Some questions

[AeonKen]

Hello,
 First of all, incredible project! I started drooling when I saw all the possibilities flash before my eyes.

I have a few questions as I want to heavily modify my build:

1. Is it possible to buy the PCB only? If not, Could I have the Schematics or the pinout from the raspberry (as in what goes to what pin for the code to work) for personal use with no copyright granted? I already own most of the components and have no need for more.
2. Can other SPI LCDs be used? What needs to be considered for easy compatibility (controler,resolution, etc.)? I want to use a bigger display and give it more of a Cyberdeck feel, to make it much easier to read durring gigs. And what files would need to be modified to accomodate that?
3. No question is too stupid: I'm not familiar with the output of the audio boards. Can other pedals be connected after the pistomp, or is it needed to be last on the signal chain before the amp?
4. How could one add a DI output and balanced outputs for PA systems? Can a standard switched circuit be placed between the output of the audio board and the jacks?
5. What about headphone outputs for monitoring/readjusting on the fly by ear? Is it enough to have a switched jack to connect/disconnect the output signal to an amplification circuit?
6.  Did I understand this correctly: There is no output buffer.   Most pedals I know and have build have an output buffer. Why was it deemed unnecessary in this project? Built in on the DAC audio circuit?

EDIT: While on the topic Cyberdeck, is there any way to use the SPI display data to run into or be converted to HDMI? Maybe a smaller SPI display? I'm thinking a head mounted display

Thanks a lot for all the great work!

[Randall (Admin)]

Thanks for your interest.  My whole reason for creating pi-Stomp was to empower others to create something cool.  I highly recommend you read the customization guide:

https://www.treefallsound.com/wiki/doku.php?id=customization_guide


Here are some answers...

1. Is it possible to buy the PCB only?  No.  You cannot use any just any jack, pot, encoder, op-amp etc. and expect it to work as if the proper parts were used.  I offer full support on everything I create/sell, but will not waste that time on someone who didn’t use the correct parts and do not want to encourage a useless piece of e-waste to be created.

I've found that most people who ask for a board think that I’m charging too much for kits because they haven’t done the math to calculate all the shipping and tax required to obtain the parts.  Basically, the margin I keep is simply the discount I’m able to obtain by buying parts in lots of 100 or more and combining shipping, plus about $5/hr for my time maintaining stock and putting the kits together.

That said, if someone can tell me about their project and convince me they can build something that'll work without certain parts, I'll consider putting together a custom/prorated kit.

If you wanna try to create your own, that's cool, just know that I spent months and at least 4 board iterations before I got noise issues under control.  The RPi power rail is noisy AF.

See the bottom of the customization guide for RPi pinout

2. Can other SPI LCDs be used? Yes.  There is pi-Stomp firmware supporting various display drivers and aspect ratios.  Most of the displays thus far are smaller so don't support all the content of the recommended display (240x320 ili9341).  The one I've designed the enclosure around is a 2.2", but a 2.8" works great too with no hardware or software mods required.  2.8" is pretty easy to read from standing height:
https://www.amazon.com/gp/product/B073R7BH1B/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

3. Can other pedals be connected after the pistomp?  Sure, it has adjustable line level outputs just like most effects.  I wouldn't put a Fuzz Face after it, but...

4. How could one add a DI output and balanced outputs for PA systems? See the customization guide

5. What about headphone outputs for monitoring/readjusting on the fly by ear? Is it enough to have a switched jack to connect/disconnect the output signal to an amplification circuit?  No, line level will not drive most headphones.  But this little guy will: https://www.ebay.com/itm/184214756448   Or if you use the AudioInjector Zero card, it has a 1/8" headphone output jack.  In general though, I much prefer the IQAudio Codec Zero card and have standardized on it for future development.

6.  Did I understand this correctly: There is no output buffer   Most pedals i Know and have build have an output buffer. Why was it deemed unnecessary in this project? Built in on the DAC audio circuit?  Why would line level output need buffering?  It's already low impedance and higher output than most effects.  I suppose some pedals might have a high-ish impedance output to emulate a guitar pickup, but I doubt many do that.

[AeonKen]

Thanks for the quick reply!
I think I can work with the RPI pinut. Thanks for adding that!

What about smaller SPI Displays or SPI to HDMI?

TL;DR: Your prices are reasonable. I Want to save waste and heavily modify.

I don't know about the supply and cost you take, and honestly, its not a bad thing to make a profit. When compared to "competitors", you offer reasonable price and leave the joy of building and customizing to the user. I haven't done any math on how much it would cost me to get the components, as I already have most them in my bins... With the exception of a 3V relay, an audio interface board and the specific debounce chip. Luckily I can get all at a single store, and pick it up within my city (saving on shipping alltogether), as their warehouse (and former walk-in shop) is located here. But this is of course an exception. And even comparing a few of those components with the price listed on your BOM, its more expensive. I can confirm that.

Keep being Awesome!

[Randall (Admin)]

Thanks.  Good luck on your project.

