Hexaphonic Pickup Project

[Cycfi]

Hi!

I designed and built the carbon-fiber-bamboo guitar. Currently, I use DiMarzio Injector pickups. On my next iteration of the prototype, I would like to experiment on hexaphonic pickups for hex processing.

The Cycfi Six Pack Project is an ongoing Open Source Hardware project for the development of an active hexaphonic pickup system designed for hex processing. The pickup has six low impedance coils. The hex pickup is active with six differential low-noise, low-power pre-amplifiers —one for each coil. It has the same footprint as the ubiquitous Strat single coil, with a very low profile: 8mm (0.3 inch).

Eventually, the goal is to have hexaphonic sustain drivers as well. That, and with extensive processing for each string, will give us musicians full control over the dynamics of the guitar. I know hexaphonic sustain has been done in the past with the Moog guitar, but that was a very expensive gear. I want something more affordable. And I want a system that can be adapted to just about any guitar. This IMO is the holy grail and I know this is very difficult to do right, but every journey starts with the first step.

As an Open Source Hardware project, all the designs (schematics, PCB layout, software, bill of materials, CAD drawings and source code) will be freely shared, 100% free.

I would very much love to hear your thoughts and gather ideas while the project evolves!

[verhoevenc]

I can't say I know ANYTHING in the realms that could help you. But I did enjoy your site. Really fun to read through. I love innovation in the industry. Keep up the fun stuff!

Chris

[Cycfi]

Thank you, Chris. I'm glad you enjoyed the site. Building innovative stuff is my passion.

[Prostheta]

Really good to see you here, Joel! A few members have used composites in their builds and I know a few people have shown an interest in the same kind of processing ideas. You're well qualified to do this so seeing the process would be a pleasure!

[Cycfi]

Thank you, Prostheta! This is indeed a nice forum. Thanks for inviting me. I've just started reading through the posts. I see the looonnnggg thread about the "Sustainer Ideas" dating back from 2004. Wow! Anyway, reading through the posts, I believe I have some new ideas to share on the subject. For instance, I intend to use high-efficiency, low-power class D drivers (instead of linear amplifiers) and do digital signal processing (DSP chips are very affordable now!) on each string to have full control over attack, decay and sustain level. Again, as I said, I wish to share all the designs to the community.

[DropTop]

Your site is indeed a great read.I have been more or less on the same page about tonewoods but only in theory.Hope to start building soon . Hope you achieve your goal.

[Prostheta]

I think you would do well to add your site's address into your signature Joel. Very much a project to be proud about (and to shout about!).

I also noticed that DSPs are ridiculously cheap these days and companies like TI are falling over themselves to give samples out for people to create new applications.

In some ways your six-pack concept reminds me of the construction of WAL bass pickups, except the WALs are single-coil-per-string which are then electrically connected rather than individually processed. It makes me think about how compact the pickup is, or at least how much room is left in there for "bonus toys". If two coils were stacked on top of each other in place of the one or perhaps one large wind was tapped halfway to make two coils per string, it would be feasible to maintain the hexaphonic capability whilst adding in a WAL-like full-width coil to the same package. I can see that you are quite committed to the design already however!

How difficult in your opinion would it be to turn a guitar into a Bluetooth 3.0 compatible device? The specification of 2.0 means that transmitting a reliable hexaphonic signal at quality higher than 44,1/16 would be impossible without extending the bandwidth to that which 3.0 offers. Bluetooth Low Energy would be interesting also as you could run a micropower op-amp and transmitter for a very long time off nothing more than a coin cell or a pair of AAAs. I kind of get the idea that offloading the processing external to the guitar makes the instrument standalone so all manner of external receiver/processors can be designed and used without guitar surgery.

I love this stuff! However, if you were to send a man to the moon you would do it in a calm, controlled, precise and considered manner like a good engineer. My pilot would be liquified on takeoff after not writing anything useful on the drawing board other than "ROAR" in big letters and the time/place of the afterlaunch free bar and pool party.

[RestorationAD]

Glad to have you on the forum.

Really like carbon fiber and I have read and enjoyed your site. I think you have a lot to offer on designs and innovation and look forward to checking your stuff out and reading your contributions to the site.

As a minimalist guitar/pickup designer I am not into active electronics I am still excited to see how this one works out.

Pro I am all in for drinks.

Cheers!

[Prostheta]

You betcha you are, RAD.

[Cycfi]

I think you would do well to add your site's address into your signature Joel. Very much a project to be proud about (and to shout about!).

