BSTJ 60: 7. September 1981: Digital Signal Processor: Receiver for Touch-Tone Service. (Boddie, J.R.; Sachs, N. ; Tow, J.)

4.5 MB·English

Digital Signal Processor:

Receiver for TOUCH-TONE” Service

By J. R. BODDIE, N. SACHS, ana J. TOW
(atanuscrt recive July 2, 1860)

‘thie paper describes the design of a singleackane, all digital
‘receive for TOUCH-TONE® service implemented by using a digital
‘Signal processor integrated cieut. The eerie i patcatarly sled
for systems that operate on signals that hace been. encoded indo
“digital pulse cade modulation format. Pha program canta in the
Aigital signal processor mos designed ty ematate the signal proces
few functions of « centred office xrade receiver. Measurements of
performance confirm the equivalency.

1. rRopUCTION

‘This paper deceibes the design of a ingle-package, all-digital re.
ceiver for TOUCI-TONE® service implemented by oaing a digilal
signal procasor (nap) intagracel etek: ‘The receiver fs puciculasly
‘uted fr systoms that opernte on signals thet have been encoded int
‘digital pulseredesnaatin (PCM) farm

TOLCH-TONE! service is a voice fmapremy’ Signaling system in
which ony one of 18 digits may’ be transmaitied by simultaneously
terling two tones, The frequency of one of the tones may be ether
687,77, 869, oF $41 Fe (ealed the low group) and the frequency of the
other rane may be either 12, 136, 1477, oF 129 He (ele the high
troup. The receiver mass lemate frequency shifts inthe tranemiter,
‘operate over a wide dynamie range in the presence of noise, and be
insnsitive to spol (dig simulstion)”

"The program contiined in the mst” was designed to emulate the
signal processing functions ofa contra ofce grade receiver. Meanure-
iments of performance confirm de equiva

1573
I DESCRIPTION OF THE RECEIVER

‘The model forthe ns oeciverwechileture was the analog type-H
service receiver ‘Ihe typo-H receiver i used in many central offices
day ond hae relatively high esiivty and noise immunity ws well
as good digit simulation preformance
“A block diegram ofthe sh receiver ia shown in Fg. 1. At tho inpot
‘a filter which reduces inerference from power line and precise dial
tone components. This Eller proves last 3B, 180, 350 and 440 Fl,
Andi establishes a passband hetwoen 650 and 3000 Hz Its fourth
‘order high-pase fiver ins of »seth-order bandpass flter asin the
lanalog receiver, boctust the gia reowiver dows not have to remove
able test tones abowe 10 kEtz The 40 dal have been converced from
[naog sights thae are band-limited to les thn 4 KH
"The output af the input Filter is then split by ewo sith-order band
liminaton fiver. ‘The low-growp wow sTimination leer (cos)
provides lass only in the frequeney baad from 600 to 1950 He. "The
Figh-zroup ber! elimination flter (2GBEF) provides les fm che high
troup frequency passband and gain in the band above 1900 He. The
be of Bers is imporeant for good digit simulation performance as
tice lle
“Bach uur ourpat follow by » Timite which serves exo functions
ies, for signal levels aliove » rnnimum value, toe limiter gives ast
‘utpat thac ia mdependent ofthe input signal amplitude. The second
funetion of che limilur bs» provide digit simulation protection, ax
nat driver a eet of eooond-urder
vise nd

shat is great than «fixed threshold. The threchold is sel to 2 4
‘low the peak signal hve that sould be present if ine wave of the

nina ffequeey asl amplitude ware ioput to the Limiter. 1 the
‘inal tothe detector dropa below the chreehold, the devoeturenntinues
(clinica detection fora ied hold time

‘The outputs of Ue detectors are packed into an it word and ore
applied to the Foutine which implements the digit validation lg.
‘This logic chucks for a valid detector output and apliee » Lining
tilerion, Ifthe detects show the presence of tone pair for &
‘continous valdarion interval che loge indicat a valid digit with the
Aigtpresene (oe) flag and outputs «coe word forthe dgit"The loge
then require the paw to be continuously ubsent for the validation
ingerval before indicating no digi. An wiry deloet (FP) Mag is also
[prvi to ineate chat a tone pain the process of being vlad,