For pi-Stomp, the LCD, audiocard, encoder, MCP3008, debounce chip, etc. are simply connected as that pinout shows.  Those who understand how to create a simple buffer and supply regulator, could probably deduce the rest just from seeing the bill of materials.  It's all pretty simple and textbook.   If you want pieces of schematic,  I'm fine sharing that with ya.  I'm just not gonna publish the whole thing cuz that encourages those who don't know enough but think they do to create something that might likely suck.

The one bit of magic to deal with supply noise, is to have a separate ground for all of your audio circuitry.   Even the regulator that feeds it, gets its ground reference Only from the ground of the audiocard.

Another tip if you're rolling your own.  Consider if you really need true bypass.  I sent a survey to all pi-Stomp builders asking for improvements, features they like, features they don't or don't use, etc.  A very high percentage never bypass their pi-Stomp.  I designed in a relay because I wanted that and the original audio card I used (AudioInjector) didn't have internal bypass.  The IQAudio Codec Zero (which I highly recommend) does though.  It has a digitally controlled analog bypass path which is probably good enough for all but true true-bypass purists.

What about smaller SPI Displays or SPI to HDMI?
Sure, you could probably use most any SPI display.  There is firmware for 5 different LCD's already in the code, see the files that begin with "lcd" here:
https://github.com/TreeFallSound/pi-stomp/tree/master/pistomp

Different pixel counts and aspect ratios mean different content though, so there is less info displayed on smaller displays.

One could use HDMI, but that was not my vision for pi-Stomp.  Having a general purpose display with window manager just doesn't make sense on a small display, IMO.  Every pixel is precious, so display what's important, no more.  Maybe if you had a 7" screen, then yeah, display the whole MOD-UI in a browser.  There is at least one or two pi-Stomp variant builders who have done something like that.

[AeonKen]

Thanks!

I think its better to have some form of bypass and not use it than not having it and needing it. Ill looked into the internal bypass on the Codec Zero... well, the DS7271 Audio Codec used in it. It should be enough to satisfy most, even some purists. Although it does pass through an amp I assume to be a form of buffer? I find the latching relay a good idea, especially with the transistor drivers you used. That not only takes off the strain from the digital pins, but may also reduce some noise. In my experience digital pin outs have noticable noise for audio circuits, and the coils in the relay may induce some on the circuit. ADCs dont care about that though, as long as you use seperate grounds.

Always a good idea to seperate the power input ground from the audio circutry, as most power supplies (epecially those designed for digital devices) are noisy as hell. Just compare with an el cheapo USB audio interface, which uses the USB (digital) ground.

Im currently going back and forward between the Codec Zero and the HiFiAudio DAC+ ADC. The hifi audio has a better signal to noise ratio (90/100 vs 110/112 -ADC/DAC respectively). But it is more expensive, and has no LINE amp included. On the other hand, the codec zero has a lower max sample rate at 96 Hz vs 192 Hz, doesnt have a GPIO pass through for stacking, and only has the right channel on one of its outputs. Couldnt find any data on weather the DAC+ ADC has an internal bypass tho.

I dont want to use the HDMI for displaying the MOD-gui. That would be a bit... unnecessary? A tablet or a phone can do that just fine. But i was thinking of maybe implementing a wearable display (kinda like google glass or EDITH) which displays whats on the LCD. Thats why I was asking about SPI to HDMI, since almost all available units use HDMI. A smaller display with a prism may also work tho...

[Randall (Admin)]

You've got some cool ideas and it sounds like you know your stuff, so I'm sure you'll create something awesome.

Both the Berry and IQ have mic inputs, but the Berry is stereo and seems to be better implemented, IMO.  The stackable GPIO on the Berry is pretty cool.

The Alsa controls available for the HiFiBerry do not include a bypass feature like the IQAudio.  The IQAudio actually has tons of features available via Alsa.  Routing is super flexible, there's a global eq, Automatic Level Control, Noise gate and Programmable gain amps for just about everything.  It's impressive.  Soon, once we revamp the pi-Stomp LCD UI, we'll be exposing many of those IQAudio alsa controls.  So if you want to leverage the audiocard features that we'll be adding in the near future, I'd recommend the IQ.   

Regarding 96kHz vs 192kHz, I would wonder if you'd really use 192.  It'll be a CPU hog.  For reference, I can run a CM4 pi with HiFiBerry at 96kHz/128 Frames (2.7ms latency) just fine on most of my 10 to 12 effect pedal boards.  CPU will hover from the high 30%'s to mid 40%'s.  If I switch to 192kHz/128 frames, I can only run about 5 effects without overruns.  If your running on a pi3 your CPU will have even less headroom.  Do you really need less than 2.7ms latency?  Personally, I run at 48kHz/128 frames (5.4ms latency).  I can just barely feel the latency then so it's a happy compromise to not have my CPU taxed.

[AeonKen]

Thats some great info! So I assume the IQ has a DSP built in? Thats quite cool. Yeah, Im looking forward to the updates. IQ it is!

[Randall (Admin)]

Yeah, does.  You've probably seen these docs, but in case you haven't:

https://cdn.sparkfun.com/assets/e/6/5/a/9/20201209_IQaudIO_v32.pdf

https://www.renesas.com/us/en/document/dst/da7212-datasheet

[AeonKen]

I wasn't really able to find that user guide, because I was looking for a datasheet. Thanks for sharing!