Digital BBD concept looking for comments and beta testers

[Cabintech]

We are experimenting with a "digital BBD" concept (not an entirely new idea, there are pedals out there now using digital BBD emulation). The idea here is to make a dedicated, configurable module that is a very flexible but easy to use CV controlled delay device. Digital at its core, but analog in and analog out. It is only a delay device, not an entire effect. All feedback, mixing, and filtering would be done by analog circuits like any BBD circuit to make various effects (chorus, flanger, reverb, etc). No DSP programming or microcontroller needed to use it. From a system design point of view it is a CV controlled analog delay line, nothing more. We have a breadboard version working on our bench but no beta modules yet.

Looking for some thoughts on features and functions, and ultimately beta testers. The genesis of this device was some of our customers’ frustration at the ridiculously high price and short supply of MN3011 BBD chips. If you are not familiar, it is an unusual BBD with 6 outputs arranged at specific points along the BBD chain. Using those multiple delay taps in various feedback configurations can make some complex and interesting effects. But the MN3011 has been out of production for many years and there is no current production clone.

We then expanded on the MN3011 idea to a more general CV programmable multitap delay that could emulate the MN3011, but also do some cool things not possible with silicon BBDs (like move the tap positions by CV, change delay times without changing sampling frequency, etc). A single module would be limited to 4 analog outputs, but they could be (digitally) chained to make longer delays and more taps. 2 modules could make the 6 taps of an MN3011 (or 8 taps if you want to go crazy).

Interested? I’ll post some more details and thoughts. Would love to get some feedback and if things look good we will ask for beta testers and give you some test modules to experiment with.

 

[bean]

Yeah, I'd be down for this.

 

[Gordo]

Ditto.

 

[JTEX]

Do you have a target power consumption? (thinking of battery use). How about the size/form factor?

 

[Robert]

Make the module and we'll come up with some projects for it.

 

[bean]

I mean, I was closing in on my final analog delay projects and you drop this bomb. Kuddos, but also, goddangit.

 

[Passinwind]

Interested? I’ll post some more details and thoughts. Would love to get some feedback and if things look good we will ask for beta testers and give you some test modules to experiment with.

Very interested, and ready and able to whip up some bespoke PCBs that I would happily share with the class if that might be helpful.

 

[Cybercow]

Yep. Interested as well.

 

[analogdevice]

While you're at it, can you cook up a SAD1024 drop in?

 

[passtheducky]

It’s a cool idea. I think it would be useful in a lot of designs. I would want to be sure that modulation was as smooth as possible to make it widely applicable to chorus and vibrato style applications.

 

[Cabintech]

Do you have a target power consumption? (thinking of battery use). How about the size/form factor?

Power is a bit of a guess right now, still deciding on codecs which are a big part of it. My guess is 300-400mA. @ 3.3V.

One design alternative requires 5V and 3.3V supply... is that OK or too hard to accommodate in a pedal design? Should the module take 9V and make its own 5V/3.3V supply (adds cost and size). A pure 3.3V design adds several $ to the cost. Its always a game of tradeoffs.

No good feel for the size yet. Pedal design space can be tight, so as small as possible. The module will be a small PCB with (I suppose) through hole pins. Is there a way to direct connect PCBs that is more space efficient than DIL-spaced through-hole pins?

 

[JTEX]

Power is a bit of a guess right now, still deciding on codecs which are a big part of it. My guess is 300-400mA. @ 3.3V.

Oy... 300mA pretty much rules out battery power for me. Personally, if I had to think about designing a BBD replacement (and had the skills to do it...), I'd try to match (or improve) the original's power envelope, footprint, cost and ease of use, while greatly improving the signal/noise and sound quality. A VERY tall order, I know... Which is why I won't think about it

Is there a way to direct connect PCBs that is more space efficient than DIL-spaced through-hole pins?

Yes, SIL (vertical) might work better for a small board

 

[Cabintech]

Oy... 300mA pretty much rules out battery power for me. Personally, if I had to think about designing a BBD replacement (and had the skills to do it...), I'd try to match (or improve) the original's power envelope, footprint, cost and ease of use, while greatly improving the signal/noise and sound quality. A VERY tall order, I know... Which is why I won't think about it

Haha, if we were designing a 100% custom chip we might hit a few of those, even then only a few

Yes, SIL (vertical) might work better for a small board

Good idea. Also occurred to me we could use castellated via on the edges like a lot of small radio and similar modules have.

 

[Cabintech]

So one of our design ideas is to make the module configurable, probably with jumpers. The base components have these resources to work with

• 660ms of delay memory
• 4 analog in
• 4 analog out
• 6 CV in

Here are some configuration ideas

1. One delay line, up to 660ms, with 4 independently controllable tap points.

2. Instead of 1 delay line with 4 taps, split the device into 2 independent delay lines, each with 2 taps, each with their own delay CV control. Each line could be up to 330ms. (Equ to a pair of MN3005 with 1/3 more delay range)

3. Same as #2 but the 2 delay lines are linked and controlled from a single set of CV inputs (e.g. they will have identical delays and taps).

5. Make a stereo output pair (one less tap point) by adding a fixed delay (~15ms) on one side of the pair (Hass effect). You could do the same thing with careful setting of CVs, but this would be a fixed delay offset unaffected by changes in overall delay time.

6. 4 independent delay lines, 1 tap each, up to 165ms delay on each line (Equ to four MN3008 with 1/3 more delay range)

Other ideas?

