Vc24 Digital Filter - Sony SCD-1 Technical Background

2.11. VC24 Digital Filter.

We have just described the SACD playback
process from laser to analog output. However,
the SCD-1 and SCD-777ES are also CD players.
Compact Discs get the benefit of most of the
technologies we have mentioned (servo DSP, RF
processor, S-TACT, Current Pulse D/A Converter
and GIC low-pass filter). However, exclusive to
CD playback is Sony's VC24 Digital Filter. The
design gets its name from 24-bit processing and
Variable Coefficient design.
The filter's 24-bit process is 2 bits more powerful
than Sony's previous best. But this actually
understates the case. The new filter also con-
ducts twice as many operation steps and per-
forms direct oversampling with three additional
bits. When viewed in terms of operation steps,
two bits yields 2 2 or 4 times as many steps, twice
the operations yields 2 times the number of
steps, and 3 bits yields 2
steps. All told, the VC24 filter handles 4 x 2 x 8 =
64 times as many steps as Sony's previous best.
Since the launch of Compact Disc, each new
digital filter has had a fixed filtering coefficient.
It's been designed at the factory and offered to
home listeners with no user controls. Sony's
Variable Coefficient digital filter is a dramatic
departure. The VC24 actually offers five distinct
settings, representing different filter coefficients,
different filtering methods and different objectives
in reproduced sound.
Table 1 Comparison of the VC24 and Previous VC Digital Filters
Category
Number of operation steps [steps/44.1kHz]
Operation data word length
Coefficient word length
[Direct 8 times oversampling filter]
Maximum number of orders
[Direct 8 times oversampling filter]
Maximum number of orders
[Direct 4 times oversampling filter]
Coefficient word length
[3-stage cascade 8 times oversampling filter]
Maximum number of orders
[3-stage cascade 8 times oversampling filter]
Maximum number of orders
[2-stage cascade 4 times oversampling filter]
3 or 8 times as many
VC24
Previous VC
512 256
24 bit 22 bit
16 bit [ 2.0] 13 bit [ 2.0]
224 112
112 56
27 bit [ 2.0] 27 bit [ 2.0]
171 + 35 + 19 171 + 35 + 19
171 + 35 131 + 27
• Std. The standard setting meets the classic
digital filter objectives of the steepest possible
roll off above 20 kHz, with the greatest possi-
ble suppression of ultrasonic noise.
• df-1. This setting incorporates smoothing inter-
polation. Conventional 8x oversampling uses
three cascaded stages of 2x oversampling. In
contrast, df-1 performs the full 8x oversam-
pling process in a single pass. The result is
smoother processing and clearer sound.
• df-2. This setting maximizes the digital word
length. In each stage, it adjusts the word
length of the input number to match the noise
shaper in the next stage. This eliminates all
non-linear portions in the computation process
and suppresses quantization noise.
• df-3. This is an all-new 224-order design with
an even number of filters. Unlike conventional,
odd-number filters, the original data samples
are retained as-is, without alteration by the
oversampling process. Frequency response is
flat to 17 kHz, while noise suppression at 26
kHz is 80 dB.
• df-4. An enhanced version of df-2, this setting
increases the computation word length by four
bits, increases the filter degree twofold and
employs an even-number filter. Frequency
response is flat to 18 kHz, with a gain at 20 kHz.
Table 2 Positions for 24-bit VC Digital Filter
Position Filter type
STD standard Sharp roll off
df-1
Slow roll off
[No. 1 Clear]
df-2
Slow roll off
[No. 2 Plain]
df-3
Slow roll off
[No. 3 Fine]
df-4
Slow roll off
[No. 4 Silky]
Cut-off characteristics
20kHz
20kHz
20kHz
20kHz
20kHz
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