BSTJ 60: 9. November 1981: Simultaneous Transmission of Speech and Data Using Code-Breaking Techniques. (Steele, R. ; Vitello, D.)

11.5 MB·English

Simultaneous Transmlasion of Speech and
Data Using Code-Breaking
Techniques

fy R. STEELE end D. VITELLO

onus racalves Apr 19, 1961)

A egntem whereby speech is used as a data carrier ix proposed. The
speech sampled ui 8 hia, is divided into Blocks of N samplen, and
provided the correlation confisient and mean square vatur of the
amples exceed ayatem threshold, data i allowed tobe transmitted.
{he data ina login, the samples are sont without modification;
Drowever, if logical 1 is provnt, frequency inversion scrambling of
the samples oceure, The reveiner performs the inverse process 0
recover both the apeesh and data, Data rates of 700 bs ware achieved
Iuthout date errors a7 speech distortion ba en Edeal chanel. The
effect of eitte background and channel noise were investigated,
Gnd the etem seas shown Io operate ae 126 b/s with mo data errora
(then the additive noise wos as high aa 10dB below the mean square
tain of the speech signal

1. wermopucTION

‘There are numerous scheres"* for analog scrambling of speech
igual, bt they all require a srmlling key. Wor example, we may
Sample the apeech aba rate it exceas of ita Nyquist rate, parcel the
Saimplea into locks, and rearrange che blocks rine tn «wansmission.
"This rearrangement of the blocks bretls wp the mhythm inthe speech
‘making it dificult for an eaveclropper (9 comprehend the cooverea-
thon. ‘The shufing of he Black posivions ix done under the auspives of
the scrambling key, and provided tho receiver knows this key and,
hence the descramtbling Key, the blocks of speech can be correctly re-
positioned and Taade intelligible lo he desied veipier.

Te is ot awe purpose to describe the mumerous scrambling tach:
rigues, bu rather Co suggest method whereby epevch and data can
be ansmitad sinullanenusly aver the chanel ly using scrambling

2081
strategies. 'The principle is very simp, The serambling key hesomes
the data to be tranamitted. The roveiver adopts the role of exes
Dear. Bvery time the receiver corrouily guesses the key and brews
tho vod it recovers both the apeech and the date For the schore to
Jnnve any significance, the receiver must break the cvde success at
early every acempt, Therefore, we minbeelectacrambling heya which
le enay 10 bronk, and this means that me ave not aiming for peech
paivacy although a degree of privacy may be achieved as a by:
product). The sermhling procersi, therfore acatalyer which enables
the data ta he trantmited.

"AL Gira sight, it might appear that we azo gutting something for
rcching, With care we can arrange forthe data to be ransmittd at
fegligibie errr vate, the specch faithfully recovered, wad a sna
bandwidth expansion ofthe transmittod signal compared to the argi-
ful speech. ‘These rewards are derived from the inherent redundancy
{nthe mpecch signal Indeed, we emphases thal. the method wil work
‘sth any signal that corrcltive aie uch a8 speech, alien,
facsimile, and analog-plant concol signals, ike pressure and temper.
ature vazitions, ete

I. SIMULTANEOUS SPEECH AND DATA TRANSMISSION USING

FREQUENCY INVERSION SCRAMBLING.

‘Aa adomonstration of the concep we describe the transmistion of
dsc ning the simplest of acrambling methods, frequency inversion.
In hie method, apeech, band-limited to JA KH, is sampled at 8 KHz
tnd 1 samples are praceaed atime. Tt ut represent these samples

Srey tists oti a
“To invert the frequency components associated with these samples,
al we need lu co ie (o aller the polarity of every other sample,
namely,

a e
Noven,

Th frequency inversion acrarmbling (Ps), aqueres So would alivays
be cunamitted, btn our sehome, sequence Seis ws whan we decide
‘oman data and, further, che dacn it @ logical 1, Obeerve only one
bit per speech samples ie ceanseitt

