Pure 1-Bit “DSD” DAC
Why 1-bit?
When Cayin announced the sixth and final R01 Audio Motherboards (May 2021) for N6ii DAP, we surprised the DAP community as we made the R-2R technology more affordable than ever. The N6ii with R01, or its limited edition sibling N6ii-Ti, becomes the first streaming-ready R-2R DAP. Since then, we have received tons of suggestions/requests all over the world, asking for an updated design that will explore the R-2R technologies further, and with a newer Android system. While Cayin takes user feedback seriously, we also examine the choice of technologies from a professional perspective, and always look out for opportunities to push the envelope for the benefit of our users and the industry.
As R-2R become a very popular choice and we have seen numerous R-2R products in different sizes and prices, covering a wide variety of applications, including our RU6 Dongle DAC which remains the smallest and cheapest R-2R DAC product right now. R-2R quickly becomes the equivalent of a “resistor network” DAC. When users noticed rails of resistors on the circuit board of a DAC, they’ll call that an R-2R DAC immediately.
When Cayin decided to develop a new DAP that takes advantage of our R01 experience, we noticed that there is a hidden gem in our pocket: the 1-bit DAC, or to put it bluntly the pure DSD DAC, that is designed to natively decode DSD format efficiently.
We conducted a technical feasibility study and compared the R-2R and 1-bit DAC technologies head to head and found out that the two technologies are more different than common. From an implementation point of view, the biggest hurdle to implementing R-2R DAC is the extremely demanding precision and temperature tolerance requirements on the resistors. On the other hand, while the 1-Bit DAC technology has relaxed these requirements relatively, it requires a lot of work in noise shaping and is very sensitive to the quality of the power supply. Nevertheless, we decided to defer the R-2R development and give 1-bit DAC a chance because we honestly believe that this is comparable to the R-2R and deserved to be known and appreciated.
1-Bit DAC is rooted in High-End Audio
The 1-bit decoding technology is frequently found in high-end Compact Disc/SACD players and digital amplifiers since the 90s. There is a famous saying that "DSD is analogous to the music's waveform" (Johnathan Scull, 2000), and it is much closer to the analog experience than any other digital format. There are different approaches to handling 1-bit bitstream, with Sony and Philips, the two co-founders of the Super Audio CD (SACD) optical disc format playing the lead role to promote this format. One of the widely adopted implementations, the Philips TDA1547, is a dedicated Switch Capacitor Network chipset. It is one of the references to 1-bit DAC that were frequently cited in Audio Engineering, R&D, and audio reviews. You can find TDA1547 in many mid-range CD/SACD products at that time while the high-end implementations combined two pieces of Philips TDA1547 in dual differential mode and added one piece of SAA7350 (noise shaping and oversampling, on-chip DACs did not use) as input bitstream PDM converter. This combination is known as DAC7, this is Philips’ legendary 1-bit DAC solution and without a doubt, it was extremely well-received and highly regarded by vendors, media, and users.
Among the first-generation high-end players, Marantz SA-1 and Sony SCD-1 were the two giants that competed for head to head for the title of “King of SACD player” back in 2000. Both players were top-notch, representing state-of-the-art technologies of their time. For the record, Marantz SA-1 was designed around the DAC7 with 4 x TDA1547 1-bit Dual Bitstream DA Converters and an 8X oversampling digital filter (for PCM playback).
If you think 1-bit DAC is nothing more than a historical moment in high-end audio, I would like to draw your attention to the continuous development of 1-bit DAC technologies in high-end audio industries. Since we mentioned Marantz SA-1, this is a natural starting point. SA-10 is their current flagship SACD Player. It is co-designed by Marantz’s digital guru Ken Ishiwata and Rainer Finck, an ex-Philips TDA1547 Audio Engineer. The two have developed the MMM (Marantz Musical Mastering) technologies which is a modernized 1-bit DAC solution of Philips’ implementation. It is composed of MMM-Stream, which converts PCM inputs to DSD256, and MMM-Conversion, which produce the analog output from the DSD-256 bitstream. The SA10 is retailed at US$7,500, and it is one of the most competitive SACD players in the $10K bracket.
Going up the $20K ladder, let’s check out the offer from Playback Design. CEO Andreas Koch is one of the designers who put together the Sonoma (Sony One-bit Mastering Audio Station) for Sony SACD recording studio, so we know he is a strong believer in 1-bit DAC. When you playback a CD with his MPD-8 Dream DAC, the bitstream will go through 16x oversampling to 705.6kHz, and then transcode to DSD128, and oversampling to 50MHz before decoding in their proprietary discrete 1-bit DAC circuit. As the current flagship DAC of Playback Design, the MPD-8 retails at US$22,000.
Last but not least, EMM Lab, another major player in 1-bit DSD DAC, is founded by Edmund Meitner. He started using 1-bit recording in 1978, 10 years before the DSD format was announced. He was a major contributor to Sony when they formulated the Super Audio CD format. His current reference DV2 DAC retails at US$30,000 and it is the world’s first fully discrete 1-bit DA converter with an internal conversion rate of 1024fs (16xDSD). This is the current holy grail of 1-bit DSD DAC.
