Building a PC as a High Performance Digital Stereo Source

This all started as I came to realize that my reliable and still functioning G&D Transforms CD transport is about 10 years old, and on its second laser diode. My time is numbered here, and I need to figure out just exactly what I’m going to do next. Recently I picked up a CD player with an SPDIF out to use as a bench tester for outboard DAC’s. The unit, in digital domain did not go to 0 dbfs, as it had an amount of attenuation permanently installed. This is not what I want in a bench test unit, and I considered this a suitable warning about the state of consumer audio for stereo. Some of my friends and customers of R.E. Designs had converted over to their PC as their digital source, and were rather enthusiastic about the results. Theoretically I see no reason why using the PC could not be a viable source as it’s really a question of implementation, and for the long run, parts availability. For us long time CD users, parts replacement, especially those laser diodes that seem to die after about 6 to 7 years is a real issue. Repair cost is one thing, and parts availability is another. I note that CD/DVD ROM drives are cheap, readily available, and do not appear to disappear in the near future, in addition they also have a reputation for being rather reliable, & accurate. (More on this later) That’s a very real consideration for the long term.

I started with my CD ROM drive and observed that it had an SPDIF out and said to myself, why do I need a sound card for CD playback if I just connect my SPDIF out of CD ROM drive to the SPDIF input of my DAC? A few emails to Plextor, and a phone call or two yielded a null result. The support tech simply emailed me back saying that the SPDIF connecter was on the right hand side, and I should contact engineering. Well, I tried to contact engineering but without a name to contact you are sent elsewhere. Catch 22 is alive and well.

I made up the cable connection, started playing a CD and I observed that my error light is coming on and off. Out comes the schematics of the DAC and the data book for the Crystal CS8412, the digital interface receiver chip, which is signaling the error. I should also note that that error signal of the D.I.R is connected directly to mute function of the digital filter! After a brief investigation with an oscilloscope and referring to the data book, the problem is that the phase locked loop is out of lock. I am mighty puzzled by this, as this DAC has never had this kind of issue with any CD player no matter what the price. My next step is to connect the SPDIF out of the CD ROM drive to my back up to another unit which is a Behringer SRC 24/96.

This works for about 10 minutes before the Behringer gives up and the lights on the front panel indicate an out of lock condition. At this point I get on the phone to an engineer that I know who is on this side of the business and I learn from him that the SPDIF out of the CD ROM drive is different from the SPDIF of the rest of the world, and this is due according to him as a cost cutting measure as the CD ROM drive market is pretty cut throat. This tells me that I will need a sound card or some kind of unit that can take the wide PLL issues of the CD ROM SPDIF out, clean it up and send it on to my DAC.

After a bit of research on reclocking units I come up with a Digital Interface Processor unit from Monarchy Audio. I make a phone call and tell them what my application is and they say it should work.

I get the unit and connect it up, put on a CD, and the lock light goes on but I am getting some pretty heavy distortion and a boatload of noise when I put the CD ROM drive in pause or stop. At this point, I open my DAC to probe the input to se what the DAC is receiving and I find a 3 volt peak to peak data stream. SPDIF into 75 ohms is supposed to be one volt peak to peak so the Monarchy unit is saturating my input and causing the distortion. I also note that the Monarchy unit is putting out a data stream of its own when no data is coming from the CD ROM drive. I then call Monarchy to tell them this is not acceptable and return the unit the next day. For this I am charged a restocking fee of some thirty dollars, plus the cost to ship it back.

Well sound cards manage to take the SPDIF out of the CD ROM drive and process it correctly so I go searching for something appropriate to my needs. My needs are pretty simple as I am only interested in the digital section, and really have no need for an analog section, besides, I can’t think of only one worse place to do sensitive analog than in a computer, and that would be a radar transmitter. The market, however, has a different opinion. I consulted with folks who are more in the know than I am on this subject as was told to stay away from the consumer issue stuff as they had a tendency to convert the sample rate and sometimes the bit rate, and besides I am also not interested in, at this time, in all that analog for a 5.1 or 7.1 system.

So it’s on to researching the pro audio sound cards, which is a dizzying process, as they have more capabilities than I can possibly use at this time, and deal with issues that I have no plans on encountering. Yes folks, the recording world has much different needs than this old fart who just wants some decent playback. After hours of combing through web sites that give insufficient data I finally get some answers from the folks at Marion Digital Audio. I bought their Marc 2 sound card as it appeared to have a decent digital section, with actually more capabilities than I really can use at the moment, and their rep got me some info that was not on their web site. The card uses the CS8427, as probably many do, which is a digital transceiver. The jitter attenuation is respectable but not great, and the RCA SPDIF inputs and outputs are transformer coupled to isolate the ground connections. I consider the transformer coupling of high importance as computers are not noted for having the “cleanest” of grounds. There is also a connection on the card for the “SPDIF” out of the CD ROM drive. The card also has optical SPDIF inputs and outputs, and analog inputs and outputs.

