I'm sorry, I know you are trying to help me in the best way possible and I don't mean to hinder you.
I will try to be as clear as possible about what I want to do.
To begin with I think it is important to clarify the objective I am looking for. My idea is to build an active noise cancellation system, like the one built into headphones such as the Airpods.
However, the part I want to build is the electronics, what is known as the electronic subsystem, the core that is responsible for processing the noise cancellation.
I don't want to take anything for granted, so I will make the complete explanation, to produce the cancellation of a sound it is necessary to emit the inverse wave, that is a basic principle of physics, but when it is transferred to the real world sounds the thing gets very complicated.
In the earphones it works effectively because the working area is very small. To be able to cancel a sound first it is necessary to know which sound has to be cancelled, so it is necessary to capture it.
As I mentioned, my specialty is not in electronics, and the idea I have behind the project is to make it accessible to anyone without great knowledge in the field, so using Teensy was a very practical solution, as it makes it much easier to develop projects using the Arduino IDE that has been widely used for years in the education of students in high schools, at least here in Spain it is something that is already commonplace.
It's simple and extensively documented, and the Teensy audio shield boards make things much easier.
The basic scheme of what I am trying to do is as shown below ( roughly):
*What I am trying to represent with the audio output jack is that the output channels I took were taken as I indicated there, the tip is the left, the ring is the right and the sheath is the ground, not that there are wires connected like that.
The two audio shields are connected in quad channel, they have each Line In, so the four channels, and the sound is routed to both outputs of the shields because I didn't bridge the pin 7 with pad 32 of the Teensy, because I didn't need 4 channels for the output. The channels are connected as shown in the scheme.
On one side we have a stereo audio source, the signal is taken, inverted (with the mixer gain as -1) and emitted to cancel the sound.
On the other hand we have another stereo audio source, it is the one that has to pick up silence, which indicates that the cancellation has been effective. If it is not, we work with that signal and the original one to correct it and achieve an effective cancellation signal.
Audio sources are literally that, sources with a left channel and a right channel.
The way to capture that information is somewhat indifferent, the most obvious is to use a microphone, but microphones are mono, they only have one channel, so technically you would be capturing either the sound of the right channel or the left channel.
If I have not understood incorrectly, a stereo microphone is actually composed of two microphones, one captures what would be corresponding to the left channel and another corresponding to the right channel.
Then, as what I need to perform the cancellation is a sample of the left and right channel of the sound to be cancelled, for example, a 200 Hz sine tone, it is somewhat indifferent how to “capture” this sound, it can be recorded with a microphone, it can be an mp3 file and can be played directly on the Teensy via USB, etc...
That's why I decided to use the line inputs, but I didn't take into account that the line inputs are not capable of feeding a condenser microphone like the ones I usually use, specifically this one in the image:
It hasn't any technical specifications like is impedance, just is described as a microphone.
Okay, so, if I have not misunderstood how the microphone inputs work, in order to “hear” the sounds it picks up, it is necessary to provide it with power, that is why the Mic Line is supplied with about ~2V (usually) on its positive connection, which is the MICBIAS, but the Line In inputs expect to receive much more powerful signals, which is just what you were telling me about earlier, so they do not supply voltage to either the R channel or the L channel.
In order to capture the sound of both channels, it would be necessary to use two microphones, one for the R channel, and another for the L channel, that is what you were telling me when you referred to the fact that I have to use 4 separate microphones each one with its amplifier:
I know your project is running out of time, but you're only going to waste more of that little remaining time going down a fruitless path if you try to somehow connect 4 mics without 4 separate amplifiers.
Reading the schematic of the audio board I realized that the MICBIAS reaches the MIC connection point, that's why I thought that it might not be necessary to introduce any amplifier between each LINE IN channel and each microphone if I made a bridge between MIC, R and L, and that was my main doubt, since if that is not feasible what I understand is that I have no choice but to introduce an amplifier in between.
The attached image is a simple amplifier that I found on Amazon that serves to preamplify microphones (that what the title says), according to the technical specifications can be feed with a DC voltage whose value can be between 3.8V and 15V, and uses as is indicated an operational amplifier AD828 and is suitable for two channels, R channel and L channel.
Given the low voltage required I understand that I can supply 5V from the Teensy board connection to both amplifier boards if it is necessary to use them, but of course, if it is possible to supply them with power from MIC and it is adequate, then I would not have to introduce those amplifiers.
I am sorry for the length of my explanation, I hope I have been able to provide you with enough information on this occasion to make it easier to understand what my objective is.
I just wanted to avoid having to use additional amplifiers in order to keep the cost low and the package as compact as possible.
Overall, thank you very much for everything, just the fact that Teensy exists has given me the opportunity to work on a project that I am really motivated just for closing my studies.