Sounds cool. I tried that, but it seems I am not allowed to have URLs in my signature. Anyway, I donated to the site. Perhaps I'll be allowed as soon :-) Thanks, Carl!

[Cycfi]

In some ways your six-pack concept reminds me of the construction of WAL bass pickups, except the WALs are single-coil-per-string which are then electrically connected rather than individually processed. It makes me think about how compact the pickup is, or at least how much room is left in there for "bonus toys". If two coils were stacked on top of each other in place of the one or perhaps one large wind was tapped halfway to make two coils per string, it would be feasible to maintain the hexaphonic capability whilst adding in a WAL-like full-width coil to the same package. I can see that you are quite committed to the design already however!

Re: size. Yes, It's very compact. I'll post some pictures soon for the first prototype. The coils + PCB + SMD Op Amps is just tad under 10 mm. The distance from the pickguard to the strings of a Strat type guitar is something like 5mm.

I'm not sure what you mean by WAL-like full-width coil. Could you explain a bit? Do you mean having 2 coils for humbucking purposes?

For prototyping, we are molding our own bobbins using a CNC'd mold and casting urethane, but I am also in contact with some manufacturers for mass producing them (expensive!). I've searched everywhere, but couldn't find off-the-shelf bobbins like these. So, yes, some aspects of the design (such as the bobbins) are already committed. However, I am pondering on having some flexibility. For instance, I can imagine 7-string or 8-string, or even bass guitar applications. For that matter, I am also considering single units complete with active preamps. I can imagine crazy ideas for combining and using such things.

[Prostheta]

Crazy ideas are the ones that break moulds though. The WAL pickups are two rows of four/five coils on a PCB so share a degree of their construction with yours. Not sure if there are applicable patents I should make you aware of though. My thought process was on how to get a "two-in-one" stacked set of differently configured coils.

I guess the op-amp debate is an old one by this point seeing as you are in the fourth stage of design? It would be interesting to rejig my own pre-amps with more modern components.

[Cycfi]

How difficult in your opinion would it be to turn a guitar into a Bluetooth 3.0 compatible device? The specification of 2.0 means that transmitting a reliable hexaphonic signal at quality higher than 44,1/16 would be impossible without extending the bandwidth to that which 3.0 offers. Bluetooth Low Energy would be interesting also as you could run a micropower op-amp and transmitter for a very long time off nothing more than a coin cell or a pair of AAAs. I kind of get the idea that offloading the processing external to the guitar makes the instrument standalone so all manner of external receiver/processors can be designed and used without guitar surgery.

It's cool you asked. I've been looking into wireless options sometime last month. I haven't yet found something suitable yet (e.g. BT 3.0). There are a couple of modules for BT 2.0, but as you now, the bandwidth is not sufficient (though they can theoretically go to up to 1382400 baud; Asynchronous: 2.1Mbps(Max) / 160 kbps, Synchronous: 1Mbps/1Mbps). Link: http://tinyurl.com/9sd596j.

Another option is TI's pure-path: http://tinyurl.com/d9v6g74, but that is for 2-3 channels uncompressed and 4 channels compressed. There's also the NRF24L01 module which can (theoretically) go up to 2Mbit/s: http://tinyurl.com/9s8kc7h.

So bottom-line, I have no good leads on that matter yet.

[Prostheta]

Ultimately those are best case. Real-time signalling with low latency would benefit from any additional spare bandwidth going so perhaps BT4.0 might be the only one that offers the reliability needed to make it truly useful. I'm very interested in the ultra-low-power specifications in the proposed BT4.0 standard however the signal from the pickups would be too hot for these supply voltages which is kind of self defeating!

[Cycfi]

Crazy ideas are the ones that break moulds though. The WAL pickups are two rows of four/five coils on a PCB so share a degree of their construction with yours. Not sure if there are applicable patents I should make you aware of though. My thought process was on how to get a "two-in-one" stacked set of differently configured coils.

I guess the op-amp debate is an old one by this point seeing as you are in the fourth stage of design? It would be interesting to rejig my own pre-amps with more modern components.

Those are interesting ideas! No, the op-amp debate is not set in stone yet.

[Cycfi]

Ultimately those are best case. Real-time signalling with low latency would benefit from any additional spare bandwidth going so perhaps BT4.0 might be the only one that offers the reliability needed to make it truly useful. I'm very interested in the ultra-low-power specifications in the proposed BT4.0 standard however the signal from the pickups would be too hot for these supply voltages which is kind of self defeating!

