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GENESIS | ||
Genesis Technologies Digital Lens Robert Harley Let's say you play a CD on a poor-quality CD transport and store the digital audio data in a massive computer memory. You then repeat the process, but this time play the CD into the memory from the finest CD transport extant (say, the Mark Levinson No.31). A week later you feed the two sets of data from the massive memory into a digital processor and listen to the music. Would the CD transports' sonic signatures be removed from the signal? Could you hear a difference between the transports a week later? I believe that the two reproductions would sound identical. Because the memory's output clock bears absolutely no relationship to the transport's clock, the sound would take on the characteristics of the memory's output clocking circuitry. Any jitter in the transport and the digital cable driving the memory would be completely removed from the digital audio signal. [I have performed a similar experiment, storing the datastreams from nominally identical but different-sounding discs on a large computer hard-drive. When the two sets of data are played back from the hard drive, they now sound the same.-Ed.] That's the theory behind the new Digital Lens from Genesis Technologies. Designed by Paul McGowan, the Digital Lens uses not an hour's worth of memory, but a few seconds-long enough to isolate your digital processor from any jitter or speed inconsistencies in your transport and let you listen to CDs in real-time. This is why Genesis Technologies calls the Digital Lens a jitter eliminator rather than a jitter-reduction device. While Paul was at it, he took the opportunity to add other interesting features to the Digital Lens, including selectable output word length and dither generation. What it does The Digital Lens is housed in a slim, attractive chassis with a beveled ?"-thick front panel. A display section takes up about a third of the unit's width. No controls are provided on the front panel; the Digital Lens is operated exclusively by the supplied remote control. (This remote also controls the woofer servo amplifier on Genesis loudspeakers.) Because the Digital Lens operates entirely in the digital domain, the unit has only digital inputs and outputs. One of every input type is included: AES/EBU, coaxial on a BNC jack, coaxial on an RCA jack, ST-Type optical, and TosLink optical. The output complement consists of AES/EBU, Coaxial (RCA), and ST-Type optical. In addition to its primary function as a jitter-reduction-er, jitter-elimination device, the Digital Lens performs what Genesis calls "resolution enhancement." In this mode, the Lens adds dither (a small amount of noise) to the output signal and increases the output word length from 16 to 20 bits. Unlike Audio Alchemy's resolution enhancement in their DTIPro and DTIPro 32-which attempts to calculate the additional bits using Digital Signal Processing techniques-the Lens simply adds dither to the digital signal (see Sidebar). Dither mode 1 operates only when the Lens is outputting 20-bit words. This setting activates bits 18, 19, and 20. In other words, Dither 1 tacks on three bits of dither to the end of the 16-bit samples read from the CD. Dither mode 2 adds dither at the 15-bit level. This mode can be used in either 16-bit or 20-bit output. No matter what the dither mode selected, the Lens never changes bit 16 so that the control code buried in bit 16 on HDCD discs will pass uncorrupted through the Lens to your digital processor. You can select between dither modes and output word length from the remote control. Genesis says Dither 2 softens the sound of older CDs that may be hard and bright. Most DACs, according to Genesis, like the 20-bit mode with Dither 1. A green LED illuminates in the display when in 20-bit mode, and another part of the display shows which dither mode you're in. Pushing a certain sequence of buttons on the remote will engage Dither 1 and Dither 2 simultaneously (dithering bits 15, 18, 19, and 20). You can hear for yourself the difference between the two dither modes by putting your ear up to a loudspeaker's tweeter; Dither 2 produces an audible hiss/buzz. You'd think that a digital processor that will pass only 16-bit data wouldn't benefit from Dither 1 and 20-bit output words; these processors truncate (cut off) any bits below 16. According to Genesis, however, that isn't the case: 16-bit processors, and even processors using 1-bit DACs, sound better when fed dithered 20-bit data. Genesis doesn't know why, and there's no theoretical basis for believing it's true. However, as we'll see, stranger things can happen. Dithering the DAC chip in your digital processor isn't the exclusive province of the Lens. Some processors that use the Pacific Microsonics PMD100 HDCD decoder/filter take advantage of the PMD100's dither-generation function. The PMD100 can be programmed to add one of seven levels of dither to the signal. Only a few processors use this feature, which requires a microprocessor to program the PMD100's dither generator. The Lens displays the CD's subcode information on its front panel. Subcode is non-audio data-such as track number and track time-recorded on the CD along with the audio signal. When you see your transport or CD player counting track time, it's getting that information from the subcode (footnote 1). This part of the Lens display simply duplicates the transport's display. Footnote 1: A CD has eight subcode channels, designated "P" through "W". Channel P is simply a bit that goes high two seconds before the start of a track. The first generation of CD players used the P flag to find track beginnings. Subcode channel Q contains all the track number and time information, as well as the emphasis flag and other housekeeping data. Subcode channels R through W were originally reserved for graphics encoded in a now-defunct format called CD+G (Plus Graphics). Each subcode channel has a data rate of 7.35k bits per second. When measuring the jitter in digital processors, it's not uncommon to see a spike of jitter energy at 7.35kHz. This jitter is induced in the digital interface by the subcode. Simply throwing out this unneeded subcode reduces jitter in digital processors.-Robert Harley