Marion Marc 2 Sound Card

I get the card and plug it in to the PCI slot, and proceed to load the software. I naturally get it wrong the first time and come up with the German language version. I manage to get it right the second time and I pop in a CD. Lo and behold, no error light flashing on and off, and the DAC is locked at 44.1 kHz.

This is looking good, and it’s sounding good too. In a quick A/B comparison I think there’s a very small difference between the computer drive and the CD transport, but testing like this is at best difficult as the similarities far outweigh the differences one might perceive initially.

Now at this point I am going to diverge a bit and talk about software issues for essentially two computers, the first will by my six year old unit, and the second will be recently purchased unit.

On my old unit I found I could bypass the direct sound & k mixer on Windows Media 9 for real time play back of CD’s. I just used the SPDIF out of the CD ROM Drive to my Marc 2 Sound card, and it’s related software. Copying your CD’s to the hard drive or playing back of wav files is a different story. Microsoft can make things very convenient, but not always what you want for fidelity. I don’t want compression for my copies to the hard drive and I don’t want the Microsoft K mixer or any re-sampling involved. Fortunately there are two pieces of freeware that will resolve this issue.

ASIO is a freeware package that you can download from a number of places on the net. ASIO will allow you to go directly from the hard drive to the sound card. Please note that most Pro Audio soundcards are ASIO compatible, so there is no issue with compatibility there. I was unable to use the ASIO with Windows Media 9, but I was able to load the ASIO plug in to a Winamp output plug-in file and configured it using about half of the buffer capability of the sound card. This is one of the main advantages of good soundcard, there is hardware and software on the soundcard to not only buffer your data stream, but re-clock it as well. A good soundcard will require next to nothing of the CPU, or the RAM, and allow you to do just about anything you want on the computer so you can have uninterrupted playback. I found Winamp very easy to use here for this particular application.

The other piece of freeware for copying CD’s to your hard drive is called Exact Audio Copy. This will allow you to copy your CD’s with or without compression at user selected “exactness”. There will be a bit of trial and error here, but I use the test and copy in secure mode, which takes twice as long to copy a CD. I prefer seeing things like no errors in the log file, and matching CRC code that one sees after the copy is done. The software will automatically limit the read speed of the CD ROM drive and add error correction when needed. Comparing the Exact Audio Copy from the hard drive vs. real time playback has been interesting. Here again the similarities are obviously going to be far more prevalent than the differences. I found the differences really very, very, small at best. I find this to be good news, because my initial goal was to equal or better my present level of system performance. However, the funny thing about all of this is that I seem to prefer the playback from the hard drive.

My “new” computer really represented some tough new challenges when it came to bypassing Microsoft. The new computer uses a new Motherboard and the one of the Intel Dual Core CPU’s. After we installed all the software and got a bunch of updates for Windows Media Nine I found that after a few days of banging my head against a wall that the “new” Windows Media Nine was not going to allow us to bypass it’s direct sound software, and k mixer for real time playback of CD’s. We gave up on Windows Media Nine and used Winamp for this too, which certainly was not as convenient as the “earlier version” of Windows Media Nine, but I do have my preferences when it comes to these things.

So I am pretty much back to the state that I want things in, no Microsoft k mixer and re-sampling, but using Winamp for real time playback is certainly not quite as convenient as the Windows Media Player. I guess I’ll just have to suffer.

Storing and filing can be another issue for many of us. I gave up on the readily installed software that comes with either Winamp or Windows Media Player. All of my copies are in folders by artist or band, and when I have a number or CD’s from one artist they are put into one “master folder”. This way I can keep track of my music as I am more accustomed. Winamp does make this very easy to access, which I truly appreciate.

Quieting down the acoustic noise of the computer is another challenge, and one that I am still working on. My “new” computer with Dual Core CPU runs pretty cool if one can believe the monitoring software, also the large copper heat sink with heat pipes and a low noise fan will help here also.

