design shop

designing and fabricating front panels for pcbs (using kicad and lightburn)

i’ve spend the last two weeks house-sitting at my parents place while they are on holiday. this has given me time to deep dive into some larger topics i have been meaning to get around to.

one of these topics was the design and fabrication of panels for underscores circuits (having unlimited access to a laser cutter over this time helped!). i made a half vlog / half tutorial style video to document this exploration – you can see on videos.scanlines or on youtube:

types of panels

sometimes i get asked if there are enclosures/front-panels available for the pcbs i sell (there actually have been a few community contributed designs for the recurboy and i_n_c_u_r circuits) but so far nothing from me. this will change soon with the addition of two enclosure options – stand-alone sandwich and eurorack front-panel for the following circuits:

  • sync_ope
  • recurboy
  • _rupture_
  • two_comparator_effect
  • i_n_c_u_r

originally i planned to prototype with laser-cut MDF (and share these files so others can laser-cut also) but only stock the FR-4 versions in the shop (since these would be cheaper and tend to look quite professional) – however after seeing the MDF i think i prefer them to FR-4.. i wonder if there would be any interest in stocking MDF (or even bamboo) lasercut panels too ?

euro-friendly designs

although definitely not eurorack modules themselves a number of these underscores circuits were designed to be euro-friendly, in the sense that you could mount them in eurorack case with the appropriate front-panel if you wanted to.

all the interfacing parts of these circuits are vertically mounted and the pcbs are at most 100mm tall. all circuits run on single-rail power (mostly 5v) but some have footprints for euro-power headers anyway and the others you could mount a vertical barrel-plug on the bottom of the pcb allowing a small eurorack-to-barrel-jack converter to sit inside your case and power from beneath:

as i mentioned in the video there are a few other things to consider when adapting to eurorack – the small slide switches i usually use sit a bit too low for comfortable switching (i would love to try 3d-print some switch-caps for these when i get a printer!) and i would recommend choosing longer shafts on the pots than the ones usually supplied with my kits…

its fun to see them mounted together in a rack like this however this conversion is not the same as an actual eurorack module version of these circuits – which i do plan to design one day (closer to that day now that i finally have a euro-case built) the true eurorack versions will:

  • use smd components and be designed to conserve hp
  • have interface parts chosen to fit panels better rather than prioritizing what is cheapest
  • layout of interface optimized for rack use
  • have proper eurorack power headers
  • hopefully expose some more of the controls with CV

my workflow for creating panels

this is shown with more detail in the video, but here is a written overview:

copy the pcb footprint into a new project and delete everthing except for the edgecuts, silkscreen and any footprints of interface parts

then replace each interface footprint with a edgecut hole to access it from the panel (based on a table of known hole sizes)

arrange the silkscreen however you want the panel to look – 3d viewer in kicad is useful!

when you are happy with how the panel looks you can export it – use the usual GERBER export if you want to get it fab’d in FR-4. my method for getting designs into lightburn was to first export from kicad as SVG and then open these SVGs with inkscape and save a copy as DXF (it seems like lightburn didnt like some of the SVG format that kicad creates)

once in lightburn put the edgecut and silkscreen imports on different layers – you can add thickness to the silkscreen using the offset tool

then set your laser speeds, check the material is lined up with the cutting area and make the cut!

design shop

introducing the two_point_one_colorizer

we have a number of new projects in the works – im hoping to release three more circuits before the end of this year – heres a preview and small overview of one of them!

underscores presents an adaption of Rob Schafer’s two_point_one_colorizer:

history of the project

the original circuit was designed by Rob Schafer on breadboard and shared on the facebook group Video Circuits :

since then Rob has moved on from working with composite video – (check out his youtube channel to follow his exploration of HD analog video) – but my interest in this circuit remains!

how it works

the way this circuit creates colour is both genius and extremely hacky!

the first stage is very similar to another underscores circuit adapted from Rob Schafer – the two_comparator_effect :

the signal passes through two comparator stages – reducing the signal down to this posterized effect with only white, grey and black visible – two_point_one_colorizer is similar but instead creates two colours plus black (hence the two point one)

colour on composite video is created with a sub-carrier signal – an extremely fast sine wave that runs over-top of the luminance signal:

the hue is selected by the phase difference between this sub-carrier signal and the colour burst reference.

on the next stage of the two_point_one_colorizer a sub-carrier signal is approximated using high-speed monostables and overlaid over these ‘white’ and ‘grey’ portions of the comparator outputs:

monostables generate square waves with variable pulse widths -which at this speed can approximate the sub-carrier signal with variable offsets (hue) – inverters are also used to shift the offset by 180 degrees – since this approximation is not perfect some colours are easier to generate than others – and some displays are more or less picky than others.