"To minimize the number of bite recive in error, we proceed an
fotles "he ealeulation

®

20e2 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1081
fs made on the original speech soquenee S; and called hore the
‘corrcation coefficient, and the mean square value

“i

‘nthe Hock ofypeech aamplesiaalzo found. Notice hat the corelation
‘mafficient py ofthe scrambled sequencs Sis ~p. Figure 1 shows the
block diagram of the system, Mean square value of and conelation
‘nefficent pare vompared wich system threshold parameters T and X
in comparators COMP Land COMP 2, reqpoutvely. Pameters Tand
may be oelocted wh that 02> 7 and o > K generally impli the
absence of unvoiced speech and silence, ansuming Uhre ano addkive
beckaround noise. This strategy nit in veducing the number of r=
celved bit erore when traralting through noisy channels, Later we
tl ive decile of how T and Kayo aeleccod.
‘Data is only tranumitied when the Beolean equation

7-6, o

ond

cox [local p= K,
”* LNogical 0fa-= x”

r a] a

[oe be 6

a tr

Ig. drm of he a erin he tng on ot

SPEECH AND DATA TRANSMISSION 2083
The dala eaence In allowed to alec 8 oF Sif eq. (S) i sataed.
‘Thus, ify = 1, the switeh in Hig. 1 is set in position A or B if the data
islogical 0 or 1, spectively be, a soquonce Spis generated according

Sy, data = logical

so (egret ®

Whenever, ithe unsorambled speech. The sequence So is
ppropriaialy Geral au transite as the combined spel ure

da signal

"To luatzate the efor of the imposition of data onthe speech signal,
swe show the wavefors in Fig 2 Tr(a) el (0) of Pgure an arbiter
ferment of spooch and the correnponding craneclted inal containing
Adm for 120 hooks ate show, regpeedvely."The envslopeof the signa
J barely changed, and blocks conveying weras ure nol serambled,
lence, the Lrunsmilled signal is perceived usa distorted version of the
input apevch ineligible ut titan tae Ttener, Asmuller segment
Lu Ove original spect signal, and Ube resulting ened sal Cor
{he logical values ofthe daca shown are displayed, reepectively, ia fe)
‘and (@) of Hig. 2. There are now only 24 blocks and the frequency
inversions are apparcat when the dat isa logical 1

J Heth HTH HEE
te ws

Seite cetera! serene
lvls ef ic dota ageless ft “he Senate ial "

2004 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1961
‘The signal emerging from Ue tranenssion channel i sampled at 8
els to give $5, where a caret (=) above the symbol signifies its
resence at the receiver, In the abeence of channel impairments
Sem Si, the power #3 and correlation coeffcicnl of the soquence Sr
in the block of Vaemples i computed necording to oq. (3) and (8).
‘The operations associated with eq. (S) ure implemented, and the
following processes are performed until a decision is reached
(@) IF 9 sg logical 1, data is sscumed (Co be transmitted of value
logical 0, and $+ = $s the recovered speech sequence,
it) Fis a logical, data may oF may not be present. To determine
whether data b present, every ather sample in Sy i inverted and the
scrambled correlation eoeMicien. es coraputed, Then,

{a) if § remains alogieal 0, i is decided that no data was sent.
‘The recovered upeach suyence is Urerefore, the exginal reelve
soquence Sr

(if ocomes a Topical 1, it is decided thac data is present of
value logical 1, and the recovered speech sequence is the scrambled Sy
Observe that ifthe conditions are not correct for the conveyance of
‘ala, or if a logical 0 is transmitted, the speech is dispatched without
being serail. Only when a logical 1 is tranemitad is serarbling
performed, and this s doe twice, once at che transmitter and once at
the eursiver. Should a data error eceur, the pech atthe eutput ofthe
receiver may be exronsoutly acrembled, The veulting eeoraarapos in
the bloc of length N have a race of 1 KH, and magnitudes double
‘that of the oviginal speech samples

