HDCD: Keith Johnson, Pflash Pflaumer, Michael Ritter Page 4

Johnson: Because recordings made with the encoder sound better on standard playback equipment, there's no hindrance to the recording industry for using HDCD, even though the decoders are not out in large numbers yet. And since HDCD-equipped playback equipment makes standard CDs sound better, there's a real advantage to putting the decoder chip in playback equipment, even though there aren't a lot of HDCD-encoded recordings out yet.

The combination of those two things, combined with the magic that's there when the encoded recordings are decoded, will allow HDCD to enter the market from both ends. HDCD can move into the industry easily with nothing but benefits for everyone involved.

Harley: Many digital audio workstations used to prepare CD master tapes change the data as a matter of course, which would destroy the encrypted control channel and prevent HDCD decoding. It this a significant concern?

Ritter: Yes. But luckily, all the available equipment has modes that pass the data uncorrupted. If you start throwing dither on the LSB, and that sort of thing, it's goodbye to the control code. [When I compiled the master for Stereophile's Test CD 3, which has one HDCD track, I was very careful to do all transfers in the hard-disk editor's bit-for-bit mode, thus preserving the integrity of the LSBs.—John Atkinson.] If the HDCD light comes on [on a digital processor's front panel], you know you have data integrity.

As we increase our marketing and sales effort, we'll be talking to the manufacturers of editing workstations to stress that the data can't be corrupted.

Harley: How do jitter-reduction boxes, DSP-based loudspeaker and room-correction systems, and other consumer data-manipulation devices affect HDCD decoding?

Pflaumer: Some of the dejittering boxes that use interpolation filters internally will end up destroying the ability to fully recover the HDCD.

I should say something about the tendency to try and make comparisons between HDCD decoded and HDCD "not decoded" by somehow messing up the thing so the HDCD light doesn't come on. These tests are not really valid because not all of the decoding depends on the presence of that buried channel. There's no way to completely shut off the decoding in the HDCD decoder chip. Even if the light doesn't light, there are still complementary processes going on without the control code.

The only true test of HDCD vs non-HDCD is to compare playback of an HDCD-encoded disc through a D/A converter that exists in two forms: one with our decoder chip in it and one with a standard digital filter.

Ritter: When you're running HDCD-encoded information through these devices, it's best if they leave everything alone. If they're reclocking in a way that doesn't alter the data, that's fine.

Pflaumer: That's exactly right. If you want to reproduce HDCD to the highest level of fidelity, don't monkey with the signal. If you want to stabilize the time base, that's fine, but don't alter the data in any way.

Harley: One thing that surprised me about HDCD-based processors was how much better they sounded than NPC-filter–based processors on conventionally coded discs. What's going on in the filter section of the HDCD decoder chip that makes such an improvement on standard CDs?

Pflaumer: There are a number of factors, but we can't talk about all of them just yet. We can say that the calculations are performed to a higher precision than they are in most other filters. The coefficients are actually equivalent to 27-bit accuracy. It turns out that those last few bits, which add components that are on the order of 115–120dB down, create differences that are clearly audible.

Ritter: We have excellent stop-band rejection—better than 120dB. Another thing that the HDCD chip has which the NPC and other monolithic chips don't is a wide selection of dither on the output of the chip. There are eight selectable dithers that are designed to overcome the quantization problems of going through the filter, and to optimize the performance of the DAC.

If you inject fairly large amounts of dither, it randomizes the DAC nonlinearities. Instead of having DAC nonlinearities which are correlated to the music waveform, the DAC nonlinearities become random.

The dither we use is all above 22kHz. Since you have an oversampled signal at the DAC, you can put the dither energy outside the audioband, where it gets filtered out by the analog low-pass filter which follows the DAC. Using dither like this dramatically improves the sound from multibit DACs.

We also provide variable timing of the de-glitch signal, which can be used by some DACs.

Harley: It seems with all these variables that the HDCD decoder would be more implementation-sensitive than an NPC filter—you'll get a wider range of sonic quality depending on how well the designer has used your chip.

