What limitation was characteristic of early synthesizers built in the 1960s?

The correct answer is The could produce only one note (tone) at a time

This means that you couldn't press numerous keys and have notes play at the same time like you could on a piano so those who played them had to have high skill in order to make them sound amazing. This was fixed very soon as people understood the need of a synthesizer that could produce numerous tones at a time.

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