Classé Omega Super Audio CD player Page 2
The Classé Omega is said a no-compromise design. All data and control signals from the transport mechanism are connected to the main circuitry via high-speed optical isolators, so that digital switching and power-supply noise from the transport don't contaminate the sensitive circuitry in the D/A section. The transport and D/A grounds are kept completely separate, which Classé says lowers the noise floor, reduces jitter, and improves sonic performance.
The Omega uses separate signal paths for PCM (standard "Red Book" CD) and DSD (Super Audio CD) datastreams. Treating the signal paths separately, as laid out in a technical white paper available from Classé, means no compromises need be made for either datastream, and that the best performance is extracted from both formats.
The separate signal paths begin with the two laser pickups in the transport mechanism: one for the "semi-transmissive" SACD layer, one for the CD layer. The wavelength of each laser is optimized for the track pitch and pit size of the respective format, says Classé. The 780-nanometer CD laser also reads CD-Rs. The two signals are processed separately until the last stage of digital/analog conversion.
To decode standard CD signals, the Omega uses a Crystal CS4397 filter and DAC chip. This "state-of-the-art converter"—the latest from Cirrus Logic, trumpets Classé—offers performance "near the theoretical limits for dynamic range and linearity with both PCM (standard CD) and DSD (SACD) signals." It performs up to 8x interpolation "with moderately slow rolloff characteristics." The white paper goes on to explain that typical brick-wall filters with steep rolloffs "trade time domain performance for improved frequency response." Classé feels that the CS4397's moderate-rolloff interpolation filter delivers a good balance between frequency response and time-domain response.
A Pacific Microsonics PMD-200 is used to decode HDCD recordings. Realized with a Motorola DSP 56364 chip, the '200 can also perform 8x oversampling and digital filtering. "However," Classé enthuses, "the filter in the Crystal CS4397 DAC provides even better filtering, so the PMD-200 is used in the Omega SACD only for HDCD decoding."
Nor is that the end of the story. "Placing the HDCD filter before the sample-rate converter would have avoided this problem, but would have required that the HDCD filter be engaged at all times, and would significantly have increased the complexity of the design. This would also drive up the cost of the unit, and degrade performance during normal listening. Since for most audiophiles HDCD discs represent the minority of their listening, favoring HDCD recordings at the expense of standard CD recordings was not an acceptable tradeoff.
"Another possible solution would be to use the Pacific Microsonics PMD-200 for interpolation filtering as well as HDCD decoding. However, this too was seen as a compromise, since the performance of the interpolation filter in the Crystal CS4397 DAC chip in the Omega SACD is sonically superior to the interpolation filter in the PMD-200."
When playing standard "Red Book" CDs, the user can tap a button on the remote, light up the SRC indicator, and listen to 16/44.1 upsampled to 24/96 by a Crystal CS8420 chip. The switch to SRC and back again during 16/44.1 play was always accompanied by a rather sharp electronic crackling noise through the tweeters, but not when using the digital data outs.
Classé is quick to point out that one of the main advantages of the Sony SACD system is that the DSD signal requires no interpolation filtering before being converted to analog. "The Omega SACD takes full advantage of this by routing the DSD signal clear of all digital processing and connecting directly to the delta-sigma digital-to-analog converters."
D/A conversion for both SACD and CD is accomplished by an array of three Crystal CS4397 chips. As each CS4397 sports two identical converters, the Omega SACD actually carries a total of six converters. Each channel uses one DAC chip to generate the balanced output: one internal converter for positive, the other for negative. The remaining DAC chip is used by both channels to convert single-ended signals. The outputs from these DACs are routed to dedicated balanced and single-ended analog circuitry. "This way," says Classé, "both balanced and single-ended outputs don't interfere in any way with each other."