4574 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1981

“The digit simulation preformance of the receiver relies on limiter
guard action, Iq signal with two or more frequency components is
resent at the input of @ limiter, dhe mugnitute of each individual
ignal component a dhe output ofthe Limiter is rely ta Yes than it
‘would bef only that corpnent were present atthe input. When vali
ones are transmitted, each group limiter receives only one signaling
component and the magoitai ofthat eomponent a dhe limiver ouput
{is above a threshold, w= mearared by the channd filter and detector.
When speech is input to che receiver, the group limiters get many
Frequency compunenta—somne of which mizht be in he range of valid
Signaling tones. To this oa, the Timer quagd action reduces the
iagnitude of nny signaling component atthe output ofthe limiter eo
‘sve las ten the dececeor threshold. The we of mers increases the
numberof fexquaucy components from specch that would reach the
Jnputs tothe limiters

2.2 Diptaeinal processor program

‘Figure 2 shows the organization of the nse program. After inital
zation, the pst exoeutts the main receiver routine which consists of a
Toop thet is traversed ace every 250 ux (ar ceo Ble sample periods
‘The main rating cal Mller subroutine mice per loop so that the
digital flkaring operons are petformed once every 125 uz. The

J H

Fig >From ogni,

1876 THE BELL SYSTEM TECIINGAL JOURNAL, SEPTEMBER 198:
‘operations done in the main routine include packing the eight detector
‘outputs into a single word and perforaing the timing valdacin lox
‘Tho program wes organized in rhiswny because aloft filtering and
digit validation eannot be done in w single 125 us incervl, However,
the digit validation loge dors not have to be done at an @4H race
and can be spit over two ot mare intervals

The initialization routine set the at? control register, ato, for
rounding and overflow protection. The 1/0 control register, 100, i se
for format 0,8 bi, passive input and 2.5-Mb/s active output. "The Ras
is cleared aoe some register pointers ares,

‘The first operation af the main rie is to call the filter subroutine,
‘Tho iter subroutine waits for an input and then converts it from y
255 to Linear. The input and REPS are ehown in lack diagram in Fig
3. All of the secom-order structures use only for multiplies ‘The
‘signal levels are adjusted ac the ipputa tothe BE,

"The group liaiters are realized hy Une following code

pelimesrys; “rya poincs ro YH"

‘Tho ical multiplication of a faci (lim) and che output of the
sees (YH shown here) limits the dynamic range ofthe seciver. The
resulcin the accumulator isshifted up and tenaterred tothe w register
Simale inthe range of O co ~28 dB wll cause the w register
stuurate because of ovcelow, ‘This limiter action uring the overdone
protcet function is very similar to the behavior ofthe analog receiver

"The outputs of the limiters are spiel to the eight tae ters
‘These filters are reali as shown in Fig a, which illustrates the 70.
Hew.

‘Tho detector algorithm for each channel output is shown in Fig. 4,
‘This isthe equivalent of che comparator eieuitin tho analog receive,
‘The output is compared with » threshold valuc (th) for every sample.
che sample is greater than the threshold, th dlector oarpt is se,
10-8 positive qumber (ewe) and decremented, otherwise, the slelecior
oulput is just deeremente. The value of en is such that when the
‘gnalflls below the threshold, the detector output wl bea postive
number for the detector held time, ‘The 8H 1477 and 1396 Te
detectonseompare the rectified channel ter ousju: withthe threshold
to improve the detectability of these frequent TLis possible to
sample sine wave al these frequencies whose emplitude is above the
‘threshold in sucha way that none of che positive snp for one eye

RECEIVER FOR TOUCH.TONE® SERVICE 1677
Fig 3-tnou sd ban ping ter

a

41575 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1881
a OG OTE
TO
IS ec MOT a 1

ols

are shove the threshold. The rectification operation swmrantees that
ar least ane sample per eyele will be above threshold i the aunplivude
fof che gine wave in dB, shove thy threshold. The fer subroutine
then returns to the main routine

In the main routine, the outputs of the low-group detectors are
packet into single word using the “sgn” function whieh tests the sign
of the detector outputs. The word is tested to see if there ism tone
Aetecte inthe low group, The high-group detector outputs are packed
fd tested in a sila fashion, ‘The lowe aad high -eoup outs ns
then combined to form an address tha ean be used to access the
sired output cede for the daectad tone pale from a table i ROM,

‘After calling the filter subroutine again, the main soutine does the
timing validation function and provides xD and or fags on the s bite
of the os, The lavest detected tone pair (VE) is compared withthe
tune pair that is under validation (OLD). ‘The reeue of the validity
test ean be followed with the flowchart in Fig. 6. there is no detected
tone pair or the detection doos not match the previous decected pai,

