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Portable speakers roll for the iPod

Posted: 27 Jun 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Teardown? Richard Nass? Logitech? iPod? mm50?

Maybe not everyone owns an iPod, but a large percentage of consumers claim they do, particularly those in the 15-to-30 age bracket. These days, at least according to Logitech and some of its competitors, it's not enough simply to own an iPod. You also must own a companion set of speakers that you can take on the road.

That's the philosophy Logitech took in designing the mm50 portable speakers for the iPod. Simply plug your iPod into the base of the speakers, and operate the player as you normally would.

The mm50 comes with a wireless remote, so iPod settings can be changed without getting off the couch. Going a step further, Logitech designed the mm50 to run from a battery, so listeners can enjoy the system anywhere. It's slim enough to fit in a briefcase or backpack, making it truly portable. The speakers retail for $149.

This is the first integrated iPod dock from Logitech, and the company calls it the top-selling product of its kind. So expect to see follow-ons from the company.

The mm50's design began in the summer of 2004, and the final product shipped in the fall of 2005. The design team described that design time as conventional for such a product, noting that it's basically all new technology, with nothing borrowed from other designs.

The design work for the product blended three engineering disciplineselectrical, mechanical and acousticalso Logitech employed three teams to do the work. "The biggest problem we faced was finding board space and getting the boards into this form factor," said Wells Brimhall, product manager at Logitech. "EMI [electromagnetic interference] was not an issue for us, as we have a team of in-house compliance engineers and an EMI chamber we've built. We always tackle that issue early in the design so it doesn't come back later."

The mm50 is designed with two active and two passive speakers. The passives act as ports that extend the bass response. The ports are tuned to the point at which the frequency starts to roll off. The back half of the wave is inverted and brought back out, boosting the bass beyond what could be achieved with a normal driver.

Another feature of the mm50: It portrays 3-D stereo audio, which is essentially a method for expanding the sound stage so that the speakers seem to the listener to be placed farther apart than they really are.

Logitech's mm50 speakers

Key parts
The mm50's two key components are a Texas Instruments TPA3004D2 Class D amplifier and a Zilog Z8 microcontroller. The TI part helps Logitech maintain a high audio quality with a long battery life using a rechargeable 3.7V, 2,150mA-hour Li-ion battery.

"The other reason they chose a Class D amp is that they didn't want to use an external heat sink," said Greg Davis, TI's portable audio marketing manager. "Our packaging provides a pad that dissipates the heat into the pc board itself. But it's not much heat to begin with, because we have a low package temperature, even when you turn the volume way up."

TI claims the TPA3004D2 can run at nearly 95 percent efficiency. The reduced heat lets the system more easily fit the desired form factor.

Because a lower power level is needed, the transformer needn't be as large, or as heavy. That has a secondary benefit: Because the systems are typically built overseas, the lighter weight reduces the shipping costs. In addition, because of the Class D amp's high efficiency, the capacitors and other filtering normally required for a Class AB amplifier aren't needed. Smaller filtering capacitors can be used at the supply and on the board itself.

The filtering on the output of a Class D amplifier is usually a consideration because these types of systems must pass Federal Communications Commission qualification for EMI. The EMI generated by a Class D amplifier is typically higher than a Class AB amp's EMI because of the pulse-width-modulated (PWM) output.

But that's not the case with the TI TPA3004D2, since it requires just a small ferrite bead. In fact, depending on the cabinet geometry and on the grounding that exists within the cabinet, it's possible that no filter would be required. If the cabinet is primarily plastic or there isn't a large ground plane on the pc board, however, a filter must be used.

Note that interference is typically measured externally to the amplifier, at the system, rather than board, level.

Keeping wires short
Another important design consideration was keeping the wires between the speakers and the Class D amplifier as short as possible. The longer the wires are, the more they're going to radiate, just like an antenna. That's another factor that dictates whether a filter is required.

The differential inputs on the Class D amp result in common-mode rejection. While they can be operated in a single-ended fashion on the input, most designers opt for the differential inputs. As a result, if there's a similar signal coming in on a line, such as interference from a phone that uses the Global System for Mobile Communications standard, that signal is canceled out.

The Z8 that's used in the mm50 is one of Zilog's first Z8 flash-based microcontrollers, part of the Z8F1621 series. Compared with previous Z8s, it's an improved model. The core was revised for this version, and all the peripherals were completely updated.

The 20MHz processor contains 16Kbytes of flash program memory; an eight-channel, 10bit analog-to-digital converter; three 16bit timers with PWMs and capture and compare; and a watchdog timer. The 20MHz frequency is achieved without using clock dividers. With a 1:1 relationship, the result is a maximum 20MHz execution internally. The fastest instructions execute in a couple of cycles, meaning that the peak performance is about 10MIPS.

"We've got parts that span from 1- to 64Kbytes of flash, so there's some scalability for our customers, and that's one of the things that Logitech liked about our parts," said Bill Soto, a senior field applications engineer at Zilog. "They seem to flop between 16- and 32Kbyte versions, and we give them that flexibility, usually with the same pinouts and footprints."

With the flash, Logitech can instantly program the parts using their own programmer. Or the company's designers can solder the parts down on the board so that those devices can talk to the flash through a small connector. Using a single-pin debugger, the firmware can be completely reflashed. All the emulation is also handled through this pin.

- Richard Nass
Editor of, part of the EE Times Network

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