BSTJ 60: 7. September 1981: A Comparison of Three Speech Coders to be Implemented on the Digital Signal Processor. (Cox, R.V.)

5.4 MB·English

ans anon gt Tn

A Comparison of Three Speech Coders to be
Implemented on the Digital Signal Processor

By. ¥. COX
(tarsi ecnied June 24, 1960)

“Tha recently developed digital signal processar ina device used for
Iinplementing ow: to mdium complexity speech coders, It is cur

rently being used in implementing adaptive differential pulse-code
‘modulation (abPcm) coiling, two-band sub-band coding, and four

brand sub-Band coding. This study was performed to determine opt

‘mat parcaneter ues for the ta eub-band coders in preparation far
thet implementation onthe digital signal processor and fo determine
thee performance relative to ADPCw. (The actual implementation of
the anpew and wo-band sub-board algorithms are discussed in other
Dopere in Part 2 of thes isue of the Bell Syst Technical Journal
Performance was Judged on the bas of segmental signal-tonoise
ratio and 1 forced-choice, aubjective comparison tet of the clr.
AU three coders were simulated at bitrates of 16, 20,24 28, and 32
‘his. The simulations were performed on a Tahoracory computer.

1. mrRopucTION

‘The recently developed ost ia & devioe for implementing low.t0
_muaium-oomnplesity speech coders, Three oolens are currently being
{mplemenied. ‘The simplest coder ix adaptive diferentat palse-code
‘iodalation (appen) and is diteussed in Fef. 1. The other two are in
‘the aub-band coder (ste family. OF these, the simpler onc owo-band
fub-band coding (28-4n¢), featuring quadrature mirror filtering and
‘Bro equal bord, Its discussed in Ite, 2. ‘The other coder—the mest
complicatet—is fourhand sub-band coding (48-880), featuring four
‘squal bands Its implementation ia atin progress,

"This report discuss the inl dasign paramler for the acter 80
coders ant the relative performance ofall three. Segmental signal-to-
foie ratio (een) aeasirements were made on all three coders via
‘omputer simulation a fvedillerent bitrates, 16 2, 24, 28, and 32
‘hI ation, 12 subjects raed the coders in a compan es.
simulations reported here were earrid out on a laboratory com=
paler a preprsn forthe implementation athe S80 coders the
Section I reviews the design of anreat and discusses the design of
the ceo sub-band coders, Section IM discusses the remlte of the
subjective testing experiment, and Section IV gives the conclusions of
this etudy,

1 DESIGN OF THE cones:
2.1 Design of ADPCM

"The aDrcM design simulated here i based on the design of Cum
riskey etal” A block diagram of the apres coder described below i
shown in Hig. 1. Tho most significant change from the design in Het.
248 that only to muliplier valor are ae in ennging the sepia,
‘yale of the bil rate, This sed on he ADPOM implemented by
‘ohnoton and Goodman* This version of AnPex has alreedy bees

2g sage

i. a
Be presi

Fig !—Adgpve difereatl uecade dean cer ed in sition

1412 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1901
implemented the nse and is desrib in Ref. 1."The anvem design
‘Was also used for quantizing the sub-band signals in the olhur eriors.

"To siete 20 Kb/s and 2¢Why/x ees, alternating quantizers were
used, For 20 kb/a, the (wo quantiens used are for 2 and 3 bite. Since
the sterrsne is adapted baced only on the most significant magnitude
bit, the same step-sive adaptation algorithm is used forall samples,
‘The ratio of 2 to Sil quantizer map-sae is held constant. ‘This
requires one additional muliplisation to eonvert from the 2- to &-bit
topsite

2.2 Twobane s6C

[All sub-band coders are male ftom a fow fandamental building
‘ocke'The frais Linear Altering co divide the signal ino two oF more
sul-handa, Theo sub ands ean chen be decimated toa lower sampling
‘rate than the original signal. Some form of quantisation must be used
tn encode and quantize exch band, Tnerpolation and additional linear
filtering is used to bring each band back co the orginal sampling rate
fnd te is original anaes inthe frequency spectrum. At this point "hey
fan be added together to prosuce-an outpat signal

"The quedrature mirrrftering erfnique is ani wellknown for its
tase with sub-hund coders, Each pair of quadrature mirror filters (917)
produces two eub-bands of equal width in frequency. Jobson has
Compile a collation of diffrent kegth aus" The possible quantizers
‘ohlch can be used are adaplive delts modulation (xDM), aDeest, and
{daptive puse-code modulation (AFEO). Each of hese techniqies is
fairly well known and understood. Likewise interpolation and deci-
Imation are also well understood, So what remains is Une task of
‘combining these building blocks in such a way aso i on the nse and,
ils, give che best puis performance. One ofthe vss of thia study
tras te choovo gon candidates for implementation.