 

[Cabintech]

Other possible useful features?

1. Is there any need/usefulness to a bypass (0 delay no matter the CV inputs). Technically there is still some small delay from A/D and D/A conversion, about 0.02ms.

2. Is the chaining concept useful, e.g. is there a need for > 660ms of delay or more than 4 tap points? (The MN3011 has 6 tap points, so there is that, although the overall delay is less than 300ms).

3. Provide a global scaler CV input. Right now, the delay CV input is 0 (no delay) to 3.3V (660ms delay). Is that too narrow of a CV range for such a broad range of delay times? A change of 8mV in the CV results in about 1.5ms of delay time change. A scaler input could be set at the half way point (1.65V), then the full scale delay time range would be 0-330ms. That limits total delay time but maybe easier to generate a smooth CV modulation?

Those are just some ideas, always interested in what people think might be useful...

 

[jubal81]

Power is a bit of a guess right now, still deciding on codecs which are a big part of it. My guess is 300-400mA. @ 3.3V.

One design alternative requires 5V and 3.3V supply... is that OK or too hard to accommodate in a pedal design? Should the module take 9V and make its own 5V/3.3V supply (adds cost and size). A pure 3.3V design adds several $ to the cost. Its always a game of tradeoffs.

No good feel for the size yet. Pedal design space can be tight, so as small as possible. The module will be a small PCB with (I suppose) through hole pins. Is there a way to direct connect PCBs that is more space efficient than DIL-spaced through-hole pins?

My input would be to include the voltage regulators on the device, even if it ups the cost. It's a good tradeoff for convenience.

I also like the idea of jumpers or some other setting to quickly configure the module for different applications (parallel, series, stereo, etc.).

 

[phi1]

I’m curious, is there a general consensus on whether the BBD chips themselves impart any tone characteristics, or if the tone of analog delays (etc) is entirely derived from the filtering in the rest of the circuit?

I like the idea of having more options for time based effects, and I think the drop in chip that still relies on analog circuitry for modulation, feeadback, etc is cool.

 

[Cabintech]

I’m curious, is there a general consensus on whether the BBD chips themselves impart any tone characteristics, or if the tone of analog delays (etc) is entirely derived from the filtering in the rest of the circuit?

I like the idea of having more options for time based effects, and I think the drop in chip that still relies on analog circuitry for modulation, feeadback, etc is cool.

There are some audio effects imparted by the BBD. Some would call them defects, other call them "classic BBD sounds". Depends on what you are trying to achieve. Three significant effects are pitch shifting, inaccuracies caused by low frequency sampling, and noise.

Pitch shifting happens when the clock rate is changed (modulated). While the clock is changing there is a shift in pitch of the sounds already in the pipeline. When the clock is stable there is no pitch shift. A subtle pitch shift can be desirable in some effects.

When delay is increased (by decreasing clock frequency) the fidelity of the reproduced signal suffers and it becomes unable to reproduce higher audio frequencies. Heavy filtering is often used to remove the HF noise, but that results in a muffled sound, so there is some trade offs there.

BBDs are also inherently noisy, S/N ratios of 70dB are common. This leads to many designs using companders (compression before, expansion after) to improve the S/N, but those devices impart their own inaccuracies as well.

The digital BBD device also has the pitch shifting, but does not suffer fidelity or HF loss at longer delay times and the S/N ratio is typically >90dB. You could still filter the output to get the muffled sound if that is what you want, but it becomes a design choice instead of an inherent requirement.

Bottom line, does it make a better effect? That remains to be seen (and to define what "better" might mean).

 

[jwin615]

I think on the onboard regulator vs off board, you have to figure where this will be used.
Will it be in an otherwise analog device or in a shared digital device?
If you're using an Arduino, attiny etc to generate CV, then you likely already have 5v/3.3v rails.
The other tradeoff is pin count. With onboard, does it allow a smaller footprint with less pins/pads? Or does it necessitate more real estate regardless?

Edit add
If you do onboard, there needs to be a 3.3 output to loopback to CV.

 

[Cabintech]

I think on the onboard regulator vs off board, you have to figure where this will be used.
Will it be in an otherwise analog device or in a shared digital device?
If you're using an Arduino, attiny etc to generate CV, then you likely already have 5v/3.3v rails.
The other tradeoff is pin count. With onboard, does it allow a smaller footprint with less pins/pads? Or does it necessitate more real estate regardless?

Edit add
If you do onboard, there needs to be a 3.3 output to loopback to CV.

Yes it depends a lot on what kind of system it is going into. A basic analog pedal is not likely to have 5 or 3.3 supplies whereas a synth system or hybrid analog/digital is quite likely to have at least one if not both. Eurorack modules have +-12V and 5V defined power rails, would still need to derive the 3.3V for that.

So the answer is "all of the above" . I am thinking to either (1) have a small optional plugin modules that goes on the DBBD module. If not present then the system supplies +5 and +3.3. If it is present it takes in +9 to +18V and derives the needed +5 and +3.3 (and as you pointed out, makes the 3.3V available externally). Or (2) make it a separate module and save space on the DBBD, at the expense of a little more system design. I don't want to have multiple flavors of the DBBD module to support different power scenarios.

Seems like someone must already make a 9-18V to +5,+3.3 converter that is reasonably quiet and filtered (and not a linear regulator that would itself consume a lot of power).