James Croft has fun reviewing a Controlled Passive Radiator patent awarded to inventor Tony Doy, on behalf of Sonos (Santa Barbara, CA.) The patent details multiple techniques to control a passive radiator, including a device buffering successive samples of audio content and predictive techniques on active designs. With this extended review, Voice Coil readers will gain a valuable position on key possible innovation areas. This article was originally published in Voice Coil, October 2018.
Controlled Passive Radiator
Patent Number: US20180103313A1
Inventor(s): Tony Doy (Santa Barbara, CA)
Assignee: Sonos, Inc., Santa Barbara, CA
Filed: October 6, 2016
Published: April 12, 2018
Number of Claims: 21
Number of Drawings: 9
Abstract from Patent
Example techniques may affect controlling a passive radiator. An implementation may include a device buffering successive samples of audio content. For sets of buffered samples, the device predicts excursion of the passive radiator caused by playback of the respective set of buffered samples by active speakers via a model. The device limits excursion of the passive radiator to little than an excursion limit erstwhile certain sets of buffered samples are predicted to origin the passive radiator to decision beyond the excursion limit. The device plays back the successive samples of the modified audio content via the active speakers. The device measures excursion of the passive radiator erstwhile sets of buffered samples are played back via the active speakers. For sets of samples, the device determines respective differences between the predicted excursion and the measured excursion and adjusts the model to offset determined differences between the predicted and measured excursion.
Independent Claims
1. A playback device comprising: 1 or more active speakers; a passive radiator; 1 or more processors; and computer-readable media having stored therein instructions executable by the 1 or more processors to origin the playback device to execute operations comprising: buffering successive samples of audio content; for sets of 1 or more buffered samples, predicting, via a forward prediction model, excursion of the passive radiator caused by playback of the respective set of buffered samples by the 1 or more active speakers; limiting excursion of the passive radiator to little than an excursion limit erstwhile certain sets of buffered samples are predicted to origin the passive radiator to decision beyond the excursion limit, wherein limiting excursion of the passive radiator comprises modifying the audio content to lower sound force levels of the buffered samples that are predicted to origin the passive radiator to decision beyond the excursion limit; playing back the successive samples of the modified audio content via the 1 or more active speakers; measuring excursion of the passive radiator erstwhile sets of buffered samples are played back via the 1 or more active speakers; for sets of 1 or more samples, determining respective differences between the predicted excursion and the measured excursion; and adjusting the forward prediction model to offset determined differences between the predicted excursion and the measured excursion.
8. A tangible, non-transitory computer-readable average having stored therein instructions executable by 1 or more processors to origin a playback device to execute a method comprising: buffering successive samples of audio content; for sets of 1 or more buffered samples, predicting, via a forward prediction model, excursion of a passive radiator caused by playback of the respective set of buffered samples by 1 or more active speakers; limiting excursion of the passive radiator to little than an excursion limit erstwhile certain sets of buffered samples are predicted to origin the passive radiator to decision beyond the excursion limit, wherein limiting excursion of the passive radiator comprises modifying the audio content to lower sound force levels of the buffered samples that are predicted to origin the passive radiator to decision beyond the excursion limit; playing back the successive samples of the modified audio content via the 1 or more active speakers; measuring excursion of the passive radiator erstwhile sets of buffered samples are played back via the 1 or more active speakers; for sets of 1 or more samples, determining respective differences between the predicted excursion and the measured excursion; and adjusting the forward prediction model to offset determined differences between the predicted excursion and the measured excursion.
