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This sineCW vst vsti plugin,  acts just like a hardware code practice oscillator.   It is "keyed" by a MIDI MESSAGE via your own MIDI HARDWARE or MIDI INPUT using VIRTUAL MIDI CABLES or a VST HOST with direct connections.   You can adjust the pitch, the RISE and FALL time of this keyed sine wave software CPO.  SineCW also has its own cw audio bandpass filter.  You can adjust both the Center and Q controls to set how narrow or wide you want your own cw audio tones to be - while cleaning up and filtering "out" the cw keyed edge noise.  Please see this ongoing article for more info:
http://qrqcwnet.ning.com/forum/topics/home-brewed-software-code-practice-oscillator-vst-plugin
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How to use TEXT TO MORSE CODE MIDI FILES to practice QRQ CW.  Two advantages for using MIDI FILES -  1.  VARIABLE SPEED playback at the same desired cw pitch.  2.  Very small file size compared to wave or even mp3. 

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A HIGH PERFORMANCE CW LISTENING STATION using free software

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There is some unique audio qrm,  that becomes increasingly problematic as you start to listen to qrq cw above 70 wpm.  Filtering out the lower harmonics that are created by the percussive barrage of these qrqQ cw elements themselves, in spite of the rise/fall time of the waveform, really helps to clean up the qrqQcw audio.

BASS NOTE DRONING of qrq cw above 70 wpm.

Above 70 wpm, in spite of passing the qrq cw audio through a good cw
bandpass filter, a significant amount of bass note droning was heard while listening with headphones.  The impact of each cw element has enough energy by itself, even after going through a narrow cw bandpass audio filter, to energize a bass note droning from the collective strumming effect of the cw elements themselves.  Here are the bass note frequencies vs speed,  that I heard, and measured,  while using my headphones:
70wpm      ~       31 hertz
80wpm      ~       35 hertz
90wpm      ~       39 hertz
100wpm      ~      43 hertz
110wpm      ~      47 hertz
120wpm      ~      51 hertz
130wpm      ~      55 hertz
140wpm      ~      59 hertz
150wpm      ~      63 hertz

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Here is one of the best variable speed qrq cw audio players I have used for qrq cw copy training while listening to a high speed morse code audio file -  MIXXX 
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