Krell DT-10 CD transport
The digital interface between CD transports and digital processors is a perfect example of this dilemma. The Sony/Philips Digital Interface Format (S/PDIF) standard was designed so that connecting two digital products required only one cable. This single cable carries left and right audio channels as well as the timing clock essential to making the system work.
The problem lies in embedding the clock with the audio data. As described in the October and November '93 issues of Stereophile (footnote 1), transmitting the clock in the same data stream as the audio data causes jitter to appear in the digital processor's clock. The result is degraded musical performance from digital audio.
The simple solution is to have separate cables for the audio and the clock. In this scheme, the audio would be transmitted through one cable and the clock through the other. Interface jitter would be eliminated as a source of sonic degradation.
Someone apparently thought that asking consumers to connect two cables instead of one between digital audio products was asking too much of them. Consequently, audio purists and serious music lovers are stuck with a flawed standard created for the Walkman generation.
But there's a simple solution to this problem: provide the clock signal on a separate output from the transport, and make a processor that will accept the clock input and ignore the clock embedded in the audio data. This technique eliminates the interface as a sonic variableand ends the debate over which interfaces and cables sound the best. Unfortunately, such a system works only between transports and processors made by the same manufacturer; no standards exist for separate clock transmission.
Fortunately, we can enjoy the benefits of separate clock transmission in the gorgeous new Krell DT-10 CD transport. The DT-10 provides what Krell calls a "Time Sync" output that directly drives the clock input on their processors. Hearing what the Time Sync function could do inspired my opening railings against the established S/PDIF standard. In fact, if you want to hear how bad S/PDIF can sound, connect a TosLink cable between a DT-10 and the Krell Reference 64 processor. Listen first with the Time Sync function disengaged (a front-panel button), then with the separate clock line activated.
If only all transports and processors had a separate clock link...
The DT-10 transport is a cosmetic and electrical match for Krell's Reference 64 processor (reviewed elsewhere in this issue). The two units share identical cosmetics, making them an attractive combination. The DT-10 is finished in brushed gray metal, with black side panels and a black, front-loading drawer. Because the DT-10 is a front-loader, it can be placed inside an equipment rack rather than requiring top-shelf placement.
The usual features are provided, both on the front panel and on the remote control. There's a numeric keypad on the front panel, as well as a display that shows track time and number. The display can be switched between elapsed time and remaining time, or shut off completely. The DT-10 also features a form of Favorite Track Selection (FTS) that automatically remembers track programming for each disc and repeats that program every time the disc is inserted. The remote, beautifully made from machined aluminum, incorporates some of the control functions of Krell's KRC preamp and KSA-series power amplifiers, allowing you to control an all-Krell system with one handset.
All four digital outputs (coax, AES/EBU, AT&T ST-type optical, TosLink) are included as standard. An additional ST-type optical jack marked "Time Sync" sends a separate clock signal to Krell processors to reduce jitter. Specifically, the Time Sync carries an 11MHz clock from the transport to the processor directly, bypassing the interface and input receiver. This technique eliminates the interface and input receiver as sources of jitter in the processor. Note that the Time Sync works only with Krell processors with a Time Sync input.
Inside, the DT-10's left side is all power supply, with the transport mechanism in the middle and the decoding and control electronics on the right. The power supply features a large toroidal transformer and four regulation stages, some of which use heavy-duty TO-3 devices. Most of the decoding electronics are Philips chips, with transport control and the user interface handled by Krell's custom software. To reduce vibration in the unit, the chassis is mounted on four large compliant feet.
The transport mechanism is a Philips CDM 4 Pro mounted in Krell's custom housing, clamp assembly, and drawer. The CDM 4 Pro is radically different from the CDM 9 Pro used in many other transports. The laser diode assembly, swing arm, and motor are all unique to the CDM 4 Pro, and the CDM 4 Pro mechanism is mounted in a large, heavy-duty cast assembly (instead of a stamped frame as in the CDM 9 Pro).
The laser pickup is encased in a Krell-designed machined-aluminum housing attached to two rods running on either side of the housing. These rods form a linear bearing on which the transport mechanism moves in and out for disc loading, driven by a large motor at the rear of the chassis. When the drawer is closed, a sturdy-looking clamp mechanism above the disc descends to lock the disc in place. This is by far the most elaborate and beefy drawer-loading mechanism I've seen.
Footnote 1: "Jitter and the Digital Interface," by Rémy Fourré, Vol.16 No.10; and "A Transport of Delight," by Robert Harley, in Vol.16 No.11.Robert Harley