Indeed! I've heard accounts that the BT module I linked to can only go up to 700+kbps in practice, at best. I also don't have a good experience with BT audio devices, for example, headsets. Move a few meters away and you get a choppy signal.

As for the dynamic range, I see no problem in scaling down before transmission (then a corresponding gain after, and prior to the amp). In the end, it's the resolution that matters and that there's enough to capture the transients. 16 bits (or even 24 bits) is pretty good enough. What am I missing?

[Prostheta]

Significant headroom to fall back on is almost mandatory, for sure. 44.1kHz/16-bit was decided on by Sony for stereo CD-quality audio so I myself would see it as my sworn duty to smash their "standard" and create one tailored to the application instead of to Sony's standards.

Would 32kHz be just as adequate, if not even 22kHz? It would cater for frequencies in the order of 16kHz (according to the theory that "sampling rate should be at least double the highest frequency"). A guitar is generally around four octaves with the lowest fundamental frequency of 80Hz and a top frequency of 1,2kHz or thereabouts so I am sure that 16kHz is still quite excessive for even the squealiest of higher-order harmonics from a clean pickup. Trying convincing somebody these days that a 22kHz sampling rate is a good thing is probably a hard sell though despite mathematics and science.

I just tried resampling one of the 44.1kHz/16-bit sample audio files from the Seymour Duncan site of a bright Strat pickup played "clean" at the bridge and neck/middle. Resampling to 22kHz showed vague difference and I am sure that these samples were coloured with higher end harmonics from the amp used in recording anyway. Since you mention that dynamics are far more important than overall frequency range (you're right) is it possible to implement a floating point method of describing bit depth rather than a fixed bit scale? I am sure that ultimately this will be down to the ADC/DAC chips available, and more than likely this might be the dealbreaker.

It's all well and done hammering out what is ideal from the application standpoint but it is still trying to hammer a guitar-shaped peg into a fairly small Bluetooth-shaped hole using a market-led range of analogue/digital converters! I calculate that mashing six channels of 24-bit audio at 22,050Hz requires a good 3,2Mbit connection. Nowhere near the 700kbit real-world result. Even dropping to 16-bit leaves 2,12Mbits. What's the state of play with regards to compression and the impact that might have on latency? I don't know if these two added layers of data conversion are at a state where they butterfly with high performance realtime audio.

It might well be that hashing in the concept of wireless transmission of six channels of fast audio in a reliable manner to the six-pack pickup is out of bounds until the real-world reliability of BT4.0 can be quantified. That really does endanger the immediate idea of processing outside of the instrument. If processing is kept onboard, perhaps control of that could be done offboard? Ideas ideas ideas.

I do love how our traditional little foible of strings stretched over (usually) wood still has the capability of snapping at the heels of cutting edge technology.

[Prostheta]

For fun I recorded my own guitar playing a variety of chord, note, harmonics, etc. to capture a good cross-section of the raw spectral information from a pickup (admittedly, after tone/vol, all the recording stages, converters, etc.) at 44.1kHz/16-bit.

Here are two plots of the spectral content of the original audio before and after downsampling to 22kHz. There is noticeable dropoff in relation to the original signal right about the 11kHz mark which is bang on what is to be expected. That is at about -120dB though!

Taking the 44.1kHz sample and applying a harsh EQ curve around the 11kHz mark to dump everything that the 22kHz sample reproduced left a harsh and nasty set of frequencies I would only wish on a drummer's cymbals. Those frequencies are above and beyond "presence". Even pick attack noise is only at 4-5kHz!

[ScottR]

I do love how our traditional little foible of strings stretched over (usually) wood still has the capability of snapping at the heels of cutting edge technology.

Amen brother.

SR

[Cycfi]

It's all well and done hammering out what is ideal from the application standpoint but it is still trying to hammer a guitar-shaped peg into a fairly small Bluetooth-shaped hole using a market-led range of analogue/digital converters! I calculate that mashing six channels of 24-bit audio at 22,050Hz requires a good 3,2Mbit connection. Nowhere near the 700kbit real-world result. Even dropping to 16-bit leaves 2,12Mbits. What's the state of play with regards to compression and the impact that might have on latency? I don't know if these two added layers of data conversion are at a state where they butterfly with high performance realtime audio.

