# DIY USB microphone?



## beamrider

Has anyone ever looked into building something like this? I was thinking that it would make a nice mate to the USB DAC I just built, and figured I'd ask here first before trying to build one......


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## robzy

I dont see why not, commercials USB soundcards have inputs.

 Basically a DAC with a ADC instead.

 Rob.

 [edit]: Yeah, sorry, i know im not much help, im just stating that its probabvly possible


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## rickcr42

USB is just the method of getting the digital signal out of the box so can in some cases be piggybacked onto an already present AD converter stage with mic pre/buffer input.check the TI (not BB) offereings in the TI USB Zone for data sheets and eval board pdf schematics.

 another possible option depending on the quality level required is an all in one solution with a DSP chip/CODEC design which can allow for the use of Volume Control/EQ/Compression in the digital domain.

 Many options depending on requirements and need


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## beamrider

Well, nothing too complicated I guess, but a level control would probably be a good idea, I think. Don't really need any EQ or compression tho, but looking for decent sound quality, as this would be primarily used for internet gaming, ventrilo, etc.

 Geez, upon checking prices on eval boards, looks like it's gonna be cheaper to buy a $20 USB headset and rip it apart...........lolol


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## rickcr42

check out the Maxim site.they are beginning to get into the portable audio arena and i beleive they have an all in one solution.Not positive but i think so.


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## beamrider

Checked ti and maxim, and unless I'm missing something, I can't find a chip that does just the microphone input. But I have seen standalone USB mics, so there has to be a chip out there somewhere, unless they are using a combo chip and somehow disabling the DAC functions.............?


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## rickcr42

I think you may be a bit off here man.USB can *not* pass an _analog _ signal so any analog device (microphone) to USB intreface has to do a the analog-to-digital conversion or you have an unusable signal.


 You can not just add mic to a "USB" adapter and be up and running.that is why for simplicity i suggested the all-in-one chips.Far easier to use something that already has the circuit blocks figured out than it is to assemble multiple analog and digital chips onto a workable board unless you have done digital audio designs before.

 The quick and dirty way to go and the one I have used many times in the past is to choose the part then hunt up the evaluation board pdf.They know their own product and so the layout will be optimised for the part.
 The eval acrobat reader files normally include a 1:1 scale pc board layout which can be copied.Being an evaluation board that must be set out from the beginning for testing purposes there will be a lot of extras that you do not need so they can be left out of the final design.

 Until you have done digital audio layout it is the best way really and it is how i learned to do it myself-by doing it from the already engineered layout.

 BTW-when doing analog-to-digital input circuits accuracy and a stable clock are essential and the noise level tough to get right (I still can not get my ADC noise input down to 24 bits) but when doing digital-to-analog it is all about the _sound_ and that takes precedence.Special attention to the sonics is actually useful once going back to the analog world.Sound means absolutely nothing at the input so choosing "designer" caps and resistors would be silly and an uneccessary expense.More you need to shoot for electronic accuracy and tightly specced parts.

 just a suggestion


 BTW-if it was ME doing the USB mic i would add an analog mic buffered "pass through" so the mic signal could be tapped off before the inteface electronics allowing analog use s for the mic also.A simple dual jfet buffer or bipolar pair ( potentially lower noise level) should suffice for the connection to the outside world.

 BTW ll-you also need to figure out your bandwidth requirements and then add filters for the analog input high and low cut off points.Trying to reproduce a bass down to Dc is dangerous and over the sampling frequency impossible if you want to avoid trouble.And if for voice only applications there is really no reason to go outside of 100hz-10khz or so.Will save you headaches later.Even for live music it is a real good idea to have filter turnovers in a range no sane person would accept if it were an amp or speaker but recording is different (as is PA work ) and you MUST have set cutoffs depending on application or you will be wishiung later you did.

 BTW lll ( 
	

	
	
		
		

		
		
	


	




 )-input levels : You will never hear worse sounding digital than if you exeed the input level overload point and that even on peaks.Analog can distort gracefully whendriven to hot but digital is either GO or NO GO.all or nothing.So even though the _potential_ is there for a huge dynamic range the reality is you never approach the upper voltage input limits out of fear.Where you get dynamic range is the low noise floor of digital which allows for signals that in analog would have been lost in the system noise floor and would have to be brought up or tossed out.
 i would add and adjustable input pad and 3 LED mini meter (go-almost no go and approaching trouble-overloaded, SCREWED)to the mic input PLUS a volume control at the minimum but really a limiter is the way to go and some of the chips have this already built in to the design.Actually both

 Or all the out solution if voice only is not what you are looking for : Analog Devices SSM series balanced mic preamp/compressor chips mated to a USB ADC.set the upper limits of allowable signal and 

 just some more options thrown your way and something to think about.

 as you can see the all-in-one chip can really simplify things,especially if mated to a DSP chip to handle all the EQ/Compress/volume/etc


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## beamrider

Oh yeah, I know you have to have at least a CODEC/ADC to do this, what I meant was I've seen mics that are USB, and that's all. So I don't know if they are using a chip designed to just perform the CODEC/ADC functions, or it they're using an all-in-one chip that also has stereo DAC , and just disabling the DAC functions..........


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## rickcr42

the only thing that would make it a "mic only" input is because that was the selected mode of the company producing the device.Being an anolg signal means it could just as easily been "just a line input" and nothing other than input Z and gain would change-same basic device running the interface.


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