DigiSpeaker is a cached music system. It is intended to be used to play music that has been previously stored in the system. Because DigiSpeakers are distributed throughout a home and buried in walls and ceilings (often inaccessible), it is important that all DigiSpeakers be connected in some manor as to make loading music simple and convenient. The obvious choice, of course, is to network DigiSpeakers together. Furthermore, it seems obvious that each DigiSpeaker appear as an Internet node so that they can be accessed inside or outside the home (depending on router settings, of course).

Since DigiSpeaker is a cached system, all music scheduled for playback exists in DigiSpeaker before it is played. As mentioned in previous posts, each DigiSpeaker will have ample storage for music (provided by plug-in flash sticks or memory cards). Also, as previously mentioned, several DigiSpeakers in an installation will 'band together' to form one large storage in such a way as to make music stored on one node accessible to all nodes in the home.

So, networking is important to DigiSpeaker. But at a glance, it would appear that the required network throughput is not all that high. Each DigiSpeaker need only be able to acquire (either from its own cache or from another unit) a new song in the time it takes to play the previous song. If that new song needs to come over the network, then the required throughput is low. For example, if an MP3 file containing a 3 minute song is around 3MBytes in size, then the required network throughput is only 17KBytes a second. Hardly taxing to modern networks even given dropouts and lost packets.

However, experience has taugh us that you can never have too much bandwidth. We anticipate DigiSpeaker to be no different. As the feature set of DigiSpeaker grows, so will its bandwidth requirements. For instance, if DigiSpeaker is ever used to stream live events (like sports or concerts), then a system of caches will add unacceptable delays to the sound. Those delays will be noticed by the user watching a video of the event and expecting the sound to be in sync. We expect this type of feature growth to be common with DigiSpeaker so it suggests a design goal for network throughput (and processing power in general). That is, we want to add as much bandwidth to the system as we can afford. In this case, we think we can afford about $10.

Another thing experience has taugh us is that networking devices in places that do not have networks can be problematic. This is the exact situation in homes. Most homes do not come wired with Ethernet cables in the walls. To get around this, of course, people use wireless or powerline connections. However, we notice that any of these solutions (wired, wireless, powerline) have 'challenges'. Wireless connections are sensitive to interference and building structure. Powerline connections have trouble transmitting across phases. Wired connections are impractical and/or unattractive if they haven't been built into the walls. Seeking to avoid these shortcomings suggest another design goal for the DigiSpeaker network. That is, each DigiSpeaker should have as many connection modalities as possible to give the user multiple connection choices. In this case, it means that each DigiSpeaker should be capable of communicating over wired, wireless or powerline connections. To avoid skyrocketing the cost, the choice of connection could be specified at manufacturing time so as to avoid including connection choices that the user knows they do not want.

Always mindful of cost, we went searching for ways to implement these three connection types on the MPC5200. Our goal was to find parts that could be afforably added to the design (once again, affordable generally means about $10 or less to us).

Starting with the wireless connection, we discovered that most wireless chipset manufacturers would not return our calls. Digging a little further, I found somebody at Broadcomm that explained to me that they don't work with independent hardware vendors (read: little guys) because most were not capable of passing through the worldwide licensing requirements for certifying a wireless device (hmmm). They suggested that we look into adding a prebuilt wifi module that presumably had already been blessed by the certification process. Fine enough... we went looking and discovered that they aren't affordable (by our standards, anyway). However, we noticed that USB modules, which are banged out in the millions per year, do seem to be at least reasonable (around $30 or so retail). Of course, cheap USB dongles are hardly appropriate to be the cornerstone of our network, so we went looking for an alternative.

That alternative appeared in the form of the Intellon INT5500 powerline mode chipset. This chipset is capable of transmitting information over powerlines in homes at up to 85Mbps. It is compatible with HomePlug 1.0 and doesn't interfere with Insteon (yet to be verified). Best of all, Intellon returned our calls and agreed to sell us chips.

The Intellon INT5500 connects to the MPC5200 via its MII interface. However, each DigiSpeaker is supposed to retain a wired connection. To get around this, Jon found an Ethernet controller with a PCI interface. It is the KSZ8841-PMQL from Micrel. This chip is affordable by our standards.

So, to achieve our goals of affordable bandwidth and varied connection types, we have decided to: 1) add a second USB connector so that any Wifi module can easily be added. 2) add the Intellon INT5500 chipset so that any DigiSpeaker can network with another over powerlines, and 3) add an Ethernet connection via PCI so that any DigiSpeaker can be networked directly.