It might well be that hashing in the concept of wireless transmission of six channels of fast audio in a reliable manner to the six-pack pickup is out of bounds until the real-world reliability of BT4.0 can be quantified. That really does endanger the immediate idea of processing outside of the instrument. If processing is kept onboard, perhaps control of that could be done offboard? Ideas ideas ideas

Update: I forgot to mention this technology: BlueGiga 12MBPS module. So... there's still hope

I wonder about the range though since it's listed as class-2 BT (10 meters). Class-1 (100 meters) would have been better. But the lower power is very welcome.

[Cycfi]

Significant headroom to fall back on is almost mandatory, for sure. 44.1kHz/16-bit was decided on by Sony for stereo CD-quality audio so I myself would see it as my sworn duty to smash their "standard" and create one tailored to the application instead of to Sony's standards.

Would 32kHz be just as adequate, if not even 22kHz? It would cater for frequencies in the order of 16kHz (according to the theory that "sampling rate should be at least double the highest frequency"). A guitar is generally around four octaves with the lowest fundamental frequency of 80Hz and a top frequency of 1,2kHz or thereabouts so I am sure that 16kHz is still quite excessive for even the squealiest of higher-order harmonics from a clean pickup. Trying convincing somebody these days that a 22kHz sampling rate is a good thing is probably a hard sell though despite mathematics and science.

I don't have the figures right now, but I seem to recall seeing strong overtones from the 10KHZ..20KHZ range at the attack transients, when hitting the strings hard (e.g. slap bass). Not quite sure though. But you might want to check it out.

I just tried resampling one of the 44.1kHz/16-bit sample audio files from the Seymour Duncan site of a bright Strat pickup played "clean" at the bridge and neck/middle. Resampling to 22kHz showed vague difference and I am sure that these samples were coloured with higher end harmonics from the amp used in recording anyway. Since you mention that dynamics are far more important than overall frequency range (you're right) is it possible to implement a floating point method of describing bit depth rather than a fixed bit scale? I am sure that ultimately this will be down to the ADC/DAC chips available, and more than likely this might be the dealbreaker.

About ADCs, my electronics and C++ friend shared this article about Multi-channel ADC choices (specifically delta-sigma or SAR). He has very good points about SAR.

[Prostheta]

This is where my working knowledge craps out Joel. I don't do this at any level for either a living or as a hobby but as a "more knowledgeable than as not" interested observer. I love the depth of this technology however there are but 24hrs in a day!

You're more than likely correct about the slap bass and string attack noise, and generally it is unrelated to the resonating note in terms of harmonic content. Realistically however, a 22khz sampling rate results in anything above 11kHz just not being reproduced with fidelity. If we were recording the high impact high frequencies off the edges of cymbals then certainly!

Is there possibility for variable bit rates controlled by say, some kind of quality of service protocol? If bandwidth drops, sampling frequency is altered to fit within it? Certainly, the bandwidth offered by 12Mbit/s is just short of six channels at 96kHz/24-bit which is far beyond requirements. 48kHz/24-bit is more than adequate plus you could probably send the signal from any "traditional" magnetic pickups seperately to any switching or summing. Given a two-way communications path, it might even be feasible to control the sustainer from a pedalboard....

Bringing this all back to earth, the pickup design is lovely and compact. I'm really looking forward to how it will sound in practice. My only reservation about these types of coil is that they sound quite hifi which may be a difficult thing for most people to get used to. The common complaint about active pickups and other low turn pickups is somewhat of a false one ("oh it sounds compressed!" or "oh it sounds sterile in the treble!"). I think people often are not used to hearing what they are missing. Same problem when people use a wireless pack and suddenly there are no cable losses....

Is the testbed instrument going to be something along the same lines as the previous bamboo/carbon guitar?

If it sounds like I am talking off the top of my head, it is because I genuinely am.

[Prostheta]

Oh yes, my brain stopped working after reading through the information in the links you presented....perhaps if I had decided to go down the path of becoming an electronics engineer beyond my college qualifications I would be able to comment more usefully than my hack attempts and clumsy generalisations. Certainly when I was in this stage of my education the hottest thing on the block was a i486DX2-S@66MHz loaded with 16MB of RAM. That thing was a total hit with the girls, honestly. It wasn't my bass playing that's for sure.

I fully respect your completeness however I have to wonder as to when this becomes a case of diminishing returns. Maintaining a balanced signal path throughout is definitely a good thing to stick to, however I think that the beauty in our "brass and wood" instruments comes from their organic flawed nature which is very much a mix of fighting and co-operation between the player and the instrument. All that work in producing a flawless detail of a signal is soon lost when it hits an amp to add in the much-needed colouration.

