Nuvoton ICs with Integrated Klippel Controlled Sound - New Possibilities for Speaker Design
I have consulted for some of the leading chip vendors over the years (TI, NXP, Maxim, Fairchild, and more) in amplifier and audio signal processing applications for speaker optimization, and we consistently ended up "rediscovering" Dr. Wolfgang Klippel’s patents for smart speaker protection with feedforward distortion nulling. While Dr. Klippel is famed for his transducer analyzer test instruments, back in 1991 a younger Dr. Klippel at JBL Pro was issued a patent (US5438625A) for "Arrangement to correct the linear and nonlinear transfer behavior or electro-acoustical transducers." About 30 years ago, DSP processing had too much latency and cost and there was still much to be learned on speaker failure modes. Dr. Klippel went on to found Klippel GmbH in 1997 with the focus on audio test instrumentation. This fall, Klippel GmbH will celebrate its 25th anniversary and that will provide a good reason to revisit interesting and less known stories from the company, including its path toward the Klippel Controlled Sound (KCS) that is now starting to get recognition. The extensive published research and the implications on the speaker industry for much of Klippel's work deserve to be revisited. But there's also a lot more history on amplifier protection circuits and why they became important in transistor designs.
Direct coupled amps that were transformer-less without coupling caps could lead to burning out the speakers if the amplifier had a meltdown – especially in the earlier generations of high-power and switching-mode amplifiers. Most protection circuits were really about protecting the amplifier, not the speakers. Some protection schemes even ended up chattering ultrasonically, frying the tweeters/compression drivers as they attempted to protect the amplifier's output stage from problematic speaker loads. Conversely, smart speaker protection from being overdriven by the program signal is more about excursion limiting for the bass combined with voice coil thermal limiting in mid and high frequencies. Pioneers in these techniques were Meyer Sound and Apogee Sound in the early 1980s with their processor-controlled touring sound systems. These were closely matched controllers and speakers that not only stayed out of trouble in the field, but never distorted no matter how hard they were over driven. Standalone speaker management systems from dbx and others followed, but ideally the speaker and amplifier designers would work together for defining the boundaries in integrated solutions. The cost of these rackmount processors were in the range of $500 and up (affordable for pro audio applications...), depending on how many amplifier channels they could handle. Smart amp integrated circuit chips with just about everything the professional speaker protection processors accomplished were introduced more than a decade ago, essentially designed for mobile phones, laptops, Bluetooth speakers, and TVs. Astonishing at the time, all this was combined in a chip along with a few watt amplifier – for less than a buck! Those chips have been trickling up the product development chain into more expensive "smart" home audio products - first with smart speakers intended to promote the ecosystem for voice assistants, and now with a large focus on soundbars and speakerphones.
Serve and Protect Advanced speaker protection processing can be a two-sided sword. We want to increase the maximum clean acoustic output without creating new failure modes by inadvertently crossing the line. Specifically, the concern is transducer fatigue issues by dancing at the edge with a high duty cycle (especially on ring tones), creating new fatigue failures that could cause smartphones to be returned by unhappy users (which Apple, Samsung, and Huawei, might find annoying). NXP, Maxim, Texas Instruments, Cirrus Logic, Infineon, and Qualcomm have succeeded with their Smart Amps (sometimes also called "speaker amps"), all of which are a couple of watts - more than enough for smartphones, considering battery drain and what the speaker can handle. At this point smartphone speakers can play as loud as possible without rattling, buzzing, or most importantly without damage — allowing everyone in the business to sleep soundly. I have always pressed the product managers at the Smart Amp companies to consider a version of this dynamic protection processing in a form that could be used from soundbars to subwoofers to concert sound to military hailing and civil defense voice warning systems. To further enable the expansion of Smart Amps is the prevailing trend of integration of the signal processing, amplifier, speakers and enclosures into finished products – not just smartphones, soundbars, subwoofers, but also self-powered pro sound speakers and automotive sound OEM. Closed systems enable close-coupled protection to do its job effectively. But what more could speaker system designers want? How about where this article started: not just to protect the speaker from overload, but enable the speaker to play louder with reduced bass distortion. For functionality, aside from saving the speakers from damage, how about we drop distortion? And for applications where there are full-duplex signal paths with acoustic echo cancelers, add graceful barge-in, providing significantly wider gain margins before echoes?