The diagram below (Figure 1) shows a simplified block diagram of the Squeezebox Boom audio system. For the purposes of this paper, we'll break it down between CPU section and audio section. The CPU section contains all the traditional Squeezebox components, such as the CPU, Ethernet, display, IO, and user interface. This paper will not cover the Squeezebox CPU and IO in any depth, but will be focused on the audio architecture and processing.
Figure 1: Squeezebox Boom Block Diagram. This paper will discuss the audio design, starting at the I2S & I2C interface between the CPU and Audio sections and follow the signal path through to the speakers, line-in and subwoofer-out.
As seen in Figure 1, the audio section includes the primary digital audio processor chip (TI TAS3204), the subwoofer/headphone DAC (Wolfson WM8501), the power amplifiers (TI TPA3100D2 and TPA3101D2), and the loudspeakers themselves.
Digital Audio Processor
The TI TAS3204 digital audio processor is a high-performance DSP optimized for audio applications, combined with high-performance (over 100 dB signal-to-noise ratio) DACs and analog-to-digital converters (ADCs) built in. It also can send and receive multiple channels of I2S to talk to secondary DACs or ADCs. The DSP processor itself is a 135-MHz, 48-bit DSP, with 28-bit coefficients and a 76-bit accumulator.
The secondary DAC, the Wolfson WM8501, is used to drive the subwoofer/headphone port. It gets its digital signal from the TAS3204 via an I2S interface. The DSP software is configured to process the signal differently depending on whether there is a subwoofer or a headphone plugged in to the output jack. The user can select from the user interface the type of plug-in they are going to use.
High-performance class-D amplifiers power the woofers and tweeters and can easily deliver full power to the speaker drivers with minimal distortion. We chose high-quality class-D amplifiers for both woofers and tweeters. It may be a bit atypical to use class-D amplifiers for tweeters, but we found there to be no significant sonic difference between the class-D and class-AB amplifiers we tested.
We designed Squeezebox Boom, working closely with our industrial and mechanical designers, to simplify the assembly process and minimize risk, yet maintain top-notch audio performance in an attractive package. The basic configuration is a sealed enclosure consisting of a rear cup-shaped case and a front panel assembly, where the speaker wires pass from inside to outside through a single rubber grommet.
Figure 2: Exploded schematic view of Squeezebox Boom assembly.
The drivers we chose were custom developed by Logitech's audio engineers to produce the best sounding products while maintaining reasonable costs. The woofers are 3" long-throw drivers with woven cloth cone and a rubber surround. They have a flat frequency response of between 100 Hz and 4 kHz. The tweeters are ¾" soft-dome drivers that have a flat response of between 1200 Hz and 20 kHz. There is almost no signal loss all the way to 20 kHz. The woofer-tweeter crossover is set at 2 kHz.
Figure 3: Grills and front panel removed.
Figure 4: Main board removed, exposing the rear of the main board and acoustical chamber.
Figure 5: Acoustic housing disassembled. The woofers share a rear acoustic chamber. Production units have foam around the wires to prevent buzzing.
Figure 6: Loudspeaker Drivers. Each channel uses a woofer and tweeter.