“The 2n-anc desig in based on the 28-#Rc commentary grado coder
of Johaston an! Crochiere. That coder was developed with the object
of maintaining a high-quality aM radio smal. Is parameters were
(ned 9 musi rather than to speech. ‘This section describes param
‘ler for a pesch bandwidth version. "There are five possible bic raves
fnvisioned. Hor « mone detiled discussion of subsband coding in
‘general and the exec implementation ofthis coder refer to Ref. 2

igure 2 is. block diagram of the 28 sic. The input speech has been
bandlintd fromm 200 t0 3200 Hz hy 1 sharp bandpass filter and
sample al 8000 He. A S2-tap qb designed by Jobnston is ured for
Separating the digitised speeh ilo the Ewo sub-bands” Afr 2-0-1
‘ecitmation on both bands, we found average correlations for speech of
(07 and ~0.45 forthe low and high bands, respectively

"The two bands are chen coded using ether abPCw or ADM. The ADM

SPEECH CODERS 1413
ww Le ae Lo

ES 4°

in used only for the higher band at low bic rates. Ite based on the DBE
of Jayant The proticion ovoicnes wae for these coders were the
‘correlation values mentioned above. Sine the high band has a negative
correlation, it was frequency inverted before quantastion by the a3,
TDeeause this ADar requires a positive correlation for ite adaptation
mechanism to work properly. Since Trequency inversion just meens
changing the sign of every other sump, thsi avery minor operation.

"The next step was to determine optimal bit alloeations for Une Tow
and high bands. After experimenting with different bit llocaions and
covalunting them on the basis of segmental SN measurements and
informal listening, Une following bit allocations were adopted forthe
five bit tes

18 Kb/s low: 3 bits high: 1 bit (aus)
20 kb/s low hits high: [bit (An)
24 kb/s low Gils high: 1 bit (ana)
25 kb/s low 6 bite high: 2bita
$32 kyo 6 ite igh 3 bia

‘Some alternative designa were very close. Por instance a (4,2) allo
‘ation for 24 kh/a ia almost ax good as (, 1) for the speech it was
tasted on. Perhape ifthe speech were les sharply bandpaas-Sltered

1414 THE SELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1081
and if there were more high-frequency content (auch asin telephone

pooch) che better allocation woul he (4, 2) for 24 kb/s

2.3 Fourband SBC desion

‘The dnsne design described here i new, although it is a loxical

extension ofthe 26 ment

1d above, It aera with the tome 180

fub-bands ap the two-hond design. Hoth of these bands are then
fivided into to new bans, yielding a total of four equally spaced
bande, The filter used for Ue additional division in each band ie the
1S-ap coer of Johnston designated Cin Ref 5, Figured whose» Hock

iagram for this vader,

sor Llp se fy

cornell 2a

re

aefcee

re moet |

gree LT

sameeren |
el ea
Tb

md

TA.

:

SPEECH CODERS
Once more the bands are quantized using aDecit or ADM, Our
rmensurements of average correlation for epeech data showed correla:
tions of 04,0, 0, and 8 for che four hands goin trom lowest a highest
in fequency” The fourth band (2000 to 1000 Ha) has actually Deen
Dandpasefiltered to cut off ac 2200 Ha. As a reeul, ie conta Hite
poser and can bo ignored for low-bit vate coders. The comelations of
‘tho two middle bands are zero, reflecting thatthe long-term average of
the speech spectrum from 1000 to 3000 Hx is Mat. If « prediction
cocticient of zero is used with abredt, the vault i aves. Thos, the
fen middle banvia are arew-enooded, The lagest amount of power is
In the firs band; therefore, It receives the mowt bite

"The bit allocations fou to be the best by the same segmental sv
mensuroments and cama Hscening paves were a fellows:

16 kb/s 4,20 (hands 1 to 4)
20 kb/s 5.2.2. (abu on band 4)
2a kbs 5420

‘The greatest amount of error occurs in Uh Tote ean. ven at the
high Tater (28 and 32 kb/s) this error is still percepcible a8 a low
‘rumbling noise. However, it was found that a high-pass filter with a
‘cutoff af 200 He eliminated this problem, "The filter ured wan 8 12-tap
‘in fter. Table L gives the costcints, and Fg. ¢ shows the frequency
sponse, A much smaller un iter could also be used todo the same
job! Note that the above bit amignmenta were made without using
the ri filter. With w high-pes fer, fower bits could bo alloerted to
the Towa al and mar to nel wo ad three

24 Relate complexity of deaigne

‘The anreat designs for 16, 24, and 82 kb/a have alresdy been
‘implemented on the DeP. The combined encoder and decoder algo-
rithms use 48 percent of the ust real-time capability for a sampling

‘Table I~Costficents for symmetric Fn high-pagsflter

1416 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1981
ig. —Frequney rsp pas ler ar 8,

