A conceptual diagram of DigiSpeaker Prototype IV is pictured below. Unlike previous versions, this design does not include analog/digital I/O and is capable of driving stereo speakers or bi/tri-amping a single speaker.

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1. Control

Like previous versions of DigiSpeaker, Prototype IV has the MPC5200B from FreeScale at its core. Even though this processor is a little expensive ($18ea. in 1000s) we still like it. It is well supported in the Linux kernel. It has a wide range of peripherals. It consumes little power yet has a lot of punch (contains an FPU also). This might change in the future, however. NXP has developed the LPC32xx family of processors. These processors are enhancements of their LPC3000 series. We previously considered these processors but rejected them because they lacked I2S interfaces. However, the new family of processors has a couple I2S interfaces. They are comparable to the MPC5200 in power yet they cost half the price (that is just a rumor at this point). For now, we are sticking with the MPC5200 until the LP32xx processors are in production.

We have dropped WiFi support in favor of a powerline modem chip set from Intellon (INT5500CS). WiFi modules are expensive and can be 'tricky' depending on the installation. We noticed in the Sonos forums that the major complaint about their unit is WiFi reliability (or lack there of). Sonos has recently elected to step up the 802.11n to try and fix the problem. This plan has a chance but is yet again, more expensive and the vendors that sell the chips are even harder to work with. So, we have decided to use the powerline modem approach. Notice that we still have USB in our system so it would be possible to slip in a USB WiFi module if it is required.

We still plan on using Insteon as our wall control. The idea here is that users will outfit their homes with Insteon compatible switches that can command the DigiSpeakers for things like volume control and playlist selection. We are also keeping our IR control link (not pictured) so that any 38KHz remote control can be used to control the unit. Also, we are keeping our browser interface to an installation for full control. Finally, Jon has been playing with the iPhone and iTouch devices from Apple. He likes them a lot and will probably build an interface into these devices (browser based?) as well.

2. Amplifier

The major difference between this prototype and previous versions is the amplifier. We are still using the TAS5504 as our PWM processor and the TAS5142 (or TAS5342 if available) as our power stage. Both are from Texas Instruments. However, we now configure the output stage as Bridge-Tied-Load (BTL) and drive the speakers differently via software depending on the product (more on the products in a minute).

At 96KHz and below, the TAS5504 has 4 independent processing channels that can be configured to accept either channel (left or right) from any of the I2S interfaces. Above 96KHz, the TAS5504 runs out of processing power and can only process 3 channels (see page 11 in this document). The processing pipeline for each channel is impressive. Each channel has seven bi-quad filters, base, treble, volume, loudness and dynamic range control. Also, each channel has an output mixer with gain control to mix any of the processing channels at output. With this flexibility, it is possible, via software, to create several products from the proposed design.

1. Simple Stereo Mode. In this mode, a single DigiSpeaker is used to drive two speakers in stereo. The speakers will have crossover circuitry to handle the separation of tweeter and woofer frequencies. In this configuration, only two of the TAS5504 processing pipelines will be used (PWM 1 & 2) for left and right channels of a stereo signal. The bi-quad filters are used for music shaping (country, rock, rap, jazz, ...etc). The treble, bass, loudness and dynamic range control are used in the traditional manor.
2. Full Stereo Mode. In this mode, a single DigiSpeaker is used to drive two speakers in stereo. In addition, a subwoofer channel is driven from a second TAS5342 built into the DigiSpeaker. In this configuration, two TAS5504 processing pipelines are used for stereo processing (PWM 1 & 2) and a third is used for subwoofer processing (PWM 4). The bi-quad filters are used for music shaping. Treble, bass, loudness and dynamic range are set as the user prefers.
3. Bi-Amp Mode. In this mode, a single DigiSpeaker is used to drive the tweeter and woofer of a single stereo channel (either left or right). Two DigiSpeakers are expected to be used in a zone to complete the delivery of both left and right stereo signals. Two TAS5504 processing pipelines are used (PWM 1 & 2) but they both are given the same input signal (either the left or right stereo channel). The bi-quad filters are used to implement the speaker crossover as well as music shaping. Treble, bass, loudness and dynamic range control are used as the user prefers. Notice that using two DigiSpeakers in a single zone doubles the power delivered to that zone. In fact, it is possible to add any even number of DigiSpeakers (2, 4, 6, ...etc) to increase the power delivered to a zone.
4. Tri-Amp Mode. In this mode, a single DigiSpeaker is used to drive the tweeter, mid-range and woofer of a single stereo channel (either left or right). Two DigiSpeakers are expected to be used in a zone to complete the delivery of both the left and right stereo signals. Three TAS5504 processing pipelines are used (PWM 1, 2 & 4) but they all are given the same input signale. The bi-quad filters are used to implement the speaker crossover as well as music shaping. Treble, bass, loudness and dynamic range control are used as the users prefers. Like the Bi-Amp mode, this mode can be used to increase the power delivered to a zone simply by installing more DigiSpeakers.

The MPC5200 requires an external clock chip to reach speeds above 96KHz. So, there are potentially two grades of each of these products depending on whether the clock chip is included or not. In addition, the addition of the second TAS5342 could also be a product differentiator.

3. Power Supply

The power supply for DigiSpeaker is still an important component. Power supplies tend to get more expensive above 100W where high efficiencies are required to keep the heat produced to acceptable levels for installation in a wall or ceiling. However, power supplies in the 60W to 70W range are pretty cheap because they are commonly used for TVs and Laptops and they produce less heat. The new DigiSpeaker design could potentially use a cheaper power supply when configured in Bi-Amp or Tri-Amp modes because they are powering less speaker. Deploying two DigiSpeakers in a zone could add up to the same power as a single DigiSpeaker in Stereo mode.

For now, the power supply remains the same as previous version. We will try to deploy a 130W power supply that can be used to power the logic and deliver 100W of power to the speakers. This is probably overkill for he Bi-Amp mode (and possibly the Tri-amp mode), but will just be comfortable for either of the Stereo modes.