What's your take on the drive for ultra-high fidelity? I fear that medical and scientific instrumentation will run and hide in the shadows whenever this pickup system is in town!

[curtisa]

When you're experimenting with the sound difference in sampling rates in this application you should be applying the sampling rate conversion at the source, as it would be applied in real life.

Try this instead - plug your guitar direct into an audio interface (preferably one with a high-Z input) and record it dry and direct. Replay the recorded file back through an amp (or software amp sim) and listen to the results. Take the original recorded file, change the sample rates/bit depths/encoding etc and replay it through the amp/amp sim and compare.

My gut feel is that even with 22KHz sampling rates through a guitar amp you'll not notice much sound quality difference. Most guitar amp speakers and pickups do not offer much beyond 6-8KHz. Even if you consider all the high-end fizz associated with ultra-saturated distorted tones which ordinarily generates a lot of HF, 90% of this is generated after the guitar signal is in the amp. A guitar amp is nothing more than a dirty great big (but hopefully good sounding) signal processor. I have an old Boss half-rack digital delay unit that is 22KHz 8bit which fine when run through a guitar amps' effects loop. Even though there's some additional processing going on inside the unit to expand the dynamic range of it (compress/expand) to get past the low bit depth, the low samplerate does not introduce such a severe loss of HF that it destroys the natural sound of the guitar, also helped by the natural post-filtering effect of a guitar amp speaker. When the output of the delay unit is run direct you only notice a slight loss in top end with guitar signals.

Of course, if your goal is recreating a new kind of high fidelity DI-ed sound, experimenting with lower samplerates may not be useful.

[Cycfi]

Is there possibility for variable bit rates controlled by say, some kind of quality of service protocol? If bandwidth drops, sampling frequency is altered to fit within it? Certainly, the bandwidth offered by 12Mbit/s is just short of six channels at 96kHz/24-bit which is far beyond requirements. 48kHz/24-bit is more than adequate plus you could probably send the signal from any "traditional" magnetic pickups seperately to any switching or summing. Given a two-way communications path, it might even be feasible to control the sustainer from a pedalboard....

That's certainly a neat idea! I'd say cross the bridge when we get there. For now I'll start with a wired connection. As you say, there's finite time. There are lots of things to work on still. My strategy is to make it work as quickly as possible. Refine, rinse repeat

Bringing this all back to earth, the pickup design is lovely and compact. I'm really looking forward to how it will sound in practice. My only reservation about these types of coil is that they sound quite hifi which may be a difficult thing for most people to get used to. The common complaint about active pickups and other low turn pickups is somewhat of a false one ("oh it sounds compressed!" or "oh it sounds sterile in the treble!"). I think people often are not used to hearing what they are missing. Same problem when people use a wireless pack and suddenly there are no cable losses....

Indeed. My observation is that in general guitar players are conservative when it comes to guitar sound. I think that is the reason why digital emulations of amps cabs, pickups, guitars, etc., are quite popular, for example. Take the Roland VG guitar. You see emulations of the strat, the les paul, acoustics, all sorts of amp and cab emulations, all sorts of pickup emulations (single coil, double coil), effects emulations, etc. The same can be said of all the current generation of plugin effects available for computer based recordists.

My opinion? We've stagnated! Emulations are good, but they are, ehm... emulations. If I want to have a strat sound, I'll get a strat. If I want a marshall JCM800, well, nothing beats the real thing. But what's sad is that we always tend to go back to the same tried-and-true path. The les pauls, the strats, the marshalls, the 50s style humbuckers and single coils. Bottom line is: we all sound the same! There's nothing new these days that gives me the OOHHH that is new! Like, I would imagine, what people thought when they first heard Jimi play a soaring riff through an overdriven marshall.

What I really want is to explore new sounds. Not the tried and true. Not the strat, Not the Les Paul. Not the Marshall. Something totally different with a totally unique signature. For example, I'd really love to have complete control over attack, decay and sustain. I'd die for cello or violin like expressivity, dynamics and sustain without resorting to distortion and without sounding like a synth. IMO, expressivity (through sustain) is the very reason why the fuzz, overdrive and distortion were invented. I'd like to traverse different avenue for exploration with that same goal in mind.

Is the testbed instrument going to be something along the same lines as the previous bamboo/carbon guitar?

For the test bed, I resurrected an old strat copy to start with. I'll probably incorporate the results later in another prototype of the bamboo/carbon guitar.

Edited April 6, 2013 by Cycfi