According to Zero One's website, "The Mercury CD/HD Player combines the functions of the Ti48 CD/HD transport and an internal DAC based on the Ar38 design so that it will function as a standalone CD and hard-disk audio player. As with the Ti48, the Mercury has a software-enabled upsampling processor built-in that allows users to choose the sampling frequency and digital filter. The internal connection to the on-board DAC uses our custom I²S link and the sampling frequency can be set at various rates (e.g. 48kHz, 176.4kHz) up to a maximum of 192kHz. An S/PDIF (coaxial) output is also available and when using this connection to an external DAC, the Mercury can transfer data up to a maximum sampling rate of 24-bit/96kHz.


"The digital filter choices allow you to tailor the sound to your system or room to some degree. There are five filter options ranging from "no filter" to "brick wall". In addition, there are various dither options which help to optimize the performance of some DACs when the S/PDIF connection is used. The Mercury has 250GB of standard hard-disk capacity which enables it to save nearly 400 hours of uncompressed CD data. The Mercury can store either an entire CD or selected tracks. These can be organized according to artist or album name. Using these classifications, play lists can be created containing preferred tracks. The Mercury has a database that recognizes the album titles, track and artist names of nearly 2,000,000 CDs. This database can be easily updated with a CD-ROM (available from Zero One). The DAC section of the Mercury is based on the Ar38's DAC design, with multi-bit DAC architecture, fully regulated power supplies and a single-ended, zero feedback, discrete (no opamps in the signal path) output stage."


The terms upsampling and oversampling are the source of much debate and confusion in audio circles. The fact that several manufacturers either inadvertently or deliberately obfuscate the definitions only muddies the water further. Zero One's explanation is blessedly straight-forward:


"Upsampling and oversampling are forms of sample rate conversion (SRC), where one sampling rate (like the 44.1kHz of CD) is converted to another. With oversampling, the source rate is stretched out to an integer multiple (e.g. 4 times to 176.4kHz) by mathematical interpolation. The term upsampling tends to be used when conversion of the audio data is to a non-integer multiple of the source rate (e.g. 96kHz). SRC produces more output than input bits so devices such as the Ti48 increase the bit depth of the data to improve the accuracy of the processing and output.


"Neither upsampling nor oversampling adds data that did not already exist on the CD being played back. The benefits of the process mainly result from pushing the digital images that arise as part of all digital sampling processes further away from the audio band. With a sampling frequency of 44.1kHz, digital images of the data would appear directly after the valid audio data (at 22.05kHz). A very steep filter (commonly referred to as a "brick wall") would be required to remove the ultrasonic mirror images perfectly but the ringing that such filters introduce may affect the sound quality.


"By upsampling to for example 176.4kHz, the digital images of the data would be pushed out beyond 88.2kHz, allowing the use of a digital filter with a gentler slope to reduce the ringing and phase changes. Another possible benefit of having less steep digital filters is that these filters allow some ultrasonic noise to reach the D-to-A conversion chips. The ultrasonic noise may be acting as dither which would help to optimize the performance of the chips."

If the above wasn't sufficient enough, Zero One Audio's Alvin Heng emailed me a terrific technical overview of the Mercury. I wish every manufacturer was this forthcoming:


"1) Transport Section
This is based on the same (6moons) award-winning design as the Ti48 CD/HD Transport. It is a PC at heart running on Linux, with a custom user interface and our own digital processing software suite. It has been mentioned before in 6moons but may be worth repeating that our custom ripping software has been compared to the best currently known ripping software (Exact Audio Copy) and is as good as that and bit for bit accurate.


"The digital processing is done using software algorithms with 64-bit floating point maths so it is far more accurate than any off-the-shelf upsampling/oversampling or digital filter chip used in 99% of all CDPs. The digital audio then passes to the pro audio 24-bit sound card which is used purely to transfer the data via I²S to the internal DAC (there is no audio processing performed in the sound card).


"To ensure that the sound card is not the weak point, we feed it with a regulated, filtered DC supply. The card is also mounted on a separate isolator PCB which removes a significant amount of noise coming from the motherboard's electronics.


"2) DAC Section
This is a multi-bit design using the famous Burr Brown PCM1704 chip. There are no digital filters or upsampling chips on the DAC board. All the required digital filtering and upsampling is done in the transport section with our custom software. The digital audio is fed directly to the PCM1704 chips by internal I²S running at sampling rates of up to 24 bit/192kHz.


"The output section of the DAC is fully discrete, single-ended pure class A, with zero global feedback. What this mouthful means is no opamps or caps in the signal path and the design is as inherently linear as it can be without negative feedback. We like this approach because we feel that it maintains the harmonic richness of real music without any of the bleached, hyper accurate sound of some digital designs.


"3) Design Philosophy
We have had several customers compare the Ti48 and Ar38 to some very highly regarded, far more expensive CDPs or transport/DAC combos (often their own or belonging to friends) and all have said that the Zero One kit was either as good as or superior to these other makes. We think the reason is that we focus on achieving the playback qualities of musical fluidity and natural harmonic textures and tones. We don't worry too much about extreme treble or earth-shaking bass or soundstaging. It's not that our products are not accurate - we think they are true to the recording, just that nothing is emphasized over any other quality.


"Having achieved most of our goals with the Ti48 and Ar38 designs, we wanted to put them both into a single chassis which many of our customers had been asking for. But to get them to fit in one chassis, we had to throw out a few things. So some of the extensive internal shielding had to go and we had to cut back on some of the comprehensive supply regulation. We think the result is that the Mercury retains the most important aspects of the Ti48/Ar38 sound and cuts back slightly on the other more audiophile qualities.


"Lastly, and perhaps most importantly, we do not consider the Mercury to be an "audio server". We get lots of enquiries asking why the Ti48 or Mercury cannot be configured for multi-room and why they do not have modems to connect to the internet and why you can't rip to MP3 etc. The Mercury is a stereo, high performance CD audio player (just as the Ti48 is a stereo, high performance CD audio transport). To this end, we deliberately disabled the modem functions (to avoid viruses and electronic noise); we threw away the monitor (again, electronic noise); and we focused all the PC's processing power on just one thing: stereo sound quality (no glitches trying to stream to multiple locations). So sound quality was the key issue but you do get the bonus of having all your CDs in the hard drive."

Specs are as follows:
- Reads CD, CD-R, CD-RW
- 250Gb hard disk drive ; higher capacity HDDs available on request
- Digital output : 1 S/PDIF coaxial (RCA)
- Analogue output : 2V RMS; 200 ohm impedance