What is DDX (Direct Digital Amplification)? Encyclopedia of radio electronics and electrical engineering

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Any audio system that is defined today as digital is actually digital-to-analog. The digital signal from the information carrier is fed to a digital-to-analog converter (DAC, in English terminology - DAC). In mass audio systems, further signal processing (filtering, volume control, timbre, etc.) is performed by analog methods. This is a potential source of distortion and noise, especially when processing low level signals. In high-end audio systems, signal processing is performed by a digital signal processor before the DAC, which can significantly reduce distortion. However, these differences in the structure are not fundamental - in any case, the signal after the DAC goes to the analog amplifier. The sound signal is especially vulnerable at the stage of analog processing and amplification, which is why the character of the sound depends so much on the components used.

This problem is in principle solvable, the only question is the price. We will not touch on the problems of coordinating the components and their interaction with each other - we will restrict ourselves to the most critical links. It's an amplifier and a DAC. Without going into details of the various methods of digital-to-analog conversion, we note that each of them gives the sound its own shade. Amplifiers, as you know, also have their own "handwriting", determined by the class of operation of the output stage and the general circuitry. Economic class AB or B amplifiers introduce significant distortion into the signal, class A amplifiers provide high sound quality, but are uneconomical. If a digital amplifier is used in the path, the inverse conversion of the input analog signal into digital form is carried out in it, which introduces additional distortions. Therefore, digital amplifiers are still used mainly for subwoofers, since the requirements for signal quality are not so high there.

To improve sound quality and maintain high cost-effectiveness, the audio path must become completely digital - from the signal carrier to the speaker system. Apogee has proposed DDX (Direct Digital Amplification) technology for digital amplification paths, in which the conversion, processing and amplification of an analog signal is completely excluded, which significantly simplifies and reduces the cost of the system. The digital signal after processing in the digital signal processor goes directly to the digital power amplifier, without additional interfaces and conversions (Fig. 1). The processor supports up to 96x oversampling and can handle sample rates up to 24kHz. A 8-bit signal representation is used; XNUMX-bit devices have been developed for speech transmission systems.

Rice. 1 (click to enlarge)

Strictly speaking, Apogee was not a pioneer in this area. The idea was in the air and various options for fully digital paths have been known since the late 80s of the last century. However, it is in DDX technology that a new approach to the process of signal processing and amplification is expressed. The topology of small-signal and power microcircuits is different, and combining them on a single chip presents significant technological difficulties. The separation of the processor and the powerful output stage made it possible to implement them in the form of separate microcircuits, which significantly reduced the cost of the system. The chipset consists of a DDX processor and separate power amplifiers by the number of channels. Both two-channel and multi-channel systems are available. Developers define the application range from computer multimedia devices to home audio systems and MP3 players.

The main advantages of DDX are as follows:
• High efficiency
• No DAC
• High noise immunity
• Low noise level
• Low cost
• No feedback
• Good speaker damping

Therefore, the concept of damping factor can be applied to this technology only with some reservations. When measured by traditional methods, the value is small - about 16, but the degree of damping of the moving system is not worse than that of an analog amplifier with a damping factor of more than 100. By disconnecting the load during signal pauses, the new technology also increases the signal-to-noise ratio.

Fig. 2

In addition to improved damping, the efficiency of the amplifier increases when amplifying weak signals. Compared to analog class AB amplifiers, DDX efficiency is approximately two to three times higher, traditional class D amplifiers are inferior to them in the small signal region by approximately 20% (Fig. 3).

Fig. 3

Another important advantage is the absence of general feedback in the amplifier. Together with the bridged organization of the output stage, this significantly reduces the effect of changes in the power supply voltage on the signal parameters. Without feedback, the stability of the amplifier also increases: microcircuits designed for car amplifiers develop power up to 100 watts at a load of 0,7 ohms. And this is at a supply voltage of 12 volts, without converters and massive radiators! The creators are optimistic that the widespread adoption of the new technology is not far off.

But what about analog media? The demise of the compact cassette is postponed indefinitely, and digital broadcasting will not soon become available to the public (for reasons more economic than geopolitical). Although the DAB standard is recommended for use in Russia and the CIS, things did not go further than talks and broadcasting experiences. And in other countries, analog broadcasting is not giving up yet. However, there are no major obstacles. For analog sources, you will have to use analog-to-digital converters, and new possibilities open up. For example, it becomes possible to use already existing digital noise reduction systems and other signal processing devices. Such a solution in Hi-End digital systems has become standard, no one bothers to apply it in automotive devices.

Apogee currently supplies audio equipment manufacturers with DDX amplification solutions in the form of semiconductor components, OEM devices, and licenses its technology. Apogee is constantly developing new products and technologies and expanding cooperation with other companies working in this field. The latest news is the signing of an agreement with STMicroelectronics (formerly called SGS-THOMSON Microelectronics). This company designs, develops, manufactures and markets a wide variety of integrated circuits (ICs) and discrete devices for a wide range of applications, including telecommunications, computer systems, consumer systems, automotive, industrial automation and control systems. The result of this collaboration is the release of a new line of chips, including a 4.1-channel DDX controller and two DDX amplifying chips, providing up to 100W of output power.

For more information about Apogee and DDX products, visit apogeeddx.com.

Author: A. Shikhatov; Publication: bluesmobile.com/shikhman

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