What ever happened to the digital audio transducer and the commutated voice coil ?

[smowry]

The moving coil audio transducer as we know it was invented by Rice-Kellogg in 1924. Notice that in more than 100 years, the asymmetric topology appears remarkedly similar to audio transducers of today.

Is that the GENELEC coaxial's patented flat surround et al?

Note that the digital audio transducer still seems father away than Mars, even though IBM and Bell Labs tried and failed to develop one in the 1960's.

I found three (3) expired US Patents that claim the Commutated Voice Coil as an invention.
1. US8284982 Positionally sequenced loudspeaker system by Adam H. Bailey, filed 6 March 2007, status: Expired - Fee Related
2. US5748753 High power audio subwoofer by Robert W. Carver, filed 2 January 1996, status: Expired - Fee Related
3. US4531025 Loudspeaker with commutated coil drive by Thomas J. Danley and Charles A. Rey, filed 19 March 1984. status: Expired - Lifetime

I was unable to identify any active commutated voice coil US Patents.

Having said that, I thought I would turn to the ASR community for a discussion on why the loudspeaker industry has failed to make progress on the development of the digit transducer or to even implement any commercial products containing transducers with commutated voice coils regardless of claims of invention.

 

[Salt]

Perhaps it was not 'Worth‘ it?

 

[smowry]

The analogy of the commutated voice coil is the class A amplifier to the class B amplifier. Transducer efficiency and large displacement linearity could be significantly improved. Note that the typical (class A) audio transducer efficiency is very low, ~1%. Most if not all transducer engineers would love to commutate the voice coil of their subwoofers.

 

[sergeauckland]

I'm not convinced that a sliding commutated contact would be silent in operation, without noise, sticktion or steps as the contact slides over the coil. A conventional voice coil has no contact with the stationary part, only sometimes ferrofluid for tweeters, never for bass or midrange units. The cost of engineering a very smooth sliding contact wouldn't be trivial so as Salt said above, not worth it for doubtful benefit.

S

 

[egellings]

I'm not convinced that a sliding commutated contact would be silent in operation, without noise, sticktion or steps as the contact slides over the coil. A conventional voice coil has no contact with the stationary part, only sometimes ferrofluid for tweeters, never for bass or midrange units. The cost of engineering a very smooth sliding contact wouldn't be trivial so as Salt said above, not worth it for doubtful benefit.

S

I could see that sort of a sliding contact arrangement wearing out soon. Flex lead pigtails are the way to go.

 

[smowry]

I was wrong; Danley and Ray's patent went the full 20-years. The other 2 listed above expired due to nonpayment of patent fees. But patents can discourage R&D.

 

[kemmler3D]

If Audio-Technica's marketing is to be believed, they actually did ship a "digital transducer", https://www.audio-technica.com/en-us/blog/technology-behind-ath-dsr7bt-ath-dsr9bt-headphones which I assume is driven with a raw DSD-like signal or something?

I don't think the transducer itself is actually anything special, and I don't know if this is what you actually had in mind.

 

[smowry]

Well the ATH-DSR9BT appears to utilize a digital audio transducer; there's no D to A and there are 4 voice coils.

• Exclusive 45 mm True Motion Driver, featuring 4-core twisted OFC-7N voice coil and diaphragm with DLC (diamond-like carbon) coating, extremely accurate audio reproduction with extended high frequencies

This sounds really exciting but the product has been discontinued. It's still sold on eBay , Amazon, and with a local online retailer in Thailand, Shopee for about US$599. I searched for a patent which would contain complete disclosure but I could not find anything.

 

[kyuu]

It's not always (in fact almost never is) a conspiracy. Sometimes the juice isn't worth the squeeze.

 

[Randyman...]

Doesn't a servo-controlled sub basically do this (the servo is a form of indirect commutator) - but is still technically an analog domain thing...

 

[smowry]

Isn't servo-controlled a closed loop feedback control system?

 

[Randyman...]

Isn't servo-controlled a closed loop feedback control system?

Oh OK - I think I'm following this a bit better now. I was thinking it was using a commutator as part of feedback or something (reading the comments about mechanical noise that would likely result) - but it's trying to use a commutator in a similar way a motor does. Seems unlikely to materialize IMO - and with complex audio-frequency waveforms, seems unlikely to be able to track smoothly/continually (or "gapless-ly") across the coil or whatever the conductor/flux generator is...

 

[smowry]

The analogy of the commutated voice coil is the class A amplifier to the class B amplifier. Transducer efficiency and large displacement linearity could be significantly improved. Note that the typical (class A) audio transducer efficiency is very low, ~1%. Most if not all transducer engineers would love to commutate the voice coil of their subwoofers.

If we look at Carver's Patent images contained below, we can see two coils.

