You can have sustain added to the Strings with the 'Left Hand' Voice with the "Hold" button activated.  This will allow you to have the Left Hand Voice 'Hold' or, for another word, 'Sustain' indefinitly until you press a Right Hand Voice Note or Chord.  The Right Hand Voices such as Strings, etc. will Sustain after the hand[s] are released from the Keys and the Sustain Pedal continues to be pressed; for roughly 4 seconds with a gradual fadeout from second 1 to total fade out at second 4 {approx.}  Limitation??  I'm not sure.  Feature???  Possibly.  Because with the Left Hand "Hold" feature you are essentially getting what you want, ie., 'String sustain {indefinite} after you release the Sustain Pedal then you go up Octave[s] hit some Keys and then the String sustain from the Left Hand Voice is terminated whilst your Right Hand Voice[s] will Sustain with the press of the Sustain Pedal and will have the Sustain fade-out when the Pedal is released.  With good coordination with the Left Hand Voice and Right Hand Voice[s] whilst playing with 'both' hands and the "Split Point" of the Keyboard set optimally for your Style of playing you could really achieve some great sustained full sounding Strings.  Another often overlooked feature is the 'Harmony' button used in conjunction with Sustain and also with the Left Hand Voice 'Hold' function.  You might be surprised at just how good your Strings can sound.  Although the 'Harmony' will only work when the Style Accompaniment section of the Keyboard is playing a Chord (left side of Style Accompaniment Split Point) first.  You can also get it to work by playing a left hand 'Note' "first" (then) playing a right Hand melody Note and or Chord.  PS:  This info is what is produced when the Chord Fingering in the Function Settings is set to Full Keyboard mode.  Other fingering modes may produce different results.
Best regards,
Mike
PS: I should mention some Keyboard effects on the Tyros such as an 'Octave' "effect" when playing certain Voices, eg., 'Grand Piano', does NOT require left hand chords or Notes to be played first before the Harmony effect is added.  You would be able to get the Harmony effect by pressing the Harmony button and simply play right hand melody notes and or chords.  There would be no need to play Left hand chords or notes first to get the 'Octave' "effect".  There are some other Voice effects other than the 'Octave' effect that also do NOT require left hand Chords or Notes to be played first to get the Harmony effect added to the Voice[s] depending on the Voice.  Others may be able to add their opinion and knowledge to this topic also.
[This message has been edited by keybplayer (edited 10-30-2003).]
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Yamaha Genos, Mackie HR824 MKII Studio Monitors, Mackie 1202 VLZ Pro Mixer (made in USA), Cakewalk Sonar Platinum, Shure SM58 vocal mic.