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Current-mode signaling and its application for audiophile audio design

A new approach to audio system design

Contemporary audio component design relies heavily upon complex component and microprocessor technology to obtain broad multi-functional capabilities. These, often complex designs derive however from the fundamental physics of digital and analog signal processing and years of refinement in design of circuits to preserve the integrity of audio content in dac and adc conversions. In addition, handling analog and digital signals, the process of converting them to each other is based on many years of research results of transmission and communication technology and information engineering. This document examines our most fundamental assumptions regarding digital/analog signal conversation and the possibility of creating a more efficient and loss free technology obtaining a new level of excellence in signal quality. In the design of high precision communication and test instruments more precise signal integrity has been obtained by employ modulated “current” rather than modulated “voltage”. This same current-mode signaling (alias current transmission, current modulation, current-mode interconnection) approach has recently been applied in audio system design to obtain remarkable results in signal quality and efficiency. Compared with traditional “voltage-mode” technology, “current-mode” of an audio signal provides a wide spectrum of advantages deriving from the underlying principles of circuit design.

Re-examining the basics of voltage, current and resistance

Above figure depicts a simple circuit consisting of a battery, conductor and lamp (miniature lightbulb). Current (I) flows from the positive (+) terminal of the battery, through the conductor illuminating the lamp and returning to the negative (–) terminal of the battery. The “circuit” is the flow of current from the battery, via the conductor through the lamp and back to the battery. Under Ohm’s Law,

wattage (total power consumption) = voltage × amperage,

For example, a 100-watt lamp operating on a 100-volt circuit will result in a 1 amp current across the circuit. In the same manner, in our example of the battery circuit, a 1.5-volt battery supplying a 1.5-watt lamp and 1 amp of current flow and in both examples, the resulting voltage drop will be 100% of the supply voltage dropping the voltage across the circuit to zero.

Ohm’s Law

As we noted above, Ohm’s Law describes “power consumption” in terms of Watts (Volts × Current = Watts). Referring to our diagram the resistance in our lamp (expressed in ohms) is the load. When the power supply of the voltage E, the current flowing through the I, the power consumption P is,

P = E × I, means power [W] = voltage [V] × current [A]

Here voltage is volt [V], current is expressed in units of amps [A]. When passing a load resistor R to the current I, voltage drop will occur that it proportionally E. This is called the “Ohm’s Law”,

E = I × R, means voltage [V] = current [A] × resistance [Ω]

Across a load, voltage drops in relation to resistance as the formula above illustrates. In the small battery powered bulb, with a 1.5-ohm resistance voltage drops to zero in the circuit. The same effect would occur with a 100-volt circuit feeding a lamp with a 100-ohm resistance at 1-amp. What is often not recognized in this fundamental review of Ohm’s Law is that while voltage drops in direct relationship to load resistance current is not affected: current remains constant while voltage is reduced by load. The fact that current in our circuit remains constant independent of voltage drop provides incredible opportunity for efficiencies in signal transmission. In an audio system, the modulated signal constitutes and AC circuit. For example, a CD player produces a digital pulse (ones and zeros), processed through a D/A converter to produce a low power modulated signal, this modulated (AC) signal is then amplified and routed to the loudspeakers. While the traditional approach to D/A circuit design has focused on voltage-mode signaling, our model suggests that many variables and inefficiencies might be avoided if we focus on the current potential of the circuit (current-mode signaling). Fast of all, what does it means the voltage-mode and the current-mode signaling?

Transmission of the AC signal

Sound itself constitutes and analog data flow. From a microphone or similar input, it is expressed and a waveform of varying wave constantly changing in frequency. As illustrated below, in the audio waveform, changes in voltage or current result in modulation of the wave amplitude (AM). Also, audio signals express constantly changing frequencies which are described in the waveform as frequency modulation (FM). The analog waveform is represented as a sine wave (sinusoidal wave), such as the following equation.
a (t) = Am sin 2πft, where Am: amplitude (voltage or current), f: frequency [Hz]
a (t) = Am sin (ωt + φ), where ω = 2πf: angular frequency [rad / s], φ: phase angle [rad]

At a resting state, any audio circuit has a voltage of zero, as the transmitted signal includes both positive and negative voltage with reference to these zero states we refer to the + and - voltages expressed in the signal as “phase”. The change in amplitude (customarily expressed as positive and negative voltage levels) in the waveform expressing the characteristics of the audio signal can be referred to as “voltage-mode signaling”.

