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The Development of "Advanced AL24 Processing":
Taking On the Challenge to Reduce Digital Distortion
DENON led the world in developing a practical PCM recorder in 1972. The shift to digital technology solved numerous problems such as noise, wow, flutter,
and frequency response. The delicate nuances of sound from CD up to the least significant bit (LSB) are lost, however, due to the 16-bit quantisation and
band cutting at fs/2 (half the sampling frequency) during 44.1-kHz sampling. DENON was determined to tackle the problem of quantisation noise that
remained in digital audio, and developed ALPHA (adaptive line pattern harmonised algorithm), a technology that reproduced 16-bit data with 20-bit quality
and became a favourite with audiophiles around the world.
This development later progressed to 24-bit quality with the evolution of "AL24 Processing," and as support for higher resolutions and sampling continued to
improve, DENON developed "AL24 Processing Plus" for the 192-kHz sampling of DVD-Audio.
DENON's most recent achievement is "Advanced AL24 Processing," a proprietary high-speed signal processing technology that expands the volume of data
along the time axis.
"AL24 Processing" for Bit Expansion,
"Advanced" Circuitry for Greater
Data Volume along the Time Axis
In AL24 Processing, the internal processing
circuit generates 8 lower bits of data which are
added to the original 16 higher bits for 24-bit quality
output. The circuit then interpolates the digital
data so that it as closely as possible reproduces
the smoothness of an analog waveform and allows
recordings on CD or other digital media to sound
the way they should in the natural world.
[Figure 1]
1 kHz, -90 dB sine waves (CD playback)
Conventional system
AL24 Processing
Compared by impulse response (24-bit, 96 kHZ)
Conventional system
AL24 Processing
Figure 1 shows the playback sine wave of con-
ventional 16-bit data and how an audio signal that is
only output in steps of one LSB is smoothed as if it
were 24-bit data. Of course, these differences are
also clearly audible: distortion that causes discomfort
is reduced and the sound enjoys a superior S/N, free
of noise. In addition, Advanced AL24 Processing
uses proprietary high-speed signal detection tech-
nology and high-speed processing technology to
perform high-sampling up conversion on the time
axis and produce a sine wave that is even closer to
that of an analog signal. Besides expanding the con-
ventional number of bits, Advanced AL24 Processing
uses high-speed arithmetic processing algorithms
developed by DENON to observe and analyse sam-
ple groups of the original data and make interpola-
tions associated with up sampling and frequency
range expansion. (Figure 2)
Figure 3 shows the features of arithmetic pro-
cessing. If we observe the sine wave of a relatively
large attack signal assuming an actual music signal,
we can easily see how different it is from conven-
tional processing. While considerable ringing
occurs with general FIR filters both before and
after the attack signal, there is little ringing with
[Figure 2]
output
Input data
Sample group import
Signal analysis,
Detection and
Algorithm A
Timing generate
Arithmetic unit
Control unit
output
Output data
Advanced AL24 Processing and we can see how well
suited this technology is for the reproducibility of sine
waves. Since greater efficiency in processing and
higher processing capacity allow data samples to be
processed in one stage across a wide range, signals
can also be interpolated with greater accuracy com-
pared to methods such as multi-stage configurations
that use conventional digital filters. In addition, the
oversampling rate has been boosted to 16fs from 8fs
to facilitate the expansion of data volume that contains
a more detailed, natural sound. The sonic result for the
audiophile is the ability to enjoy musical recordings
[Figure 3]
Playback sine waves with attack sounds
Conventional system
AL24 Processing
14
time
AL24 Plus Bit Extension
Algorithm B
Algorithm C
Arithmetic unit
Arithmetic unit
Output unit
time
with the full ambience of the concert hall replete with
its spaciousness and the movements of musicians.
Greater Resolution
for the Adaptive Digital Filter
The adaptive digital filter is one more brain of
ALPHA. In the conventional ALPHA, adaptive
digital filters were used to prevent the occurrence
of ringing by extending the pass band with respect
to pulse data. For Advanced AL24 Processing,
however, DENON developed more highly adaptive
filter algorithms. Since this filter varies the inaudi-
ble frequency range using the most appropri-
ate algorithm for the pulsive musical data or con-
tinuous high-frequency sound, adverse influences
caused by repeated noise or reduced response in
the high frequency range are avoided, enabling
optimum sound reproduction.
Memory
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