COMPACT HELIUM SPEECH UNSCRAMBLER USING CHARGE TRANSFER DEVICES
Dr M.A. Jack* Dr A.D. Milne, Dr L.E. V i r r . A
ABSTRACT
This paper describes the p r i nc ip l e , design and performance of a compact helium speech unscrambler using charge t ransfer devices. Helium speech is the term commonly used for the d is tor ted speech uttered by deep-sea divers breathing in an atmosphere which consists of a mixture of helium and oxygen gases. The unscrambler considered here operates in p i tch synchronism with the input helium speech waveform and uses time-expansion signal processing to achieve a considerable improvement in the i n t e l l i g i b i l i t y of the speech s igna l .
INTRODUCTION
Deep sea divers breath a mixture of helium and oxygen gases in order to overcome the tox ic and narcot ic problems produced when breathing high pressure a i r . However, speech produced in such a high pressure helium/ oxygen mixture suffers a unique and severe form of d i s to r t i on known as 'helium speech 1. The main features of the helium speech d is to r t i on can be summarised as:
* A l inear s h i f t in vowel formant frequencies {1,2} * Minimal change in fundamental frequency (p i tch) {3} * A non-l inear s h i f t in low frequency formants {2} * Minimal change in formant bandwidths {4} * Reduction of unvoiced sound in tens i t i es {4}
Present helium speech "unscrambler" designs take into account only the f i r s t two of these properties {4 ,5 } . In these systems, the d i s t o r t ions present in the helium speech waveform are reduced by signal processing techniques based on spectrum compression - by means of waveform time expansion - in p i tch synchronism with each p i tch period {5} . Such signal processing requires electronic storage and current systems gene ra l l y employ d i g i t a l memory for th is funct ion.
The unscrambler design considered here uses s imi lar time expansion techniques but employs analogue charge t ransfer devices fo r wave storage with CMOS d i g i t a l c i r c u i t r y fo r control functions and potent ia l rea l isa t ion as a single integrated c i r c u i t . Min ia tur isat ion of the system would permit the unscrambler to be carr ied by the d iver , leading to improvements in through-water, d iver - to -d iver communication systems.
Dr M.A. Jack, E.E. Dept., University of Edinburgh, EDINBURGH. Dr A.D. Milne, Wolfson Microelectronics I n s t i t u t e , EDINBURGH. Dr L.E. V i r r , Admiralty Marine Tech. Est . , PORTSMOUTH.
376.
PRINCIPLE OF OPERATION
Figure 1(a) shows the basic form of a voiced helium speech waveform where the amplitude peaks correspond to the s t a r t of the p i tch i n t e r va l s . As indicated in the i n t roduc t i on , th i s p i tch in te rva l which is determined by muscular propert ies of the larynx, shows minimal change from normal a i r to high pressure helium/oxygen mixture. Here the helium speech waveform suf fers a s h i f t in (vowel) formant f r e q uencies and the rate of decay of the i n t e r - p i t c h waveform is corresponding ly more rapid than fo r normal speech. The unscrambler technique considered here consists of s tor ing those sections of the i n t e r - p i t c h waveform Which contain useful in format ion, f igure 1(b) and subsequently time-expanding th i s stored waveform to the general form expected in normal ( a i r ) speech, f igure 1(c ) . Figure 1(c) shows that in the long term there is no time-base change (the p i tch in te rva ls on input and output remain equal) however, in the short term, between p i t ch peaks, the time-base is expanded. The discarded sections of input waveform cause l i t t l e degredation to the speech i n t e l l i g i b i l i t y .
The maximum durat ion of the stored segment is governed by the maximum expected p i tch fo r the input helium speech. The design considered here stores a segment of durat ion 3ms corresponding to a maximum voiced fundamental frequency of 330 Hz.
The degree of i n te r - p i tch time-base expansion is governed by the spec i f i c helium/oxygen mixture being used by the speaker. However, the maximum required time-base expansion is of the order of 3 : 1 . Thus, in order to el iminate loss of any section of input signal which contains meaningful in format ion, four para l le l storage channels are required such that one channel is always avai lab le to store the s igna l . More than one channel may be producing an output at any one t ime, a feature which is acceptable since in normal speech, the sounds produced during succesive p i tch in te rva ls tend to superimpose.
SYSTEM DESIGN
Figure 2 shows a schematic block diagram of the compact helium speech unscrambler considered here. The helium speech input is fed to a high-gain p re-ampl i f ie r which incorporates high frequency pre-emphasis to compensate fo r the rad ia t ion losses produced at the mouth of a speaker in helium. The pre-ampl i f ie r also incorporates an AGC f a c i l i t y (30 db) The pre-ampl i f ie r output enters the analogue delay l ines (Reticon SAD 512), each of which is capable of s tor ing N =256 samples of the i n put waveform. As indicated prev ious ly , four storage channels are requi r ed to permit a maximum time-expansion of 3:1 without loss of information bearing s igna l .
The pre-ampl i f ie r output fu r ther appears at the p i tch detector . In view of the reduced envelope decay times of the speech waveform in in hel ium, a simple peak detector c i r c u i t can be successful ly employed fo r p i tch synchronisat ion. For unvoiced sounds, characterised by a no ise - l i ke waveform, the p i tch detector operates cont inuously.
