Simple Idea of "Fully" Digital Microphone and Guitar

[StevenStanleyBayes]

I am interested! I have a few ideas on the preamplifier. I will try to write them up but in a long while.

Originally Posted by shanx

Hi Steven, I think the PCM2902C could be a good start too.
I have a couple of local suppliers I deal with , who may be able to get a few if you want and we could work things out with paypal. When the next component buy comes around I will let you know. I would be interested in this from a ''DIY kit'' perspective, so maybe some other DIYers here would be interested also with a little development. Let me know what you think as it is up to you.

[shanx]

I am interested! I have a few ideas on the preamplifier. I will try to write them up but in a long while.

Cool. I was planning to build a FET based preamp shortly, and that could incorporate the codec in it, for experimenting. I will let you know when I get on it.
For some people who do not want to modify their guitars, the preamp-converter could be built as a standalone unit. There are some units commercially available($$) like that but I think it is very interesting to make DIY version.

[StevenStanleyBayes]

OK. This is great.

Can you get some optocouplers too, please. I think, a good idea would be to disconnect the digital signals by optocouplers. Thus the PC power supply would not touch the ADC nor the preamplifier and cannot spread electromagnetic noise.

On the pre amplifier: the guitar humbuckers pickup coils, whatever they call them, differential or not, are just coils, right. Just a piece of copper wound up. Just a simple electromagnetic induction caused by the moving string near a coil and a magnet. This woud run current. The current can be converted into voltage by a resistor. Or by a current to voltage converter which is an inverting amplifier without R1. Wouldn't any of these work for you. Either a resistor followed by a buffer or an amplifier or a current to voltage convertor. You can also continue with currents in case you so prefer. You can still do so by amplifying the coil current with the current to voltage converter but with a resistor in series to the output before the feedback. This is a combination of the standard current to voltage convertor and the standard voltage to current convertor. I do not know how to call this. A current amplifier? A current to current convertor which ensures 0 load to the source?

The same applies to microphones with coils. ( A. k. a. dynamic). There is a point which I am not very clear with, though: some people may claim some microphones are "optimised" to work the best only with a given load and nothing else. Sounds strange.

I do not understand fully what you mean but, I believe you would like to get the signal from the humbuckers, pre amplify this signal, digitise this signal with the said ADC, encode the digital signal and send the encoded sigmnal to the PC.

Does the ADC with USB have encoder? I had a quick look and I did not see any. Do you plan to get a separate ADC, encoder and USB interface. ( The last two may be available together too. Or all of them. ) You can do encoding with PIC, too.

I am puzzled why you need encoding. I guess for error correction. Or for compression. Do you really need these? This is a guitar. What when there are errors. There may be another reason. You may need to send a specific format not to a PC but to a recorder or whatever they have there. I have heard a lot of people record with recorders as opposed to a PC for whatever reason, most likely noise. Seems to be very fashionable: recorder, then PC to work the sound out a bit ( a lot when I sing and play ), then a CD, then... What happens then? Do you perform in a pub or get gigs so you can sell your CD while performing? I wish you do and make a lot of money!

[shanx]

For purposes of adapting the outgoing signal of the pickup into the ADC, a buffer preamp is necessary. There is a thread in instrument/amp section of DIY discussing pickups and such
Impedance matching for DI to Mic Pre

The PCM2902 series can run as a 'standalone' codec chip in USB input/output mode. You don't need to worry about ADC to USB output conversion. That is why I suggested it for a simple interface. There are some codec chips that can do some Digital Signal Processing on board as well but you need to get involved more, like you say with PIC control etc to access the functions.

If you want optical isolation then the S/PDIF with Toslink type optical interface is the standard for that method. You would need a couple of more chips to do that, but the PCM2902 is S/PDIF capable.
More info on the protocol and standards here
S/PDIF - Wikipedia, the free encyclopedia

[StevenStanleyBayes]

I haven't read the impedance matching but I wonder whether the simple voltage to current convertor in the attachment wouldn't do some kind of a job.

[shanx]

Not really looking to convert the current of the pickup, with high enough input impedance on a buffer we are just looking at a low level voltage signal coming from the pickup.

[StevenStanleyBayes]

I have just read your reply. An instrumentation amplifier may do the job. Have a look at the attachment. In case a differential current input wouldn't do the job, either an impedance can be put on the front or a voltage instrumentation amplifier with two buffers upfront can be used.

[StevenStanleyBayes]

I read your reply after I posted. Sorry.

[StevenStanleyBayes]

I have copied the PCM2902 ready to use schematic from their manual and connected 2 simple pre amplifiers as well as 2 simple post amplifiers.

The post amplifiers are to be used as pre amplifiers to a pair of amplified speakers / headphones. These can be buffers.

The idea is to have the microphone, the guitar and the PCM2902 with amplifiers on the side where the musician is and some digital cables going to the PC / Digital Recorder. The PC will send the information back through digital cables and play whatever is being recorded through a pair of monitors and / or headphones.

The return may be unused to reduce noise.

All digital should be disconnected from the PC through optocouplers and a separate power supply should be used for the PCM2902 and Amplifiers, not the PC.

For some applications, optocouplers need not be used, the simplified PCM2902 schematic as per the manual may be used and the PCM2902 and Amplifiers can be powered through the USB. I haven't read the power consumption diagram but I am sure this will be well within the limits of USB. This is why they have mede PCM2902.

[shanx]

Yeah, it should be a pretty straightforward circuit to put together, being careful about isolation of digital and analog supplies and the component layout.

Steven, I wanted to post some suggestions for you and others who may be interested.
Once you have the guitar or mic signal digitized, and sent to the USB input (or optical) of the computer..welcome to the world of VST plug-ins.

Here is an example
I am using a different one but this is kind of freeware open architecture programs that will take the digital USB and process it with ''virtual'' effects, amp simulations and speaker simulations. Can be used for digital recording or live playback.