RESEARCH DEP.AI-fl'JYlEN'T

IMroI.sIVE INTERFEREJ:;OE: OOMFA...'Usi)NOF FM REOEPI'ION'-WIT'H AM REOEPI'IO!-£

usnm NOISE LIMITERS.

Report No" C-'-.. 044 Serial No" H49/l0

Research Department Marah, 1949.

REl?ORT NO.. G,044 Serial N9. 19~9LlO

.IMPULS NE INTE:RFl?RENCE:

Figs. Nos.G. 044.1 ' to G.044.14

COMPARISON OF FM RECEPTION WITH AM RECEPI'ION USING NOISE LIMITERS.

1.1. Summary

This report desoribes tests 0arri6d out to dGtermine the relative merits of FM and .AM ~¥hen the ..AM re;;.ei"var is fitted with a noise limiting circuit. Objective mea'surements of signal-ta-noise ratio show that although the addit ien of a limiter improves .AM' reception verY considerably) lITvI is still superior to AM. Subjective cQmpari.son of the two systems res'Il1ts in ]'M: being always at least 10 db. better than »'1 with a 11miter. 'Two types of limi tors ,W3l'e tested and both we:ro, found to be very

, ,

effeotive in reducing impulsive ir:terference in a wide band ~i receiver. The effect of pre-emphasis and 50 ~~ecde-emphasis on the AM system did not appear to alter the results very appreciably.

2.1. Introduction

3.

The development of impulsive noise limiters has raised the question of whether AM can be made as free from interference as FM by fitting a noise limlter to the AM reCeiver. It was therefore decided to compare an AM receiver, fitted with either of the two best types of 11miter available, with an ~I receiver under practioal oonditions of broadcast reoeption during impulsive interference. ' '" Both the types of limiter usod require that the RF and IF bandwidths of the receiver be considerably wider than that normally required for the reception of ~I., It was arranged that the ~I receiver tested had the same bandwidth as the FM receiver used for comparison.

Description of Tests.

3. J.. General Descr.iption,

The tests were first made comparing pre-emphasised ~1 with" normal JJ1l and were thEm repeated using pre-emphasis on both systems. Two signal generators, one frequency modulated and the other emplitude mod-Alated, were ,used to supply the same programme to the two receivers. The FM modulation was pra-emphasisod beforebe1ngapp11ed to the

2 ...

signal generator and the FM reoeiver had a 50 ~seo de-emphasis network inoorporated. he-emphasis and de-emphasis oould be switched in or out of the AM system as required for partioular tests. Both "transmitters" were set up aocording to standard B.B~C. line up prooedure as used for AM and experimental FM tra~smissions. when pre~emphasis was used on AM the modulation was reduced by 3 db. as in the FM oase. A sohemati(} diagram of the apparatus is shown in Fig. 6. For subjeotive tests the modulation was provided by gramophone discs. High quality B.B.C. recordings were used and replaoed whendiso noise became apparent. The impulsive interferenoe was fed to both reoeivers through hold-off resistors.

3.2. Reoeivers and Lim~ters.

The receivers used for this investigation were manufaotured as experimental FM and.·,AM reoeiversby G.E.C. Research ·Laboratories, (Wembley)~ and had IF bandwidths of approximately :80 kC/so The·

AM reoeiver inoorporated a delay line type limiter developed by Murphy Radio Ltd~ A fead-back type limiter was construoted in the laboratory from a oircuit diagram supplied by Murphy Radio Ltd. This type of limiter is used in certein Murphy television receivers. The feed-back type limiter is described 1n Appendix l~ The delay line type limit er is described in Appendix 2. When de-emphasis was used on the AM receiver this followed the limiter stages.

3.3. Objeotive Tests.

These oonsisted of meesuring the r.m.s. output of the receivers due to impulsive interference with various depths of tone modu­lation on the received carrier without measuring the modulation tone itself. This was achieved by using 8 high pass filter before the meter to pass the impulsive noise from 250 o/s upwards but reject the modulation tone which was 60 c/s.

