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Author:Jokinen, Emma
Title:Adaptive post-filtering of speech in mobile communications
Puheen adaptiivinen jälkisuodatus mobiililaitteissa
Publication type:Master's thesis
Publication year:2010
Pages:x + 55 + [6]      Language:   eng
Department/School:Elektroniikan, tietoliikenteen ja automaation tiedekunta
Main subject:Akustiikka ja äänenkäsittelytekniikka   (S-89)
Supervisor:Alku, Paavo
Instructor:
Electronic version URL: http://urn.fi/URN:NBN:fi:aalto-201203131510
OEVS:
Electronic archive copy is available via Aalto Thesis Database.
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Location:P1 Ark Aalto  807   | Archive
Keywords:speech enhancement
post-filtering
fomant
puheen ehostus
jälkisuodatus
formantti
Abstract (eng): Speech enhancement is needed to improve the quality and intelligibility of speech degraded by noise.
In this thesis, a post-filtering approach for the mobile communication environment was designed.
The purpose of this post-processing scheme was to enhance certain frequency regions of speech, so that when it was degraded with a very high level of noise, the speech could still be understood.

The post-processing worked by locating the formants of a voiced speech frame by extracting the peaks of the LP spectrum.
After this, the first formant was attenuated and the second one enhanced.
The idea was to move energy to higher frequencies where the energy level of the noise was lower.
The coefficients of the formant filter were optimized with informal listening tests, and the possible tilt of the filter was compensated with a first order low-pass filter.
The performance of the post-processing algorithm was studied by analyzing its effects on different voiced sounds and by comparing the filter to other post-filters.

It was concluded that the post-processing worked as intended and improved the intelligibility of speech.
Some unexpected behavior, such as shifted formants, was also encountered and needs to be further studied.
The advantages of this approach are its more adaptive and tunable structure compared to the other methods used for post-processing in high noise levels.
Abstract (fin): Puheen ehostusta tarvitaan kohinaisen puheen laadun ja ymmärrettävyyden parantamisessa.
Tässä työssä suunniteltiin matkapuhelimiin tarkoitettu jälkisuodatusalgoritmi.
Tämän jälkiprosessoinnin tarkoituksena oli korostaa joitakin taajuusalueita puheessa siten, että sen ymmärtäminen olisi edelleen mahdollista hyvin kovassa kohinassa.
Jälkiprosessoinnin alussa soinnillisen puhekehyksen formanttitaajuudet haettiin tarkastelemalla sen LP-spektrissä olevia piikkejä.
Tämän jälkeen ensimmäistä löydettyä formanttia vaimennettiin ja toista vahvistettiin.
Ideana oli siirtää energiaa korkeammille taajuuksille, jossa kohinan energiataso olisi matalampi.

Formanttisuotimen kertoimet optimoitiin kuuntelukokeen avulla ja sen mahdollinen kallistus kompensoitiin ensimmäisen asteen alipäästösuotimella.
Lopullisen jälkisuotimen suorituskykyä tarkasteltiin sekä tutkimalla sen vaikutusta erilaisiin soinnillisiin äänteisiin että vertailemalla suodinta muihin jälkisuotimiin.
Saatujen tulosten perusteella voitiin päätellä, että toteutettu menetelmä toimi halutulla tavalla ja onnistui parantamaan puheen ymmärrettävyyttä.
Tarkasteluissa tuli kuitenkin ilmi myös yllättäviä piirteitä, kuten formanttien siirtymisiä, jotka vaativat lisätutkimusta.
Verrattuna muihin jälkisuodatussysteemeihin, jotka on suunniteltu toimimaan kovassa kohinassa, työssä kehitetyn algoritmin etuna ovat sen adaptiivisuus ja säädettävyys.
ED:2010-08-20
INSSI record number: 40198
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