音響魔術師 Audio Alchemy DTI Plus 時基校正器
AUDIO ALCHEMY DTI PLUS
JITTER ANALYSIS: CD TRANSPORT-TO-DAC VIA AUDIO ALCHEMY DTI PLUS
One of many ‘little black boxes’ in the Audio Alchemy range, the DTI Plus offers inputs optical sources and provides a single coaxial output. Crystal’s evergreen CS8412 interface chip is run in ‘test mode’ with its free running PLL (Phase Locked Loop) offering jitter rejection beyond its 20kHz bandwidth. AA then provides a second stage of dejittering with a discrete crystal-based PLL that has a claimed bandwidth of just 5Hz. Once activated, a yellow LED lights up on the black fascia.
This master oscillator is used to re-clock the CS8412’s digital output via a D-type flip-flop and buffer. In practice, however [Fig 1a and 1b], the common ground by its AC-coupled Input and output aggravates a massive increase in fundamental data induced jitter (13) and random noise (14) at frequencies well beyond the 5Hz suggested in AA’s literature. By way of comparison, a figure of 12304ps is nearly 100-times higher (worse) than that incurred by the best of today’s budget CD players.
In reality the bandwidth of the crystal PLL must be several hundred Hertz because, via the optical input at least, the suppression of jitter only starts to take hold beyond the 3rd (15) and 5th (16) harmonic sidebands. If only the DTI Plus had been a transformer-coupled unit, then this unhappy result may well have been avoided.