UI PERFORMANCE PARAMETERS FOR DATA TRANSMISSION

From data transmission point of view we are interested in che
Arnel bit vate (rmR) and the total bit error rave (rae). Data
will only he transmitted when y of eq. (3) is @ logical 1, and the
elficeney of che system co transmit data is given by

seta data rate

ine arate
from which

«

‘where {is dhe sinpng ate ofthe sptch signal. tor bita are thowe
bits Kenerated incorrectly at che receiver, and che number of bit rors
per second i the-nprn. Let the measure ofthe deficiency of che system
that result in erroneous data at the output of the receiver be known
‘the data tanamission defcieney,

yx iat ror rate
possible daca ras o

‘Then
ao)
‘ran — nen + mn, ay

where nr isthe conventional biterror rat thet relates to those bite
‘ranamitted that were erroneously received. The term Yui the fale
bit rte that is arcociated with the generation of bits atthe receiver
vwhen none were actually transmitted, and the declaration at the
receiver that ne bits were Lraramued when Chey really were, Repro-
tenting the states when the traninitter dace not tranamit data, and
‘when the receiver dectas that no dala waa trananitied, by the symbol
ind using the logical data symbols of 1 and 0, we are able to
‘onstract Table I which shows all the posible aca-error eanditions,

Let us connder the cate of no adiilive noite to the speech input
signal, and an Weal channel. Tn this case, the flee bits are always a
Togleal 1 and gecur when no daca (1) wax amaitced. This estate A
in Table I Thete erurs occur when the power in the block is above
the threehold, o! = 7, and the correlation pie below ite threshold,
2<K, prohibiting tunsmission of data. Now K isn positive number,
‘nd the bit error wil accor fp te negative having a magnitude Ki, 487,
that Se greter than HAC the receiver, Sy = 8, = logieal 0 and,
Thence, the received sequence is rambled, Because the correlation

Table |—Data eror table and output
‘speech status
coofficient of the scrambled sequence isp, = “6 = +K;, und Ki > K,
‘Fis now a logical 1, and data is deemed (a le prement having a value
logical 1. Thus, the probability ofa false bit being generated is very
low, being the joint probablity that of = T and p =< =.

‘When the speech signal inn a noiy environment, the symbols x1
representing speech in eqs. (1) to (4) are replued by 23 = 5 + mise
‘where it (he noise component and the “above the symbols meuns
noize contamination, The effect of the noise ia to inerease of and
‘decrease p' and ax both of: and” must exceed their thresholds [seo
‘oq ()] for date to be transmitter, the tax decrease. Provided the
‘channel i dos, the TAFR wil depend on the corelaive properties of
the received epeech, and the only zource uf errors derives fromm sae
Avi, THER = FRR

"When clesn speech is used and the chunel is nvsy, the Tam it
‘unaffected, However, the THER incroases with channel noise power
‘because the noize decorrlates the received signal, causing the receiver
‘to sometimes erroneously presume chat no data was ransnitied Thus,
‘eaten D and F apply for this condition, and ws the existence of other
‘raves oovurs with much lower probability, eho received bitrate ix
‘approximately the difference between nau and ran,

‘Dispersive channela alter both the power wd coreelatian of the
covered signal The most common sate ia D which occurs when
B= [RL State C occurs when dhe scrambled speech avvives with «
Coreclation j = K, causing L to be interpresed ne nO. State F ecure
when i <1K | oF! <7, or when boch <|K|undf < 7. The other
nes were found to rarely happen.

