Posted on April 11th, 2014 No comments
I get asked quite often about how I approach video bending projects, so here are my thoughts. Because I am trained as an electronic engineer my approach is theoretically and technically driven as opposed to the experimental approaches that other people may use. I find this works particularly well with video bending because you need to retain the structure of the video signal if it is going to display properly.
When starting out on a project the first thing I do is to find all the documentation I can about the unit. If I can obtain schematics or service manuals then these can be a great help. For an interesting unit I am happy to buy service manuals if they are not available for free because it saves so much time and hassle. So far it has always been a good investment. If I do get hold of good documentation then examining the circuit will often give me ideas about how things might be modified. Basically I am looking for places where key signals, like the separate RGB colour levels, can be intercepted and modified. There is more discussion on that below.
If I don’t have a good manual then I will look at the circuit on the board and try and reverse engineer the key parts. This normally starts with identifying the main chips on the board and through the datasheets and knowledge of the normal operation working out how the signal flows through the circuit. Once you get an idea of the signal flow then you can start to focus in on areas that seem to offer the possibility of modification.
Finding bending opportunities is partly science, partly art and partly gut instinct. You want to find a signal or processing function in the circuit that is amenable to modification. A composite video signal consists of the video information (luminance and colour) combined with synchronization information (horizontal and vertical). In most cases you don’t want to over-distort the synchronization information because this will prevent the signal displaying. Finding opportunities to just modify the video information is important.
For the video signals then a lot of units will separate video and sync information internally. If you can pull out pure luminance, hue or RGB signals from the circuit then you can make these available to process and distort through other circuits without damaging the synchronization. Just additively mixing in audio or other video signals on top of a video signal can be interesting. Some, cheaper, circuits don’t separate the video and sync information and it can be hard to bend the signals for these.
Many video mixers also contain various gate signals that control how different parts of the same picture are processed in different ways. These are used to implement things like wipes and colour fills. Pulling out these gate signals or being able to inject new gate signals can create fantastic effects. Try xoring two gates from different mixers together and then injecting the result back in to the original circuit.
As well as going after the signals you can go after the processing functions. A simple trick is to modify the circuit to remove the limits on how strongly a processing function affects a signal. For example if you have a circuit that controls the colour saturation then it might be possible to boost the gain of this beyond what is intended and create super-saturated and unstable colours in the output.
Another thing I like to do is to see if it is possible to replace manual controls with control voltages. In this way you can sweep control values rapidly, even within one frame of a video, under the control of an external circuit. A lot of effects just come from feeding an audio signal in to a control voltage that varies some aspect of a video signal.
Intellectually I find it more interesting and more satisfying to work with primarily analogue video equipment, but I guess I should add a word on digital equipment. In circuit bending on digital circuits a common technique is just to ground certain address or data signals so that the circuit starts to misbehave. The results are unpredictable, but fun. This approach carries quite a high risk that the outputs of the digital circuits will be damaged as they try and drive a grounded signal “high”. I suspect this is the cause of a lot of equipment damage caused by circuit bending attempts. One reason you can get away with it on some older equipment is that they used NMOS logic. A feature of NMOS is that it has no active pull-up device (just a resistor) so grounding the output is acceptable.
People often ask me if I have ever blown anything up while circuit bending. So far the answer is “no”. I think this is largely because I am normally working with a reasonably good idea of how the circuit is meant to operate and I understand what common electronic circuits will and won’t tolerate being done to them. However, circuit bending is a full contact sport. If you can’t tolerate the risk of destroying what you are trying to bend then you shouldn’t have opened the case.
Personal safety is your responsibility. My very strong preference is to only work on equipment that is powered from a low voltage source or, if it has an internal power supply, where the high voltage elements are fully protected against accidental contact. I recommend you don’t work on designs where high voltage components might be touched.
Older equipment will contain lead, and possibly other unpleasant chemicals. Always take appropriate precautions including through hand-washing before handling food.
I don’t know how much it is possible to teach circuit bending. Particularly with video it is a black-art and relies heavily on experience and luck. Hopefully these hints will help those that want to give it a go though. With a lot of old analogue equipment being sold cheap just get something and start exploring.
Posted on June 2nd, 2013 No comments
Warning: this post contains maths
I can never find on the web or in my text books the general equations for op-amps used as combined multi-input summing and difference amplifiers (ie they have several positive and negative inputs). It makes designing mixers for synthesizers annoyingly awkward as I have to rederive the equations each time. So, to save myself having to work everything out from scratch again, here are my derivations and notes on multi-input Op-Amp circuits. I will also take the opportunity to point out some interesting parts of the results.
So, here’s the setup:
We have an op-amp circuit with “N” negative inputs and “M” positive inputs as shown above. All the positive and negative inputs are identical.