The internal resistance of the power supply

Returning to our example of the battery and the miniature lightbulb, the resistance of the bulb provides a matched circuit. While Ohm’s Law suggests the possibility of a theoretical “short circuit” which could be obtained by directly connecting the positive and negative terminals of our battery, this is not quite possible in the real world. Every circuit in the real world will present some resistance by virtue of slight impurities in the composition of conductors, internal chemical resistance in our battery attributable to design and construction limitations, and loss across components comprising our audio circuit. The battery has a load that consumes power in its own, it refers to it as the “internal loss” or “internal resistance”. On the evidence, because the battery was not short-circuiting will be hot, it is why is converted into heat and power. The internal resistance is not a conspiracy of the battery manufacturer that tries frequently to buy a battery, due to the chemical composition of the internal battery, we will not be able to completely 0-ohm. In general, the larger the battery that can handle a large current, the internal resistance is smaller. Shorting a large battery for a motor vehicle, since the terrible thing, please do not absolutely imitate. Speaking of which, there was a jet aircraft that caused the fire in flight, the cause was that leakage around the battery. To focus again on our battery example, we can illustrate the internal resistance of the circuit in the following diagram. In this figure, RL is our load (the bulb), r2 is the internal resistance of the battery which is an attribute of its composition and r2 is the parallel internal resistance inherent to battery design (this is why even an unused battery will eventually loose voltage). The other of the internal resistance r1 is, gradually will increase the use of the battery, because coming down the voltage, is what comes from the change of the interior of the chemical composition (deterioration). Assuming that the internal resistance and the 1-ohm, even if short-circuit the terminals of the battery 1.5-amp only current flows.

Voltage and current sources

Because the voltage source, by the power source having an electromotive force and internal resistance as the battery, even if current resistance value as a load is changed, the terminal voltage means the power of the substantially remains constant. Insofar as the battery itself contains both supply power by virtue of flow across the terminals and by design possesses and internal resistance, while changes to the value of r1 will changed the voltage across the circuit, the current will remain relatively constant. In particular, r1 is 0-ohm, referred to as the “ideal voltage source”, the voltage source of r2 is infinite.
In a traditional audio system, the input is of relatively high impedance (e.g. 10k-ohms) and output of relatively low impedance (e.g. 600-ohms). Amplification occurs through the modulation of internally supplied voltage by the low voltage analog input resulting in a high-power voltage modulated output.
The current source against it, because it does not buy at a nearby convenience store like a battery, but is a little hard to understand, is one of the important concepts that are basic in the electrical circuit. The current source as defined, also changes the resistance value to be a load, current in and out of the between the two electrodes is called the power of nearly remain constant. As shown in the figure, in the current source is enclosed symbol with a circle arrows, the inside of dotted lines is the configuration, including the internal resistance. r1 is infinite contrary to the voltage source, referred to as the “ideal current source” in the case of r2 is virtually 0-ohm. In a circuit with no resistance current has the potential to obtain a theoretically infinite value, however all real-world materials and circuits contain inherent resistance. If our circuit within the dotted lines is viewed as the current source and RL is the load, changes in the load against the constant current flow will be reflected as changes in voltage within the circuit.
That is, since no matter how flowing a constant current by connecting a large load resistance, you can increase the voltage (potential difference) generated plenty at both ends of the load. It does not exist in practice such ideal current source, but please understand as a concept. Even with a relatively large load, current will remain relatively constant.

Voltage-mode and current-mode signaling

In the case of the AC signal, it can be thought of in the same way as DC current. For example, D/A converter as a signal source are often designed to output a current proportional to the value of the digital signal, and converts it to temporarily receives the some k-ohm resistor to a voltage signal of 1 ~ 2Vrms, further capacitor it sent out to cut the DC component. Then, the cable to the weak current (as at most 10 ~ 100µA) flows, it occurs again receiving side as a voltage signal which is obtained by subtracting the loss of the signal path. This is called a “voltage-mode signaling”. Pretty in a roundabout way, yet it is not doing a poor thing. Still appears to be the transmission of voltage, but in fact we are sending secretly converted to a weak current in the signal path. In this way, there is a fundamental problem in the voltage-mode signaling.
On the other hand, if you connect to the ideal current source can be regarded as the signal source from the sent current signal (about 1 ~ 10mA) to 0-ohm of load (input impedance), it is of course voltage to 0-ohm so the receiving side does not occur, but still current it can be transmitted accurately. It is the same reason as the return to the batteries from the 0V side of the miniature bulb. This is called the “current-mode signaling”. Contrary to the voltage-mode, becomes a “high Z-output, low Z-input” in the current-mode, the “voltage” on the receiving side in the case will not be transmitted, but the “current” will be transmitted. In other words, the difference of voltage-mode and current-mode signaling, whether in the delivery side and the receiving side of the impedance is higher or lower, only the difference in the magnitude of the amount of current to be transmitted in other words. It turned out exciting the current signal from the ideal current source if you take it in the current input amplifier with input impedance 0-ohm, you can achieve a strong respectable current transmitted to the noise. Even just thought of it, we feel that it is better audio system.

to be continued...

Noise Protection

We, audio listeners live in an increasingly crowded technological environment. As our lives become filled with technology, every lamp (LED, fluorescent, etc.), cell phone, computer or other technical gear contributes to the electrical noise around us. Also there are many kind and wide bandwidth noise source around audio instruments. However, interconnect cables and chassis is one of effective RF (radio frequency) antenna! Then the audio circuit also becomes "noise radio", because these RF noise raise inter-modulation distortion and make sound signal dirty. If there is no noise, the audio circuit will work more completely (ideal). So we have studied how to reduce noise from external environment, common impedance and internal RF pulsive devices. On this viewpoint, we develop our products.