LOGITECH Squeezebox Boom Audio Design User Manual

Logitech Squeezebox Boom

Audio Design

Caleb Crome

Sr. Hardware Engineer

Logitech, Inc.

Introduction

Ever since I bought my first Squeezebox from Slim Devices several years ago, I wanted a version with built-in speakers. I was not alone. Members of the extremely active Squeezebox community have built quite a few homebrew Squeezebox "boom boxes" over the years, mashing together Squeezebox hardware with amplifiers, speakers and power supplies from a variety of sources. After Logitech acquired Slim Devices and I was hired on, I was excited to be part of the team that would build the Logitech® Squeezebox™ Boom all-in-one network music player.

Over the past months, our team has worked extremely hard to build a compact, self-contained, high-performance network audio system. With advanced digital signal processing, a high-quality bi-amplified speaker design, an easy-to-use user interface, line input and subwoofer output, Squeezebox Boom is a system that can go in any room of the house and sound great.

After a quick tour of the high-level architecture and disassembly photos of the Squeezebox Boom, this paper will describe the audio architecture starting from a digital PCM signal (after any MP3, OGG, FLAC or other decoding), and will follow the signal through the digital signal processing (DSP) chain, digital-to-analog converters (DACs), power amplifiers, the speaker drivers and acoustical enclosure.

The DSP signal flow in Squeezebox Boom contains several processing stages that optimize the sonic experience. The primary DSP stages that will be discussed are: volume processing, bass management, StereoXL™ stereo enhancement, woofer-tweeter crossover, subwoofer processing, and driver protection.

Although we believe Squeezebox Boom sounds great today, because of the extremely flexible and upgradeable nature of the Squeezebox architecture, we can roll in new features on an ongoing basis to meet our customers’ needs. Some of the future enhancements could include: automatic loudness filter for low-volume listening; dynamic range compression for low-volume listening; multi-band equalization; dynamic range compression for high-noise environments; and whatever else we can think up to help improve the overall experience.

System Overview

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.

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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.

Secondary DAC

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.

Power Amplifiers

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.

Hardware/Mechanical Design

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 cupshaped 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

Loudspeaker Drivers

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 !” 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 woofertweeter 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

Audio Design

Squeezebox Boom is a bi-amplified design, using digital crossovers and independent DACs for each speaker, with a second independent crossover for the subwoofer output. The crossovers and equalization are implemented in software on a digital signal processor (DSP). This is the same technology that’s found in high-end studio monitor speakers. Obviously, the Squeezebox Boom doesn’t compete in bass performance with high-end studio monitors, but because of its advanced signal processing capabilities combined with very high-quality drivers, we believe we have created one of the best sounding products in its class.

Typical desktop speaker systems will be a 2.0, or occasionally a 2.1 system. Very few desktop speaker systems use true tweeters, and thus the high end will either be nonexistent or it will ‘beam’ with much more energy coming from the front of the system than off axis. This is a fundamental property of sound propagation. For the best quality sound, it’s critical that loudspeakers be as omnidirectional as possible.1 The result is more unified and balanced sound than can be achieved with other architectures.

A Multiple Regression Model for Predicting Loudspeaker Preference Using Objective Measurements, Sean E. Olive,

AES Convention 116, paper numbers 6113 and 6190. http://aes.org/e-lib/browse.cfm?elib=12847

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