Onkyo DV-SP1000E - Manual (Page 3)

VL Digital Technology 11
VL Digital—A Quest for the Perfect Digital Sound
Another potential attraction is that low-frequency reproduction places little
load on the power supply. Analog recording techniques have limitations when
recording low-frequency sounds. However, digital recording, which has
become the dominant method for storing and reproducing audio data, has
eliminated these limitations. For this reason, more and more of today’s music
is based on powerful low-frequency sounds. These recordings contain bass
power in all its intensity.
Onkyo’s Approach to Digital Amplifiers
Based on the research of Onkyo’s development team, we believe power
supply is essential to achieving quality sound from digital amplifiers, even
though their efficiency far exceeds that of analog amplifiers. If we go back to
the basics of amplification, we want to reproduce sound that you can feel, not
just hear. For this purpose, we need a power supply with the lowest possible
impedance and superior transient response.Very few manufacturers are
building digital amplifiers with power supplies that follow our concept.
A great deal of attention has been given to power supply performance in
every Onkyo digital amplifier. In fact, in our digital amplifiers, we have taken
this concept even further by including large-capacity transformers.
Pulse Width Modulation (PWM) and Onkyo’s VL
(Vector Linear) Digital
In digital amplifiers, there are two methods of pulse conversion: pulse width
modulation (PWM), in which analog quantity is represented by the width of
the pulse, and pulse density modulation (PDM), in which analog is
represented by the number of pulses. Onkyo uses the PWM approach for a
number of reasons:
1) PWM produces far less digital noise in the higher frequencies than PDM.
2) PWM is more efficient than PDM in terms of delay relative to the pulse input.
3) PDM is dependent on a large amount of negative feedback (NFB)—approaching 100%. Even in an
analog amplifier, a lot of NFB will negatively affect the sound.
Up to now, PWM has been used as an efficient method of amplifying audio
signals. Theoretically, this method should
result in accurate analog-to-digital
conversion. In reality, a digital amplifier
generates a lot of “noise spikes” from
sources external to the modulator circuitry.
This spike noise introduces errors into the
inversion timing, making accurate conversion
into pulse widths impossible. So, to further
improve the precision of amplifiers, we’ve
had to push even further. Our response is a
highly accurate analog-to-digital conversion
circuit—VL Digital—that is unaffected by
noise in the analog signal.
Onkyo’s VL (Vector Linear) Digital technology comprises a vector generator,
an integrator (like a charger) and an inversion trigger generator. When the
analog input signal is received, the vector generator outputs a current
proportional to the size of the analog input. This current is sent to the
integrator, where it is “charged”. When the charge quantity reaches a
specified value, the trigger operates and inverts the output pulse. Circuits
charge and invert alternately, performing pulse width modulation proportional
to the analog signal.
The upper and lower portions of the spike noise waveform are symmetrical,
so they have the same area. Therefore, if the analog signal contains spike
noise, their charge quantities will cancel each other out. This will ensure
accurate pulse width
modulation at all times.
Onkyo’s third-generation VL
Digital technology includes
an inverted Darlington
circuit that goes beyond
earlier versions to accurately
produce a current flow
based on the input voltage.
The Difference Between Analog and Digital
Amplifiers
Understanding the amplification process helps to explain the difference
between analog and digital amplifiers. In an analog amplifier, the analog input
signal is amplified without any modification. In a digital amplifier, the analog
input signal is converted
into a pulse (digital) signal,
and then converted back
into an analog signal using a
low-pass filter. An analog
signal is constantly changing
within a range extending
from zero to a maximum
value. However, a digital
signal is comprised of
“pulses”—a series of zeros
and ones. The significant
difference between analog
and digital amplifiers is the
basic principle used for
amplification.
In an amplifier, the power supply circuitry (actually, the capacitors) collects
electricity. A transistor (valve) opens when an input signal is received, causing
some of the collected energy to flow out through the output jacks. This
process simply defines how amplification works. Analog amplifier signals
continuously change: the transistor must adjust the size of the “valve” opening
to match the constantly changing input signal. On the other hand, with a
digital amplifier, the signal consists of either a pulse (1) or no pulse (0)—there
are no intermediate values. The “switches” in a digital amplifier are completely
open (switch is on) when there is a pulse or completely closed (switch is off)
when there is no pulse.
Why the Interest in Digital Amplifiers?
First of all, we should consider an analog amplifier, where the signal always lies
between zero and a maximum value. Therefore, the amplifier elements
function as variable resistors that adjust the amount of electricity supplied by
the power supply to match the input level. Electricity that does not flow
through when the amplifier elements are closed is lost. For this reason, analog
amplifiers can only achieve a maximum power efficiency (relative to the
power supply) of about 70%. This large amount of energy loss means that a
substantial amount of heat is generated.
In a digital amplifier, the signal level is either 0 or 1, and the amplifier elements
function as switches with two states, ON and OFF. The amount of power loss
is very small. Consequently, digital amplifiers typically have very high
efficiency—90% or so. Very little energy is generated, so heat-dissipating parts
such as heat sinks can be smaller and the amplifiers can be more compact.
Possibilities of the Digital Amplifier
At Onkyo, we are not only interested in higher efficiency and a more
compact size, we also believe that there is a great opportunity to build a
digital amplifier with improved sound. When a digital amplifier’s signal value is
1 (the current is flowing from the power supply to the speakers), the
amplification elements in the output stage remain completely open. Broadly
speaking, there is little resistance that consumes power between the power
supply and the speakers. Consequently, there is no loss of power. In contrast,
with analog amplifiers, there is always some resistance between the power
supply and speakers because of the manner in which the amplifier operates.
Furthermore, since the output elements are used as switches in a digital
amplifier, properties such as linearity (crucial in an analog amplifier) are not
particularly significant. By reducing the number of parameters that the
amplifier must control, it is easier to ensure that the elements will be driven
as intended in all circumstances. We believe that the potential of digital
amplifiers lies in more accurate signal reproduction.
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