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Old 1st August 2008, 10:00 AM   #1
Bidule is offline Bidule  France
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Default PCI I2S External NOS DAC

Post #1
This is my first post ever, i'm not as skilled as most of you by far but i spent a lot time in learning from your post. My turn...

I have an project for a High quality non oversampling PC based DAC for which i will need positive help an critical thinking.
Here is my plan (after much research):

1) Ripping my music to a PC media server. Why?
- Because it is so much more convenient to have music on a PC, it can be sorted using a jukebox program, you dont need to constantly select & insert & put away CDs, your CDs never get scratched, the music is available on other PCs via the network.
- More over, providing you use a good ripper (EAC), you have bit accuracy, you could implement software oversampling and finnally PC based system is future proof in terms of hi rez music format.
- You could perform moderate digital loudspeaker/room EQ.
- One could also perform digital crossover (personnaly not my approach).

2) Now that your music is in a big Hard drive (in RAID) you may decide to perform (night) software oversampling (ANYTIME from : http://www.fmjsoft.com/ (other similar/better software welcome). The advantage of software based oversampling is that oversampling may be done with much more precision than that performed with ASRC or DIR filters.
If you decided to go with oversampling, your files are now two (88.2 KHz) or four (176.4 KHz) times larger (big disk doesn't cost anymore expensive).

3) Use a software player that supports upsampled WAV and is able to bypass the internal Windows audio processing engine Kmixer. The player needs to support Kernel/ASIO Streaming mode to bypass Kmixer and allow the PC to use the native bit perfect PCM stream without processing it.
The Foobar (www.foobar2000.org) player will do this.
Might be that SoftSqueeze would also work (your thought ?). This approach would be much great as more looking friendly and web remotable with Squeeze controler or any PC runing SqueezeCenter.

4) If you are still with me so far, we have stored and can play a bit perfect (oversampled or not) copy of the CD on the PC. Now we have to get the PCM out of the PC and convert it to audio using as high a quality DAC conversion as we can. The conventional approach is to do this using a soundcard like the Creative Audigy cards, or even better some of the pro music cards like the Delta 410 or RME cards. Note that these pro cards in a Digital Audio Workstation (DAW) process a fair chunk of the music produced today in the audio production process. However I'm proposing that we do things a little different (this is a DIY site after all) and that we host the DAC external to the PC and that we use non hard ware oversampling and all the other great techniques articulated in previous posts about DAC design.

Why? Because the PC is a noisy environment to be doing the digital to analog conversion, there is heaps of EMI boucing around the place and the PC powersupply is no place to hang a sensitive DAC chip to (having said this, the pro audio cards do a great job). An obvious option here is to use a decent soundcard and use the SPDIF output into a DIY high quality DAC. I think we can do better that this because SPDIF is such a crummy interface and hard to do correctly, and surely with a PC we can create an I2S feed that can go straight into the DAC and forget about jitter?

So how do we do it?

5) I plan to use an E-MU 1010 PCI card along with its daughter board (other similar/better board welcome). This board provides SPDIF output and SPDIF SYNC.
There is also a SYNC Daugther board availlable that allow WORD CK SYNC from external high quality clock.

To go one step ahead one should be able to get I2S out with word clock sync from the DAC Clock.
The advantage of this approach is that the data is buffered in the pc memory until the dac needs it, so fluent "streaming" whatever sampling rate should result. And the external SYNC approach along with I2S should provide the DAC with JITTER FREE ACCURATE BIT.

6) The DAC itself wold be based on a differential PCM1704 DAC that if i well understood woul allow to play native 176.4/192 KHz material.

I'm not sure who on this board is interested in this approach (high end audio & PC convergence) but I think this will make a decent and unique high end DIY project. I'd expect the quality to be stunning.

I'm interested to hear if anyone else has any thoughts on this approach, I'll post the results, schematics etc. as this project progresses.


Regards,
Jean
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Old 1st August 2008, 10:29 PM   #2
deandob is offline deandob  Australia
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Jean,

I'd be interested. I use a PC as a transport, use Foobar and have an EMU 1820m (has the 1010 PCI card & daughter card). I use a dual TDA1541 double crown NOS DAC via SPDIF and its pretty sweet, but would be interested in a potentially better NOS DAC, especially one that avoided SPDIF. Yes, the 1010 card and WORD SYNC can be used to avoid SPDIF jitter, and may not need conversion to I2S for transport if you can re-sync at the DAC.

