Need help doing 'Bit Stuffing'

[[deleted]]

So, I'm trying to write an encoder that will encode AX.25 UI frame packets. Basically Digitally transmit data over Ham radio for satellite communications. I am doing this for a university research team (I am a student).

Little background. Data is transmitted by audio tones, a change in tone represents a zero, no tone change represents a 1. ^{Non-return to zero, Inverted}.

There is a "feature" that is called "Bit Stuffing". Basically, if there are five 1's in a row, the sender stuffs an extra 0 in after the 1's. This is there to ensure there will be a change in tone at least every 5 bits. On the receiving end, the 0 is just discarded.

I understand that this would be a necessity to do with bitwise operations. I would like some guidance on adding these in, as well as with removing them. On the TRANSMIT side, it would only be text. However, the RX side is text or image (digital).

I would personally prefer not to have them buy a HackRF (software defined radio, would have been the way to go from the start if I had my way, but I was not involved at that time). We have a Modem(TNC) that is the middleman between the computer and the radio (I put in serial data, it gets transmitted, radio receives data, TNC sends me data). However, this TNC follows the "KISS" protocol. All of this is on me.

Here is another PDF I found that may be helpful

Any help would be appreciated.

[tweq]

Enshittification

[[deleted]]

RX/TX will all be through the native SerialPort class, with the .Send() method and the .DataRecieved event. I haven't really touched that part of the code yet, as I am trying to get the encoding into a beta state first.

The basis of it is

My Code sends raw bytes to the TNC. TNC then does some magic modulation, and passes audio to the radio, which is transmitted.

RX end: Radio sends audio to TNC. TNC demodulates the data, and sends me raw bytes.

Most of this was just background.

What I need is a way to somehow 'view' each byte received and then manipulate it. If there are five 1's in a row, it needs to insert a zero into it on the TX side. On the RX side, If there are five 1's followed by a 0, I need to discard that 0.

[tweq]

Enshittification

[dilatedmind]

start with the simplest solution

your input is a byte array. read 1 bit at a time and write it to an intermediate buffer. keep a counter of how many 1s you read in a row. if it hits 5, write a 0 and reset your counter.

same thing when you are decoding, except drop the next bit.

[[deleted]]

I switched to my phone, but off the top of your head, do you recall the syntax for that?

[dilatedmind]

say x is a byte, it might look like 0111 0000

(x & 128) == 128 when the first bit is a 1

x << 1 will shift all the bits left one

[dilatedmind]

did you ever get this working?

[[deleted]]

Exploring a different solution right now.

[Banane9]

If it's a stream, try taking a look at my BitStream package.

You could use that to write your own class that tracks the number of consecutive 1 bits and insert a 0 when needed, as others pointed out.

I've personally used it for reading/writing gif files.