has anyone tried PS Audio perfect wave duo

Thanks for the good explanations, Shazam. You should replace whoever writes their literature!

I do agree that the file-based read, and the I2S interface (which avoids multiplexing the data and clocks together) sound like excellent approaches. Hopefully, as you say, the I2S approach, or something similar, will be adopted by others.

What I was taking exception to as misleading, though, is exemplified by this paragraph at their site, in the description of the transport:

In a standard CD player or Transport, the master clock is synchronized to the optical disc reading mechanism. This means you are basically relying on a mechanical spinning mechanism and all of its correction systems to give you a perfectly stable, fixed clock to feed the DAC. It does not work and it is not stable.

Here's the problem. Optical disc readers are constantly changing the rate at which the data is coming from the disc. Sometimes it comes faster and sometimes it comes slower than the fixed speed of an asynchronous clock. If that data is coming in faster than the clock, you get a traffic pileup and the system crashes. Too slow and nothing comes out.

The Digital Lens has a large and smart memory storage buffer. It's big enough to handle any speed variation of the optical disc reader.

That would appear to indicate that in other transports and players the timing of data to the dac is subject to fluctuation identical to the fluctuating timing of the data coming off of the disk. It was sufficiently misleading, in fact, to have apparently misled one of our most intelligent and experienced members, with whom I was having the discussion above. :)

Best regards,
-- Al

I am a PS Audio dealer and am anxiously awaiting the demo set to be brought by from the rep firm who is local. When I have actually heard the set in my system I will give a report based on comparison to my current reference room demo set of the Bel Canto dac3 and CD2 combination.

I agree that the literature is poorly written and doesn't explain it well. I actually spent a good 30 minutes in the PS Audio room at the RMAF last fall chatting with Paul and others about the technology and I think I understand it pretty well.

A typical CD player does have a buffer, but it basically amounts to a fraction of a second in most cases (good players will have a second or more). This is to allow for the variations in spin speed and movement of the laser changing the timing data being pulled off the disk. So long as the data pull stays within the margin of the buffer you are fine in this regard. But pull too much data and the buffer doesn't have room for it (buffer over run). Pull not enough data and the buffer empties out and the digital stream stops (buffer under run). This is basically the second paragraph Al quotes above.

The Perfect Wave has a significantly larger buffer which provides much more room for error. Let's say the PW holds a minute worth of music and begins to generate the digital stream when it is half full. This gives the laser 30 seconds either way to keep the buffer filled with enough data to keep music going.

The second part of the equation, however, is the important one. A CD laser is a single pass reader and uses error correction (ECC) to try to clean up dropped sectors - it has one chance to get it right and one chance to clean it up (with imperfect correction data no less). The PW laser operates like a computer drive, which will read a sector multiple times if necessary to assure it has the data right. If you read up on the technology in the ripping software Exact Audio Copy (EAC), you will get a better understanding of the principle at work here. In fact, if I heard Paul correctly, the PW actually uses EAC to read the disk and generate the buffer.

The CD player has to spin at a constant rate that basically reflects the stream rate (this is "1x" speed in CD-Rom speak). The Perfect Wave can spin up significantly faster because it's building a significanlty bigger buffer data file (think a CD-Rom that runs at "16x" or "24x" speed). This is how, when they demo the player, they will eject the disk after about 30 seconds and the music continues. The PW is capable of pulling the data much faster because it is acting like a computer drive to create what amounts to a file, which is then turned into the digital stream.

Finally, the PW generates the digital stream like any other CD player, but if you take the stream out to the PW DAC, it keeps it in I2S format all the way to the DAC chip. A traditional transport device must convert the stream to a different format to accomodate Toslink, SPDiff, or USB, and then the DAC converts it back to I2S to send to the DAC chip. By doing this, all sorts of timing errors and re-clocking problems are introduced which creates jitter and hash. This is where we spend money on external re-clockers (like the Pace Car) to better manage this process. Using the PW combo eliminates this problem because the data is always in the optimal I2S format.

Does that make sense?

Mostly, yes. I'm still of the opinion that the design is a very good one. if you like the sound of the DAC, it should work well.

I'm not convinced yet that, other than support for higher resolution, it is leaps and bounds above other player or in particular server options at least for redbook CDs.

Isn't it a bit ironic the prices charged for "good" CD transports when a computer is seemingly so much better equipped and cheaper, at least nowadays?

I suppose when CD players first came out the technology needed to read and forward the data correctly was pricey, but I am certain it is not nowadays.

Shazam -- Yes, it makes (bit-)perfect sense! Thanks very much.

So the benefit (or at least a benefit) of the large cache is that it allows time for the large number of retries which may be attempted, at least with marginal disks, which in turn would essentially eliminate the need for error interpolation.

One question that brings to mind that I would want to assess is how well-controlled the acoustic noise produced by the drive mechanism is, considering that it is running at considerably faster than 1x rates, and it will occasionally shuffle around and backtrack for the re-reads. I'd assume that is addressed well in the design, but it seems like a relevant question to raise.

Another point relating to their descriptive literature, though, would be that claims that no error correction is used are probably mis-stated. EAC's site, in fact, states that with the EAC software "if there are any errors that can’t be corrected, it will tell you on which time position the (possible) distortion occurred, so you could easily control it with e.g. the media player." In other words, I would envision that the PerfectWave Transport does away with the combined error correction/error interpolation provisions that are provided by more conventional cdp drive units, but then makes use of the Reed-Solomon error correcting codes in its own processing, to correct all errors which can be bit-perfectly corrected. There would seem to be no reason not to do that. The difference relative to a conventional cdp is, if I am correct, that no error interpolation (i.e., estimating of what the sample value should be) is performed -- that is what is eliminated by the multiple re-reads.

FYI, re your statement about the PW using EAC, one of the pages at their site indicates that their MREC (Multiple Read Error Correction) process is "similar in concept to EAC."

The benefit of all of that, of course, will vary with the physical quality of the cd, and perhaps the age and condition of the laser. But there is no question in my mind that the I2S interface approach is vastly preferable to the conventional interfaces that multiplex and then de-multiplex clocks and data, and that is perhaps the most significant advance provided in this design.

Thanks again for the good explanations.

Regards,
-- Al