On the outside, it looks a lot like an iPod and acts a lot like an iPod, performing the same functions as an iPod. But peel back the cover and Microsoft's Zune reveals it's not an iPod; Microsoft has gone to great lengths to configure the Zune in their own image.
The first-generation Zune, designed in partnership with Toshiba, on the inside at least, looked a lot like Toshiba's own player. The second-generation player was defined from scratch by Microsoft, including all the hardware, software, and mechanics.
Microsoft did a good job specifying exactly what they were looking for and not settling for anything less. The Zune is designed with an easy-to-use touch sensor, a 1.8-inch color display, and a WiFi connection. And all those features are packed into about 3.5 by 1.5 inches, a feat Microsoft achieved through an innovative board design that also left room for the battery.
The model that I took apart was powered by a Freescale i.MX32 microprocessor, as shown in Figure 1. While this series of microprocessors is in the Freescale general catalog, this particular version is not available on the open market. It's only available to special high-volume customers.
Similar to an i.MX31 (now the standard product), the i.MX32 tries to balance high performance with low power. It's designed with an ARM11 CPU core.
“For a product like the Zune that employs a feature-rich operating system like Windows CE, it's important to have a lot of headroom in terms of performance on your host processor,” says Boris Bobrov, Freescale's manager of applications engineering. “That way you don't spend extra battery life for high performance.”
Lots of processing power
My immediate reaction to having an ARM11 in an MP3 player was that it's overkill, and you don't want to pay for processing power that you're not using. But after further analysis, it became clear that the user interface on the Zune requires all the available performance, with its snappy, feature-rich graphics. That's one of the features of the Zune that separates it from the competition.
“It's pretty common that the requirements of the user interface drive the choice of the processor,” says Dan Loop, i.MX product manager at Freescale. “Being able to browse large content libraries is something that OEMs are pushing for in this class of device.”
The Zune's different subsystems operate at a variety of voltage levels. One of the nice features of the i.MX processor is that it always defaults to the lowest voltage possible for the required performance level. To ensure maximum performance, the system designers take advantage of techniques like dynamic temperature compensation, which measures the processor's temperature. Dynamic frequency scaling, depending on the CPU load, reduces the frequency, thereby reducing the voltage. The processor's main frequency operating points are 66, 132, 266, and 532 MHz.
The software side of the design presents its own challenges. For example, the design team had to integrate the multimedia codecs into Microsoft's software environment. “There's so much content out there that our customers have to support, that running the performance tests and dealing with tens of thousands of audio and video streams from various sources is a big challenge,” says Loop.
The codecs themselves are provided by Freescale and are optimized for the processor. Freescale then works directly with the OEM to make sure that the highest performance level is achieved in the final software environment.
In any audio-based design, there are some important factors to be aware of, such as the annoying audio pops and clicks that occur when you first turn on the player, pause a song, or adjust the volume. Far too many systems ship without eliminating this problem. In the case of the Zune, that particular element was addressed very late in the design cycle. Hence, a lot of optimization and scrambling had to take place toward the end of that development.
The audio optimization occurred later than it should have because the real focus was on power management. It was important to optimize the power consumption around the player's various “on” modes, especially when running video.
“We worked closely on the software with Microsoft, because we had to integrate software from various places, including ourselves, Microsoft, and Freescale,” says Paul Wilson, a product-line manager for audio products at Wolfson. “It all had to integrate seamlessly. There are three different development teams there, and they all have to be in sync with revisions of both hardware and software. For us, it was about delivering a set of functionality on the agreed-upon schedule. For example, Microsoft wanted different elements of the design operational at certain times.”
No pops, no clicks
That first-generation Zune employed a Wolfson codec and a Freescale power-management part. A Maxim chip was used just to suppress audio pops and clicks. The second generation targeted better audio performance than the first generation and is at least as good as the main competitor.
The Wolfson part, the 8350, eliminates most of the unwanted audio, but it's not completely pop free. When you've got a capacitor in the line, there must be some element of charge-discharge. A lot of optimization went into the power-up sequence, and that involved some engineering trial and error, as this is somewhat of a black art.
From the hardware designer's perspective, it's vital to follow the placement and layout rules. And he must select the proper capacitor values. For example, capacitor values can fluctuate if the device gets hot or the voltage isn't exactly what's expected.
Wolfson provides the core-layer drivers, like the initialization code for its 8350. They then ensure that it operates seamlessly with the Freescale processor. That keeps Microsoft from having to write code at that level. Microsoft's focus can be where it belongs, on bringing the whole system together.
Wolfson claims to have a new technology on the drawing board that includes an integrated charge pump. Hence, they can eliminate the need for external dc capacitors in the headphone mode, reducing cost and saving board space.
Wilson adds, “Another thing people forget is that these capacitors can have a lot of yield problems in production and if you eliminate them, you eliminate potential yield losses.”
One of Zune's features that I liked was the FM tuner. In this case, it was a Silicon Labs Si4703. The part makes use of a standard antenna. Alternatively, Microsoft could have opted for a tuner that used the headphone as the antenna, but that requires the platform to employ a wired antenna. Although a wired antenna is the typical user model for the Zune, you want to be designing with an eye toward the future and the potential for wireless headphones.
“This is something that we talked to Microsoft about extensively,” says Wade Gillham, a senior marketing manager for broadcast products at Silicon Labs. “We have a technology on our chip that lets you put an antenna inside the enclosure of a very small, dense device, like a Zune, and still get good FM performance.”
In general, dealing with radios that operate on different frequencies doesn't present a problem for the FM radio. You may get the occasional spikes, but it's more a function of the internal frequencies of the system's ICs, and whether those ICs have emissions that could potentially interfere with the WiFi or Bluetooth signals at 2.4 GHz, among other issues.
With respect to the FM radio, the Microsoft designers had to decide whether to include a digital output in addition to the analog out. Digital output is becoming more common for recording FM and then using that information for time-shifting the content. Features down the road could include FM transmit (in addition to receive) and AM receive.
Gillham says, “Transmit allows you to have more of a sharing and social networking experience. You can use the same embedded antenna to do receive and then you change the chip's mode to do the transmit. And AM has its own challenges. There are more challenges on the AM side because that band sits right where all the spurs tend to sit.”
The transmitters that are available today are typically analog solutions, and they're prone to have different peaks across the FM band. To avoid violating FCC regulations, you must program your output power so that the peaks are either at the high or low end. But that results in a less efficient output in the other parts of the FM band.
The Si4703 is also designed with a digital compressor that gives the audio a richer sound. The part also uses less external components than competitive devices. For example, the VCOs and inductors are all integrated.
Rounding out the Zune is a Marvell 88W8686 WiFi transceiver; an ASIC developed by Synaptics to handle the sensing; a Silicon Motion SM267 flash-memory controller; and Hynix Mobile DDR SDRAM and MLC NAND flash memory.
Richard Nass is editor in chief of Embedded Systems Design magazine and editorial director of TechInsights' Embedded Group. He can be reached at .