You do know that if you software oversample and you use a NOS DAC you somewhat negate the idea behind NOS . However an advantage is that you can use a less severe filter to get rid of the out of band artifacts from the analog conversion. Also if you use 88.2Khz you don't need to rebuild your music offline, as it is a simple sample interpolation 2x you can do this realtime in foobar easily.

Do you have a differential PCM1704 NOS DAC design in mind?
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Old 1st August 2008, 11:58 PM   #3
flshzug is offline flshzug  Antarctica
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flawed thinking
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Old 2nd August 2008, 02:34 PM   #4
Bidule is offline Bidule  France
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flshzug,

Could you be more acurate ?

deandob

"software oversample and use a NOS DAC, you somewhat negate the idea behind NOS ."

That's true, Howerver, seems to me that :
1/ Pure NOS have other trade off in the folowing amplifier because of the feedback loop.
2/ Might be a good compromise in terms of keeping good timing.

"Also if you use 88.2Khz you don't need to rebuild your music offline, as it is a simple sample interpolation 2x you can do this realtime in foobar easily."

I'm hoping that software interpolation can be much precise as it allows for more points to be taken into account and more algorythm to be tried. Try Anytime (free evalution), just to see how muc time it takes to perform oversampling as soon as you add more points.

"Do you have a differential PCM1704 NOS DAC design in mind?"

Yes but not fully detailed, still have to make some decisions.
I will be out for about 3 weeks, i'll try to post something then.
Do you know of a free schematic design software ? Any way i will need help and advices.

Thanks for your support

Jean
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Old 2nd August 2008, 05:34 PM   #5
flshzug is offline flshzug  Antarctica
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"There is also a SYNC Daugther board availlable that allow WORD CK SYNC from external high quality clock."


and the brand new from factory AK4115 Rx/tx chip is specified 300 ps jitter when used with wclk . Look for peufeu's post, there s no need for PCI.
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Old 3rd August 2008, 02:27 AM   #6
Bidule is offline Bidule  France
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flshzug,

Thank you for your answer.
I had seen Peufeu's post.
1/ His project is above my personal skills.
2/ Its multi chanel minded.

Originally, i was planing to mod a SqueezeNox. So, it was a two chanel Peufeu's similar approach. I changed my mind for the following reasons :

1/ 44.1 KHz limitation, and Price for Transporter too high just for adressing this limitation
2/ Full computer looks more future proof

I like the PCI approach as it seems to me the more fluent, the more flexible and the least intrinsic jitter added.

Only draw back that i could see would be less insulation.

Do you think i should consider to add ISO 150 between PCI and DAC chip ?

Jean
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Old 3rd August 2008, 11:09 AM   #7
flshzug is offline flshzug  Antarctica
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it doesnt matter if PCI or Firewire or USB, the drivers are more important than that , Audio Precision has USB connectivity for example. This is no beginner project either way, and even , if you want call it high quality ,you have to source 2 values of low jitter clocks for both 44/48khz with their own seperate regulated supplies etc. Also, to transmit i2s to a distance, you need another protocol besides spdif, thats the tiniest issue, there are some cinnercial DAC 's using LVDS inbetween PCBs.
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Old 3rd August 2008, 11:09 AM   #8
flshzug is offline flshzug  Antarctica
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"commercial..." can't edit because im moderated
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Old 3rd August 2008, 11:09 AM   #9
flshzug is offline flshzug  Antarctica
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and, I think i read that squeezebox has somewhat stupid (but moreso economical hehe) clock generation scheme, it has one single clock thingie that generates both frequencies for 44.100 and 48000 . There is a modding topic on audiocircle about this.
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Old 3rd August 2008, 12:50 PM   #10
phofman is offline phofman  Czech Republic
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Unfortunately, most "modern" sound cards use only one crystal and derive the required frequencies via PLL. In the old PCI world two crystals were a standard, nowadays I do not know of any PCI-express card with two crystals. Apparently, manufacturers consider 44.1kHz a history (and they are basically right ).
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