15. A method comprising: a playback device buffering successive samples of audio content; for sets of 1 or more buffered samples, the playback device predicting, via a forward prediction model, excursion of a passive radiator caused by playback of the respective set of buffered samples by 1 or more active speakers; the playback device limiting excursion of the passive radiator to little than an excursion limit erstwhile certain sets of buffered samples are predicted to origin the passive radiator to decision beyond the excursion limit, wherein limiting excursion of the passive radiator comprises modifying the audio content to lower sound force levels of the buffered samples that are predicted to origin the passive radiator to decision beyond the excursion limit; the playback device playing back the successive samples of the modified audio content via the 1 or more active speakers; the playback device measuring excursion of the passive radiator erstwhile sets of buffered samples are played back via the 1 or more active speakers; for sets of 1 or more samples, the playback device determining respective differences between the predicted excursion and the measured excursion; and the playback device adjusting the forward prediction model to offset determined differences between the predicted excursion and the measured excursion.
Reviewer Comments
Active, powered loudspeakers have a variety of advantages over full passive implementations. In fact, I would propose that 1 day engineers will look back and think it was beautiful silly that we worked so hard to get these primitive passive loudspeakers to behave alternatively of making all loudspeakers active.
In the realization of an active loudspeaker, 1 of the many useful techniques that has been applied is that of “motional feedback” (MFB)—the sensing of the motional output of the transducer diaphragm to monitor the device’s behaviour as compared to the mention input signal and realize a difference signal to be utilized to find an appropriate compensation for any correctable errors.
While the approach can be applied to compensate for many tiny and large signals, linear and nonlinear effects, the top subjective impact is frequently derived from the ability to correct for large signal nonlinearities that happen at the overload limits of the transducer.
The method literature is rich with investigations into applying MFB to loudspeakers, with 1 of the seminal mention papers being that of Egbert De Boar, “Theory of Motional Feedback,” IRE Transactions on Audio, January–February, 1961. 1 of the earliest papers describing an actual simplification to practice of a applicable MFB loudspeaker was “A Motional Feedback loudspeaker System” written up in the Philips method Review (Volume 29, No. 5, 1968) by J. A. Klaassen and S. H. de Koning of Philips Electronics.
Philips is widely credited with introducing the first loudspeaker product incorporating MFB in 1973 (the RH532, known affectionately as David for its tiny size and powerful sound). But, it was actually Panasonic (Matsushita), applying Luxman’s SQ-65 MFB, vacuum tube, motional feedback electronics in loudspeaker systems in the early 1960s — models: 12P-X3 tri-axial and EAB-8M2 talker Systems — that appear to be the first commercial product to incorporate MFB. Those of you who enjoy specified things will be entertained by the thorough operating manual, including schematics, operational principles, and mill assembly line pictures, which can be found here.
Try uncovering an owner’s manual like that with any product made currently. Audio hobbyists were seriously active with all method aspects of their equipment in those days.
Of course, as with so many audio technologies, the first inventor did not see his creative ideas realized in production and that besides seems to be the case with MFB loudspeakers—for which the earliest patent appears to be US Patent 2,194,175, “Distortion Reducing Arrangement” to Karl Wilhelm, filed just 1 year after the 2 celebrated 1937 patents on negative feedback in power amplifiers, by H. S. Black and H. T. Nyquist were granted.
Even though there are dozens of patents and papers on MFB loudspeakers, the vast majority address only sealed, acoustic suspension-type loudspeakers. Even with specified a simple device as a closed-box loudspeaker, with a singular resonant frequency, there are already difficulties in dealing with the behaviour of a feedback-based corrective system.
One of the primary issues is that the phase shift of the strategy at, and near, the fundamental resonant frequency can convert a (acceleration-based) feedback strategy into a feed-forward strategy and measures should be put in place to address this condition. It is beyond the scope of this review to supply a complete discussion of this issue, but for those interested, it is covered in almost all paper on feedback and explored in a peculiarly clear and concise manner on pages 103 to 106 of the 3rd edition of John Borwick’s comprehensive reference, Loudspeaker and Headphone Handbook.
