OpenEuphoria

Re: alogorithm to convert MIDI to Wav

• Posted by Raghavendra M L <raghavendra.ml at BFLSOFTWARE.COM> Mar 29, 2000
• 804 views

David ,

yes .. it was a very comprehensive answer to my question

Thanks a lot ,
Raghu

David Cuny wrote:

> Raghavendra M L wrote:
>
> > I was searching for an algorithm to convert a
> > MIDI file to Wav file. can any of you can help
> > me with that ..
>
> This isn't entirely trivial. MIDI specifies information about the state of a
> musical keyboard, such as:
>
>    instrument number 42 was selected on channel 7
>    the c3 key for channel 2 was hit at a velocity of 32
>    the expression pedal 2 for channel number 5 was pressed
>    the pitch bend wheel for channel 2 was moved 12 units
>    the c3 key for channel 12 was released
>
> The General MIDI (GM) specifies a specific mapping of instruments. For
> example, instrument 1 is a Grand Piano, instrument 9 is a Celesta, etc. So
> to create a waveform, the general algorithm would be:
>
>    what waveform would instrument X generate at pitch Y?
>
> Again, note that there's no information about an instrument specified in
> MIDI - just the state of a musical keyboard's keys, pedals and levers. It's
> up to the MIDI player to supply that data.
>
> This is typically solved by storing digital samples (looped waves) in a
> lookup wavetable. Since the timbre of an instrument changes over it's range
> (that is, the spectral composition of a low note is much different for a low
> note than a high note), an instrument is typically multisampled. That is,
> several waves are captured over it's range. Since it takes a lot of storage
> to capture a sample, it's not practical to sample each and every pitch of
> the instrument. Instead, most instruments have samples spread across the
> instrument, from a couple notes to a couple octaves. If the pitch D4 is
> selected and the closest sample is Bb3, it's the job of the software to
> scale the pitch of the Bb3 sample to D4.
>
> Scaling a sample is done by writing out every nth sample point. For example,
> if you write out every 2nd sample point of a sample, you effectively double
> it's pitch, and raise it by an octave. The best samplers will interpolate
> missing sample points from the data, typically with a quartic spline.
>
> In addition to instruments being sampled over the pitch range, they are
> often sampled over the velocity range, since playing an instrument harder
> will also change the timbre, in addition to changing the volume.
>
> The sample player typically applies some sort of ADSR envelope
> (attack/decay/sustain/release) to the waveform to make it more natural
> sounding. For example, one instrument might fade away, while another might
> just end abruptly when a MIDI note off event is received.
>
> For more information on converting samples to sound, you might want to look
> at MOD players. Pete coded on a while back in assembly for Euphoria, but he
> might still have the 'pure' Euphoria code.
>
> You can find free collections of General MIDI samples on the internet in
> SoundFont, DLS and Inst formats, ranging in size from 512K all the way up to
> 80Meg. These files are typically a pain to parse; you can find information
> about them in Wotsit's File Formats (do a Web search).
>
> Hope this answered your question.
>
> -- David Cuny