Future Research 121

As   stated   above,   the   primary   focus   of   this   research   was   to   expand   on   existing   methods   of   voice   source   modelling   in   order   to   explore   more   natural   sounding   voice  synthesis  techniques.  There  are  many  potential  avenues  towards  achieving   this   goal,   many   of   which   fall   outside   of   the   scope   of   this   project,   or   were   considered  only  after  the  work  described  herein  was  completed.  The  following   section  summarises  the  main  considerations  for  future  work  in  this  area.  

5.3.1  Issues  within  LF-­‐Model  implementation    

Some   extensions   to   the   LF-­‐model   that   were   included   in   this   application   were   either   intended   as   a   proof   of   concept,   or   found   to   be   far   more   complex   than   originally   considered.   Because   of   this,   fairly   rudimentary   implementations   of   these  extensions  were  used,  leaving  room  for  expansion  on  pre-­‐existing  methods   and   ideas.   One   such   area   for   improvement   is   the   f0-­‐dependent   voice   type   modulation  technique.  This  method  would  benefit  from  further  primary  research   into  voice  type  modulation  in  natural  speech.  As  no  voice  source  recordings  were   made   specifically   for   this   research,   voice   type   information   such   as   pitch   range   and  timing  parameters  was  taken  from  secondary  sources.  It  would  be  beneficial   to  probe  the  effects  of  pitch  on  voice  source  through  inverse  filtering  or  similar   techniques,   in   order   to   ascertain   the   ranges   in   which   certain   voice   types   are   present.  This  would  also  go  towards  defining  the  behaviour  of  the  voice  source   as   it   modulates   between   voice   types,   so   that   a   more   complex   technique   than   linear   interpolation   could   be   employed   at   the   thresholds   between   voice   types.  

This   could   also   be   used   to   develop   amplitude-­‐dependent   voice   source   modulation.  

Two  parameters  for  ‘vocal  tension’  were  made  available  to  the  user:  SQ  and  ta.    

Both  of  these  were  shown  to  have  a  noticeable  effect  on  the  spectral  qualities  for   vocal   tension   as   outlined   in   [14].   However,   the   use   of   vocal   tension   in   natural   speech  is  clearly  a  complex  issue,  as  it  relates  to  perceived  emotional  content  of   speech,   as   well   as   being   affected   by   factors   such   pitch,   amplitude   and   vocal   pathology.   Further   research   into   vocal   tension,   perhaps   through   detailed   analysis   of   inverse-­‐filtered   speech   waveforms,   would   allow   a   more   complex,   rule-­‐based   vocal   tension   parameter   to   be   incorporated   within   the   model.   At   present,  the  inclusion  or  exclusion  of  vocal  tension  in  the  voice  source  model  is   defined  only  by  the  user,  whereas  a  thoroughly  verified  rule-­‐based  system  would   prevent  inappropriate  use  of  a  ‘tense’  or  ‘relaxed’  voice  source.    

5.3.2  Further  Extensions  to  the  Voice  Source  Model  

This   implementation   of   the   voice   source   model   focused   purely   on   voice   types   that  could  be  modeled  as  a  single  pitch-­‐period  of  an  LF-­‐model  waveform  variant.   This  excludes  other  voice  types  such  as  whisper  and  harsh  voice  [15]  as  well  as   accurate   vocal   fry   representation,   whose   pitch   period   is   in   fact   made   up   of   several  weaker  LF  pulses  following  the  initial  pulse  (fig.  5.1).  

Figure  5.1  –  One  pitch  period  of  an  idealised  vocal  fry  waveform  displaying  two   secondary  pulses  

It  is  clear  that  the  current  implementation  does  not  cover  every  possible  voice   type,   so   it   is   an   intuitive   extension   to   add   to   the   existing   software.   This   may   involve  the  inclusion  of  a  wavetable  synthesis  method  resembling  that  described   earlier.  A  stable  wavetable  synthesis  approach  would  allow  for  any  waveform  to   be   stored   and   resynthesised,   as   opposed   to   only   those   which   can   be   modeled   using  the  LF-­‐equation.    

It  has  been  acknowledged  since  the  early  days  of  voice  source  research  that  voice   source  qualities  can  be  altered  by  supraglottal  factors  such  as  vowel  choice  and   other   articulations   [7].   A   worthwhile   extension   to   the   existing   voice   source   model   would   be   the   inclusion   of   these   dependent   variations   in   voice   quality.   Again,  this  would  benefit  from  primary  research  including  analytical  recordings   of  the  voice  source  under  a  variety  of  conditions.    

5.3.3  Multi-­‐touch,  Gestural  User  Interfaces  

Whilst   skeuomorphic   interfaces   made   up   of   on-­‐screen   buttons   and   sliders,   usually  with  a  one-­‐to-­‐one  relationship  between  control  and  parameters  affected,   are  the  most  immediately  intuitive  to  use,  it  has  been  suggested  many  times  in   HCI-­‐related  research  that  this  approach  does  not  directly  lend  itself  to  intuitive   performance   in   real-­‐time   [80].   With   a   subject   as   complex   as   the   vocal   system,   providing  individual  and  independent  user  control  of  every  possible  parameter   would  not  make  sense  in  terms  of  modelling  voice  source  behaviour  in  real  time.   An   alternative   approach   would   be   to   develop   a   gestural   touch-­‐screen   interface   that  departs  from  the  instantly  recognisable  skeuomorphic  layout  in  favour  of  a   highly  cross-­‐coupled,  expressive  interface  such  as  that  presented  in  figure  5.3.  In   formant   synthesis,   this   approach   has   already   proved   effective   with   the   HandSynth  [81]  software  and  hardware  which  allows  a  variety  of  vowels,  pitch   and   amplitude   to   be   controlled   using   a   single   touchscreen   interface   and   a   pressure-­‐sensitive  stylus  (fig.  5.2).  This  method  could  conceivably  be  adapted  to   control  an  array  of  voice  source  features  concurrently.  

Figure  5.2  –  HandSynth  stylus  operated  touchscreen  interface  for  articulatory   synthesiser  [image  taken  from  [81]]  

Figure  5.3  –  Proposed  design  for  multitouch  interface.  Position  on  x/y  plane  could   control  pitch  and  amplitude,  with  multitouch  gestures  such  as  ‘pinch’  (i.e.  distance  

between  two  touch  points)  could  be  used  to  control  parameters  such  as  vocal   tension  

5.3.4  Implementation  of  Dynamic  Impedance  Mapping  within  the  2D  DWM  

In  [30],  a  method  for  simulating  vowel  articulations  with  the  2D  DWM  vocal  tract   model   is   described.   This   allows   for   multiple   vowels   to   be   synthesised   in   software,   with   minimal   digital   distortion   present.   Other   vocal   artefacts   such   as   dipthongs  and  some  consonants  can  also  be  modeled  using  this  technique.  As  a   proof   of   concept,   a   single   static   vowel   was   modeled   using   the   same   method,   however   a   full   implementation   of   the   dynamic   impedance   mapping   feature   would  allow  for  multiple  vowels  and  articulations  to  be  used  with  the  extend  LF-­‐ model   software.   This   would   not   only   provide   a   more   fully-­‐fledgedd   voice   synthesis  software  application,  but  would  also  allow  for  further  extensions  to  be   implemented   such   as   the   vowel-­‐dependent   voice   source   factors   mentioned   in   section  5.3.2.