When it comes to incorporating MFB into a bass-reflex system, the complexity of the variables to be addressed due to delay/phase effects is increased. Among the dearth of papers written on the application of MFB to bass reflex systems, 2 appear to supply a good representation of both the difficulties and what is possible erstwhile applying MFB to vented enclosures, those papers are: “Current Controlled Vented Box Loudspeaker strategy with Motional Feedback” by Philippe Robineau and Mario Rossi, from the 108th Audio Engineering Society (AES) Convention, February 2000; and “On the usage of Motion Feedback as utilized in 4th Order Systems” by Stephan Willems and Guido D’Hoogh, from the 126th AES Convention, May 2009. The first paper uses current drive and limits the frequency scope of its MFB capability so as to be operating likewise to utilizing MFB over the same scope as with a sealed system.
The second paper, which utilized a very advanced excursion passive diaphragm radiator as the acoustic mass in the reflex system, is more comprehensive in applying an adaptive strategy with phase rotation and by matching complex weighting functions of 3 sources of feedback: 1) active driver acceleration; 2) passive radiator acceleration; and 3) current through the voice coil, making it possible to get effective feedback correction while maintaining the same frequency consequence as the passive system. Any desired equalization was performed outside of the MFB loop.
Other than delivering unintended acoustic output at lower midrange frequencies, due to interior enclosure and/or tube resonances, a passive acoustic mass radiator in a bass reflex strategy tends to operate over a very narrow bandwidth that is associated with the Helmholtz resonant tuning frequency of the enclosure system, based on the acoustic mass of the passive device interacting with the air spring compliance of the enclosure volume.
When applying a feedback sensor to a passive diaphragm radiator, the radiator is pneumatically driven substantially evenly across its surface, specified that it is not subject to diaphragm breakup modes that an active driver is, due to the active driver being driven at the center of its diaphragm with a voice coil. For passive radiators, this is advantageous from the standpoint of monitoring diaphragm motion, as a sensor placed most anywhere on the diaphragm will represent the displacement of all points across the diaphragm surface. Whereas, erstwhile utilizing MFB on an active transducer, erstwhile monitoring the voice coil’s movements for feedback information about the diaphragm displacements, movements at the voice coil and neck of the diaphragm are not necessarily representations of what the diaphragm is doing at its outer regions, where breakup modes will ensue, peculiarly at higher frequencies of its operating range.
The patent under review is simply a bass reflex strategy with a passive radiator that utilizes a feedback sensor on the passive radiator. Excursions of the passive radiator, while frequency dependent, are about linearly related to excursion of the active talker in the shared enclosure. Passive radiators are arranged to have maximum excursion at its resonant tuning frequency, while the active driver has an excursion minimum at the same frequency, so as to substantially increase the linear acoustic output capability of the active driver at the tuning frequency. With the active transducer moving in proportion to the voltage applied to the voice coil, in a frequency dependent manner, the excursion of a passive radiator in the enclosure is (within its mechanical limits) linearly related to the voltage applied to active transducer in the enclosure.
However, the relation between the voltage applied to the active driver and excursion of the passive driver is only linear up to the affirmative and negative excursion limits of the passive diaphragm. These physical limits are imposed by the limits of the elastic linearity of the passive radiator’s suspension system.
At a given frequency where the output from the passive radiator is the system’s dominant acoustic output source, applying a voltage to the active driver that exceeds the affirmative and/or negative excursion limits of the passive radiator causes mechanical “clipping,” resulting in highly audible distortion and limiting of acoustic output.
The patent discloses techniques active in controlling the displacement of the passive diaphragm radiator. The control involves predicting, via a forward prediction model, the passive radiator’s excursion caused by playback of audio content from the active driver.
The forward prediction model is based on the linear relation between the voltage applied to the active driver and excursion of the passive driver. erstwhile certain portions of the audio content are predicted to origin nonlinear excursion of the passive radiator, the audio input level is modified to control the passive radiator to an excursion maximum that is at or below a predetermined excursion limit. In particular, the levels of those portions of the audio content that are predicted to origin audible overload of the passive radiator are reduced.
It is stated that, alternatively, another techniques specified as modifying the phase of the input signal may besides be utilized to origin little voltage to be applied to the active driver to limit passive radiator excursion, but it doesn’t state what phase is altered in relation to what signal, or how this would be advantageous over straight reducing the amplitude.
