Adam H Image Capable Digital Mode (AHICDM)

April 29, 2022 - Reading time: 2 minutes

AHICDM is a simple digital mode that transmits bits by varying the amount of silence between pulses. It is a narrowband mode that transmits slowly.

I only came across this modulation while looking at the Recently changed pages list of sigidwiki. It looked easy to implement, and didn’t have much written about it, so I decided to play around with the provided audio example.

I have never seen this modulation used in the wild, and the only mention I have seen of it is in the Signal Identification Wiki. This page is written by analyzing the audio recording provided on the wiki page.

Parameters

Baud-rate

The baud rate of this modulation depends on the content of the message. A message of all 1 bits will take longer to transmit compared to a message of all 0 bits.

Modulator

Below is a simple modulator written in Python.

#!/usr/bin/env python3
import os
import struct
import numpy as np

class AHICDM_Modulator:
    def __init__(self, freq=440, rate=64_000):
        self.rate = rate
        self.sample = 0
        self.freq = freq
        self.amp = 0
        self.phase = 0

    @property
    def time(self):
        return self.sample / self.rate

    def output_sample(self, sample):
        self.sample += 1
        sample = np.random.normal(sample, 0.01)
        sample += 1
        sample *= 128
        sample = int(sample)
        sample = max(0, min(sample, 255))
        os.write(1, bytes([sample]))

    def amplitude_duration(self, amplitude, duration):
        target = self.time + duration
        while self.time < target:
            self.amp = (self.amp * 0.991) + (amplitude * 0.009)
            self.phase += 2 * np.pi * self.freq / self.rate
            s = np.sin(self.phase) * self.amp
            self.output_sample(s)

    def send_bit(self, bit):
        if bit == 1:
            self.amplitude_duration(0, 0.3)
        else:
            self.amplitude_duration(0, 0.15)

        self.amplitude_duration(1, 0.1)

def bits(buf):
    for byte in buf:
        for i in range(8)[::-1]:
            i = (byte >> i) & 1
            yield int(i)

mod = AHICDM_Modulator()
mod.amplitude_duration(0, 0.88)
mod.amplitude_duration(1, 0.7)

for bit in bits(b"gkbrk.com"):
    mod.send_bit(bit)

mod.amplitude_duration(0, 0.7)
mod.amplitude_duration(1, 0.1)