This topic will describe how you can benefit from using an ITP over an IT file.
Let me first explain what the difference is.
IT and ITP are the same format, except that an IT will store all the samples inside the song, while an ITP doesn't. An ITP stores a reference to that sample. The big advantage is that when you use samples in more than one song, you only store the samples once, and the ITP files remain small. Downside is that you always need to have the sample song, but thats just a small downside.
Picture a situation where you do want to share patterndata on several occasions but not send the samples each time. This can be archieved by sending 2 files, a zip with the instruments (.ITI files) , and the ITP with the patterns.
You can then change the ITP, send to someone else, he edits the ITP and sends back. The ITP probably is going to be around 50k on each send, while the .ITI files easilly can expand to over 5 MB.
How to setup an .ITP song?You can choose to convert an existing .IT file to an .ITP or make one from scratch. If you make one from scratch, scroll down to where that part starts.
If you decide to convert an existing .IT file...Load in the .IT file. In the general tab, press Song Properties, and set the songtype to Impulse Tracker Project (.ITP). This format allows you to embed instrument settings in your song, or keep the settings of the original file.
If you don't check the check to embed the settings, changing them will change the .iti file itself which means that any song that doesn't have the settings embedded will be changed too. Note that you'll need to save the .iti once you made a change or it will be reset on next load.I would advice to check this setting until you have enough experience to considder otherwise.
Click ok.
Go to the instrument tab, and save each instrument to an .ITI file (assuming that you don't have an .iti file yet).
I found it useful to save those .iti files to the same directory labelled after the group project am working with. This allows me to keep it all together.In the treeview on the left, unvold your song's instrument tab and sample tab. Click each instrument and press delete to delete the instrument. Same for the samples.
Save your song as an .ITP file and close it.
Reopen your song and verify that you have an empty song with just the patterndata in tact.
This is basically the same state as when you make a new file, so I'm going to explain what to do to create a new file first, and then continue from there. So skip down to the 2nd bold text.
If you decide to create a new .ITP file...You can do this 2 ways. Either press new and set it to an .ITP file, or choose File->new->Impulse Tracker Project (.itp)
If you create a new file by clicking on the [new] Icon in the toolbar then do the following: In the general tab, press Song Properties, and set the songtype to Impulse Tracker Project (.ITP).
This format allows you to embed instrument settings in your song, or keep the settings of the original file. (open the song properties for this if you haven't done already to check the setting)
If you don't check the check to embed the settings, changing them will change the .iti file itself which means that any song that doesn't have the settings embedded will be changed too. Note that you'll need to save the .iti once you made a change or it will be reset on next load.I would advice to check this setting until you have enough experience to considder otherwise.
Click ok.
We're getting there... How to add instruments?This is the final step. You have an .ITP song ready, and only need to load the instruments.
Drag in an .ITI instrument from the treeview to the instrument tab. OpenMPT will automatically set the path info for you.
Save your song, and thats it.
COOL, but I can't add empty instruments for VSTi usage... 
why?This was my first reaction too, but it is possible. Make a new instrument, and make sure it has a sample even if that sample is empty. Save this as VSTi Controller.iti or something. Now you can drag in an empty instrument. and attach a vsti to it. Note that for this to work, you NEED to embed the instrument settings to the ITP file (see first section when
creating a new/converting existing song).
NIIIIIIIICE, but is there really a big advantage in using the ITP file over an IT if I only use VSTi's?Now thats a good question. Since ITP only saves sampledataspace and not the instrument section, the only advantage you'll gain is if you make .ITI's for each VSTi you use and not embed its settings, but even then, its probably much easier to just use an .IT. So, if you use samples, go for ITP, if you only use VSTi's, go for IT's.
So I started to make .ITP chiptunes, but I can't load them into any other player... Don't forget, you can always convert your song back to Impulse Tracker and save as .IT and have the samples in them again for distribution. It would be a bit pointless to save a chiptune as .ITP I guess as the sampledata usually is below 30k anyway but I mentioned this for the example of being able to convert it back to .IT.
What NOT to do?You are able to manually set a path to an instrument in the treeview. Do NOT set it to an existing .IT file. Although you can set it to any file, choosing another file than .iti or .xi will cause mpt to overwrite your file when you right click save. If you just load in the .IT file, you'll loose your changes on next load.
