You don't need years of music theory training to write for an orchestra. If the sounds are in your head, technology can get them out.
Once it took years of training to become a musician. For example, it takes about ten years to learn to play the violin to a standard sufficient to play in an orchestra. And you have to have the natural talent for it, otherwise it will be ten years wasted.
To compose music for orchestra takes just as long. It takes years to become imbued with the orchestral tradition. You don't just have to learn musical notation, you have to learn about all the capabilities of the various instruments. You have to be able to auralize (imagine in your head) how they sound - individually and in combination. And you have to know all the things that players find difficult - there's no point in writing something that will hardly ever be played properly.
But these days, you can short circuit all of that time and training. You will still need a good musical imagination though - there's no substitute for that. Yet.
To write for orchestra in the modern way, you need a master keyboard, a DAW and a good orchestral software instrument library. Don't record audio, record MIDI tracks so that your key presses are recorded, rather than the audio signals.
You can easily build up layers of all the orchestral instruments. Your symphony, or film sound track, will be finished in no time at all.
But a performance made up from samples isn't the same as a performance given by an orchestra. It sounds kind of orchestral, but it isn't the real thing. So you need to go to the next stage...
When you have finished inputting all the notes, the next step is to turn it into musical notation. If you can print out a conductor's score and set of musicians' parts, then you have a composition!
There are many softwares that can turn MIDI data into musical notes that you can print out. However, they often don't do it all that well. Musicians need the notes to be set out in a certain way, or it doesn't make a lot of sense to them. To do this takes human intervention and expertise.
Unfortunately, without the training and background, you don't have that. In fact, even if you do know musical theory thoroughly, unless you have actually written out music for other musicians to play and taken some feedback (often vitriolic - you would be surprised some of the words classical musicians know), you won't be able to do it well enough.
The answer is to engage a copyist. A musical copyist is a person who specializes in writing out music. Often they have another agenda, such as struggling to be a successful composer or musician in their own right, but it's the copying that pays the bills.
In the old days, they would copy out a composer's rough score into one that could be engraved and printed. These days, they take your MIDI sequence and put it into a specialized score writing software such as the wonderful Sibelius. Even Sibelius doesn't do everything automatically, the copyist has to use considerable skill to transform your date into musician-friendly music. He or she will need to consult with you on dynamics and other details too. If something looks unplayable, they can advise you.
The end result will be a score and a set of parts that can be performed by a real orchestra, whether classical or for a film or TV soundtrack.
All you need is the imagination to make the music in the first place. Technology is wonderful.
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