Discrete Resistor Networks for DSD Format
While reviewers and users appreciate 1-bit DAC technologies as natural, smooth, and realistic when compare to their analog experience in the practical world, they are inevitably not as popular as their PCM counterparts. The not-so-impressive measurements have hesitated a lot of vendors to devote their resources to 1-bit DAC. In addition, the existing solutions are far too bulky and consumed too much power for personal audio, so we didn’t have any 1-Bit DSD DAC implementation for DAP or even transportable DAC/Amp. To introduce 1-Bit DAC to our portable users, Cayin designs a micro-miniaturized 1-bit DAC circuit from fully discrete components:
- DSP Pre-processing: Enhance digital audio signal and output L+, L-, R+, and R- digital bitstream for fully balanced decoding.
- Audio Bridge: pass-through DSD unaltered, convert PCM to 1-bit bitstream before transmits to DAC circuit
- Discrete 1-Bit DAC: convert digital signal to analog signal through a resistor network composed of 128 pcs (4 x32) high precision Thin Film Resistors
- Power Supply: Sophisticated low-noise highly-isolated supply circuit to support different functions of digital and analog processing separately
As mentioned previously, 1-bit DAC is very sensitive to the integrity of the incoming digital signals, we have to perform a series of DSP pre-processing including re-clock, de-jitter, and noise shaping. The resulting bit-stream will then be passed to Audio Bridge where all incoming signals will be organized before feeding to the DAC circuit. If the incoming signal is DSD, then it will be pass-through without any conversion. If the incoming signal is PCM, it will be transcoded and upsampled to DSD512. Theoretically, FPGA is a good fit for this job, but the FPGA we adopted for N7 cannot handle (1) and (2) simultaneously, we need to off-load either (1) or (2) to other options, and after numerous studies and experiments, we decided to add a
single chip SRC (Sample Rate Converter). With this design, DSD playback will remain purely software-based DSP in (1) and (2), while PCM playback will go through the single chip SRC plus software DSP in FPGA/MCU
The Serial to Parallel Shift Registers will convert the serial data signal to a parallel data signal and transmit it to the DAC circuit. In our test, FPGA can carry out the Shift Registers functions beautifully but in view of the loading of our FPGA, we opted for a hardware solution here. The 1-bit DAC circuit is fully differential by design. Each channel is composed of 32 pieces of high precision low TCR Thin Film Resistors rated at ±0.1% (or ±0.001 or ±1/1,000). The temperature coefficient of resistance (TCR) of these resistors is also respectable. It is rated at TCR25 (±25 ppm/℃), and the resistor value will only fluctuate within 25/1,000,000 per 1-degree change in temperature. By fully differential, we need 4 rails of resistors to decode L+, L-, R+, and R- channels separately, hence 128 pieces in total.
As explained in the previous discussion, the N7 PCB layout is symmetrical on both sides, meaning we have layout the left (L+ and L-) and right (R+ and R-) channels on a different side of the main PCB. This is how they look on the actual PCB.
Last but not least, the power supply is extremely crucial to 1-bit DAC implementation. Cayin has decided on a low-noise low-interference power supply to feed the resistor network, Audio Bridge and FPGA separately, we shall review more detail when we discuss the overall power supply design of N7.
Synchronized Accurately
The sophisticated resistor-network-based DAC circuit can decode satisfactorily only when we can synchronize all digital processing perfectly and lower the jitter during the complete playback process. Low jitter minimizes digital artifacts and enhances the musicality and naturalness of digital audio playback.
N7 employs TWO Femtosecond Crystal Oscillators with extremely low phase noise (-100dBc) to facilitate high precision low jitter playback.
A brief comparison to R01 Audio Motherboard
When compared to the R-2R design, the 1-Bit DAC technology has relaxed the precision requirements of the resistor, but it is very sensitive to the integrity of bitstream and hence requires more work in noise shaping. For the record, our R01 R-2R Audio Motherboard uses ±0.01% TCR10 thin film resistors, so this will provide a benchmark to understand the different requirements we are referring to.
The basic building block of R-2R ladder DAC is a matched pair of R and 2R resistors, so we need two different values of resistors. The 1-bit DAC is relatively simpler because all 128 pieces of thin film resistors are of the same value. This allows more freedom when we determine the most appropriate value of the thin film resistors during the sound tuning process.
In addition, the R-2R circuit in R01 Audio Motherboard is single-end but the 1-Bit DSD DAC circuit is fully-balanced. With R-2R, we need 48 resistors to achieve basic 24-bit decoding, a single-ended “stereo” implementation will require 96 resistors, and a full-balanced “stereo” implementation will require 192 resistors. With the limited space available in R01, we can only design a single-end R-2R DAC with a balanced driven headphone amplifier (please refer to the
R01 Audio Motherboard Functional Diagram for more detail). We don’t even have space to fit the line out in R01 and all these are limitations we needed to live with R01 form factors. With N7, the 1-bit DAC circuit is composed of 4 rails of 32 pieces of thin film resistors, and that is a full-balanced full-discrete DAC design.
Last but not least, we can also compare the supporting circuit of R01 and N7 briefly. The LPF of R01 is a piece of OPA op-amp and the volume controller is the PGA2311A, a 2-ch electronic controlled analog volume chip. So the circuit remains single-ended up to this point. The LPF of N7 is a big improvement over R01 because we go all-in and designed a full-balanced discrete LPF circuit to perfectly fit the output of the full-balanced resistor network. The volume controller is also upgraded to a premium low-noise 4-ch resistor ladder electronic volume control from JRC, so both LPF and volume controller is full-balance design to start with. Is LPF that important? It is in our opinion but that is another topic that I’ll cover in my next writing, so please stay tuned.
