Separate 4 channel from Buffer nodejs - node.js

Question

I'm trying to separate the 4 channel that are in a Buffer i receive from a 4-mic Array ReSpeaker. I'm using nodejs and currently i use a spawn command like:
spawn('arecord -r16000 -fS16_LE -traw -c4 -Dac108')
and then pipe the output in a transformer where i split the Buffer in the 4 channels and save them into separate file for check the result
const stream = require("stream");
const fs = require('fs');
class ChannelTransformer extends stream.Transform {
constructor(options) {
var write_1 = fs.createWriteStream('ch1', {encoding: 'binary'});
var write_2 = fs.createWriteStream('ch2', {encoding: 'binary'});
var write_3 = fs.createWriteStream('ch3', {encoding: 'binary'});
var write_4 = fs.createWriteStream('ch4', {encoding: 'binary'});
options.readableObjectMode = true;
options.writableObjectMode = true;
options.highWaterMark = 20000;
options.transform = (chunk, encoding, callback) => {
let channels = [[],[],[],[]];
for(let i=0; i<source.length;i++ ){
channels[i%4].push(chunk[i])
}
write_1.write(new Uint8Array(channels[0]));
write_2.write(new Uint8Array(channels[1]));
write_3.write(new Uint8Array(channels[2]));
write_4.write(new Uint8Array(channels[3]));
callback();
};
super(options);
}
}
As result from this code i get 4 file and if I import them with Audacity i find out that ch2 and ch4 files have been correctly separated, while ch1 and ch3 are corrupted and result in a white noise file.
Am i missing something on the separation? i thought that audio was stored on the pattern :
[[ch1_0],[ch2_0],[ch3_0],[ch4_0],[ch1_1],[ch2_1],...]
Also i dont get why, if the pattern i follow is not correct, 2 of the channels where separate succesfully.
I've also tried to cast the chunk into something else like:
let source = new Int8Array(chunk);
and then in the for cicle:
channels[i%4].push(source[i])
with different Type like Float32Array, Uint8Array, Uint16Array, Int16Array
but the result are the same.
I've already tested that the 4mic is working correctly by using the command:
arecord -r16000 -fS16_LE -traw -c4 -Dac108 -I ch1 ch2 ch3 ch4
which produce 4 file as expected containing each channel.
For every test, I block with mi finger on mic each couple of seconds while speaking so i can tell the difference between every channel.
Can anyone help me? or have some hints?
Thanks!

Answer 1

OK i figured out what was the problem.
Basically i was recording with bitWidth = 16, and the Buffer object in Node is an instance of Uint8Array so the pattern i was following was indeed correct, but due to the 8bitArray i had to assign 2 element per channel. Cause the format of the 8bit Array is:
[[ch1],[ch1],[ch2],[ch2],[ch3],[ch3],[ch4],[ch4],...]
Also casting the Array was useless because the produced 16bitArray didn't merge 2 8bit element to create a 16bit element but instead each 8bit element create the last 8bit of a 16bit element where the first 8bit are 0 like this:
8bitArray=[[11111111],[11111111],[2222222],[22222222],...]
16bitArray Casted= [[0000000011111111],[0000000011111111],[0000000022222222],...]
So i have created a method that merge the 8bitArray into the correct Array to be handle correctly based on the bitWidth of which your are recording:
var bitMultipler = bitWidth/8; //so i can handle any bitWidth with the same code
let channelsMap = new Map<number, Array<any>>();
for (let channel: number = 0; channel < totalChannels; channel++) {
channelsMap.set(channel, new Array())
}
/**
For each channel i push as many element as needed based on the bitMultipler
*/
let i = 0
while (i < chunk.length) {
for (let channel = 0; channel < totalChannels; channel++) {
for (let indexMultipler = 0; indexMultipler < bitMultipler; indexMultipler++) {
channelsMap.get(channel).push(chunk[i]);
i++;
}
}
}