It is besides suggested that feedback may further improve control of the radiator. erstwhile the active talker plays back the audio content (with portions modified to limit excursion of the passive radiator), a sensor measures excursion of the passive radiator. Predicted excursion, from a forward prediction model, is compared against measured excursion for various portions of the audio content. Differences between the predicted excursion and the measured excursion can be provided as corrective feedback to the forward prediction model parameters. This feedback may origin adjustments to the forward prediction model, which may aid to minimize mistake in the model.
Figure 1: An example of 1 embodiment of the invention.
Two typical implementations are described. The first implementation includes a playback device buffering successive samples of audio content, and, for sets of 1 or more buffered samples, predicting, via a forward prediction model, the excursion of the passive radiator caused by playback of the respective set of buffered samples (see Figure 1 and Figure 2). Then, the strategy limits the excursion of the passive radiator to little than a predetermined excursion limit erstwhile certain sets of buffered samples are predicted to origin the passive radiator to decision beyond the excursion limit. The strategy then plays back the successive samples of the modified audio content through the active talker and measures excursion of the passive radiator. For sets of 1 or more samples, the strategy determines the respective differences, between the predicted excursion and the measured excursion and then the forward prediction model is adjusted to offset determined differences between the predicted excursion and the measured excursion.
Figure 2: An example of the steps of the method of operation of the invention.
A second implementation includes buffer-to-buffer successive samples of audio content, along with a forward prediction model to predict, for sets of 1 or more buffered samples, the excursion of the passive radiator. A limiter is included, to limit excursion of the passive radiator to little than a predetermined excursion limit, erstwhile certain sets of buffered samples are predicted to origin the passive radiator to decision beyond the excursion limit.
An audio phase is included to play back the successive samples of the modified audio content through the active speakers and a sensor is employed to measurement excursion of the passive radiator erstwhile sets of buffered samples are played. A processor is utilized to find respective differences between the predicted excursion and the measured excursion and adjust the forward prediction model to offset determined differences between the predicted excursion and the measured excursion. The delays and buffering time periods are disclosed only in general terms without circumstantial psycho-acoustical effects explored or utilized to quantify timing criteria.
Usually, any form of fast attack and slow release is utilized for these types of “overload avoidance” systems, but these issues are not explored, but to propose that limiters or compressors could be used, suggesting that the invention may comprise either real-time limiting or attack and release time compressors (see Figure 3). It is stated that the strategy may be utilized in multiples and in that case not having an audible time hold difference between devices was a priority.
Figure 3: An example input voltage vs. excursion limit graph
The patent suggests the anticipation of utilizing any of the standard sensor types, including optical, capacitive, and inductive. This strategy is different in having its focus be that of maintaining audible linearity of the passive radiator, as opposed to the standard approach of emphasizing the linearity of the active transducer or the full system. Most often, it is easier to plan a passive radiator to have greater usable displacement than an active driver with the same diaphragm area, since the passive radiator has only mechanical suspensions to deal with, whereas an active driver must besides be able to keep linearity of its voice coil in the magnetic gap.
Most active drivers have more mechanical linearity than electromagnetic linearity, so a passive radiator, with a well-engineered suspension, depending only on mechanical linearity, should be of lesser concern. That said, the demands on a passive radiator at the tuning frequency can easy be more than 4 times greater than the active driver.
Because of that issue, the passive radiator is most frequently made larger in diameter than the active driver, but with the proliferation of tiny systems, the packaging doesn’t always let this more optimal area differential between the 2 diaphragms. Therefore, in the case of passive radiators that aren’t able to have importantly greater volume displacement than their associated active driver, the disclosed invention should be a useful addition to the engineering choices available to minimize audible overloads. VC
For those curious in pertinent prior art not disclosed in the patent, see US5,191,619, “Bass Enhancing Device for a talker System” by Toru Hayase, Assigned to Sharp Kabushiki Kaisha.)
This article was originally published in Voice Coil, October 2018.
