Sounding Out with the OLPC XO
In January of this year, I received an XO laptop from the One Laptop Per Child (OLPC) Project, thanks to a kindly recommendation from my friend Dr Richard Boulanger, professor of music synthesis at the Berklee College of Music. Rick knows that I maintain a private teaching studio and that many of my students are youngsters who would love to play with the XO. He also knows that I have a twin interest in Csound and Linux audio development, two rather significant aspects of the machine. Thus, this article focuses on my experiences so far with the XO's audio subsystem and its sound and music software. My students have had only brief exposure to the machine, but I conclude with some remarks concerning their interaction with the XO and its audio capabilities.
There's plenty of material on the Web that describes the XO in minute detail, so here I recap only the most salient features of the machine.
The XO laptop (Figure 1) is small and lightweight without feeling flimsy or poorly constructed, and the few mobile parts are connected firmly at their joints. The display swings up from the base and can be rotated 180 degrees left or right in its upright position. It also can be tilted slightly backward. The keyboard is a single rubber membrane, designed for kid-size fingers, but ham-handed adults like yours truly can plug in a USB keyboard if necessary. A two-button touchpanel replaces the mouse, though currently only one panel and one button are active. That's not a problem, because only the pointer control and an entry button are required to navigate the GUI.
I'm impressed by the thought that has gone into the design of the XO. At every level, I find consideration for the user's experience, from the design of its battery pack to the excellence of its display resolution. In fact, when I've shown the machine to friends, they've all especially admired the handle and wondered aloud why their laptops didn't include one.
On the software side, the XO is powered by a modified version of Fedora Core with a 2.6.22 Linux kernel. The GUI is the renowned Sugar, a Python-based graphic interface that is singularly unlike the typical Linux desktops with which I'm familiar, and the Linux command-line is easily available at any time.
The XO's CPU is a 433MHz AMD Geode LX-700. The laptop's multimedia capabilities are provided by the Geode CS5535/CS5536 companion chipset. According to the Wikipedia page on the Geode, the CS5535 is a “...Southbridge for Geode GX and Geode LX...[that] integrates four USB ports, one ATA-66 UDMA controller, one infrared communication port, one AC97 controller, one SMBUS controller, one LPC port, as well as GPIO, Power Management, and legacy functional blocks”. The processor's AC97 controller is of central importance to this article, along with the possibilities afforded by the USB support, so let's consider exactly what that AC97 is and what it does.
In 1997, Intel developed an audio codec to provide high-quality audio services for motherboards, modems and sound hardware. The AC97 defines a high-quality audio architecture with a sampling rate of up to 96kHz for stereo and 48kHz for multichannel digital audio recording and playback, with bit depths up to 20 bits. The AC97 became very popular with manufacturers and is found on most desktop machines, though it has been superseded recently by Intel's HDA (high-definition audio). The codec is divided into a digital controller and an analog stream handler, effectively combining the analog-to-digital and digital-to-analog converters in a single package (an appealing feature for hardware designers). By the way, Intel's use of the word codec here refers to the encoding/decoding of analog-to-digital and digital-to-analog streams, as distinct from binary compression/decompression codecs such as MP3, Ogg or WMA/WMV.
The AC97 implementation for the CS5535 comes from an integrated Analog Devices AD1888 chipset that provides up to six channels of digital or analog audio output. The AD1888 is notable also for its direct connection to the core CPU, a cost-saving factor that accords nicely with the XO's overall design. The XO also uses another Analog Devices chipset (the SSM2211) for audio amplification.
So much for audio on the inside. On the outside, we find an integrated microphone, two integrated speakers and jacks for stereo audio output (to headphones or other speakers) and for a monaural microphone-level input. The jacks are standard consumer-grade sound-card connectors that take 3.5mm mini-plugs, and I'm happy to report that connections to those jacks are firm and steady. The jack functions also are redefinable with the alsamixer utility, but I did not experiment with this feature. See the OLPC Wiki page on the XO's audio hardware for more information about redefining the audio I/O ports.
The XO also includes three USB ports. Obviously, these ports can be used to expand the machine's audio capabilities by adding a MIDI interface or a higher-quality digital audio interface.
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