Direct Stream Digital (DSD) and pulse-density modulation (PDM) store sound as a single bit at a very high rate. The density of ones in the stream is the signal: mostly ones is a high level, mostly zeros a low one, equal numbers is silence. A one-bit stream is not read sample by sample; it is filtered, and every group of bits (the decimation) becomes one audio sample.
Rates
| Name | Bit rate | ÷ 64 gives |
|---|---|---|
| DSD64 (SACD) | 2,822,400 Hz = 64 × 44,100 | 44,100 Hz |
| DSD128 | 5,644,800 Hz | 88,200 Hz |
| DSD256 | 11,289,600 Hz | 176,400 Hz |
| DSD512 | 22,579,200 Hz | 352,800 Hz |
| PDM microphone | 1,024,000 to 3,072,000 Hz are usual | 16,000 Hz at 1.024 MHz; 48,000 Hz at 3.072 MHz |
In the explorer the Sample rate is the rate of the bits, and the audio rate is that divided by the Decimation (8, 16, 32, 64 or 128). Wrong decimation does not make noise: it plays the right sound at the wrong speed and pitch, with the wrong amount of the quantisation noise that these streams push to high frequencies still left in.
Bit order and interleaving
A byte holds eight consecutive bits. Which end of the byte is first in time is the bit order: the Sony DSF specification says its data is stored least significant bit first, so DSF is read with little-endian in the Byte order option; formats that store the first bit in the top bit are read with big-endian. With more than one channel the two families differ:
| Family | How channels share the bytes | Layout option |
|---|---|---|
| DSD | whole bytes: byte-interleaved (a byte of channel 1, a byte of channel 2 …), planar, or in runs of a fixed size | Interleaved, Planar, or Blocked with the Run size; the DSF header calls the run the “block size per channel” and its spec fills the unused part of the last block with zeros |
| PDM | single bits: a stereo pair of microphones sharing one data line sends the left bit and the right bit alternately | Interleaved (bit by bit) |
How the explorer filters it
A bit of 1 is +1 and a bit of 0 is −1. The stream goes through a fourth-order cascaded integrator-comb (CIC) filter that adds up R bits at a time (R is the Decimation), applied four times, with the gain 1/R4. The integrators run in 32-bit wrap-around arithmetic, which is exact for R up to 128. This is a plain, cheap low-pass filter: it removes the shaped noise well enough to show the waveform and play it, but it does not match the sharp filters of a mastering tool, so it has some droop at the top of the band. A stream of alternating bits comes out as silence and a stream of ones as +1 (both tested). A built first-order sigma-delta stream of a 0.5-amplitude 1 kHz sine decodes to a wave of peak near 0.5.
No other one-bit codings are offered: there is no DST (the lossless compression of DSD inside SACD and DSDIFF), and no 1-bit delta modulation with a fixed step.