I have been very happy with the Extremist DAC 1, which has been powered up for the first time 3 years ago and provided me with countless hours of musical enjoyment since. It has been a worthwile project, but I need more.

So, I'm drafting the requirements for the next one.

Designing a DAC in 2006 is problematic because :

• CD is obsolete.
• 16b/44k is obsolete.
• SPDIF is obsolete.

Let's rock.

The Source

The source shall be future proof (able to read current and future formats without replacing). Only a computer fits this definition. It is made of cheap and easy to replace parts ; a HD-DVD or Blu-Ray drive can be added when needed and it can play legal music downloads in 24/96 FLAC from magnatune.com. CD is obsolete as a media for information storage anyway. I want HDD backups and playlists.

How to get rid of jitter ? Where is the master clock ?

The master clock should be on the same board carrying the DAC chips, with the shortest path between clock and DAC(s). The source (computer) and everything else should be slaved to it. Better eliminate problems rather than trying to solve them. Of course the implementation must be good enough...

Physically the DAC will be one box. There will probably be several boards inside (digital, analog, etc). Power supply will probably be external to keep noise down.

A computer can play DVDs and will be able to play any future audio/video formats (HD-DVD, Blu-Ray) while being slaved to the master clock in the DAC, so there is no problem for video playback either. For DVD playback, a computer connected to a projector via a 15€, 10 m long VGA cable has a much better image quality than a standard DVD player connected via a S-Video cable. The computer also has a lot more features, provided by free open source software, which would cost a lot of money to purchase in consumer hardware form (like : deinterlacing, recording to HDD, etc).

There shall be two clocks, one for 44.1k based sample rates and one for 48k based sample rates, with appropriate selection via software.

It shall be the first part of a system which is not yet designed. Therefore, it shall be future-proof and evolvable.

I really need new speakers. This means active digital crossover and room correction. This means a multichannel DAC. I might switch to multichannel audio (certainly not 5.1, but maybe something else). This means more channels.

Let's provision for the craziest multichannel system with active digital XO :
- 3 front channels, 4 way
- 4 subwoofers
- 2 rear channels, 2 way

Hm, so let's design for between 2 and 20 channels.

I think I'll start with a stereo prototype, then a 6 or 8 x 24/192 channels to be able to do digital crossover for 4 way speakers and various other experiments, with an expansion connector for more channels. Also a cheap multichannel codec can be used for the subwoofers while good quality dacs like PCM1704 can be used for the rest.

I will also include a stereo ADC for analog sources : tuner, vinyl.

On Formats

SACD is a joke.
DVD-Audio does not exist.
I own many legally bought CDs, ripped losslessly into the computer, when this was still legal. I do not buy copy-protected CDs or music in closed proprietary DRM-ridden formats, because :
— I would have to crack the DRM to listen to them on Linux, which is illegal,
— The sound of lossy compressions (especially WMA) sucks,
— Copy protected CDs have embedded read-errors and generally sound worse than normal CDs,
— They are quite often compressed to death,
— Online stores selling protected tunes are way too expensive,
— There is plenty of well-recorded, good music available in free (as in speech) lossless formats to be bought online (example : magnatune).

Thus, the DAC shall handle all the relevant audio formats and sample frequencies, this means PCM at 44.1 and 48, 96, 192 kHz, in 24 bits. Oversampling 44.1 to 88.2 and 176.4 is also an option. No provision shall be made for SACD.
Legacy (16b/44k) is extremely important, too.

Connection from the DAC box to the computer.

All unflexible solutions with mandatory jitter are out : SPDIF, AES/EBU, ADAT, and both USB and Firewire audio standards.

Considering the design target of max. 20 channels, all 24/192 except the woofers and LF channels in each speaker we get about 70 Mbits/s bandwidth. This is doable.

We have :
- Custom PCI FPGA board : Out, PCI is obsolete.
- PCI Express board : Out, the dev board cost $4000
- Firewire
- USB2
- Ethernet

The connection should also allow the computer to tell the DAC which sample clock to use, and the DAC to query audio data from the computer, effectively slaving it to its clock.

USB is too short (5m max) and eats system resources.

Firewire is nice but isn't very mature. Maybe someone can comment on this ? Also, Firewire audio puts the clock in the component which is elected as bus master, which can be any component. I don't fully understand how the protocol works.

I prefer Ethernet because it's the most mature one, the easiest to program and it can be 100m. long so the PC can be in another room. Gigabit Ethernet would be tops.

Connection includes drivers. The PC will, of course, run Linux, so I will have to write either a kernel driver which will masquerade this DAC as a sound card, or a userspace JACK driver, or a BruteFIR module, which will take care of :
- data transmission
- clock management
- setting volume from the computer
- displaying the song title on the DAC LCD front panel (LOL)
- etc

Linux already has support for ethernet audio (like making the soundcard on a computer virtually appear on another) so I think this should be doable without too much headache.

I might add a SPDIF input with a software PLL, just in case, for cable TV, but I don't own a TV, so this requirement is quite relaxed. And there is always asynchronous sample rate conversion.

Implementation.

I will design the electronics around the DACs, clock, and analog output stage / volume control. However this needs to be fed data in a format like I2S. I will not design a board carrying a CPU, Ethernet transceiver et al, this is too much work. I will use a readymade development board.

The volume control will be included in the DAC and computer controlled. This is to do album replay-gain in hardware.