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it2snvgm: IT module to SN76489 (Sega Master System) VGM converter

This is a utility to convert ImpulseTracker .IT modules to VGM files targeting 
the SN76489 chip used in the Sega Master System and various other platforms. It 
was developed as an alternative to existing proprietary trackers targeting this 
chip and, while not as nice or easy to use as a purpose-built tracker, it should 
get the job done.

This initial release was developed rather hastily and is a bit rough around the 
edges, but generally supports the features it needs to.

*                                  Building                                    *

Users of Unix-like systems can build as follows:

$ cd it2snvgm
$ make

Windows users are on their own, as I haven't bothered to build for that platform 
or set up a Visual Studio project. blackt and libsvgm should be built as static 
libraries, with it2snvgm linking in both.

To convert an IT file:
$ ./it2snvgm outfile.vgm

*                                 How to Use                                   *

Input IT files need to be specially designed for the limits of the sound 
hardware, so don't expect to be able to just plug in any old module you have 
laying around and get listenable results.

Channels in the IT file are mapped directly to hardware channels as follows:

IT channels 1, 5, 9, ...    = tone channel 0
IT channels 2, 6, 10, ...   = tone channel 1
IT channels 3, 7, 11, ...   = tone channel 2
IT channels 4, 8, 12, ...   = noise channel

Lower-numbered IT channels have a higher priority than higher-numbered ones. 
Channels with lower priority will play only if all channels with a higher 
priority are silent. For example, if two different notes are playing 
simultaneously on channel 1 and channel 5, the note on channel 1 will be output 
to tone channel 0. However, if the note on channel 1 stops, the note on channel 
5 will be heard instead. This can be useful e.g. for setting up delay channels 
without having to intermix them with another channel.

it2snvgm is very simple. It doesn't know or care about your samples (in fact, it 
doesn't even load them from the file), nor does it care how you've mapped them 
to instruments. All it cares about are what notes you've entered into your 
patterns and how they're being modified by effects/envelopes.

* For tone channels, a note of A0 in the IT file outputs a note of A2 in the 
VGM, A#0 corresponds to A#2, etc., in order to maximize the accessible part of 
the tone range.
* For the noise channel:
  * Notes C0-B0 = low-frequency noise
  * Notes C1-B1 = mid-frequency noise
  * Notes C2-B2 = high-frequency noise
  * Notes C3+   = tone2-frequency noise
* In "periodic noise" mode (see below), a note of A0 corresponds to an output 
note of A-2 (i.e. four octaves below the normal tone range).

Your samples/instrument maps should be configured to match this arrangement.

it2snvgm does not care about tempo, only speed -- consoles have fixed frame 
refresh periods, and this program will not interpolate for you. Your IT should 
have a tempo of 150 (or some lower value for PAL consoles) and control playback 
speed solely through Axx commands.

Finally, note that your files must use "modern" IT settings -- legacy modes are 
not supported. Specifically:
  * "Linear frequency slides" should be enabled.
  * "Old effects" and "compatible Gxx" should be disabled.
Unless you're using a very old tracker, these should be the default settings 

*                              Feature Support                                 *

Most features of the IT format that make any sense to represent on the SN76489 
are supported by the converter, including:

* Note volume and volume commands
* Instruments, including envelopes (volume and pitch only)
* Most common command effects

Basically anything else is unsupported, including but not limited to:
  * All panning- and filtering-related features
  * Samples and sample effects
  * Most of the more exotic effect commands

The following effects commands are fully supported, and should behave exactly as 
they would in an IT player:
  * Axx
  * Cxx
  * Dxx/DEx/DFx
  * Exx
  * Fxx
  * Gxx
  * Hxx
  * Ixx
  * Jxx
  * Mxx
  * SCx
  * SDx
  * axx
  * bxx
  * cxx
  * dxx
  * exx
  * fxx
  * gxx
  * vxx
These commands are only partly supported:
  * Bxx -- Works, but may only be used to jump to an earlier order, and may not 
be used in conjunction with Cxx to jump to a row other than the first one. If 
this effect is used, the generated VGM will loop.

Other effects will not function.

Additionally, there are a few extensions used by the converter in order to 
support "periodic noise" mode:
  * S7D: When used in a noise channel (IT channels 4, 8, 12, ...), this 
registers the IT channel it appears in as a "white noise" channel, and 
subsequent notes will be output as such. This is also the default mode. Note 
that this applies only to that specific IT channel, *not* to the noise channel 
overall! This means you can e.g. have channel 4 configured for periodic noise 
and channel 8 configured for white noise at the same time.
  * S7E: Like S7D, but registers the channel as a "periodic noise" channel.

Unfortunately, periodic noise mode is quite cumbersome to use right now. I hope 
to improve this eventually.


Tool to convert Impulse Tracker IT modules to Sega Master System VGMs




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