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  #1  
Old 2011-09-29, 05:25
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Dystopia Dystopia is offline
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Electronics Project

I'm making a recording-mixer device for my final project of my two year degree. The basic idea is that I plug in my Simmons E-drum module (all inputs are 1/4" TRS), press a "RECORD" button (which may trigger a timed delay before recording), play drums for 5 minutes (hopefully for a prototype, at least!!), THEN, after pressing stop and plugging in the bass for another track, press the button again to record with any previous track recorded to play at the output to allow simultaneous record/listening. Then, repeat with rhythm guitar/lead guitar 1.

THEN, the ultimate goal is to press PLAY and pluck the strings of my guitar while an invisible band is playing behind me. The algorithm is this:

EMI input >> ADC >> MEM >> DAC >> Output: Amplifiers

EMI = Electronic Musical Instrument
ADC, DAC = Analog to digital, digital to analog, Converter
MEM = Memory

Basically I need to know a couple of things to get the ball rollin', and I need professional help as I don't want to risk damaging anything plus this has been hard as shit to google... Only one of teachers has reliable advice, but he's popular...

1) I need to know the expected output frequency... and yes, I have looked at tables (E=84.5Hz, A=110Hz), but anything you guys can tell me that will help me recreate the input signal to the output signal that is recorded. I know that the sampling frequency must be double the frequency I wish to output to human ears, but that doesn't mean that a 110 Hz sine wave should be sampled at 220 Hz, that would sound too robotic.

2) So that I can accomplish 1, how do I properly measure the signal of my guitar on an OSCILLOSCOPE?
I know that the coils of the magnetic pick-ups convert the energy of the vibrating strings into an intelligence signal to be carried over to amp, but 1/4"TRS (tip, ring, sleeve).. I look at them, and wonder: Where does the power come from? I don't think I can just plug in a cord, measure the other end of it, play some notes and see the wave-form I want.

I've asked my teachers about step 2 and I am told I need to measure it coming from the Amp, which leaves me wondering how does the signal get all the way to the amp without any power pressing it? I mean I'm pretty sure it's (Ring=Right input/ Tip=Left input/ Sleeve=ground). IS THERE power connected to it? Guitar battery?

Before I can determine the ADC that I will need, as well as memory, I'm going to have to get this. After 2 years of computer electronic engineering, I should get this, but there is no guitar-electronic engineering processes, so yeah big step for me. 8 weeks until the presentation! That's my lack of planning; NOT your emergency, but if you could give me the help I may need before someone else does, or until I eventually figure it out somehow, then much appreciated. Typing this thread helped me reprocess it in my head a little, so I at least got something out of it.
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Old 2011-09-30, 16:00
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Pr0az Pr0az is offline
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Quote:
Originally Posted by Dystopia
1) I need to know the expected output frequency... and yes, I have looked at tables (E=84.5Hz, A=110Hz), but anything you guys can tell me that will help me recreate the input signal to the output signal that is recorded. I know that the sampling frequency must be double the frequency I wish to output to human ears, but that doesn't mean that a 110 Hz sine wave should be sampled at 220 Hz, that would sound too robotic.

2) So that I can accomplish 1, how do I properly measure the signal of my guitar on an OSCILLOSCOPE?
I know that the coils of the magnetic pick-ups convert the energy of the vibrating strings into an intelligence signal to be carried over to amp, but 1/4"TRS (tip, ring, sleeve).. I look at them, and wonder: Where does the power come from? I don't think I can just plug in a cord, measure the other end of it, play some notes and see the wave-form I want.

I've asked my teachers about step 2 and I am told I need to measure it coming from the Amp, which leaves me wondering how does the signal get all the way to the amp without any power pressing it? I mean I'm pretty sure it's (Ring=Right input/ Tip=Left input/ Sleeve=ground). IS THERE power connected to it? Guitar battery?



You have almost confused me with this post. With two years under your belt for an electrical degree, you lack some basic knowledge.

Understand most guitars are completely passive(A good example this is a battery powered small tuner, i can take my guitar hook up the tuner and works just fine without an amp, there is no need for an amp the small signal being sent is enough for my tuner to work). A guitar amp amplifies the guitar by a section of the amp called the Preamp(One of a few steps to amplify but this is the very first). See Magnets and coils which are the basics behind pickups produce current when strings are plucked on the electric guitar(Magnetic Wave -> Electric Field-> Current Flows through coils), this goes way back look up "Faraday's Law of Induction".

