My apology for errors in grammar and spelling, my first language is Italian and I always been a roman history enthusiast and amateur historian.

During the imperial period there was a specialist unit evolved in centuries into roman army and navy, hundreds of signal Corp units in every legion, the main insignia of the legion with the senate eagle on top was given to the highest ranking "aquilifer". The vexillus legionis, a red flag with the number and name of it, was brought by the "vexillifer". The imperatoris imago, image of the emperor, a small bust in bronze or golden wood was duty for the "immaginifer", and so flags and recognizable other long artifacts for every smaller unit such as coortes, centuriae and turmae, vexillarii horse mounted troops did the same with cavalry. The will provide visual deployment for orders before and during the battle and they were essential also during transfer marches which were meticulously organised.

There were also sound signal soldiers called "tubicen" equipped with a long bronze or wooden trumpet (tuba), the "cornicen" with a bronze semicircular horn with a wooden handle inside to put it on your shoulder while playing, the "buccinator" with an instrument made with animal horn used to time the order for guard duties during encampment. All this instruments will be used to enforce artillery, infantry and cavalry orders and movements with specific signals, but they would play simultaneously sometime giving to order for a first attack.

Edit I encourage you to Google the names of various troops to obtain period visual references, those soldiers were considered elite troops and they were always near the command, usually suffering highest mortality rate than other corps because were seen as precious targets by the enemies, so they were a prominent subject for artists.