For an ideal op-amp the output is:
Or, in other words the negative gain is:
The positive gain is:
Positive and negative gain
The negative gain is nice and easy and only depends on the input and feedback resistors and not on any other variables, like the number of inputs. Why is this? Well the inverting input of the op-amp is a virtual ground and the voltage isn’t changed by the negative inputs. Therefore the current through each negative input only depends on its input voltage. You can have as many or as few negative inputs as you like and it works the same.
The positive inputs are not in this lucky position! Voltages at the positive inputs change the voltage at both the inverting and non-inverting inputs of the op-amp. The non-inverting input voltage changes because of the voltage drop over Rg. The inverting input voltage changes due to the feedback action of the op-amp keeping the input voltages ideally identical. This means that currents flowing through all the input branches depend on the positive input voltages and hence the complicated positive gain equation.
Limits on positive gain values
Once the negative gain is set, this configuration limits the range of values of the positive gain depending on the number of positive and negative inputs. One particular example:
If the negative gain G- > 1 and number of negative inputs N < M, the number of positive inputs then G+ < G-.
To derive this then consider that the maximum positive gain is when the input resistors R+ = 0 (obvious from the circuit and also by inspection of the equation).
Special cases and derivation
There are several interesting special cases from these equations (including the basic op-amp single input amplifiers) and the derivation is worth reading. So I don’t fill the blog with equations you can read it all in this pdf file.
Posted on August 22nd, 2011 No comments
Dorklake11 was a nice event. Great job by Alex, Greenman and co. to create organization seemingly out of nothing. We had 12 Vibrati Punk Consoles built in two workshops
Mike Challis did a very nice build of the Vibrati Punk Console as a Coffee Can Synth.
Sam Freeman made some great phat sounds by looping the Vibrati Punk Console through a Korg Monotron.
Thanks to all the builders and all the campers that put-up with the massed bleeping!
Posted on June 2nd, 2011 No comments
Retro Electronic Sound
Evening Build Workshop
Tue June 14th, 6:30pm, Nottingham Hackspace
The first batch of kits for the Vibrati Punk Console (aka “Atari Punk Console 2.0″) are in. We are going to have a building session at Nottinghack on June 14th. So if you like electronic music or want to try your hand at soldering then come along.
The Vibrati Punk Console is a beginner-friendly electronic music project that can be finished in one evening. The simple circuit generates an amazing range of retro warbles, screams, glitches, fuzzes and tone ramps.
Workshop Cost: £20 includes teaching and all build materials.
For those that can’t make it to Nottingham the kit should be available to buy on the web soon.
Posted on April 22nd, 2011 No comments
For most of my projects I either need quite dense PCBs that I get professionally made or I use point to point wiring techniques. At the moment though I am working on a new electronics kit under the working title of “Vibrati Punk Console” which is kind-of an “Atari Punk Console 2.0″. To validate the PCB design before sending the first batch for manufacture I decided to make a home made board.
With great timing Matt Little had just done a PCB workshop at Nottingham Hackspace so I had good practical experience. Armed with a UV exposure box from the last DDRC Car Boot Sale and a set of parts from Rapid Electronics I went out to make my PCB.
I basically followed the tips from Mike Harrison’s guide. For etching I used the Seno GS etching system which manages to keep 99.999% of the really nasty stuff in a bag.
I made a second board as a backup, but that was a complete disaster. Once I developed the UV board you could see that the fine lines weren’t sharp and once I started etching they disappeared all together. Still not 100% sure what the problem was there. Possibly over-exposure to the UV or the mask not being completely flush to the board when it was exposed. Anyway one board was enough so as it was getting late I stopped there.
One thing I would note for future builds – the “automatic” pad size in Eagle is too small for home made boards. Use larger pads than the eagle defaults to make your life easier.
Posted on November 1st, 2010 No comments
Last weekend I led the workshop “Electronics for Artists” at South Hill Park in Bracknell. Photos are now on line. Thanks to all the group for your enthusiasm and Martin for organising. Listen out for my wise words on the Digital Media Centre podcast.
Posted on February 8th, 2010 No comments
The crew at MzTEK have just posted this video about their project to help more women get in to technology. Lots of clips from the Electronics for Absolute Beginners workshop plus a cameo appearance from me.
Posted on January 24th, 2010 No comments
My friend Sophie asked me to do a workshop on Electronics for Absolute Beginners for the group MzTEK which ran yesterday. Hard work as a presenter, but I had a really good day. The unflagging enthusiasm and excitement of everybody was great to see and really inspiring. Also lots of fun ideas on different things to do with the circuits in the course. I was also impressed that (almost) everything worked for (almost) everybody. Good building skills all round.
For anybody that wants to see the notes – follow the link above.