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Digital audio amplification: a new approach

Posted: 11 Aug 2006 ?? ?Print Version ?Bookmark and Share

Keywords:jam technologies? analog source data? amplification?

By Larry Kirn
Jam Technologies

While previous generations of audio technology have served their purpose well, the underlying technology behind them was conceived and developed specifically for analog source data. Now that virtually all audio source data is digital, we are compelled to re-examine the basic principals behind conventional amplification to see if it is adequate for today's digital source materialCDs, MP3s, DVDs, etc.

Current industry trends in digital audio
Digital audio is now ubiquitous. That said, the requirements to fulfill the promise of crisp, ultra-clear full 16 bit audio (a minimum standard for today's audio devices) is simply not met with the current crop of digital audio amplifiers. As competition continues to lower retail prices of digital television systems, home theater, flat panel televisions and DVD systems, bill of materials costs of their audio amplification systems remain constant as components that support current digital amplifiers are many in number. This factor is also an impediment to the miniaturization and rate of market growth of hand held and battery powered devices such as cell phones, MP3 players and iPods. With almost all audio sources having migrated to digital formats and media, a new approach to amplificationone with a digital input, that consumes less power, generates less heat and saves on overall system costs is called for in order to meet the demands of consumers for smaller, less expensive and better quality A/V products.

In the beginning
Transistors, developed in the 1950s, represented a quantum leap in technology for sound amplification. Transistor technology however, presented at least two significant flaws. Even though transistors are more efficient than their room-heating tube predecessors, they quickly became entrenched as replacements only for vacuum tubes while leaving the up-stream signal basically in tact. Design techniques using cooler, more efficient digital signal processing techniques are just now making their way into many markets, including audio. A second, but less significant problem, was that mass producing transistors at barely-affordable-prices meant tolerances for these parts that had to be significantly relaxed, resulting in an increase in signal distortion, or loss of audio quality. The sonic degradations were mitigated at that time because of the relatively low quality of storage media (vinyl and tape) and the less than perfect transmission and reception technology (AM radio and VHS television). That is no longer the case.

The next generation
The development of the integrated circuit provided economic leverage for significant increases in the quality of audio and significantly aided in the miniaturization of electronic appliances as long ago as the early 1960s. Analog amplifiers are extremely linear. In an analog amplifier, the power supply is constantly on, using a great deal of power and necessitating a great deal of cooling. This inefficient use of power makes them far from ideal for today's world of battery powered devices and sleek home stereo and theater systems. Because all audio sources were analog the reproducible dynamic range was limited by an audible noise floor. Vinyl sources, for example, produced various snaps, crackles, and pops; and the better-sounding tape sources had hiss that limited the dynamic range to system 60db at best. Happily, dynamic range limitation by hiss was far less egregious than the sound of some things to come.

Enter the digital age
As time went on, new sources of audio emerged and almost exclusively entered into the digital realm. Records and tapes began to vanish in favor of compact discs. Compression techniques such as MP3 were born from the relentless market drive toward more and more music on smaller and smaller devices, Digital amplifier circuits needed to adapt to embrace these new forms of media. Digital recording equipment and output creates very accurate sound. A sixteen-bit digital recording is accurate to one part in 64,000 where analog accuracy at equivalent digital resolution of twelve to thirteen bits is only accurate to one part in 8,000. Early digital amplifiers were fine with analog sources and to a degree with digital sources, but for the most part could only produce audio at 12 bit to 13.5 bit resolutions. Advancing multimedia devices such as home theater, digital audio receivers, and computer gaming systems require full 16-bit audio reproduction to support the growing arsenal of spatial features demanded by the market. These features include surround sound, echo and concert hall ambience to name a few. Typical Class-D digital amplifiers some of which have analog inputs and some of which have digital outputs cannot meet these goals because they simply lack the resolution to create accurate sound although they are low cost and efficient. Furthermore, these parts do not even meet the needs of even more mundane but advanced devices such as CD players, Car Stereo Systems, DVD players or MP3 players.

The distribution of digital audio on CDs removed a weakness in the audio signal paradigm. Finally, the promise of studio quality sound is available in your home and even in your car. Digital audio is literally noiseless. It is processed as a series of zeros and ones, which leaves no room for unwanted noise with very little distortion. One other problem, however, continued to persistthe size, cost, and quality impact of the digital to analog converter (DAC) necessitated by analog amplification. Now, new technology makes it possible for this new pure audio source to remain in pure digital form through the complete audio processing chain. A pure digital amplifier is designed with native digital input eliminating the need for a DAC in the signal's path. A true digital amplifier, or power DAC, directly translates the pristine digital input into pristine power at speaker terminals.

