Sony STR-DE845 Training Manual page 53

In this way, ∆Σ modulation accurately expresses the level of the signal by adding a density characteristic (referred
to as shadow density) to the pulse waveform. As opposed to PCM, which only handles the computed data, ∆Σ
modulation converts analogue signals directly into a pulse stream of "0"s and "1"s. Thus, the system is properly
named Direct Stream Digital.
The shaded portion of Figure 13-6 represents quantization noise that occurs with ∆Σ modulation. If we consider
the quantization noise only, we can see that compared to when a 1-bit converter is used (ref. Figure 13-4), the
quantization noise resulting from the ∆Σ modulator is more segmented, even though the original input sine wave
is the same. Because of the higher sampling frequency, the amount of low-frequency quantization noise is
reduced, while the amount of high-frequency quantization noise is increased.
ANALOGUE
SIGNAL
INPUT
FIGURE 13-5 - EXAMPLE OF DELTA SIGMA MODULATOR CONFIGURATION
In reality, whether the process is conducted by the ∆Σ modulator or a 1-bit converter, the volume of quantization
noise (total volume of the shaded area) is the same. The difference is that the quantization noise that results
from the ∆Σ modulator is shift to the upper frequency range. This is possible because the integrator (Σ) functions
as a sort of low-pass filter. Therefore, the quantization noise floor from the ∆Σ modulation is shaped, not flat.
Because of this, this process is also referred to as noise shaping.
In general, the quantization noise floor resulting from the PCM is flat, according to the number of hits. With ∆Σ
modulation, the noise floor is subjected to noise shaping. Because the DSD method uses a high sampling
frequency of 2.8224MHz, the quantization noise is shifted to a higher frequency range. This reduces the amount
of noise in the audible range for humans, which is relatively low.
FIGURE 13-7 - EFFECTS OF NOISE SHAPING ON THE NOISE FLOOR
DECIMATOR INTEGRATOR
∆
SIGNAL
LEVEL
fm
AUDIBLE
RANGE
1-BIT QUANTISER
Σ
Q
D
DELAY
DRIVE
WITH NOISE SHAPING
WITHOUT NOISE SHAPING
FREQUENCY
fm= 20kHz
fs/2= 1/2 OF SAMPLING FREQUENCY
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13. Super Audio Cd
1-BIT QUANTISATION
OUTPUT
13.6A124 1396 1/21/02
fs/2
13.7A124 1397 11/2/01