The cycle of operation of the process is as fo l lows. The output from the p i tch detector indicates the s t a r t of a p i tch per iod. The
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input helium speech - of bandwidth up to 16 kHz - ts read into one of the four channels at a clock rate of 85 KHz f o r an in terval of I - 3mS. At the end of th is 3 ms period, the clock frequency f o r th i s channel is reduced by a fac tor which is dependent on the helium/oxygen mixture being used The stored signal is read out at a lower ra te , with an attendent bandwidth compression to the normal 3-4 KHz speech bandwidth. On detection of the s ta r t of the subsequent pi tch i n t e r v a l , the clock and signal multiplexers change over and the next channel reads in the helium speech.
PERFORMANCE
/ Q C l 9 UrSi 3 {?luS5?w s a s e c t l 0 n o f t h e input helium speech waveform (85 ra depth) with f igure 3(b) indicat ing the pi tch detector output def ining a 3 ms storage in terval from the pi tch peak. The loss of signal in the discarded part of each pi tch period causes minimal degradation in the output speech. The pi tch period in f iqure 3 i s seen to be of the order of 7 ms.
A *uf1fl?r? ?i a ) s h o w s a n o t h e r section of helium speech waveform (85 m
depth) with the corresponding time expanded output from one of the four channels. Here the i n t e r - pi tch time-base expansion is seen to be of the order of 2 : 1 .
Figure 5 compares the input helium speech waveform (85 m depth), Mgure 5(a) with the corresponding unscrambled output, Figure 5(b). Note that the pi tch in terval (approx 6 ms) remains unchanged although the in t e r pi tch time-base expansion is of the order of 2 : 1 .
Subjective l i s t en ing tests of th i s equipment were performed by a panel of non-expert ( i n both a phonetic and a diving sense) l is teners The test material consists of a section of helium speech recorded at 8b m depth which comprised 15 individual phases spoken once only The i n t e l l i g i b i l i t y to the "raw" helium speech was zero. Figure 6 indicates the measured i n t e l l i g i b i l i t y of the unscrambler output - each l i s tener * w r i t i n g down each phrase as he understood i t . Clearly the mean performance of the l is teners improved as they became accustomed to the unscrambled output and i n t e l l i g i b i l i t y f igures of 90% are indicated Table 1 compares the system parameters of th is compact analogue unscrambler with current d i g i t a l real isat ions and emphasises the engineering advantages of fe red by the charge t ransfer device design.
CONCLUSIONS
The helium speech unscramblar design reported here o f f e r s d i s t i n c t engineering and operational advantages in comparison to current ly a v a i l able ( d i g i t a l ) real isat ions and the use of analogue charge t ransfer devi technology with associated CMOS logic o f f e r s a design which is suited to rea l i sa t ion as a single integrated c i r c u i t .
ACKNOWLEDGEMENTS
The authors wish to acknowledge helpful contact with Prof H. H o l l i e n ,
c o n t r a ^ U ' S * A ' ™ S W ° r k h 3 S b e e n c a m ' e d o u t u n d e r M 0 D
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REFERENCES
1 Bei 1 , R.G. 'Frequency analysis of vowel sounds produced in a helium r i ch atmosphere1 J.A.S.A. Vol . 34. 1962 pp 347-349.
2. Holywell ,K. & Harvey, G. 'Helium speech' J.A.S.A. Vol 36. 1964 pp 210-211.
3. HollteruH, et a l . 'Voice fundamental frequency levels of divers in helium oxygen speaking environments' Biomedical Research Col. 4. No 2, 1977 pp 199-207.
4 Giordano, T.A. et al 'Helium speech unscramblers - a c r i t i c a l review of the s t a t e - o f - t h e - a r t 1 Trans. IEEE Vol. AU21. No 5. 1973. pp 436 - 444.
5. G i l l , J.S. et a l , B r i t i s h patent no. 1321313, June 1970.
FEATURE ANALOGUE DIGITAL
Input bandwidth 16 kHz 12 kHz
Power consumption 150 mW 1200 mW
Physical size 10 cb. i n . 1160 cb. i n .
Weight 0.5 lb 17 lb
Standby power 30 hrs 10 hrs
Data frame (T) 3 ms 2.5 ms
Table 1 Comparison of system parameters.
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PITCH INTERVAL
(a) Helium speech
n 3ms
(b) Stored waveform
(c) Unscrambled output
Figure 1 Pr inc ip le of helium speech unscrambler operation
H E L I U M S P E E C H INPUT
P E A K D t l E C T
. P R E - A M P
CHANNEL S E L E C T
00)
INPUT C L O C K
OUTPUT C L O C K /
C L U C K M U L T I P L E X
CHANNEL 1
CHANNEL I
CHANNEL 3
CHANNEL 4
OUTPUT
Figure 2 System block diagram.
380 .
Figure 3 P i t ch d e t e c t o r o p e r a t i o n . F igure 4 Channel o u t p u t .
(a) Inpu t he l ium speech 500mV/div. (a) I n p u t he l ium speech 500mV/div.
(b) Peak d e t e c t o r ou tpu t 5 V / d i v (b) Channel ou tpu t 500mV/div.
H o r i z o n t a l scale l u s / d i v . H o r i z o n t a l scale 1 / j s /d iv .
(b)
Figure 5 Unscrambler o u t p u t .
(a) I n p u t he l ium speech 500mV/div.
(b ) F ina l ou tpu t 500mV/div.
H o r i z o n t a l sca le 2 u s / d i v .
intelligibility ( / )
Figure 6 I n t e l l i g i b i l i t y t e s t r e s u l t s .
k —
I phrase " 1 3 5 7 9 11 13 15
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