The trumpet as a musical instrument was still quite a ways in the future during the SCA period. It would be like using a telegraph key, a walkie-talkie, or a cell phone to play music. The trumpet was a message sender, a communicator, a conveyor of information, over a distance far greater than people could yell.
Ancient literature is loaded with references to the trumpet being used to sound battle calls, calls for surrender, alarm against enemy attack, and many others. Paintings of battles frequently contain pictures of trumpets. Very often what appears to be trumpets at a feast or a ball are probably shawms or something similar.
Hunting horns, the ancestor of the French horn, a close cousin to the trumpet, were also common. They were there to send information: if a fox or deer has been killed, what color they were, how big, when to release the hounds, etc. An interesting development, somewhat later than the SCA period, was the use of horn quartets to play for the dances in the evening after the hunt was over. The same players would do both, and they must have hard very tired "chops."
The huge Swiss alpenhorns, cousin to the French Horn, and (in this author's opinion) possible ancestor of the tuba, beautifully sent information to neighboring villages. They could be heard for miles, played in harmony, and must have been (and still are) a beautiful sound wafting through the mountains.
There are of course many historical and modern examples of musical notes used to send information: the boatswain's pipe, fife and drum, the bagpipes, and perhaps the best known: the chiming clock.
This author well remembers boy scout camp in the present age where the bugle (modern descendant of the medieval trumpet,) would wake you up in the morning, tell you to get out of bed revielle, to go to mess hall for meals, Mess Call, to assemble for flag raising, Assembly to play To the colors for flag raising, to go swimming, Swim call and when to turn out the lights to go to sleep. Taps. Taps, by the way, is a relatively modern composition. Story of taps
The American civil war had very involved calls for just about everything, summon officers, water the horses, mount the horses, summon artillery, fix bayonets, commence fire, sick call, and many many others. Authentic civil war reanactor buglers have many calls to learn. There were army calls for forward march , right turn, etc. There were also separate navy calls. Here's an interesting fact that you can use to impress your friends: You know that Hollywood call (and baseball) that everyone calls "charge?" It is found under navy calls as "man overboard." One wonders how and when it found it's way into the movies as "charge?" I've run across it on some fairly scholarly web sites as "charge" so I don't know the real story. The examples are endless, and would probably make up a large book to give them all. That brings us to our next topic: using the trumpet in the SCA.
Using the trumpet in the SCA can be just like it's historical role, but with some major differences. Most obviously, while there was certainly a call for start the battle, the medieval equivalent of charge, I doubt if there was a period call for "stop the battle", "hold," "Lay on" or "water bearers on the field." (although there was a water call in the civil war.)(link: water.mid) Just like we use lot's of duct tape, drive around in golf carts, just like we don't use real swords, and just like we create our own SCA names and devices, we create our own trumpet calls to suit our own purposes. The nice thing about the trumpet is that there are only 4 or 5 notes, so just about anything you make up sounds "period."
Another nice thing about doing it the "period" way is that it was a common practice to use commonly recognized popular songs as a basis for calls, so even if "jingle bells," "ride of the Walkeries," "star wars" are not period music, their use in calls is perfectly period. If the trumpet were used to give battle commands (as distiguished from marshall calls, which has been done at Pennsic for several years,) it would be easy for the fighters to remember "star wars - charge, woody woodpecker - retreat." The possibilities are endless - and totally authentic.
Finding actual period calls has so far proved to be very frustrating, although a trumpeter I met at Pennsic told me that a lot of army calls go way back (If anyone knows of any documentation for this please contact me.) (link: nichtrumpet (host) aol.com) There are a couple reasons for this. One is that the trumpeters were a kind of guild, and they passed on the calls by ear, so they never got written down, and written music didn't even exist until the period of Gregorian chant anyway.
The other reason is that military calls were probably kept secret, and might have changed frequently to prevent the enemy from knowing them or learning them and sounding them to cause confusion. There is a collection of calls I have discovered called "La Bataille," but I haven't seen it, only references on the net.
Here's a good history site: (link: www.tapsbugler.com/HistoryoftheBugle/HistoryoftheBugle1.html) Besides battle calls there is another use of the trumpet which I suspect was done, but as of yet have not been able to prove it through documentation. That is the musical equivalent of a device or flag representing a person or kingdom. For quite a few years now I have been announcing the Kings of the East as they come into court, and each King has his particular call. (link to kings page) If they re-enter crown tourney their call is sounded as they present themselves to the reigning king. I have no doubts this was done in period, but as I said have not found proof of it yet. If anyone does come across it I would be very happy if you notified me. (link: nichtrumpet (host) aol.com)
To sum up: the proper use of the trumpet in period is to convey information. There are many fine harpists, bagpipes, loud bands, choirs, which play wonderful music during court, but we are the medieval equivalent of the telegraph, as well as being the musical equivalent of flags. Everything the trumpet plays should say something.