- Signal-to-Noise Ratio
Noise proves nothing. Often a hen who has merely laid an egg cackles as if she laid an asteroid – Mark Twain
“Noise is always unwanted,” declared Dr. Optoglass. “But sometimes it fools us into thinking its existence is warranted.”
“I see you are not a believer in noise, Dr. Optoglass.”
“No, I’m not, Bob. Why on earth would I put random noise into an image or sound? It’s random for heaven’s sake!”
Dr. Optoglass became red all over, like a chameleon caught in front of a wrong chroma key. Then he changed color back to normal again. It took a few earth-minutes.
“But noise exists nevertheless, even in our eyes and brain.”
“I know all about that, Dr. Optoglass. Nothing is perfect, Professor Sampler told me that.”
“Very wise of him. And what else did you learn from Professor Sampler?”
“Yes, Bob. What else did the good ol’ Professor fill your brain with?” Paul wanted to know.
“Nothing, dad. That was about it. There’s always some loss in everything. But we’re okay.”
“Okay, indeed.” snorted Dr. Optoglass. “I’m sure Sampler told you about signals. ”
Bob blushed. Paul flushed.
Dr. Optoglass continued: “Signals are everything, and noise is part of it. Scientists and engineers measure the quality of a signal by comparing how much of the good stuff is present along with noise. It’s called the signal-to-noise ratio. Our eyes have the good sense to keep noise away from perception. One can’t say the same thing about the crowd waiting outside.”
At that moment the nurse burst back in, breathless. “May I usher in the next patient, doctor?”
“Yes, you may, nurse. Thank you. Well, I have to stop now. But you understand how noise too can sometimes give the impression of detail being present, don’t you?”
“Because it’s so small it can be mistaken for edge contrast or acutance!”
“Brilliant, my boy. I’m glad you understood.”
“And I’m grateful you told me, Dr. Optoglass. Sharpness is a perception that really depends on resolution, acutance, contrast and noise.”
“Yes. You’ve put it perfectly.”
“Let’s go, Bob. Thank you, doctor.”
“No, Paul, Bob. Thank you.”
For some strange reason, Dr. Optoglass winked at Bob. Then he took another deep breath. Thankfully for Bob and his father, they didn’t have to wait to see the end of that one.
- Noise is unwanted detail in any signal, either image, sound or anything else.
- The ratio of the wanted signal to the amount of noise is called the Signal-to-noise ratio or SNR or S/N.
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