Building a Multi-Room Digital Music System
Like many media buffs these days, I have a media center computer in my house. It is a small VIA M10000 Mini-ITX system in a Casetronic C158 case running Freevo on top of Gentoo Linux. It sits inside my media cabinet and serves music, video, photos and various other bits of information to my TV. The small media center gets all of its content from a much larger AMD64 Gentoo Linux server that resides in a closet near the back of the house. The two machines talk to each other over a wired network using NFS (Network File System). I will be the first to tell you that it is great to have all of my music, videos and photos in a digital format and easily accessible.
In the same spirit of my media center computer, I wanted to have a few small systems that could sit on a shelf or in a drawer that would serve music to different parts of the house. Many products on the market exist that will broadcast audio from a computer and many others will play the resulting stream. The problem with most of these, to me, is that only one stream is available. I wanted the ability to play different music in each room at the same time and control it with one device. A few newcomers to the market do exactly what I wanted—namely the Sonos Digital Music System, which was reviewed in the March 2006 issue of Linux Journal. However, I wanted to tackle this one myself.
My idea was to build a fanless Mini-ITX appliance that would grab content from my file server and play it using MPD (Music Player Dæmon). To control the appliance, I use lightTPD (a PHP-enabled Web server) and phpMp (a PHP application that controls MPD), running on the appliance. This gives any computer on the network with a Web browser the power to control the appliance. My controller of choice is a Nokia 770. Using Opera, which comes installed, I can point the browser to each of the appliances and control them over my wireless network.
For the appliance, I chose to start with a VIA ML6000EA Mini-ITX motherboard. This board has many features that make it perfect for this type of appliance. The main one is the VIA 600MHz Eden fanless processor. This tiny processor is more than enough to push the applications, and it makes absolutely no noise. Other nice features include six-channel onboard audio, onboard LAN, support for 1GB of memory and a PCI slot.
Because I wanted the appliance to be silent, I decided to forgo a standard hard drive. Instead, I opted for a 256MB Flash drive that plugs directly in to a 40-pin IDE slot. The Flash drive also draws its power from a standard four-pin molex. I also added 256MB of DDR 400 memory to finish out the internals.
To give the appliance a sleek look, I went with a Casetronic C158 case. This case has a smooth front, which makes it look more like an appliance. Even though the case has room for a slim ROM drive, I did not put one in, because MPD does not have support for playing audio from a CD. The case also has audio jacks on the front right side that provide easy access for enjoying music with headphones. Another nice feature on this case is the optional CompactFlash card reader that can be mounted from the inside and accessed right below the CD-ROM tray.
Because I chose to use a Flash-based hard drive, I needed to make sure that my Linux installation was small, less than 250MB, and did not write to the disk. Flash-based drives and disks have only a certain number of write cycles that can be preformed on them before they give out. My particular drive, made by IEI Global Sourcing, supports more than one-million cycles. Even with support for this many cycles, the drive would stop working in a fairly short amount of time due to disk writes, especially if there were a swap partition on the disk. With all of this factored in, I decided an embedded Linux system was the best fit.
Embedded Linux systems can be created in a variety of ways. I chose to go with embedded Gentoo (see Resources). Because my development machine, a Dell 600m laptop, already runs Gentoo, I didn't need to create a custom build chain. I simply could get a snapshot from the Gentoo Web site and use my laptop as a host to build the embedded system.
Before I attempted to build the system, I took a long look at Building Embedded Linux Systems by Karim Yaghmour (see Resources). This book provides a very detailed view of the exact ingredients that make up a functional Linux operating system. And, as the title states, it shows the reader how to build embedded Linux systems. I also followed the instructions written by Heath Holcomb on how to construct an embedded Gentoo system (see Resources). Heath's instructions describe all the needed steps for building an embedded Gentoo system and making it operational. Without these resources, this entire project would not have been possible. I strongly recommend you take a look at them if you are considering doing anything with embedded Linux.
It also should be noted that if you want to build an appliance like this, but would rather use a hard drive and save yourself some time, you can use almost any Linux distribution. Most of them come with all of the software for this project already in their package systems.
Building the embedded Gentoo system was not too difficult. If you have ever installed Gentoo before, making the leap into the embedded realm is not very hard. Several things did make my build interesting though.
I followed Heath's instructions to the letter, and when I was finished, I had a working embedded Gentoo system. After testing it and making sure it all worked, I went on to install the rest of the software that I needed (MPD, lightTPD and phpMp). This proved to be a bit problematic. Portage (Gentoo's package system) wanted to install several software packages that I did not want or need. So, I started over from scratch and carefully installed only the dependencies for the software I needed. Then, I installed my three pieces of software with a nodeps flag to ensure that I did not get the unwanted software.
During the build process, I tested the system several times. I wanted to make sure that with each step I was making progress and not breaking something that had worked previously. To test the system, I used a USB media reader to copy the embedded Gentoo system off my laptop onto a CompactFlash card. I then plugged the CompactFlash card in to a card reader plugged directly in to my appliance computer. This allowed me to test the appliance and emulate how it would operate once everything was working.
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