Audio ADC beginner questions

Thread Starter

diamondman

Joined May 23, 2010
19
It appears I may have been completely wrong. But if you hook up a multimeter to an RCA jack, i believe the current will fluctuate while the voltage will remain the same.
This is what happened to me, but it could have been many things if i am wrong (user error, bad connection... whatever).
Assuming standard hardware using RCA for audio fluctuate the current instead of the voltage, i would need more than an op amp to prepare the signal for an ADC (since ADCs read voltage not current)
 

Thread Starter

diamondman

Joined May 23, 2010
19
@blueroomelectronics I have been doing C and C++ for quite a while and while I may not be an expert... I find i mainly get better by going in too deep and fighting my way out (with engineering and coding alike). I also know some assembly (intel), but this will be an excellent opportunity to learn another platform and learn how to do more with it.

Network streamers exist, but i am not looking to use them much... first off I really am enjoying this project even though it is clearly out of my league, and also the only decent streamer that does anything close to what i want is $400 a pop, and since i will be needing a lot of them......... yeah.

And capturing the audio (of TV, sterio, whatever) is for a project a friend of mine is wanting me to do... hoping it can be a commercial success. It is silly in my opinion but he doesn't want me to tell people what it is gonna be used for (his secret project:)), but i you are interested, once it is done I can post what it is going to be used for. For now the purpose is to generate a network stream in some sort of audio codec (leaning towards OGG VORBIS because it is awesome, but worried about processor power limitatons) based off of an audio input in the form of RCA jacks.
 

Ron H

Joined Apr 14, 2005
7,063
It appears I may have been completely wrong. But if you hook up a multimeter to an RCA jack, i believe the current will fluctuate while the voltage will remain the same.
This is what happened to me, but it could have been many things if i am wrong (user error, bad connection... whatever).
Assuming standard hardware using RCA for audio fluctuate the current instead of the voltage, i would need more than an op amp to prepare the signal for an ADC (since ADCs read voltage not current)
As I mentioned earlier, RCA jacks/plugs/cables don't modulate anything.
Virtually all audio sources are low-impedance voltage outputs. The current drawn from the source depends on the load. Are you familiar with Ohm's Law?
 

Thread Starter

diamondman

Joined May 23, 2010
19
I am... v = ir. But the RCA jacks aren't pushing speakers. I know there is some sort of internal resistance inside the receivers that make it so there isn't an infinite current (i know it won't actually go infinite).

With that said, power = iv, so increasing either will up the power in the speaker and move it(I will be embarassed if i am wrong about this) and from what i have read you can either leave the voltage constant and fluctuate the current to change the power in the speakers, or you can go the other way.

My results with a multimeter tell me that rca uses the first one (changing current while holding the voltage constant) which would mean I would need extra circuitry to detect the current changes with an ADC, but i hope i am wrong on this because then i could prepare the signal with as little as an op amp and some resistors.
 
Well the line out audio jacks on your TV are voltage.

Streaming over TCP/IP isn't like broadcasting over the air, you need to open a stream for each IP address trying to listen. Your AVR will run out of horsepower very quickly.
Get a dedicated stereo audio ADC IC, these will have the filters and possibly codecs you need built in.

Personally I would recommend a proof of concept.
Take your PCs sound card and connect its line input to the line outputs on your TV. Write a complete working version of your code in whatever you're familiar with, preferably ANSI C (as C++ or objective C won't be available on your microcontroller).

If that seems to difficult or you can't get it working then stop now. An embedded solution will be far far more difficult.
 

Thread Starter

diamondman

Joined May 23, 2010
19
I was worrying about a large tcp stack killing the AVR and it's puny little 64k of ram, but that is why i am gonna multicast it. No tcp overhead and verification of packet transmission. Just fire and forget ;). It should be able to handle that. It will also stop broadcasting if there is no one listening to it to ease up on network use. (possibly a ping like function which asks if any devices are listening to it's stream, but this is still being drawn so i may find that something like RTSP has something better for this).
And thanks, I may do that. Although I believe the gcc compiler has been ported in all it's glory to the AVR, so c++ should be an option (http://www.societyofrobots.com/robotforum/index.php?topic=5135.0).
 
Like I said, much cheaper and easier to do it on a PC to see what sort of grief you'll run in to before outlaying cash and designing circuitry. You should be able to port much of your code over to boot.
 

Thread Starter

diamondman

Joined May 23, 2010
19
Cool. I might as well do that while i wait for the hardware to come in (plus side are the chips are like $10). But I have one last question, kinda the one i had at the beginning. Will the circuit proposed by kubeek work for RCA? (I feel I should know this by now, but i just don't want to find out I am wrong after i build it.)
 

Ron H

Joined Apr 14, 2005
7,063
My results with a multimeter tell me that rca uses the first one (changing current while holding the voltage constant) which would mean I would need extra circuitry to detect the current changes with an ADC, but i hope i am wrong on this because then i could prepare the signal with as little as an op amp and some resistors.
Well, if you measure an audio signal with a voltmeter, the voltage will follow the "sound". If you short out the audio signal (which is what an ammeter does), the current will follow the "sound".
As Bill (and I) said, the line out from your TV is a voltage. It certainly will have some source resistance, but this will be relatively low, probably 1kΩ or less.
 

Thread Starter

diamondman

Joined May 23, 2010
19
Thanks. I appreciate everyone's input and help on this problem. I will post updates on progress in case someone else can benefit from it (or i can make a project post and link it...)
 

kubeek

Joined Sep 20, 2005
5,794
rca uses the first one (changing current while holding the voltage constant)
I don´t think you are too familiar with ohms law afterall. You have voltage, current and resistance in a relation. For anything else than 0 ohms you cannot change the current while keeping the voltage consatnt, if the load resistance stays the same.

Anyway as others said, 99.9% of all RCA jacks are voltage sources. The fact that it can't drive a speaker is because of the internal resistance of the driving circuit, it could be around 100ohms and the unloaded voltage reaches say 2V at max. Now when you connect and 8ohm speaker to it, you create a voltage divider which lowers the voltage to 8/100*2V. Putting 0.16Vpeak through an 8ohm speaker is about 1.6mW and that certainly is not enough to be even heard.

And if the source were a true current source, you would just load it with a known sutiable-value resistor and measure the voltage. Resistor is an instant I to V converter.
 

R!f@@

Joined Apr 2, 2009
9,918
For Pete's sake.

RCA is a plug. It does not do anything to any audio signal but helps in interconnection.

OP is confused over so many things. He dunno what RCA actually is. He has no Idea on how to digitize an audio signal. 10bit audio suites for landlines phones, nothing more, Vocal range so to say.

All OP has to do is find a ready made component that samples audio and converts it into a serial data stream.

Googling will definitely hit more than enuf audio ADC's
 

Ron H

Joined Apr 14, 2005
7,063
For Pete's sake.

RCA is a plug. It does not do anything to any audio signal but helps in interconnection.

OP is confused over so many things. He dunno what RCA actually is. He has no Idea on how to digitize an audio signal. 10bit audio suites for landlines phones, nothing more, Vocal range so to say.

All OP has to do is find a ready made component that samples audio and converts it into a serial data stream.

Googling will definitely hit more than enuf audio ADC's
If OP already knew everything, he wouldn't come here for help.
 
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