Audio transmission at the quality level of a traditional landline call from the telephone companies.
Although toll quality has been touted as the gold standard for telephone call clarity, it is not really that extraordinary. Due to limitations in analog signaling years ago, the traditional range of frequencies allotted to the human voice for telephone calls was 300-3400 Hz. It was later discovered that this is a root cause of people not differentiating between "s" and "f" and "m" and "n." When the range is doubled, listeners can identify sounds much better. See
MOS and
wideband audio.