CPU Heat Sink with Low Noise Fan

I purchased low noise fans for the rear of the computer case, also shown in the photo above and the front of the case. I also purchased gaskets that go between the fan and the case to reduce vibration. In addition I installed some “acoustic foam” with its own adhesive backing on the inside of the both of the side panels, top panel and bottom panel. All of this does help, and I do need to get a noise reducing hard drive enclosure, but even though the acoustic noise has been reduced I still consider this the only drawback to implementing this system. They do advertise such things as very low noise computers, but they also come with an interesting price tag.

Noise Reducing Acoustic Foam on the interior of a Side Panel

Data for these units was taken by a DL1 Digilyzer which is a hand held meter that plugs straight into the SPDIF line. I found no serious errors using the good old CBS CD 1 test disc when I measured the output of the G&D transforms transport, CD ROM drive through the sound card, and the hard drive through the soundcard.

I did however find some differences.

1. The G&D Transforms transport had a 1 volt p-p output into 75 ohms, and the sound card had a 1.5volt p-p output into 75 ohms. The 1.5volt p-p is higher than what is specified for SPDIF but not enough to cause any distortion/ timing issues with the input receiver of my DAC.

2. The G&D transforms transport clock frequency was 44.1 kHz +/- less than 2 ppm. The soundcard clock frequency was 44.1 kHz +/- 40ppm. Since this is re-clocked by the DIR in my DAC this is not an issue. This was not enough to cause my DAC to go out of lock or cause any noticeable problems.

3. The G&D Transforms bit stream showed the validity bit to be in use and according to the DL1 manual this is due to an active error correction scheme in the unit. The computer/soundcard showed that the status bit indicated that channel A did not equal channel B when I was testing both right and left channels with the same signal. Is this an issue? Not really; the data stream is intact and according to one engineer who made a survey of assorted pro and computer equipment is that there are standards, but they are generally ignored if they appear to be of little use, or not worth being paid attention to, or not worth the time invested to do correctly. Given my initial experiments with the CD ROM drive I thought that statement was a bit conservative. The good thing is that nothing of major import has been altered; the important data is intact.

What I really wish I had was the instrumentation to measure the jitter and it’s spectrum from each of the units. I think I would readily find a difference there, and I suspect the following: The jitter spectrum from the sound card is mostly of high frequency nature and therefore the DIR and PLL filter will attenuate this enough so it will not be an issue. I also suspect that the jitter from the transport may have some small amount of low frequency jitter that my present DIR and PLL filter does not attenuate. This is sheer speculation on my part, and it might be the reason why I perceive there to be very small differences between the two units. I should reaffirm again that there is far more in common here than there is different, and the data and listening confirm that.

I would also like to emphasize to users that if you are using the SPDIF coax or optical digital outputs from soundcards to be sure that what you connect these to have competently designed digital input receivers with reasonably well filtered PLL’s. I also would recommend transformer isolation on the coax SPDIF line to insure that the computer ground is isolated from the DAC ground.

To sum things up here: Jitter in digital domain is not an issue, it is only an issue at the point of conversion; so if there is anything to be learned here it’s that the timing can be “sloppy”, but it needs to be cleaned up before it hits your digital to analog converter chip.

Conclusion

At the beginning of all of this I said to myself, “If I can equal the performance of my present system using a computer I’ll be a happy guy.” So not only am I a pretty happy guy, but it’s nice to know that I can get this level of performance relatively cheaply. If it’s not worth it monetarily to replace a laser diode in a CD ROM drive, than, I can pick up a new one cheap: In fact I just got a back up CD ROM Drive out of Tiger Direct for under $30.00, which includes shipping. I have approximately 400 CD’s in my collection, and I also note that I can now purchase a 750 gigabyte hard drive for about $300.00 which should easily store double the amount of CD’s that I have with no compression.

And all of this is a good thing, as I expect the CD as we know it today to start to go into obsolescence sometime in the near future. Given the recent plunge in sales of music CD’s, the move to obsolescence could happen much faster than some us originally anticipated. The only real drudgery here is that I get to copy all of this to my hard drive, but I’ve been through worse. So far all of you hardcore fuss budget music junkies there are two lessons to be learned here, there is no sacrifice in sound quality, and it won’t break your budget.

Dan Banquer
www.redesignsaudio.com

The author would like to thank Pioneer and the folks at G&D Transforms for making a solid & reliable CD transport that is still working after 10 years with a laser diode replacement as its only repair. Also thanks to two friends and former co-workers who helped me through the software learning process. Thanks to Dave Moulton for loaning me his Digilyzer unit, and last but not least thanks to my stepson Michael (the family computer IT person) for his help and patience through this entire process.