Product Review -
Audio Alchemy Compact Disc System - August, 1996
By John E. Johnson, Jr.
|Audio Alchemy, Inc.|
|31133 Via Colinas, Suite 111|
|California 91362, USA|
Audio Alchemy Compact Disc System: DDS Pro Digital Drive System; Single CD transport (non carousel); Digital outputs - I2S, coaxial BNC, AES/EBU, Toslink optical; Jitter - I2S 5ps, S/PDIF 35ps; Output impedance 75 Ohms BNC, 110 Ohms AES/EBU; Size 3 3/4"H x 13"W x 13 1/2"D; Weight 23 pounds including separate power supply; $1,595; DTI Pro32 Digital Transmission Interface; Three digital inputs - coaxial BNC, Toslink optical, I2S; Four digital outputs - coaxial, AES/EBU, I2S, ST glass; Power supply - Power Station Four; Jitter - < 20ps; Size 2"H x 8 1/2"W x 6"D; Weight 1.5 pounds; $1,595; DDE v3.0 Digital Decoding Engine (DAC); Three digital inputs - coaxial, Toslink optical, I2S; Two outputs - digital coaxial, analog left/right (RCA); Resolution 20 bits; Input impedance 75 Ohms; Output 3.6 V max; Size 2"H x 8 1/2"W x 6"D; Weight 2 pounds including power supply; $799
I remember when CDs were first introduced, a little more than ten years ago. The CD players were about $600 (probably $1,000 in 1996 dollars). The sound was noticeably metallic and harsh, but the technology was so exciting, I didn't care. I wanted one, so I plunked down my money and gave away most of my LP collection. After a year or two of getting acquainted with CDs, I realized I had made a big mistake. CDs didn't have the rich sound stage that my LPs had, and they were not indestructible after all. Scratches and dust DID make a difference. Throughout the last decade, I have been trying . . . hoping . . . to find that CD technology could improve to the point that I would be able to put my LP regrets behind me. Last year, after purchasing our first reference CD system for the lab, I rested easily. Now, with digital transmission interfaces, I can say that CD sound is so close to providing the same pleasure as LPs, that I no longer regret giving away those LPs in the slightest (except that I can't remember the exact recordings I had, and I would like to get them in CD form).
Audio Alchemy started out making Digital-to-Analog Converters (DACs), and have become, in a very short time, one of the world leaders in this technology. Based on that astounding success and reputation, they went on to design transports and digital interfaces (video stuff shortly down the road). I saw the prototype for their new transport at the CES in January, 1995, and have been on my knees, begging for one ever since. Courtesy of Mark Schifter, Peter Madnick, and a few others at AA, I obtained not only the DDS Pro CD Transport, but the DTI Pro32 Digital Interface, and the DDE v3.0 DAC. I donned surgical mask and gloves for the unpacking, since I had waited so long for these precious gems, I wasn't taking any chances.
All three units have their own power supply which accepts 110 v AC, and delivers DC voltage to the respective products. This minimizes interference between the AC supply and the sensitive electronics. Indeed, the hum was so low as to be inaudible. Of course, isolating the AC source also removes a potential RF interference that is present in most AC supplies. We used the Tice Audio Power Block III as further protection, connecting all associated electronics (preamp and power amps) as well as the AA equipment.
The AA package has I2S cables that can be used to connect the transport with the digital interface, and the interface to the DAC. It is not really a proprietary connection, since this 5 conductor system was designed by Philips some time ago. Anyone can utilize them, but only AA has done it so far, for CD components. The I2S allows the audio bits to be separated from the master clock, word clock, and bit clock, which maintains an extremely low jitter rate (5 picoseconds - ps).[ Click here to see diagram of bitstream and jitter ] If one uses the standard coax output, the jitter is somewhat higher (35 ps), but this is still quite low. We used the I2S connections for all tests of the AA system. Two I2S cables came with the units for complete hookup. No other cables were required, except for the analog interconnects and speaker cables after the DAC.
The digital transmission interface (DTI Pro32) performs three separate and distinct functions. First, it is a dejittering filter. It applies a two stage phase locked loop filter to the word clock signal and references it to an ultra-high precision voltage controlled oscillator. Secondly, the now jitter-free digital data are fed to the Digital Signal Processor (DSP) where custom algorithms (mathematical formulas) perform resolution enhancement by a very sophisticated form of interpolation on the signal. The DSP applies as much enhancement as possible given the complexity (amplitude and frequency) of the signal. It will always get an additional two bits or so, but in some cases can get as many as eight. This results in an output word length which can vary between 18 and 24 bits. Thirdly, the data are dithered (through the addition of a special type of noise known as Triangular Probability Density Function) to the selected word length. It is necessary to match the DTI Pro32s output word length to the input word length expected by the product connected to its output, in this case the DDE v3.0, which is looking for a 20 bit word. The DTI Pro32 can output words of 16, 18, 20, 22, and 24 bits. Interpolation has added bits in excess of the dither setting, and then the dithering will truncate (cut off) the word in such a way as to preserve the maximum amount of significant bits. If the word is shorter, then the dithering adds 0s that the DAC ignores. Also, the DSP can be shut off entirely (transmitting unprocessed 16 bit words), for the case when HDCDs are being played.