1, DATA TRANSMISSION PERFORMANCE

‘The simultaneous speech and dala transmission using frequency
inversion scrambling, D/P, described here, was investigated using
the sentences "Live wires should be kept covered” and “To roach the
fond be noeds much courege”apoien by a male and feral, respec
tively. Th specch Signal was sampled at 8 kil to yield 38912 samples,
‘a number suicendy large ta give 2 good indication of the system's
performance. The amplitude ofthe speech samples was confined
the range excending from ~G000 eo “+6000 arbitrary wns, and the
‘ean square value ofthe samples averaged over both sentences Was
MS, — 109 x 10" or 64 relative to a mean aquare value of unity.
“The time waveforms fr these ewo sentences and an expanded version
of the magnitude ofthese meech samples to give the time variation of
‘the low-level sounds are shown in Big 3

‘Our objectives were co determine how to select X, 7 and N for high
‘ran and low or negligible TARR, and to study how the performance

[SPEECH AND DATA TRANSMISSION 2067
ry

x. 9Tine gin "Liv sould he ap eve" and “Fa each e
sal eel ge a tae Ra epee oie

Aeteriorated in the precenos of addlive nine on the input speech and
on the transmitted sspr/s signal. We assumed that block synchro
‘uation between transmitter and receiver was correctly maintained at
sa Limes

2088 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1901
4.1 Selection of

"The two sentences were processed wequentialy. The speech aemples
‘wore divided into blocks of N samples, where N coald be either 8,15,
2, 64, 128, 0r 256, reeulting in 4554, 1432, 216, 608, 304, and 152 blocks
‘of samples, reepectively. For each value of the probabty density
function (ror) was computed for the correlation coeficent p. and
plotted in Fig 4 "The pars sere found to have similar shapes for N =
16 r0 256, although the shape marginally altered for N= 8. Por smaller
values off, cheve ina translation in the peitin of the Po¥ to lower
values of p. This aziaeo because ofthe definition ofp given by oa. (2)
"The maximum possible value of » for N= 4,8, and 2 ia 0.806, 0.77,
and D8, respectively. We will subsequently show (hal N= 4 isthe
small lock sine of interest inthis transition ayatems therefore,
‘we do not display wows in Fig. for <4

"in the sanr/inaystem, with the threshold T ast nero, the signal
used to tranamit data is the orginal speech signal, for which the curves
in Fig. 4 upnly. However iT > 0, more blocks of apeech are reiocted
for the conveyance of daca, Therefore, mae decreases, andthe resulting
blocks available for data transmacion have wows for the correlation
‘oeficent tha are diferent fom thore in Fig. 4. AU Unie stage, we will
confine the discusion to the case of T'= 0, ie. where TBR has its
highest values and the curves in Fig are relevant

=
7

SPEECH AND DATA TRANSMISSION 2008
Returning to theas curves, we deaw attention to their most negative
correlation coefficient, pn, vues, 28 they ean havea significant effect
‘on the numberof bie error, The variation of gay and the maximum
‘correlation coeticion! ns 8 function of Nie daplayed in Fig. 6. We
Fecal fom our discus in Section IM, tha ip < K,o¥°> 7, no data
istrancmtied, Assining an ideal channel, and given that) = p< K,
‘the ayelom foe into believing logical 1 was temsmicted and abit
for octure Clearly, i Kis selected much thal p< K dose not exit,
then no bit errors are possible over an ideal channel. To avoid bit
errors we arrange for

K>|omals aa

and the choice of K to avoid bit errns aa a function of NV mus,
therefore, be below the curve [tml gs for N = G4, K > O49, For
1-16, pas and pee Both decrease with decreasing Nand for = 4
‘re have to interesting eituation that [pon = om which means that
if inequality eq, (12) in ated no data wil be Wansmitted as > K
fannot exist The value, A= 4 therefore, wav the lower iit of the
bloc ze for comined speech and daln transmission over an ideal
‘channel without the occurrance of bit exors

‘Heducing K fr pra incresies the number of speech blocks that
can be considered forthe conveyance of binary dats, butt K=|pmil,
bit errors enaue. ‘Thus in onder o transmit the grentest amount of

fe i

lifer ne

ove) ae mii) omen eid,

12000 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1061