This scale classifies how "robotic" or "human" an artificial voice in fiction sounds.

Text to Speech

Robots sound like a consumer-grade voice synthesis program such as Microsoft Sam. Ironically, this is the most "realistic" depiction since that's what computer voices sound like today, but it's almost never seen (or heard) in fiction. When it is, it's often Played For Laughs, and may include references to real-life programs, such as the infamous SWAH bug.

• Auto in Wall E is voiced by an actual text-to-speech program.
• Justified when Stephen Hawking appears in fiction; he uses a TTS program to talk in real life.

Monotone Human

Robots sound like a human saying their lines with very little emotion or inflection. Expect to hear phrases like "does-not-compute" a lot. This was common in early sci-fi movies and TV shows, but nowadays is a Discredited Trope and is mostly Played For Laughs. This is what people often mean when they describe someone as "robotic"-sounding.

• Battle droids in Star Wars sound like this in the first two prequels.

Borderline Human

Robots sound like humans saying their lines in an idiosyncratic or automated-sounding way, like a prerecorded message.

• GLaDOS in Portal is voiced by a voice actress who based her delivery of each line on an actual text-to-speech output, after the dev team discovered the comedic potential of text-to-speech voices calmly saying malicious things. She sounds more human near the end and in the sequel.

Filtered Human

Robots sound exactly like humans speaking through a voice filter.

• C-3PO in Star Wars has a recognizable, human-sounding voice run through a filter. Presumably, this is important for his protocol and etiquette-related duties.

Perfectly Human Sounding

Robots sound just like humans and there are no audio cues to indicate that they're robots. This includes both emotional and emotionless voices, provided the voice itself sounds human.

• Bender in Futurama sounds human, although there are also robots of the Monotone variety for comedic value.
• HAL 9000 from 2001: A Space Odyssey.
• Gertie from Moon.
• Wheatley in Portal 2.