‘rate of fia, An even lower poreencage of xan and Hox memory is
tied, The Onan based onthe design parameter reparced hore has
iby been implemented on the psp chip. Tt usee 88 parent of the ral
time enpabilcy and 78 peroent of (he RaNt memory. Ie includes an i
‘bandpass fiver for the input, ‘The 4@-#8c algorithm is planned for
implementation in the near future. Since all ofthe major portions of
the dn-sue have been programmed already forthe 2u-s8¢ impleren-
tation, itis possible to projert how much of che se willbe use Both
the tranmtter andthe receiver will roquiro « nap and ch vil ws
bout the same fractions of realtime capability and RAM a8 the
complete 2u-sbe algoitm. ‘Therefore, 20 we might clisiy Ube Cree
‘oder as having commpleices of 05, 1 nd 2, respectively.

I, RELATIVE PERFORMANCE OF THE CODERS

‘Since the sue designs ave more compler, a demonstration of their
improved performance over prea was needed (a jusify their irae
‘mentation on the Dae. To demonstrate their rolative performance all
three coders weresimolated on laboratory computer Fach processed
fpeech from a stored file, The realls were evalusied by both an
Uhjective and subjective meneur, The objective measure was eogmen.
{al sem, while the subjective measure was » forced-choice, subjective
{A+B} comparison test in which all possible codera were compared.

‘ix phonetically balanend asntences were used for evaluating The
coders ‘Three were shen by male speakers and three by females
‘hey were recorded uring a linear microphone. They were band-
limited from 20 0 2200 Hand sump 1 8000 Ha using 2 16-bic
linear quantizer

3.4 Segmental signattowoise ratio routs

In comput sogmental sux measurements, blocks of speech of 82
ims were used, The anipem coder was compared with dhe original input
lpeech. The sn codere were compre with reassembled speech which
Imad been processed by the appropriate qMr Mering, but with no

SPEECH CODERS 1417
quantization. Thee slightly modified speech signals cannot be distin
fished from the original in casual listening, Without thena it would be
Aificale to make w fair comparison of the throe codors on the basa of
‘xn, The measurements on 48-80 were made before the L2L-tap FIR
high-pass filtering.

"The results of these measurements are summarized in Fig. 6."They
show thatthe mote cemplex she coders havea definiee advantage over
lanncar nt the Tower-it rates Tterestingly at 2 kb/s, anPcat beats
‘oll of the more complex covers The 4R-MRC mainline fatly
conscant 24H advantage over 2u-sbc. In terms of Bit raze chis tran
lites to 4 kb/e At the low rates, the 4u-sue has about # é-tb/s
advantage over ADPCM.

2.2 Subjective testing ofthe three coders

An A-B comparison tat was performed to rank the thros coders.
‘Each coder at each rate was compared twive aginst every other coder
at every rate, as wells azainst che original. Inthe owo comparisons of
the two coders, each one wan paved in fst position once There were
12 parliepants in the text and sliogether there were 240 compariaons
‘The test wan bruken down into twa part, ane with 110 comparisons,
the other with 190, The parceipants listened over headphanes in a
soundproof booth. The participansa were also broken down into 060
{rouns of wi. If one group listened to & particular A-B comparison
‘witha female speaker the other group heard a sentence with a male
speaker and vie versa. Thus, we atcempted tomake a totaly balanced
fd unbiased test,

Fi. Sermon ne mensuemens or the codes,

1418 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1861
{SIS055 sao wueBe BUN 1 UBL Ps

72pe5 1G pOUIEIGO 1099706 1vaee Nd

SPEECH CODERS
Pi. 6—Oveal peor racking fh sua,

Table Il gives the individual coder versus coder comparisons. In
sddition, an overall preference ranking was computed based on the
total number of votes received by aach coder. In al, a toal of 25)
votes could bo received by any coder. Figure & shows the percentage
‘of he 380 possible vos rocelved far each rnd. Ther rs is in god
‘aureement with Uhe resulls of Fix. 4. Fur example, both subsbaned
coders sow adantage over Apres at the low rates and ADPCM catches
[Up oF pases them atthe high rates.

‘Some ofthe more significant results ae the following

(i) "The db-sBc has an kb/s perceptual advantage over ADPCM at
the low rates, The 2i+kb/a abpoa has been used for voice storage and
playback syscoms ‘This result indicates that 16-kb/s 4u-suc could be
ubettatel at a 88 percent aavings in storage of, equlvalenly 50
pereent increase in message storage capability. Moreover at 20b/s
2u-s6c hus a 4-kh/ perceptual advancage over ADPCM.

(i) Alchough 40-s0e lost to aDPeM at 32 kb/a in sR measure
‘ments i beat aves in the subjective vests. In addition, indirect

1420 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1261