Embedded System: which OS should I use?
I am planning to build my embedded system for processing the sound of my guitar, like a pod, with input and output and so on and a system running with a program with presets, options etc in a small lcd screen should be multitouch for navigation. Now I am at the very beginning and dont know where to start and what system I should use. It should support the features I wrote above (like multitouch) and should be free. Embedded Linux, or Android or what?
3 Answers 3
Are you using off the shelf effects modules with some sort of interface to an embedded system or are you planning on doing the effects in your program as well? I assume the latter in this response, please clarify if I have misunderstood the nature of the project:
Do your system engineering.
You are going to need to deal with the analog of the inputs and outputs. Even digital inputs and outputs are analog in some respects to keep the signals clean. Even optical is going to be analog between the optical interface and the processors interface.
(I know this is long, keep reading it will converge on the answer to your question)
You will have some sort of hardware to software data in interface, ideally if you choose to support different interfaces you will ideally want to normalize the data into a common form and datarate so that the effects processing only has to deal with it one way. (avoiding a bunch of if-then-elses in the code, if bitrate is this then, else if bitrate is this then, else. if bitrate is this and data is unipolar then, else if bitrate is this and data is bipolar then, else. ).
The guts of the effects processing is as complicated as you want to make it, one effect at a time or multiple? For each effect define the parameters you are going to allow to be adjusted (I would start with the minimum number which might be none, then add parameters later once it is all working). These parameters are going to need to be global in some for or fashion so that the user interface can get at them and modify them for the effects processing.
the output, same as the input, a lot of analog work, convert from the normalized data stream into whatever the interface wants or needs or you defined it to be.
then there is the user interface. the easy part.
The guts of the software for the effects processing can be system independent code, and is probably more comfortable being developed and tested on a desktop/laptop than on the target system, bearing in mind the code should be written system and operating system independent as well as being written embeddable (avoid floating point, divides, lots of local variables, etc).
Sometimes if not often in an enclosed system with some sort of user interface on the same black box, knobs or buttons a screen of some sort, touch screens, etc. One system may manage the user interface the other performs the task and there is a connection between. not always but it is a nice clean design, and allows, for example a product designed yesterday with buttons and knobs and say a two line lcd panel, to be modernized to a touch screen, at a fraction of the effort, and tomorrow sometime there may be some fiber that plugs directly into a socket in the back of your head, who knows.
Another reason to separate the processing tasks is so that it is easier to insure that the effects processor will never get bogged down by user interface stuff. you dont want to be turning a virtual knob on your touchscreen and the graphics load to draw the picture causes your audio to get garbled or turn to a nasty whine. Basically the effects processor is real-time critical. you dont want to pick the string on the guitar, and have the sound come out of the amp three seconds later because the processor is also drawing an animated background on your touch screen panel. That processing needs to be tight and fast and deterministic, every if-then-else in the code has to be accounted for and balanced. If you allow for multiple effects in parallel your processor needs to be able to have the bandwidth to process all of the effects without a noticeable delay, otherwise if only one effect at a time then the processor needs to be chosen to handle the one effect with the worst computation effort. The worst that could happen is that the input to output latency varies because of something the gui processing is doing, causing the music to sound horrible.
So you can work the effects processor with its user interface being, for example, a serial interface and a protocol across that interface (which you define) for selecting effects and changing parameters. You can get the effects processor up and working and tested using your desktop and/or laptop connected through the serial interface with some adhoc code being used to change parameters, perhaps a command line program.
Now is where it becomes interesting. You can get an off the shelf embedded linux system for example or embedded android or whatever, write your app that uses the serial protocol, if need be glue, bolt, tape, mold, etc this user interface system on top of around, next to the effects processor module. Note that you could have all of the platforms suggested, an android version, a linux (without android) version, a mac version, a windows version, a dos version, a qnx version, an amiga version, you name it. You can try 100 different user interface variations on the same OS, maybe I want the knobs to be sliders, or up/down push buttons, or a dial looking thing that I use a two finger touch to rotate, or some other multi-touch gesture.
And it gets better, instead of or in addition to serial you could use a bluetooth module. Your user interface could be an iPhone app, or android phone app, or laptop linux or windows app. or your desktop computer, etc. All of which are (relatively) easy platforms for writing graphical user interfaces for selecting things.
Another approach of course could be ethernet, in particular wireless ethernet then your user interface could be a web page and the bulk of your user interface work has already been done by the firefox or chrome or other team. (wireless ethernet or bluetoot or zigbee or other allows the effects processor to be somewhere convenient and doesnt have to be within arms/foot reach of you).
Do your system engineering. Break the problem into a few big modules, define the interfaces between the modules and then worry about the system engineering if necessary inside those modules until you get to easily digestable bites. The better the system engineering and the better defined the interfaces between modules the easier the project will be to implement.
I would also investigate the xcore processors at xmos, they have a very nice simulator with vcd waveform output that you can also use to accurately profile your effects processing. Personally I would have a very tough time not choosing this platform for this project.
You should also investigate the omap from ti, this is what is on a beagleboard. You get a nice arm that already has linux and other things ported and running on it, but you also get a dsp block, that dsp block could do your effects processing and likely in a way that the two dont interfere. You lose the ability to separate your user interface processor and effects processor physically, but gain elsewhere, and can probably use a beagleboard off the shelf to develop a prototype (using analog audio in and out). I actually liked the hawkboard better (with the hawkboard you get a usable system out of the box, with the beagleboard you spend another beagleboards worth of money for stuff that should have been on the board), but last I saw they had an instability flaw with the pcb design.
I am not up on the specs but the tegra (a number of upcoming phones are or will be tegra based), like the omap, should give some parallel processing with a lean toward audio/video as well as gui. You only need the audio and gui (the easier two of the three). I think there is a development platform for sale that has a touchscreen on it and popular embedded OSes.
If you are trying to save money buy making one of these things yourself. Stop now and go to the store and buy one. The homebrew one will cost a lot more, even if all the design stuff is free. The hardware and melted down guitars and guitar amps are not. I speak from experience, many times I have spent many thousands of dollars on a homebrew projects to avoid buying some off the shelf $300 item. I learned an awful lot, and personally the building of the thing is more fun than the using it, I normally shelve it once it is finally working. YMMV
If I have misunderstood your question, please let me know and I will edit/remove/replace all of it with a different (short) answer.