what i love about this approach is that it is less precise and predictable than (for example) generating colours with an rgb encoder – you end up getting some really unexpected results – and also some great colour glitches as the tv decoders struggle to make sense of the out-of-spec signal:

the adaption

back in 2020 i made a start trying to adapt this design to pcb with help from Rob and others from the scanlines community – a few of the mods made to the original design include:

  • simplifying the sync and burst pulse portions by using lm1881 instead of doing this more ‘manually’
  • swapping out the monostables for another ic – similar in spec but around 1/10th of the price (can still swap out for robs original ic if you would like!)
  • adding video by-pass toggle – making the circuit more usable in larger signal chains
  • adding more control over colour selection – a master colour offset that changes both channels together and sub-carrier amplitude that changes colour saturation
  • (attempting to) adapt the circuit to also work on pal cvbs – adding a switch to select sub-carrier frequency and also logic to implement Phase-Alternating-Lines

PAL troubles

i got a bit stuck on the PAL compatible part of the mod and ended up shelving this project for a bit while i focused on working towards the shop launch and polishing other projects.. still not sure that the PAL version is perfect – the Phase-Alternating-Lines mod never quite worked how i expected and the range of colours seems a bit less (could be just due to PALs faster sub-carrier though since the monostables are already being pushed to limits with NTSC). im sure there will be some displays that wont show any colours for PAL but to my surprise the three monitors i tested it on in my studio all worked with NTSC and PAL (a crt tv, a small car-reversing lcd and a easycap capture card)

the nature of this project is hacky and experimental so it is already wishful thinking to have it running consistently on all displays – I think the current version would be a useful tool in the studio of many composite video artists and im excited to see it out in the world!


sync_ope psu revised

tl;dr – i updated the sync_ope psu design to only require single supply 12v dc to run – this version ( v1_0_0 ) is available in store now

as discussed in this earlier post i decided it would be best to upgrade the sync_ope circuit to not require a difficult to source AC-AC adapter.

the schematic for new power stage

the new design uses two LM2575-5.0T step-down regulators, with one in inverting buck-boost configuration to generate the negative rail.

Part sourcing and testing

the rest of the newly required parts are easily and cheaply sourced from tayda – only the regulators themselves proved a bit more difficult:

I first tested the circuit using these Microchip LM2575-5.0WT s from mouser, which performed well while adding ~7euros to the BOM

next I explored a budget alternative – these (probably fake) National Semiconductors LM2575T-5.0 from aliexpress for about 1/10th of the price.

these two are not the same…

when powering the circuit from my 12v_1.5A supply both versions successfully generated +/-5v rails, however when i tried a cheaper 12v_0.5A supply only the more expensive regulators succeeded.

+/-5v rails as expected

the cheaper ones sat around 1v on one rail, and the other jumped to ~2v before falling back down to zero – while also quickly overheating enough to burn my (foolishly unsuspecting) fingers

not at all as expected (also HOT!)

this behavior is addressed in the TI_datasheet:

the start-up input current of the buck-boost converter is higher than
the standard buck-mode regulator, and this may overload an input power source with a current limit less than
1.5A. Using a delayed turn-on or an undervoltage lockout circuit (described in the NEGATIVE BOOST
REGULATOR section) would allow the input voltage to rise to a high enough level before the switcher would be
allowed to turn on.

however my various attempts to use delayed turn-ons / larger input capacitors failed to correctly bootstrap the circuit with these regulators.

i found online a description of a problematic similar sounding method to generate +/-12v rails for eurorack that seems to use fake buck-boost regulators. Im not sure if this is in any way related, but still.

a nozoid eurorack supply using aliexpress regulators in inverting buck-boost configuration

im always game to try out cheaper parts from questionable sources – in many cases they can be just as good for a fraction of the price. but in this particular case, given:

  • that in general you shouldnt cut corners on power-supplies
  • it failed for me in a way that could hurt people
  • there is reports of related issues using similar parts online

Im happy to stick with the more expensive mouser sourced regulators !!

plus now i can run my sync_ope from a single 5v_usb with this cheap boost supply (i know – iv learnt nothing !)

version control

this is the first major revision I have done to an underscores circuit that is in production. it is a good opportunity to think about the process as I want all these projects to be living / always improving.