Ritter: I wouldn't say "implementation-sensitive." I would just say there's more potential for the competent designer to achieve higher levels of performance. In the default mode, our chip is just as easy to use as the NPC.

Harley: Pacific Microsonics certifies every processor design that uses the HDCD decoder chip. How does that certification process work?

Ritter: As part of the licensing agreement, the manufacturer must submit a production sample of a product—or a pre-production sample that's identical in every way to the production unit—to Pacific Microsonics for approval before the product is issued for sale.

We look at the fundamental implementation of HDCD decoding, and that the indicator light and trademark are used properly. And we also look at a variety of performance parameters on an advisory basis. We do not grade units, if you will. We might have our own opinions, but we keep them very much to ourselves. If the HDCD decoding is done properly and there are no other gross performance anomalies, then the unit is approved for sale. Of course, there's still a wide range of performance that's going to be achieved with various products.

We made a decision very early on that we weren't going to become a design service. The companies that are leaders in the industry are leaders because of hard work and investment on their parts. That's their competitive edge.

Harley: I know you developed some proprietary test signals and measurement techniques for digital systems. How close are these measurements now to predicting sound quality?

Johnson: There are still things that we hear that simply don't show up in measurements—even 120dB down with complex signals. We also have tests that reveal the conversion accuracy through clocking, which is different from looking at the clock and then assuming whatever comes out the converter is going to be timed right. In other words, we can look at an analog signal and identify how much jitter has been involved in the construction of that signal. And there are some surprises.

Pflaumer: Just having a good clock is not sufficient to end up having a properly clocked conversion. You can't do a properly clocked conversion without a proper clock. But there are a lot of other things that can go wrong. We don't want to disclose the test right now.

Harley: How significant a factor is jitter in the musical quality of digital audio?

Johnson: I've always looked at it like 10V vs 10µV, or 120dB. If you take the leading edge of a 20kHz signal and move it, how much do you have to move that waveform to create 10µV of error? It's not very much—a couple of picoseconds.

Harley: Jitter in the single-digit picosecond range is audible?

Johnson: Yes, but not necessarily. You don't have to have that kind of timing accuracy at all times and for all frequencies. The worst possible case is where the waveform is the steepest one, and where the timing event has just happened. A few picoseconds is as much error as you can tolerate to prevent hearing distortion components that aren't masked. If the jitter is higher than that, you may have the equivalent of 100% distortion because you don't hear the signal that created the distortion; there's nothing covering it up.

Pflaumer: Because HDCD has more resolution than 16 bits, reproducing the signal accurately requires proportionally lower jitter. We've gone to real extremes to be able to reconstruct clocks accurately. And it's absolutely essential. I'm glad to say that jitter is becoming widely accepted as an important factor. It's something that we've been paying attention to for many years, because if you don't pay attention to it, you cannot come anywhere close to the kind of quality that HDCD is capable of.

Harley: Did you realize the goal of HDCD?

Pflaumer: It actually surprised us that we achieved higher resolution and more transparency in the HDCD process than we were shooting for. We didn't realize what we had until we did a series of live recordings with real mike feeds, because there isn't any other signal source that's like a mike feed—especially Keith's mike feed. We did most of our research with outtakes of Keith's first-generation analog master tapes. We couldn't have gotten where we did without those outtakes.

But when it came right down to it, Keith's live mike feed had so much more information that it truly surprised us how much of that information made it into the recording. As we improve our D/A converters, it still surprises us how much more like a mike feed HDCD sounds (footnote 6).

Footnote 6: In 2000, the HDCD technology was purchased by Microsoft—see Wikipedia. Versions 9 and above of Microsoft's Windows Media Player app are capable of decoding HDCD.—John Atkinson

John Atkinson's picture
ortofan wrote:
Are there any new disc players available today that can provide HDCD decoding?

The Ayre QX-5 Twenty processor that I reviewed in September 2017 will decode HDCD recordings. Scroll down the page at www.stereophile.com/content/ayre-acoustics-qx-5-twenty-da-processor-page-2 and you can see a photo of the Ayre's display when it detects an HDCD datastream.

