system
Given the following post, respond as if you're commenting on Reddit:
human
What's in the way of creating a natural-sounding voice that doesn't rely on pre-recorded words?

In other words, when I get directions from a navigation device or the announcer on public transport there's usually either a pre-recorded message (e.g. "examplestreet") or a combination of an established set of words, such as "head" "right" "next" "turn", resulting in a rather jarred sentence.

I realized there's a lot to a voice and humans are great at noticing even the slightest mistakes, but considering how incredibly great we got at rendering facial expressions and the astonishing sound design in various media I couldn't yet figure out what's keeping us from creating such thing.

I also have no clue wether this is the correct flair, sorry.
gpt
This is an interesting combination of linguistics and computer science.

One of the biggest issues in synthesizing natural text-to-speech voices is intonation and pitch on specific segments of words. Given a single word, the pattern of sounds' pitches in that word will change depending on the surrounding words and the semantic properties of the sentence. For example, when asking a question: the last word in the sentence will often have a higher pitch than the rest of the sentence.

One would think that these types of things are easily encoded, but you can't just statically encode pitch values for each individual word in the sentence or phrase. It has to be computed dynamically, which leads to a host of other issues. One of the foremost issues there is how computationally expensive it is to determine what part of speech a word is. Others are determining whether something is a statement, question, or exclamation and  how the word-final and word-initial phonemes at the edges of words act in concert.

An example of the difficulty in doing that can be found in the phrase, "Hello, it's nice to meet you." Very often, in "meet" (/mit/) and "you" (/ju/), the /t/ and /j/ segments will "merge" into something like a "ch" (/tʃ/). It's not too easy to actually consolidate all of these problems yet, and it's an active field of development, primarily in industry.