I created a branch in git for this release and committed all the changes over the last few months to this feature branch – this ensures the changes and revisions are available yet also preventing people downloading WIP gerbers from the main page.

when everything is tested and ready to go i can then merge this feature branch into main.

on release i also need to update the project in a few other places:

  • the shop description
  • the circuit booklet
  • the circuit images (in this case decided not to bother with these)
  • the video walkthroughs – videos.scanlines is fine but youtube will not allow for editing existing videos so i either need to re-upload a new one or find a way to amend the existing one


sync_ope psu reverie

tl;dr – im thinking of making a revision of sync_ope to replace the ac-ac psu input with a standard switching ac-dc one

UPDATE: this has been implemented in v1_0_0 – read more about it here

sync_ope is a project iv been collaborating with Gael Jaton on for a few years. initially a concept dreamed up and tested on bread-board, iv worked on a fair few design iterations based on feedback and support from the wider community – we presented a prototype of the concept at FUBAR_2020, and continued improving it via discussions on the scanlines forum.

a over-simplification of her beauty

its been quite slow progress – and like many diy / community driven initiatives (who get passed around and only prioritized in short bursts & on productive weekends) has some built-in identity insecurities:

  • sync_ope is designed to be diy & beginner friendly – spaced out across all of a 100x100mm pcb- with footprints for even the specific video-ic’s in DIP packaging
  • the use case however requires already owning analog-glitch devices, and having pre-established capture flows where sync dropouts are a problem
  • the circuit is strictly analog, and more complicated/advance than many of the other ‘entry level’ underscores projects iv been prioritizing – one LT1251 ic can cost more than a single-board-computer!

Many similar projects in this realm would have fully committed to serving (only) the eurorack video synthesis community – where there’s an active base of designers and users creating complex functions with analog circuitry:

the advantage of this is you don’t need to (re)design common function blocks like power supplies and enclosures – there is even many design best-practices to follow to ensure compatibility

an (ac-ac) eurorack power-supply and case

the problem i see with this approach is that it can be off-putting for some people who havnt fully invested in the modular setup – where certain design preferences and restrains do not apply to them. by only serving the established (and well-enough off) we risk alienating those on the fringe.

the beauty of the sync_ope circuit is it can be just as interesting and useful in a mess-of-circuit-bent-mixers&diy-guitar-pedals as in a large modular synth system

the muddle

technically, the circuit requires a dual power supply to operate (unlike some of my other projects which can be run from a single usb-supply)

this is common and expected in euro-rack-land where all systems carry +/-12v rails. however it is less common for the average ‘non-modular’ hardware setup…

sync_ope’s power supply schematic

the current design has both a eurorack header – that takes +-12v straight from the rails – and a 12-15 AC barrel-jack input – that uses a Rectifier Diode Bridge to convert alternating current to dual dc.

i thought that this implementation struck a balance between serving those who may want to rack-mount it (atleast, all the interface controls are front facing!) and those who will just wanna plug it into a wall.

however as i prepare to distribute a small number of these circuits internationally im discovering that AC-AC wall-warts can be quite tricky to come by.

a search for ac-ac adapter returning a page of AMAZONS-CHOICE for dc ones doh

a different approach that i have been seeing recently is to start with a single dc supply and use a step-down regulator in inverting buck-boost configuration to generate the negative rail.

since composite video sits around the 1-2v range +-5v is plenty of headroom for this circuit to operate. we could start from any single dc 12v wallwart (or the positive side of a eurorack-powerrail) and create the dual 5v’s we need onboard.

lzx are powering their new 3rd gen modules with 12v single supplies and syntonie has optional dc_psu_boards for their cbv* line

syntonie’s stand-alone to euro-module adapter board

although the (ac-ac) current version of sync_ope is available in store now im already thinking about making another revision to the board to use single dc supply instead.

it could follow a similar configuration to Bastiens psu schematic below – instead using LM2575T-5.0 to create the +-5v rails we want. this change may increase the total BOM cost slightly – but since its already quite an expensive circuit i also think its worth ‘doing it right’

syntonie’s CBV002_PSU_Schematic.png

i can already imagine another future revision with smd parts and a more compact ‘rack first’ interface design that would better suit the modular crowd. but for now i will continue to try bridge the gap between this and a more hacky diy approach.

if you understand the ac-ac psu problem im describing and want a sync_ope now anyway then this version is for sale right now in the shop! but if you think you would rather wait some more months and hold out for an ac-dc supplied version then this is also an option for you too !

the beauty of open source is multiple forks of a project can co-exist. time will tell whether this revision will be a direct successor of what we have now or an alternative version