One coil is wound below the voice coil rest position, 0.0, while the other is wound above the voice rest position. So as the coils move out of the motor, the bottom coil is turned on and the top coil is turned off. Then as the coils move into the motor assembly, the top coil is turned on and the bottom coil is turned off.

Then it does seem possible to turn the coils on and off with a control system rather than some mechanical wiper.

 

[egellings]

A mechanical wiper would likely wear out too quickly.

 

[Randyman...]

If we look at Carver's Patent images contained below, we can see two coils.

View attachment 438189
View attachment 438191
One coil is wound below the voice coil rest position, 0.0, while the other is wound above the voice rest position. So as the coils move out of the motor, the bottom coil is turned on and the top coil is turned off. Then as the coils move into the motor assembly, the top coil is turned on and the bottom coil is turned off.

Then it does seem possible to turn the coils on and off with a control system rather than some mechanical wiper.

But the flux/gap is not a paper-thin emission that only exists "at rest" / 0-Point. There is a gradual decrease of flux density away from the rest position (not sure of the technical terms). Switching the coil does not also impact the flux and its dispersion. The coil still has interaction with the gap above or below the "resting point". Seems that turning off the opposite coil would eliminate that residual interaction the "inactive" half of the coil would have otherwise contributed to the resulting force.

Additionally, that only "turns off" 1/2 the coil - what is that really gaining WRT efficiency or any other desirable trait? How many "steps/channels" (opposed to just 2 channels of top/bottom coil) would be needed for that to be an appreciable benefit? And what about drivers with very short coils in long-ish gaps?

A regular loudspeaker might be crude and considered "low tech" to a degree - but their beauty is in the simplicity of their analog operation as a transducer.

 

[smowry]

Well focusing on the solution, rather than the problems, in practice the commutated voice coil analogy would need to be a class AB amplifier.

In the limit, the primary advance is a 6 dB increase in output related to the doubling of Beta(x) = [Bl(x)^2]/Re N^2/W vs. 2Beta(x) = [Bl(x)^2]/(Re/2) N^2/W. Where Re is the DC resistance of the voice coil in ohms and Bl is flux linkage to the voice coil in Tm and Beta is the true motor figure of merit. And yes this effectively only applies to longer overhung voice coils and would be ineffective for underhung voice coils motors. The design objective would be to maintain max flux linkage by always keeping enough voice coil in the gap without having an active segment of the voice coil outside the gap. This may require more than two voice coil segments as proposed by Baily. Then the challenge would be to trade Bl and x to increase Xmax. Note that Dr. Klippel uses 0.7Bl(0) as the 10% distortion threshold to establish the Xmax spec. Then the commutated voice coil would go from overhung to a linear well hung!

I published a discussion of motor topologies and the related flux linkage to these topologies in 2010.
https://pearl-hifi.com/06_Lit_Archi...rs/Mowry_Steve/Assessing_Motor_Topologies.pdf

Now with regards to Bob Carver's Sunfire Subwoofers, they never contained double stacked voice coils and his patent was only a concept. I had worked at TC Sounds in 1998 - 1999 and TC sounds was the primary manufacturer for Sunfire's subwoofer transducers. The voice coils were all highly overhung, longgggggg, single coils. However, I have a Bob Carver story.

When I left Bose and joined TC Sounds in the late 90's, they were manufacturing the Sunfire subwoofer drivers. Sunfire and its owner Bob Carver were a major customer of TC Sounds. So I was tasked to work with Bob on a few projects. At that time, it seemed like just about all his requests were quantified at 30%. For example, "make the spider convolutions 30% deeper". "Make the voice coil 30% taller". "Make the surround roll 30% bigger". After a short time, I asked him what was so special about 30% and I began to call him 30% Bob. Well that was it and I did not work with him after that but at least my colleagues were amused.

 

[smowry]

What about the following?

Position sensor + Dr. Klippel's system identification + DSP = Voice Coil Commutator control system

I suspect that there are folks within the ASR community that could "Commutate the Voice Coil". It may take some collaboration/team. Then we could call them The Commutators!

 

[smowry]

Looking at the geometry and material derivation of Beta may help to make thing a bit more clear. There is just flux density, volume and resistivity to consider.

Beta(x) = [Bl(x)^2] / Re N^2/W
= [Phi(x) l / S] [Phi(x) l / S] / [rho l / S]
= [Phi(x)^2] l / S / rho but Phi(x) = B(x) S
= [B(x)^2] l S / rho
= [B(x)^2] Volume of Voice Coil / rho

Where Phi is the magnetic flux (T m^2), rho is the electrical resistivity (ohm meter), S is the cross-section (m^2) and l is the length of the voice coil wire respectively (m).