Frankly, we could not detect audible differences in the level of DSP, but we could tell when there was none. I suppose that trying to tell between wonderful, fantastic, and incredible, was just too much for our ears. With the addition of any DSP, the sound stage seemed to open up, sort of like adding the rich texture of even ordered harmonics - much like a triode tube amp does, but not quite to that extent. The effect was very subtle, but indeed noticeable, and quite pleasant. The various levels of DSP are selectable from the front panel by depressing the Phase button, holding it in until three vertical red LEDs go out, and then adjusting the DSP setting by pushing the Input button a few times until the proper sequence of LED illumination is achieved (the code is in the instruction manual . . . one of by land, two if by sea, something to that effect). The input button rotates between the three inputs, and the phase button inverts the absolute electrical phase (0o - 180o), which is useful for some CDs, and doesn't affect the sound at all on others. The DAC also has an input button as well as the phase inverter button. Obviously, only one of the phase buttons should be used. When the transport is turned on, using either the front panel on/off button, or the remote control, the digital transmission interface "Lock" "Primary" LED lights up, followed by the "Secondary" LED, assuming that the transport's internal clock is precise, of which the DDS Pro's is. The secondary lock assures very low jitter. Both the DTI Pro32 and the DDE v3.0 are meant to stay powered up all the time, as there are no on/off switches. They get rather warm, but AA assures the user that this is normal. We placed them one on top of the other, and the pair on top of the transport. The transport is heavy, and so is the platter that carries the disc, which must be placed label side down. The high mass of the platter helps to stabilize the disc during play, keeping jitter low. The DDS Pro remote control has the usual features, for programming tracks, search forward, search reverse, repeat track, repeat disc, etc. It also has peak level search, for locating the loudest part of a CD when recording onto cassette. The basic controls are on the front panel, beneath a plastic membrane which keeps dirt out. An optional chip and additional remote control are available for adjusting the output of the DAC, if you want to connect it directly to a power amp.
The DDE v3.0 DAC has its own indications, including emphasis and HDCD. Emphasis represents processing that is applied in the analog stage of recording, where the frequencies from 1 kHz up to 20 kHz are emphasized by a gradual increase up to 10 dB at 20 kHz. Not all CDs have this, but when an emphasized CD is played, the "Emphasis" LED will illuminate on the DAC, indicating that de-emphasis is being applied to bring the frequency response back down to a flat level. HDCD (High Definition Compatible Digital) is a technology developed by Pacific Microsonics. Regardless of how many bits (word length) were used to make a recording, the CD has a stream of 16 bits for the music. In reducing the original recording to 16 bits, some of the signal-to-noise ratio is lost, along with clarity in the recording. HDCD attempts to restore the quality present in the original recording by coding the CD with information that allows the DAC to reconstruct the longer word length. It is proprietary in design, so the actual electronic procedure is not available for us to describe. The question is, does it work? We obtained several HDCDs, including a sampler CD that has alternating audio tracks with and without HDCD encoding. One of the major advantages of HDCD is that an additional 6 dB of recording level are available to the engineer, and this is a distinct advantage. When we listened to such audio tracks, the sound was truly amazing. However, when we listened to HDCD/ non-HDCD tracks where they were equal in volume (that is, the 6 dB advantage was not used), we could not tell the difference (single blind). On the other hand, 6 dB of additional recording overhead is significant, and the two other HDCDs that we listened to were wonderful. The inherent noise level of all the electronics used to listen to the music is correspondingly reduced by these 6 dB, which is quite a lot.
Following is the order of events that occurs when a CD is played with the AA complete CD system:
CD Transport (DDS-PRO):
- Read digital data from disc
- Pass digital data through Cross Interleave Reed-Solomon Code (CIRC) error reduction processor, correct the errors, and remove error correction instructions from data bitstream
- Reconstruct two channel digital audio into single datastream and deliver it to digital output jack
Digital Interface (DTI-Pro32):
- Reclock the incoming datastream so the bits are spaced properly in time, reducing jitter
- Perform Digital Signal Processing (DSP), enhancing resolution to whatever maximum level the audio can maintain (e.g., 19 bit word length)
- Optimize the word length so that it matches the D/A capability, e.g., dithering it up to 20 bits in the case of the DDE v3.0
- Deliver the 20 bit audio digital datastream to digital output jack
D/A (DDE v3.0):
- Digital filter the incoming datastream, including de-emphasis
- Redither the data line, this time to optimize it for the specific DAC chips used (Analog Devices 1862)
- Digital-to-analog conversion
- Analog filter (low pass) to remove everything above 20 kHz
- Deliver two channels of analog audio to analog output jacks
The sound of this complete system is truly remarkable. I don't imagine that CD sound can ever get better than this. Although the DTI Pro32 reduces jitter, the jitter rate for the transport and DAC are already low. However, the dither capabilities of the digital interface make it, in my opinion, a worthwhile part of the package, even though it is twice as expensive as the DAC by itself. The sound stage is wide and deep, the highs are replete with the smallest details, yet there is no harshness or grittiness. Sold individually, you can see that they add up to about 4 grand. If bought as a package, you save 1. Three thousand dollars is still a lot of money. Worth it? You bet. If you have been waiting for someone to score a 10 at the CD Olympics, Audio Alchemy has done it.
John E. Johnson, Jr.
© Copyright 1995, 1996, 1997 Secrets of Home Theater & High Fidelity
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