RECEIVER FOR TOUCH-TONE® SERVICE 1879
a variable, CU, is tested to 200 if no digit has been validated. If there
ise digit prow tpsicive CU vile, the OLD is updaved with the
NEW. A varible, CD, is decremented anil leslel Lo ace if a tone pair
hee been continuously absent for ¢he validation icerval. so, CD wil
be negative and the s bit are elarad to indicate “no digit” an the
variable CU is initialed (er). If a valid tone pair is detected, the
‘orale CU in docremented and tested to se ifthe pair has been
present forthe validation interval. not, thes bits are sec indicate
zo. If CU ie negative, indicating @ continuous detection for the vali
Getion intervi, she & bite are tet for nh and the CD varia is
Iniialioed (ere). Finally, if CU is exaetly cer, the code for the digit
pairs waitin in Ue output buffer, The program chen branches to the
Iheginning ofthe loop.

"As ttn, the program uses nearly 100 perorn of the time availabe
for processing. However, it could be reorganized auch that tbe main
tuting call ce iter aubrourine three or four ines per loop in ore
{odo more low-speed operations if necesstry. Only 2 percent of Une
Tow is used fore Teceiver program, The ret of the RON space cold

41500 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1961
be use for other programs that xe sccessad by a conditional branch
taken afer revel and based on the state ofthe ox bits

2.9 Hardware and interface consiserations

Because the complete receiver isin a single 40-pin package, the
hardnre requirements are extremely simple, Als, the Nexibilty of
the asp 1/0 unit allows a variety of possi incertae configurations £0
be relied, Figure 8 shows an arrangement that ts compatible with
the programmed :/o control described in the previous ection. [ei
alo the circuit that was used to lest the performance of the rociver

‘A number of connections are independent of the choice of 1/0
configuration The ose must be powered by u +5 volt source, the
intemal xox enabled, » ME clock applied, und revet controled.
‘The intemal now is enabled by connecting FX (o +5 voles, A SMEz
clock mey either be applied tothe oLaN pin (as shown) or the on-chip

RECEMER FOR TOUCHLTONE SERVICE 1681
‘seillator may be used with the addition of un extemal erytal. The
‘eset line should be held Yow (rm level fora es 0 ns afer power
‘op and chen eld high while the rssivr is operating

"The input ot up inthe program for passive, iE operation. That
ig the nub-receves clock pulses and eynchronicaion information from
the aystem providing the reat daca. Th input is continuously enabled
by the grounding ofthe CE pin

“The ouput is programmed for active, 8
‘025 Mb/s Allo the oarpat signal are generate by the ps and can
be used to drive a Ta shift riser to provide parallel digit output.
‘The outpat is also continunutly enabled by the grounding of the CTS
in,

fhe mp and DP flags are provided hy tho ose 0 and s1 pins
repel
All of the other nse pins are not weed and may be left open.

li, RECEIVER PERFORMANCE

‘The performance mmauirtmenta for the roceiver were made by
interfacing 0-255 law encoder (anulog-o-gital converter) with D-
type channel hank tert the use. The receiver cul then be checked
Using analog receiver tent facilities, A typoH receiver was tetod in
parallel

“Ths signal amplicude eenstvily nf te eneiver was measured with
‘worse cane patametars, That iy thy frequencies ofthe cones were set
to their maximurs allowable deviation from nominal, che levels of the
Iovo-tone groupe stare made differen, manimums expected dial cone
was added, acd the duration and interdigit interval were at their
Jinimum yes. The receiver ya ls est for detection ersors with
Torn i the presence of zaesian wml impulse noize. Pinay. digit
‘simulation a tasted by applying peel andl music tothe receive

nll tets the na recive performed az well as the ype receiver:

1v. CONCLUSION

‘hie paper has deseibed a central office quality receiver for
‘TOUCH TONES service which accepts sigonls encoded in a digital
‘row formar, The entire evever is implemented using che ns, a single
‘opin du inline package that ie poorered by a + volt supply. Te can
nay be customized for different input and out pat formate, and there
{is enough spare nox capacity to implement other functions when the
sdovice not being used ws & recivr.

\V. ACKNOWLEDGMENTS,

‘The authors wuld like 1 asknomdedge the contributions of J.
‘Thompson, who did the preliminary tor designs, and GT. Kraemer,

41682 WE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1821