White paper: Logitech Squeezebox Boom audio design

The volume control on the Squeezebox Boom is implemented as a true digital ramping volume control so volume changes are completely click and pop free. Figure 9 shows the envelope (about 40 ms) that is applied to a signal as the volume changes from one setting to another.

Figure 9: Click-less volume changes in Squeezebox Boom. This volume ramping profile shows how the volume gently changes from one volume to another. This shows a switch from 0 to 100% volume, but any other volume change follows a similar curve.

Regular DACs implement digital volume control by simply changing the gain of a signal from one sample to the next. Better DACs will detect a zero-crossing of the signal, and apply volume changes there. In both cases the volume change is very abrupt and audibly detectable. Using zero crossing detection is much better than without, but it's not perfect.

StereoXL Technology
StereoXL is a proprietary stereo expansion technology developed by Logitech audio engineers. It provides a significantly enhanced stereo sound field, expanding the sound stage beyond the physical extents of the speakers themselves.

I have always been wary of this type of processing for fear that it would wreck the audio quality in order to achieve the effect of stereo widening. However, once the engineers from the Logitech Audio business unit helped implement StereoXL in the Squeezebox Boom, the results were very surprising. The audio quality wasn't diminished at all, but the sound seems to come from everywhere.

That said, it can be overdone, and the quality depends on the track used and the encoding used. In order to allow for varying user preferences, and track encodings there are 3 settings for StereoXL. The best one is typically in the middle (medium).

Crossovers
To produce outstanding audio quality in such a small form factor, we needed to optimize the two frequency extremes: low frequencies and high frequencies. This necessitated using two speakers (the woofer and tweeter) for each channel. Any time there is more than one speaker per channel, a crossover of some type is required to steer the high-frequency energy to the speakers with better high-frequency response, and low-frequency energy to the low-frequency speakers.

There are many different types of crossover "alignments," with various advantages and disadvantages. For the Squeezebox Boom, we chose a very successful crossover type known as a 4th-order Linkwitz-Riley crossover.² This was chosen because of its high performance and proven track record. The extreme flexibility of the networked music player plus a completely software controlled audio architecture allows us to continue to improve the product over time and make these updates easily available to customers.

The Squeezebox Boom architecture is unique among the desktop systems we have evaluated in its ability to be optimized up to, and beyond its manufacture date. We were willing to spend more in component cost to bring the absolute best system to market possible and meet our time-to-market demands. We could have saved cost by using analog (passive or active) crossovers and eliminated the DSP processing all together; instead, we chose to build the best system we could while meeting our industrial design and budget requirements.

²Active Crossover Networks for Noncoincident Drivers; Siegfried Linkwitz, JAES Volume 24 Issue 1 pp. 2-8, February 1976. Available online here http://aes.org/e-lib/browse.cfm?elib=2649.