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Bringing multizone digital audio to life

Posted: 31 Mar 2008 ?? ?Print Version ?Bookmark and Share

Keywords:digital audio? music system? LCD? Wi-Fi?

Achieving the goal
On opening the ZP100, one of the first hardware decisions becomes clear: the team went with an IEEE 802.11b/g wireless mini-PCI card design from Atheros Communications for all its wireless connectivity.

Figure 4: The antennas (gold spikes on upper left and bottom right) were hidden inside the enclosure to make the system seem less complex.

"The wireless connectivity was the most nerve-wracking part of the whole design and Atheros gave us access to the drivers and all the knobs we needed to tune the system to get the performance we needed," said Schulert. The ZP100 design uses Atheros's AR5213A baseband/media access control (MAC) and AR2112A transceiver chips. The beauty of the SonosNet setup versus a traditional wireless access point is that the user doesn't have to worry about SSIDs and passwords, said Schulert. "We broadcast, look for devices, and add them to the household."

Also, the antenna placement is such that it's buried within the enclosure. This is allows the user to perceive the ZonePlayer as a stereo, not a Wi-Fi device, he added.

The main processing on the ZP100 is shared among a number of key chips. First up is the Renesas SH-4 microcontroller running Linux (see Figure 3 above). Linux was chosen as it allowed the engineers to get complete access to all the source code to enable debug. The Renesas SH-4 was chosen across the whole system for three reasons: it is low power, which is critical for the wireless controller; it does floating-point, which allows it to efficiently decode MP3 audio; and it supports mini-PCI interfaces.

The other main processor resides on the main amplifier board and is the Texas Instruments TMS320V5402 16-bit, fixed-point DSP. This performs real-time buffering to keep the codec full and also does audio equalization. While Schulert admits it may be a bit overkill for these functions, and if they had to do it over again they might pick a lower-performance device, he pointed out that they wanted to ensure plenty of processing overhead.

Supporting the main processors is a Renesas M16 16-bit microcontroller (bottom, center of Fig. 5white label on top of it) to tie up loose control ends, as well as a Cirrus Logic CS42416-CQZ 192KHz multichannel D/A and A/D codec with on-board PLL and a minimum 110dB dynamic range. The codec also provides digital volume control and differential analog outputs. Other support chips include a RealTek RTL8139CL Ethernet MAC, a Kendin KS8995MA Layer 2 Ethernet switch with five 10/100 transceivers, a Lankom SQ-H48W Ethernet transformer module and an Atmel AT27LV512A 512-Kbyte one-time-programmable (OTP) boot EPROM. Memory comprises two ISSI IS42S16800B-7T 16Mbyte x 8 (128Mbit) synchronous DRAMs for main program code as well as 32Mbyte of Samsung NAND flash for storing code as well as the index to the stored music archive. That can now store up to 50,000 titles.

Figure 5: Main amplifier controller board showing TI DSP bottom right, Cirrus Logic codec on left, Tripath Class T controller center (between two capacitors) and Class T interference suppression filter components on top. (Click to view larger image)

The main amplifier section is a Class T design that relies on two STA505 half-bridge FET power amplifiers controlled by a Tripath TC2000 Class T controller with proprietary Digital Power Processing technology.

Figure 6: Rear of main amplifier controller board showing STA505 half-bridge FET power amplifiers (center) from STMicroelectronics. (Click to view larger image)

Class T amplifiers combine the audio quality of Class A/B with the efficiency of Class D. The TC2000 itself is a 5V CMOS signal processor that amplifies the audio input signal and converts the audio signal to a switching pattern that's fed to the MOSFET drivers. Because of the high-frequency noise components derived from the switching, LC filters are required on the outputs to prevent the transfer of those noise components to the speakers (see Figure 5). Other power devices on board include two National Semiconductor LMS1587 3A, fast-response LDOs.

What's inside
The ZP80 is similar to the ZP100 but without the amplifier. Again, it relies on an Atheros WLAN design, but this time the more integrated AR2413A mini-PCI card.

Figure 7: ZP80 Zoneplayer insides showing Atheros mini-PCI card (top) and digital ICs with EMI protection (bottom). (Click to view larger image)

Again it has a Renesas SH-4 and M16 MCU, a TI '5402 DSP, RealTek RTL8139CL Ethernet MAC, Atmel OTP EPROM, two ISSI 16Mbyte x 8 synchronous DRAMs, the Samsung NAND (32 Mbyte) etc. However, though the ZP80 has only two Ethernet ports, it still uses a Marvell 88E6060 6-port Ethernet switch (versus the Kendin controller on the ZP100) with an LF-H20P-1 magnetics chip.

Figure 8: ZP80 main board showing TI DSP and supporting memory below Atheros mini-PCI slot. (Click to view larger image)

The controller relies on an Atheros AR2414A mini-PCI card and instead of 32Mbtye has 16Mbyte of Samsung NAND flash. The controller is also differentiated by having a ball-bearing-based motion sensor and a Sharp 1/4-VGA transflective LCD display. Again, a Renesas M16 MCU is included, but this time to also manage the control buttons and scroll wheel.

Figure 9: Controller's main board showing Atheros mini-PCI card, Renesas SH-4 controller and support memory, as well as ball-bearing-based motion detector (black square component, bottom left side). (Click to view larger image)

Future improvements
The cradle for the controller raises an interesting design oversight in that it was designed after and separately from the controller itself. As a result, when the controller was placed in the cradle, it put the controller at the exact angle of maximum sensitivity of the motion-control sensor. As a result, the controller would turn on with the slightest vibration. That problem was fixed with a software upgrade that turned the controller off when in the cradle.

However, that was the least of the designers' worries when it came to the system's design. Wireless and EMI issues took centerstage, particularly on the ZP100. That system's memory busbetween the SH-4, the NAND flash and the synchronous DRAMwas initially designed to operate at 60MHz. However, Wi-Fi interference ensued and it was discovered that the 40th. harmonic of 60MHz fell smack into the 2.46GHz region, affecting channel 11 of the Wi-Fi network. As result, the designers had to bump the frequency up to 80MHz, thereby pushing that harmonic safely out to 2.48GHz.

But that wasn't all. The ZP80 has far less metal surrounding it than the ZP100, where the casing and support structures within act as a Faraday cage. The ZP80 has only a plastic housing. Consequently, the designers encased both the Wi-Fi and digital processing circuitry in metal enclosures as there were serious EMI issues. That worked. "We found that wireless performance has to be designed from the ground up, both in hardware and software," said Schulert adding, "Wi-Fi is very sensitive to EMI: removing interference in our Wi-Fi was much harder than passing FCC [regulations]."

Sonos wouldn't comment on future upgrades, but said it was looking at other ways to move audio throughout the home. Patents surround its core technology and relate to audio performance, audio synchronization, and mesh networking. "We also have many applications around our GUI, addressing the ease of use of our product and setup," said Schulert. "Finally, we have several design applications capturing our unique style."

- Patrick Mannion

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