See the reason an amp is needed to begin with is to "AMPLIFY" the sound already made by the existing low voltage that is produced by the string+pickup(coils+magnet(s)).

Now do understand some pickups use batteries like active pickups and crap but the basic "passive" pickup does not.

I have some good ebooks on this subject which I'll be glad to offer you. I only ran through them ones or twice, because shit I already had a few books on my shelf that taught me what I needed to know. I've got some .pdfs that show you the design guitar pedals such as loopers, distortion, and etc as well. They are not the best schematics in the world, but could offer you a helping hand in the design of your project.

You may have come across a project in which you must dedicate every hour for the next 8 weeks to complete. From what I'm reading i cant understand is this thing going to be analog or digital (solid state)? You have a challenge here man between signal processing and hardware design. See you have mentioned there are no means of "no guitar-electronic engineering processes" you must take what you have learned and apply this to the current project and also do in depth research on it. We are not searching for "guitar-electronic engineering processes" but for signal processing alone be it of sound or images(both are "data" one in the same, we as designers must tell the mechanism what to do with it because it has no clue to what makes an image an image or a sound byte a sound byte to it an apple is an orange and an orange is an apple unless other wise coded or wired to tell the difference).

"I've asked my teachers about step 2 and I am told I need to measure it coming from the Amp", if you measure it from the amp or the guitar alone your going to get the same Frequency just a different Amplitude. You'll be dealing with Hertz(hz). Shit you can even measure an acoustic guitar and get the same results as long as both of the tunings are the same, with again the only difference being the amplitude. Hell, you could take a piece of plywood tap some nails in it, get a pack of guitar strings, wrap them around grab a tuner and tune to (EBGDAE) and still get the same damn frequency. Here's a schematic if need be http://i136.photobucket.com/albums/...z/schematic.png
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Last edited by Pr0az : 2011-09-30 at 17:33.
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  #3  
Old 2011-10-03, 19:22
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Dystopia Dystopia is offline
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Nice schematic there!

Yeah, the thing that was getting me is the low voltage coming from the guitar. Fuck it I'm hooking it up to an oscilloscope without power... I just couldn't wrap my head around not having a source voltage, but I now I see that it's the strings, and the pickup is just what, a transformer/coil? Doy! And the signal goes to the output without a voltage first going into the guitar, pushing it through, unless it is amplified internally with active pickups.. Okay, all making sense now.

So I just need to calculate the voltage and frequency output range of an non-amplified electric guitar. (F-range = about 60-1320 HZ for a 7-string, plus a little on the sides to make sure I get it it all... this really just has an effect on the sampling frequency, which would be an absolute minimum of 2.64kHz, which is kind of slow)

From there, I would need to divide the voltage signal into bits, and I'm thinking a 24-bit Analog to Digital converter, which would mean 2^24=16.77*10^6, or 16.7 million different possible "Sounds" that can be recorded into memory, which is the other thing I have to know... (memory type)

See, I know how to program an 8051 trainer to take information in (although the only form of INPUT information that we experienced was in the form of DIP switches), and since it was a trainer there was limited memory (the 8051's range of selectivity depends on the size of the the data-pointer, 16-bit we were using, meaning it could internally look up any instruction located from 0000000000000000 to 1111111111111111 [or 0000-FFFFhex].

Basically I'm gonna either need a PLC or a microprocessor that will move enough information fast enough to run recorded-music data through a digital to analog controller at the speed of real time so that when I put it through an op-amp to brush up the signal (which I might have to, I don't know) then that output signal will look exactly the same after being coded and decoded as it did when I first plugged it in, or at least as close as possible/enough to fool human ears.



So yeah, if there is any confusion what I mean is I'm going to have to program a microprocessor to go: "Take this byte + put it there, take second byte and put it behind that one" - basically doing that a few thousand/more times a second, filling up a memory storage (still not sure on what memory to use, we've only used small-scale memory and barely touched on how a few different types of memory work), and that process is strictly for record mode, which I will combined with other tracks (record bass while playing previously recorded drum-track, the timing of that processing that properly will be fun). The Assembly code would be something like, "MOV A@DPTR, MOV R0, A, INC DPTR, loop" or maybe something more efficient.

I also plan to do the same with a 5-string bass and a Simmons E-drum module.
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