As I wrote in my review, talking about Joni Mitchell's Shadows and Light HDCD, "the sonic benefits of HDCD may have been oversold 25 years ago, even in this magazine, but this was even better sound - more resolved, more palpable - than I remembered ever hearing from this set."

John Atkinson
Editor, Stereophile

PS: my apologies but I seemed to have inadvertently deleted Ortofan's original post, where he was drawing a parallel between HDCD and MQA.

Bogolu Haranath's picture

Hi, JA ........ Is there any interest in bringing back HDCD, do you know? ..........

Allen Fant's picture

Thanks! for sharing- JA

it would be a cool project to document all of the HDCD - capable
CD players. Great article by RH and trip down memory lane.

Ortofan's picture

... of CD players support HDCD discs.
Either one costs less than a tenth of the price of the Ayre unit.
Perhaps Stereophile can arrange to evaluate one (or both) of these products?

Further info on the HDCD encode/decode process:

Graham Luke's picture

...that's a mighty fine comb-over, folks!

monetschemist's picture

Thanks for reprinting this, and thanks Ortofan for the two links on the process. Not sure I trust software that claims to be HDCD-compatible.

My Linn Genki still plays the ~ 10 HDCD disks I acquired from Linn Records back in the day, thinking this might be the future. It wasn't.

dalethorn's picture

I have a dozen or so HDCD's, and if I ever knew that "decoding" was possible, I've forgotten that long ago. More importantly though, if I knew there was a shareware etc. software decoder that would allow me to write the expanded data to a new WAV file, I certainly would have gone for that. If anyone knows of such a software decoder/rewriter that's reliable and reasonably easy to use, please share.

John Atkinson's picture
dalethorn wrote:
If anyone knows of such a software decoder/rewriter that's reliable and reasonably easy to use, please share.

I believe dbpower amp and Foobar 2000 both have plugins that turn a 16-bit HDCD file into a 24-bit decoded file.

John Atkinson
Editor, Stereophile

KerryAudio's picture

They do but they do so by shelling out to the commandline HDCD.exe that was written by reverse engineering, without having access the official specs for HDCD. Thus the result of the decoding may not be identical to what you’d get from a CD player designed by a licensee of HDCD.

T-NYC's picture

Tops as named in the article equals T.O.P.S (Transcendental Operating System) from Centram Systems West, founded by Nat Goldhaber, later purchased by Sun Microsystems, now part of Oracle. Not named in this article for his own reasons at the time is David Fletcher, co-founder & fourth partner in Pacific Microsonics. David is a former experimental particle physicist at the Berkeley Particle Physics Labs, co-founder & Chief Scientist of Sumiko, co-founder & Chief Scientist of SOTA. At Pacific Microsonics, Dave was responsible for circuit board layout and prototype building, and later for component sourcing & specification, and purchasing. Dave is quite modest about his many achievements, and said the design-brains at Pacific Microsonics were Pflash & Keith. He also said that while selling his Sumiko and SOTA stock made him comfortable, that selling Pacific Microsonics set him right for life., and that working with this team got his wife and he to practice Transcendental Meditation, as they all did. Just a footnote to flesh-out the record.

Amclaussen's picture

Back in 2000, I purchased a Harman-Kardon CDR-30 Two-tray CD recorder (copier) and player. It came with HDCD and the few HDCD records available certainly did sound better. The one that had the most easily perceived difference was a Brazilian themes CD by Rosemary Clooney. BOY!... that was clearly distinct when played with the HDCD and then compared to the non-HDCD players, some of them costing many times the price for the Harman-Kardon ($650 USD in 2000).

On the other side, that Harman-Kardon recorder had UNACCEPTABLE design failures, because from say, for every 10 attempts to copy a CD into the special CD blanks of ANY brand (SONY, TDK, Maxell etc.), the damn machine WASTED more than 6, barely producing 4 playable discs (the other 6 had TOC errors and were unplayable in any player!. Soon one of the two trays stopped opening. And my unit wasn't the only one defective, as MANY owners complained to the manufacturer and received absolutely no response from Harman. I will never buy anything from them again! Amclaussen.