Digital noise in the power supply (Vdd) causes
the time base errors called jitter. The S-TACT design
suppresses the noise, to suppress jitter.
2.9. Current Pulse D/A Converter.
S-TACT effectively cleans time-base errors out
of the pulse train. The final step before filtering is
to clean up amplitude errors. The ACP circuit
does not eliminate amplitude errors, it simply
distributes them evenly through every DSD
sampling period. The actual suppression of
amplitude errors is handled by the Current
Pulse D/A converter.
A familiar feature of high-end Sony CD players,
the Current Pulse D/A changes the incoming
train of voltage pulses to a train of current puls-
es. Because the circuit incorporates an extremely
clean "constant current" source, the pulses
emerge with the desired flat tops, flat bottoms
and identical height. The high-speed switching
distortions of earlier stages are finally cleaned
out of the signal.
The resulting waveform is close to perfect. The
pulse timing approaches full quartz accuracy,
thanks to S-TACT. The pulse heights are remark-
ably even, thanks to the Current Pulse D/A. The
only remaining process in the digital domain is a
simple Current-to-Voltage conversion (I/V).
Thanks to a constant current source, Sony's
Current Pulse D/A Converter ensures high
precision on the amplitude axis.
Fig. 10 Current Pulse D/A Converter
Fig. 9 Comparison of Jitter in Conventional
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