The Simple Analogy That Explains Everything
Imagine you are holding a thread.
If you vibrate it very fast with your fingers, it turns into a blur. That fast vibration is the carrier frequency.
Now, while keeping that fast vibration going, you slowly move your whole hand up and down.
Someone watching the thread sees two things at the same time. A fast vibration, and a slow change in how large that vibration looks.
This is exactly how optical audio transmission using amplitude modulation works.
The fast vibration is just the vehicle. The slow movement is the actual message.
The Three Key Pieces
First, the carrier.
In electronics, this is a fast signal, for example a 15 kHz signal generated by a 555 timer.
In the analogy, this is the thread vibrating rapidly.
The carrier itself contains no information. Its only job is to create something fast and stable that can carry changes.
Second, the audio signal.
In electronics, this is music, voice, or a low frequency tone like 60 Hz.
In the analogy, this is your hand moving slowly up and down.
This slow signal is the information you want to transmit.
Third, modulation.
In electronics, the audio changes the amplitude of the carrier.
In the analogy, your hand movement changes how wide the vibration appears.
The result is a fast vibration whose strength grows and shrinks following the slow movement.
This is amplitude modulation.
What Happens at the Transmitter
You start with a fast carrier.
You apply the audio to it.
You send out a fast signal whose strength follows the audio.
In optical audio, this means the LED is flickering fast, but the brightness of that flicker goes up and down with the sound.
What Happens at the Receiver
The receiver detects a fast signal with slow changes in strength.
The photodiode and amplifier naturally respond to average light intensity. This acts like detection or rectification.
A low pass filter then removes the fast vibration.
What remains is the slow change in strength, which is the original audio signal.
The receiver does not track the fast carrier directly. It only follows how strong it is over time.
Why This Works
The receiver cannot follow very fast changes. Those average out.
What it does see is the slow change in brightness.
This is exactly like watching a vibrating thread from far away. You do not see each individual vibration. You see the blur getting bigger and smaller as your hand moves.
That blur is the audio envelope.
The Simple Test
Take a thread.
Shake it fast between your fingers.
Move your whole hand slowly up and down.
You will see the fast vibration is always there, but the range of motion follows your slow movement.
That is exactly how modulation works.
What Happens When Audio Frequency Gets Too High
For amplitude modulation to work properly, the carrier must be much faster than the audio.
If the audio frequency becomes close to the carrier frequency, the system breaks down.
There is a fundamental rule here.
The carrier frequency must be at least two times higher than the highest audio frequency.
This is the absolute theoretical minimum.
If this rule is violated, the envelope changes too fast. The receiver cannot separate fast vibration from slow movement. Filtering no longer works properly. The recovered audio becomes distorted or disappears.
In practice, engineers use a carrier that is five to ten times higher than the audio, or more. This makes filtering easy and keeps distortion low.
Using the analogy again, if your hand starts moving almost as fast as the vibration itself, the motion blends together. You can no longer tell what is vibration and what is movement.
In One Sentence
The fast vibration is only the vehicle. The slow change in its strength is the message.
Why This Matters
If you understand this analogy, you understand amplitude modulation, optical audio transmission, why receivers need detection and filtering, what demodulation really means, and why the carrier itself does not contain the information.
This is the same principle engineers figured out more than a century ago. You just understood it using a thread.
