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THE tinny fizz produced by most laptops' built-in speakers makes them far from ideal for listening to music. But plug-in speakers will soon be available that produce high-quality sound from portable PCs without the need for mains power.
The sound from existing computer-powered external speakers is often disappointing because the USB port, from which they get their power, can supply only 2.5 watts. Now the British firm NXT of Cambourne, Cambridgeshire, has come up with a USB-powered system that can deliver up to 15 watts to each speaker.

The transistor circuits in audio amplifiers draw power from a wire that in mains-powered units is held at around 32 volts. That allows the amplifier to deliver enough power to the speakers to reproduce even the loudest music. Unfortunately, a USB 2.0 port can deliver no more than 5.25 volts to a device, making it impossible to permanently maintain a USB-powered amplifier at 32 volts.

NXT chief executive James Lewis says that most of the time music is quiet enough to be reproduced satisfactorily by circuits running at just 1 volt. So NXT built its USB-powered amplifier to run at low voltage, but able to deliver higher voltages - and more power to the speakers - for the loud passages that demand it.

These peak requirements are met by tapping a pair of capacitors that store spare power from the USB during quiet passages. By monitoring the music signal a couple of milliseconds ahead of the amplifier, the system determines exactly when to raise the voltage and unleash the stored power.
The system determines exactly when to raise the voltage and unleash the stored power

"It's the first time I've heard of using the dynamic reallocation of power for this type of application. It seems like a great idea," says Andy Dowell, a director of Dolby Laboratories in the UK. "People would love to get more oomph out of the PC speakers."

In a demo at New Scientist's London office, the sound was indeed impressively loud for a set of USB-powered speakers. Although sustained loud passages risk draining the capacitors, after which the speaker volume would fall, NXT claims to have tested its system successfully on various music genres without volume problems. NXT hopes to convince consumer electronics companies to adopt the technology by the end of the year

sourcehttp://www.newscientist.com

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