Sony SCD-1 Technical Background page 8

The first element in this reproduction system is
ACP. Sony designed ACP to overcome one
practical limitation in decoding the Direct Stream
Digital pulse train. In the ideal world, DSD pulses
would be square, with crisp, vertical leading
edges, flat tops, flat bottoms and vertical
trailing edges.
Sony LSIs include the VC24 Digital
Filter, S-TACT system and
Current Pulse D/A Converter.
Fig. 4 Ideal Pulse Signal and DSD Signal
64ƒs
1
One cycle One cycle
Square Waveform Each cycle contains a value, including places
where the value does not switch between cycles
Fig. 5 Actual IC Pulse Output and DSD Signal
Switching Switching
distortion distortion
1
One cycle One cycle
Waveform of IC Output Switching distortion appears when the value
switches from "1" to "0" and from "0" to "1"
There is no switching distortion when the
value does not switch between cycles
6
0 0 1 1
No switching
distortion
0 0 1 1
The real world is a different place. In reality, at
the MegaHertz speeds of DSD encoding, ICs
permit some switching distortion. Leading and
trailing edges become slightly slanted. Tops and
bottoms begin with a switching glitch.
These distortions would be easy to correct in
later processing—except for one problem. The
distortion only occurs at the transitions from 1 to
0 or 0 to 1. When digital 1s occur in succession,
there is no switching—and no switching distor-
tion. The same is true for a string of digital 0s.
This uneven distribution makes switching
distortion difficult to suppress.
Fig. 6 Ideal ACP Pulse Signal
1 0
One cycle
One cycle DSD pulse signal
with a value of "1"
The leading edge is wider
Fig. 7 DSD Signal with ACP Processing
ACP
1 0 0
1/64ƒs
One cycle
With ACP, the DSD signal is converted into a complementary pulse which
contains a leading and trailing edge in each cycle [1/64ƒs]
Because each cycle contains the same distortion in both the leading and
trailing edges, the distortion can be easily eliminated by shifting the level
1 0
One cycle
One cycle DSD pulse signal
with a value of "0"
The leading edge is narrower
1 1