Until fairly recently, end-to-end digital amplifiers could only be found in the highest end, most expensive audio systems. Almost all early digital amplification systems required a two-chip implementation accompanied by a basket of external components. The cost and physical size of this two-chip approach largely precluded its use in the mainstream market.

Consider a new approach
Although existing Class-D digital amplifiers have provided improvements over preceding technologies, they have left significant improvements in sound quality, packaging, performance, price and core technology unresolved.

Job one is quality sound. To create accurate sound, the output transistors need to work equally well at both ends of the dynamic range to contribute exactly the correct power proportions. New architectural techniques not content to continue ignoring detail, have added an extra set of output transistors. These transistors provide a finer level of control to the audio signal output. This single chip solution merges fine control with raw power by employing a simple but sophisticated system of internal control logic to improve audio output.

Unlike all the traditional architectures requiring interim analog signals used in virtually all class -D parts this E-Bridge architecture, as opposed to more traditional H Bridge architecture has significantly simplified the amplification process by eliminating the need for complex feedback loops or DSP-intensive error correction. Because the architecture is fundamentally simpler this open loop approach also simplifies integration at a significantly lower cost and with a smaller footprint.


Figure 1: Convetional H bridge and new E bridge architectures

Switching amplifier audio output is composed of a set of voltage pulses. The width or density of these pulses determines how much power is sent out at any given moment. Because the height (voltage) of the pulse is fixed it is the variation in pulse width that determines the output power. Although simple in principle, timing control to the precision required by audio is literally a minefield. Traditional Class-D amplifiers try to produce quality sound with feedback, an analog technique that has seen incremental increases in quality but has never achieved the quality of their high heat, power hungry analog ancestors. This new approach to switching amplification adds low-voltage precision control to the output stage with an additional set of transistors. The conventional output transistors provide the crude, raw Class-D output. The additional transistors produce lower-voltage pulses that sum with the high-voltage pulses to determine the final value. By using these low voltage pulses in concert with the high voltage pulses, this new approach to digital amplification creates extremely accurate power output and therefore, extremely accurate sound. This extraordinary accuracy also means that now digital amplifiers can truly reap the benefit of spatial features such as environment simulation and surround sound.

The following analogy will make this fairly complex concept easy to understand. Let's say you had to create a specific and exact volume flow of water. The only device presented to you is a fire hose. The volume flow would be created by turning the hose on and off. To create an average flow you would need to turn the hose on and off very quickly. This is of course is very difficult and would likely result in an inaccurate flow. If to your cadre of tools you add a garden hose, you could switch the larger hose on and off and switch the garden hose on and off. Being much smaller, the garden hose presents a finer control mechanism therefore a finer, more accurate level of control over the volume of water being dispensed creating a very accurate distribution of volume:


Figure 2: More benefits of E bridge over H bridge

More benefits of E-Bridge over H-Bridge architecture
This new approach to digital amplifiers saves on heat management because the architectural simplicity facilitates assimilation of the output stage voltage regulator. In addition to increased efficiency and lower system cost, this also results in a smaller footprint which is an aid to miniaturization or, in the case of flat panel televisions, advances the cause of thinner designs.

The E-Bridge approach consumes less system power than analog or traditional Class-D amplifiers. This means that in addition to creating better sound and requiring fewer support devices, this new architectural approach will prolong battery life and are better suited for inclusion in portable devices such as cell phones, laptop computers and MP3 players.

Conclusions
Digital audio amplifiers from using this non-traditional architectural approach represent the next generation in digital audio amplification. The simple underlying output stage concept has brought the idea of a true power DAC to fruition. Switching amplifiers no longer sound bad. This technique finally makes a switching amplifier an acceptable solution for pure digital media and devices by enabling their full core capabilities. This new approach technology completes the truly all-digital audio system which now can take 16-bits in and play 16-bits out.

About the author
Larry Kirn
is Founder, President and Chief Technology Officer of Jam Technologies. After studying at Fort Hayes Kansas State College, he went on to study under a private tutor, Dr. Robert Austin of Juiliard School of Music.




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