Genesis Technologies Digital Lens Page 3

On the subject of the RAM delay, the Lens poses a problem for a Home Theater system: the audio will be out of sync with the video. Genesis addresses this problem with a "LaserDisc" mode that bypasses the RAM. Instead of using the memory for jitter elimination, a dual-PLL input receiver takes over to reduce jitter in the incoming datastream. This feature also lets you hear the effect on the reproduced sound of removing the RAM buffer.

The Lens gets its name from its function: focusing incoming data to a precise point at the Lens output circuit. That data could be jittered and have an incorrect frequency, but the Lens will always output a jitter-free signal at a precise output frequency.

Because the Lens's output circuit is active whether or not you've got a transport connected, you can have the strange situation of seeing your digital processor locked when no transport is driving the Lens. Seeing that phenomenon drives home the point that the Lens completely reconstructs the S/PDIF or AES/EBU output signal from scratch rather than trying to clean up the transport's output signal.

The Lens suffers from one drawback inherent in all outboard jitter boxes: It must convert its jitter-free output into the S/PDIF or AES/EBU format to drive your digital processor. By putting the audio data back in S/PDIF, transmitting it down a cable, and recovering the clock with a PLL in your digital processor, jitter will be re-introduced. The jitter in your digital processor may be lower in amplitude and have a cleaner spectrum with the Lens, but it will still have some interface jitter as well as the intrinsic jitter of the digital processor's input receiver. Audio Alchemy's I2S bus alleviates this dilemma, but it works only with Audio Alchemy processors.

Inside the Lens
Some letters that have been published in Stereophile's "Letters" column have implied that designing a data buffer to eliminate jitter is a trivially easy, inexpensive task. In fact, the Lens took Genesis two years to design and de-bug. However, its price is just $1800.

The power supply uses two transformers, one for the entire Lens except the output circuit, which is powered by the second transformer. The power supply to the output buffer and output clock is regulated by a custom, discrete regulation stage using four transistors. Paul McGowan tried a conventional three-pin voltage regulator chip, but found it degraded the Lens's sonic performance. The rest of the Lens's 48 chips are supplied from three-pin regulators.

Two microprocessors control the Lens's operation. One microprocessor handles the housekeeping chores (the display and remote control, for example), and the other controls the audio processing: calculating the incoming frequency, assigning RAM, and generating dither. The Read-Only Memory (ROM) chip controlling the audio control microprocessor is socketed, allowing software upgrades simply by changing a chip. It takes 2000 lines of code to run the Lens. The half megabyte of RAM is contained in two large chips.

Input pulse transformers couple the incoming datastream from a transport to a Crystal CS8412 input receiver. The Lens's most interesting circuit, however, is the output stage. Great care was taken in its design; all that work before the output could have gone to waste if the final stage compromised the signal. The output stage consists of a Temperature Controlled Crystal Oscillator (TCXO) to provide the timing reference, a waveshaping circuit, an output driver, and its discrete power supply. The entire output stage is on an isolated section of the circuit board.

Signals are electrically de-coupled from the output stage by opto-isolators. These devices break the electrical connection between their input and output, transferring the signal by light instead of electrons. Opto-isolators use photo-transistors and photodetectors inside 8-pin packages that look like ICs. Astonishingly, Paul McGowan found that the bias to the opto-isolators had a large effect on the Lens's sound quality. A row of resistors next to the four opto-isolators set the bias.

The actual output driver chip (a flip-flop) is socketed, a move that has already proved its utility. After listening to the Lens for a few weeks, Paul McGowan and Arnie Nudell visited my listening room to hear the Lens in my system. They also brought a new output driver chip that they've incorporated into standard production (footnote 3). After they'd installed the new output driver, the soundstage had more depth but instrumental textures were less smooth. We went back to the original chip before they decided to leave in the new output driver (I can't comment on my preferences—or what I think of a product—to manufacturers while a product is under review). But after about half an hour, the Lens sounded much better with the new output driver, presumably because the chip had warmed up.

Footnote 3: If you bought a Lens with serial number lower than 111209, it has the old output driver. Contact your dealer or Genesis for a free upgrade, which is simply a matter of replacing a single socketed chip. You don't even need to know which end of a soldering iron to hold.—Robert Harley
Genesis Technologies
4407 6th Avenue NW
Seattle, WA 98107
(206) 789 3400