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The Technology...

The Servobass DSP servo controller is the culmination of many years of research and development into low frequency sound reproduction. Combining the strengths of many different disciplines of acoustics, engineering and science we have achieved a breakthrough in sound reproduction technology that results in crystal clear bass reproduction which rivals the best in passive and active low frequency loudspeaker designs. The outcome of this research endeavor is a new digital control system called the Lambda Digital Control System which is designed specifically for low frequency sound reproduction applications and which improves performance by orders of magnitude.

The heart of our subwoofers is a DSP based digital servo system which is tightly coupled to the loudspeaker diaphragm through a highly linear industrial grade accelerometer. Using a floating point DSP and proprietary digital signal processing techniques we are able to achieve ultra-high loop gains whilst maintaining unconditional stability in the feedback loop. Higher loop gains in a servo system translate into lower distortion and higher accuracy of the reproduced music. The higher the loop gain the better and we have achieved a maximum loop gain of 47dB which is unprecedented compared to most other servo systems which only offer 12 dB of feedback at best. Add to that a current domain power amplifier which eliminates distortions caused by variations in voice coil impedance and the result is extremely accurate bass reproduction which is not achievable when the speaker is operated with no feedback.

The Lambda ultra high gain digital servo controller

The simplified block diagram to the right shows that our proprietary Lambda servo control system uses multiple feedback loops to increase loop gain which in-turn improves linearity and helps iron out motor distortions to negligible levels. All of this is done with the precision of 40 bit floating point arithmetic executed on a 400MHz floating point Analog Devices SHARC DSP that is capable of processing 2.4 Gflops.

In additional to this our Lambda digital servo controller drives a current domain power amplifier which is made up of a high efficiency class-D power amplifier encapsulated in it's own current feedback loop to maintain constant current drive in the voice coil independent of varying voice coil resistance and inductance modulation. This refinement alone reduces high frequency distortion by up to 6 dB and more, whilst providing immunity to changes in voice coil resistance with temperature. We achieve all of this whilst maintaining unconditional loop stability.

The DSP throw away line

These days the word DSP is bandied about by vendors a lot to try and convince the consumer into believing that they have something better to offer. But just what does DSP capability mean in terms of traditional subwoofer designs ? In most cases it usually means a subwoofer design is using a low cost fixed point DSP to do some basic equalization which could quite easily be done with a handful of analog components. Hardly impressive but to the untrained consumer it's easy to get excited. Some class-D amplifier vendors actually incorporate some DSP functionality as part of the amplification process so the subwoofer vendor touts the DSP capability as though it is actually part of the whole amplifier speaker interface when it is not.

Unlike these misleading or exaggerated claims, Servobass uses premium grade full floating point DSP's to control the loudspeaker motion in real-time as well as provide many other ancillary functions such as room equalization, Active Filters, LCD display, USB Host interface, Networking Interface etc. We know of only one subwoofer vendor that uses a DSP as part of a subwoofer servo feedback controller but it is only a light-weight 16 bit fixed point DSP that only runs at up-to 30 mips (million instructions per second) compared to the DSP our products use which processes 32/40 bit floating point numbers at up-to 1600 mflops (million floating point ops per second). Because of this we believe that when people purchase a Servobass subwoofer they are getting true DSP performance with total capability that will not disappoint.

Last Updated November 2, 2014