Galvanic Isolation
All inputs are galvanically-isolated (including USB). The USB input is self-powered and does not draw power from the USB bus. The USB section is quite impervious to aftermarket add-ons or tuning accessories. The isolation barrier is placed not in the inputs but instead between the relatively noisy digital processing section and the clock / reclocker / DAC section, so not only USB related but all digital noise is isolated.

Cutting-Edge S/PDIF
The S/PDIF inputs use the technology derived from the AMR DP-777; a brand new solid-state HD-VDi implementation, memory buffer and the Global Master Timing® clock system. Given this extreme attention to jitter reduction, the S/PDIF inputs benet little from additional tweaks or tuning gizmos.

External Clock options &Synchronisation
For the purpose of synchronization in recording studios, the Pro iDSD supports AES3id based DARS (Digital Audio Reference Signal) according to AES11. And if you happen to have a really good atomic clock (of Sanford Research Systems PERF10 quality or higher), it can be used to further elevate the Pro iDSD’s already impressive internal clock system performance. This is very much an over-specified approach, yet the iFi flagship deserves this feature in order to be as future-proof as possible.

3)  Passive Filtering and Discrete Analogue stage
Each d/a converter operates in the ‘voltage output mode’, giving >119dB dynamic range. All filtering is passive.  A fully-balanced 3rd order capacitor/inductor/capacitor filter is used to remove ultrasonic noise directly after the d/a conversion rather than active, feedback-based circuits.

Active filters struggle with the amount of ultrasonic noise and RFI they have to handle and at a few 100kHz they often lose the ability to filter noise at all, which is precisely where a lot of it is present.

Passive CLC circuitry in the Pro iDSD provides the correct filtering well into the MHz region, so that the follow-on analogue stage is not required to handle ultrasonic noise and RFI originating within the DAC processes.

An all-analogue six-track Japanese Alps potentiometer is to be found directly after DAC and filter stages. It can be bypassed if volume control is not required.

The actual analogue circuitry is more precisely a line/headphone driver stage. First seen in the Pro iCAN, it’s fully balanced and 100% discrete, direct-coupled (without coupling capacitors) and tube/solid-state user-selectable. Our design is not just op-amp based discrete but radically different – pure Class A topology. It was inspired by exceptional sounding, legendary studio equipment.

4) Power Supplies
All incoming DC is converted to a high-frequency waveform and then rectified and filtered by a choke input capacitor filter (that is straight out of classic tube design, brought up to the latest 21st century technology).

This produces a first-level DC bus from which all further voltages are derived. This circuit also generates a galvanically-isolated power supply voltage for the USB input circuitry.

The digital section is powered by a bank of ELNA Dynacap DZ (TM) super capacitors of 6.6 Farad (6,600,000uF) value in total. ELNA Dynacap DZ (TM) super capacitors have around 400x lower internal impedance (in comparison to similar products of regular grade) and are used to avoid the typical drawback of the very high internal impedance of common ‘super capacitors’ .

Individual low-noise TI LDO regulators with local LC filtering provide the final low-noise power for all individual digital sections. A total of six individual regulators cover clock, S/PDIF input and the digital section. The analogue side of the DAC is supplied with ultra-low noise super-regulators based on iFi audio’s custom ultra-low noise OV2028 operational amplifier.

The USB input section has its own separate power management system with multiple regulators and filtering operating from the galvanically-isolated voltage generated to power this section.

Higher voltages (especially for the tubes) are required for the analogue stage, which in effect operates on 60V rail providing massive potential dynamic range. These are generated from the main DC bus and are filtered via multiple inductor/capacitor stages based on great ElnaSilmic capacitors.

Every section in the Pro iDSD has received massive attention to detail. Classic, very low-noise design techniques found in tube equipment have been combined with 21st century tech to deliver extreme performance.

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