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MOORE'S LAW
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Silicon Symphony
March 2001Motorola announced in January 2001 they have developed a digital audio amplification process, dubbed Symphony, which B&K Components is using in their upcoming DA-2100 amplifier. The DA-2100 will do the Symphony processing in a Digital Signal Processor (DSP) program using Motorola's 56300 DSP family. Motorola plans to integrate Symphony processing into future DSPs, much like Dolby Digital, HDCD, AC3, and other algorithms are integrated now. A vendor need only add output power transistors and an output filter to get a high power and high efficiency digital amplifier.
What's the big deal?
The first trend to note is the proliferation of high fidelity digital audio amplification techniques and hardware fast enough to implement them. Until recently, digital amplifiers did not have enough distortion free bandwidth for use over the entire 20Hz to 20KHz spectrum and were relegated to subwoofer use only.The second trend is the integration of these amplifiers with DSP chips to create the first end-to-end digital audio solution for consumer audio. The audio signal stays entirely in the digital domain up to the speaker inputs. These inexpensive integrated modules and chipsets will allow companies to easily implement designs that have better sound and are cheaper to produce than traditional mixed analog/digital approaches. Let's refer to this type of solution as Advanced Digital Audio (ADA).
In the next few years ADA is going to be a disruptive force in all segments of the audio industry, from portable MP3 players to high-end home stereos. Moore's law, the doubling of the number of transistors on a given chip every 18 months, will quickly push down prices and improve the sound quality available from consumer audio gear.
Digital amplifiers
Digital amplifiers are a type of switching amplifier. Switching amplifiers rapidly switch the output devices on and off at 100KHz or higher, and then usually low-pass filter to recover the audio portion. Older Pulse Width Modulation (PWM) switching amplifiers, called Class-D, controlled their switching with analog circuits. These designs suffered from poor fidelity and high Radio Frequency Interference (RFI).A digital amplifier generates its switching signal using digital logic. Purists would say that digital amplifiers must accept a digital input signal and do all processing in the digital domain. By controlling the switching signal with digital logic, advanced signal processing can be employed to compensate for the switching distortion. Most digital amplifiers avoid the Class-D moniker to distance themselves from analog switching approaches.
Switching amplifiers have been pursued with interest since they offer higher power amplifiers at a lower cost than traditional class A or A/B amplifiers. Switching amplifiers' output devices are switched entirely on or off. This means that the output transistors do not have to dissipate power that is unused at low volume levels as they do in Class A and AB amplifiers. A Class AB amplifier may be 50% efficient at maximum output power while a switching amplifier can achieve 90% efficiency. The story is better than the specs indicate since at low power levels a digital amplifier could be as much as six times more efficient than Class AB. The increased efficiency allows for amplifiers with smaller power supplies and smaller heatsinks with equivalent output power to non-switched designs. Both of these components are costly and bulky, so shrinking them reduces the size and cost of the whole amplifier.
Until recently though, they were not suitable for high fidelity applications. In the past few years companies like TacT, Spectron, Sharp, and Bel Canto have released digital amplifiers with sound quality on par with traditional analog amplifiers. In fact, some feel that their fidelity surpasses traditional audio amplifiers. While these digital amplifiers cost upwards of $2,000, they are only the vanguard of the coming revolution.
Digital Stereo Amplifiers Technology
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Proprietary switching design. Analog inputs. PWM control unknown. |
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Toccata's Equibit PWM on Actel FPGA. |
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Tripath class-T / DPP mixed signal modules. Analog inputs.* |
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Proprietary 2.8MHz delta-sigma with 7th order noise-shaping. DSD input, not PCM. |
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Pulsus PS9604 chip. 24-bit/96-kHz. Onboard DSP. |
*Tripath's DPP used by Bel Canto is also in used by Sony (DAV-S300), Carver (ZR Series), Creative Labs (PC audio), Marantz (ER3000), and Audio Source (6.1T).
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AUDIO ECONOMICS 101THE CHANGING FACE OF HIFI HOME THEATERDIGITAL WAVEMOORE'S LAW
ACOUSTICAL BENEFITS OF FRONT PROJECTORS
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