All incoming data, PCM and DSD, are upsampled to 30 bits running at ten times the standard DSD rate and then back down again to double DSD for noise-shaping. The ten-times DSD sampling rate was the lowest common rate attainable through integer upsampling of 176.4 and 192kHz PCM files. An internal volume control maintains complete precision. Except for the sigma-delta modulation process itself there is no rounding; a full 50 bits are used. Consequently, there is no degradation from using the digital volume control. After the volume control, the signal is converted to DSD and downsampled to double-speed DSD (often referred to as DSD128). The double-speed DSD rate allows the low-pass filter to begin rolling off at 80kHz. A higher output rate would would have increased jitter.

Off-the-shelf DAC chips can’t perform the functions described above, so Ted used a Xilinx Spartan 6 field-programmable gate array (FPGA) chip instead. An FPGA is a computer chip that’s a blank slate; you can program it to do whatever you want, and that’s what Ted did. A single master clock is used, but it’s unrelated to the input sampling rate.

I don’t usually spend this much space describing the design and functions of gear I review, but since the DirectStream DAC is such an innovative design I thought it would be worthwhile; if you’re interested in learning more about it, I highly recommend a visit to PS Audio’s Web site. Suffice it to say that Ted Smith has completely rethought how a DAC should operate and has designed a unique and innovative DAC.


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