22-08-2014, 11:46 AM
Impedanc Glotgraphy
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Abstract
Impedanc Glot graphy is no va i e m asure nt of he tim varonfthe dgrof
conta be w n the vibrat ng vocal f ds uring vo ce pr duction. The asp ct of n act being
measurd icaleth vocalfd ntcare(VFCA). Tomeasur VFCA,the dvicuse ald
impedanc glot raph. T e d vic s al o c ed l trog aph r l yngo raph. T e princ le
of peration f devic , the wav form btained, lgorithm for det mina o f pitch er od is
discu e . EG waveform a i us vo ce qualit es, drawb cks in EG a d v rious n e
pres nt are d sc ibed. Concept f mul ichan el EG and p licat ons f el ctrog aphy re
discu e . Various c m er ial qu pment av il b e ar comp ed alongwith e quipm nt
devlop byITBoma.
Introducin
Lungs, vocaltr, ndayxre thmainorgs elatdwih genratio fsund.The lungsare
the sourc f ai low. The vocal tr is an c ousti enclosure and cts a ou tic f l er shaping the
spectrum of the g n rated soun . The sourc f most pe ch o urs in the larynx. Ther a two f lds o
the musc lar bundle k own as voc l h rds in e th larynx. Thes vocal h rds ob truc he airflow
from the lungs a d pro uce a dible v rations hat m ke h sp c . The m c anism of gen ratio f
sound i k own as phon ti . The vibrat ons c i t of hre p as n mely conta ph se, ar tion
phase ndophase [1].
Princple ofOratin ofDevic
A hig frequ ncy el trica u ent of smal vo t ge and mperag ( hysiol g ca y s fe) pas
betw n o el ctrodes itua ed on the surface o th r a e l v of the yroid ca t l ge. Th basic
configurat on f he d vic s depict n F g.1 The l ctrodes a m de of c p er, silver o g ld. They
have t form ings or ectangl s covering a e r ngi f om 3 c 2
to 9 cm
2
. A third el ctrode is
often us d a ref nc or impedanc m sure nt . I may be d signe a s p r te l c rode as
ring el ctrode ncir l g each of t e wo h r el ct odes. Th lectrod s a e u l y mounted a
flexib and whose l ngth may be djust o h ld t e c rod s in a te dy posit n a d to s il a ow
the subj ct o mf rtably speak nd breath n ural y. Someti s h el ctrodes a mounted a sm l
holder whic s pre d ag inst he roat by h nd. A signal e r to sup lie th c rodes with an AC
sinu o dal cur ent of a ltern i g frequ ncy s al r ngi f om 30 kHz to 5 MHz. This frequ ncy is
suf ic ently hig , so tha e cur nt ap ci n vely b pas e th l s condu tive sk n layer withou e
use of ad it onal c du tive past . The g n rato m y produce nsta vol ge r consti u e a const
curent source. Thsuplied curntis dfernto achprtiula devic,but snorgethan sverl
milapers. Thvoltage bwnthlecrods penthisu mpedanc butheypical vueis
about 0.5 V In ac ord n e a pow r dis pat on f ly sev ral mic owat s c ur at he l v the
subject's voalfds [3].
The s n i g el ctrode s th cur ent as i p e throug e skin a d the roa . The
perc ntage of mplitude mo lati n of the r c ived s gnal ref cts he p rc ntage ch n i t s ue
impedanc i the cur nt's pa h. T e r c ived s gnal is then d mo ulated by a sign l det c or i u t. The
typical s gn -to ise rat o f he d mo ulat r is about 40 dB. The d mo ulated w v form is then A/D
convertd ansored inacomputer [3].
Drawbcks inEG
At pres nt, EG s are b ing used at m ny res a ch l borat ies, but exc p for udimentary
ap lic t ons uch a t e m asure nt of v cal period, the c nique has not be ac pt d for gen al
clin a use. Th r a e b sical y thre ason th el c rog t aphy is not u ed mor c nly. First, ther a m ny subject for wh m t e pr viously av i ble com r ial un ts ei h r y eld no utp r one
tha is very nois a d/or ve y dif er nt f om v cal fo d c nta re . Mo sign f ca tly, in case of n isy
or dist e wav forms, the us r often has o clear ind cat on h e wav form is not valid. Extrem ly
noisy waveforms, tha is, w veforms tha re l tiv y smal in p tude an show a fine-grad
ir egula ity h d f ers om cy le to cy le, an g er l y be consider ac u te as r p e nta io f
the VFCA.onversly, verstong, ise-frwavoms franoml viceanusly betrusd [7].
2. Conclusion
Electrog aphy s e tablished t lf as v lu b e m thod f r evalu ting lary e
behaviour. In comparis n to her m t ods f glot raphy, el ctrog aphy l ows f r a bet r
rep s nta io f the clos d an closi g phase of v cal fo d m ve nt, special y of the v r ical ont c
are. Phot gl raphy se m particul y adv nt geous a f r s the d script on f he op n hase i
con er d. The EG is uper o t al her m t ods in tha s not u c mfortable o sp aker s it
complety no-ivase (txrnoifluec atonhericulaton dpruction fsud)
At pres nt, EG s are b ing used at m ny res a ch l borat ies, but exc p for udimentary
ap lic t ons uch a t e m asure nt of v cal period, the c nique has not be ac pt d for gen al
clina use.
Multichan el t chniques can be us d to pr duce an EG tha c n verify the id l ty of i s wn
outp waveform as n i d cator f he tim pa erni g of v cal fo d c nta d c n yield a sign l tha
helps t u er p o ly sit on he l ctrodes an /or t ack vertical move nts of he larynx du i g
voiced sp h or sing . The us of impr ved EG units corp ating such te niqu s ho ld make
pos ible a hig er l v of c n ide c n the r sult of res a ch into e us f lectrog aphy in t e
study ofviceprodutin, voceanlysi, andvoice tranigd vocalpedgoy.