3.3.1. AM Without De~emph8sis

For these measurements the FM signal generator wos first frequenoy modulated !75 kc/s 8t 1000 o/s modulation frequency. The AM signal generator wes elso first modulated to 100% at 1000 'o/s modu18tio~ frequency end the audio soinoe the AMreceivar adjusted until thf audio output of each receiver had the same amplitude. This r.~.s. output was noted, end the modulation frequency changed to 60 cls. The 250 c/s high pass filt~r was then connected in front pf the meter to prevent this modu18tion showing on the meter. Impul~ive interference was then switched on Dnd the r.m.s. output due to this interference was meesured on the meter for verious modulation percentages of the two signal genergtors. Mes.surements were made using the following four systems.

(1) FMwlth 50 \ls de .. emphasis. (2) AM with no limitsI'. (3) AM with delay line l1miter.

'. (4) AM. with feed-back limiter.

The meter used to~this measurement was a square law meter developed by this De:partmen~precedeG. by a 0.0. LF. nei;work. . The noise was measured with various amplitudes of input interfering waveform end wi th the foil6wing conditions of lr.odulation:-

(a) No modulation. (b) 40% modulation {SOc/s} (0) 80% modulation (60c/s)

The results of these measurements are discu,ssed in paragraph 4.1. and plotted in Figs. 1,'4 and 3 respectively.

3.3.2. De-emphasis on both Systems.

The tests described in the previous paragraph were repeated for the ' following systems.

(1) FM with 50 \.Ls de·:emphasis. (2) JJ:JI wi th_ 50 ~ts de~·()mphasis but with no limit.er. (3) ~I with 50 \.Ls de~emphasis using a delay line

limit.er •..... (4) AM with 50 I-ts de-emphasis using a feed-back lim1ter.

The following conditions of modulation were used.

(a) No modulation. (b) 80% modulation (SO o/s)

Thexes~lts are plotted in Figs. 11 and 12.

3.4. Sub~eotiveT~sts.

3.4.1. AM without de-emphasis.

For these tests an observer was seat.ed in front of a loudspeaker which could be swi tohed to reproduoe the audio output o'f wither the FM or J4M reoeivers. The audio amplification of each receiver was adjusted to produoe equal programme amplitude from the loudspeaker. The prograninre was the same for each system and consisted of high quality speech recordings which were ohanged when the disc noise beoame apparent.

:I: Patent applioatio'n 19804/48.

4 ;...

The programme volume was adjusted to a oomfortable listening level ~or the observer. Twenty~nine observers took part of whom fourteen were radio engineers and fifteen v!ere chosen from other occupations, 1.e. typists, olerks, mechanios end draughtsmen. Each observer took part in four measurementsi The results are shown in Fig~4 and disoussed in paragraph 4.2. For these tests the following grades of interferenQe were Used:-

(1) ( 2)

( 3)

IP interference imperceptible. JP interference just perceptible with very

careful listening. P interference perceptible wlthoutpartioular

attention. (4) SD interference slightly disGurh,ing. (5) D interference disturbing. (6) VD in+'erference very dis-';urbing.

Apart from lP, the grades are in approximately 10 db, stops.

Test 1.

The interference amplitude we s adjusted until the obsorver decided that on AM with no limiters the grede was "percepti.ble". Each lim1ter wes then switohed into circuit And the oreE.n~V\:3r estimated the grade of interferencG. The loudspeeker w8s'Ghen switched to FM and the interference againostimai;ed. ThOse grades were recorded.

Test 2.

For this test the input interference amplitude wos adjusted with the observer listening to AM with no limiters until the observer decided that the grade of interference was V'slightly disturbing". The estimeted gredes of interference for the two limiters ?nd FM were again reoorded.

Test 3.

The test wes then repeated with the input interferenc€;1 level adjusted to give en audio interference grade of "di :3turbing" on AM with no limiters.

-Test 4.

In this csse the. interference was set to a grade of /'very distllrb-1ng~ on AM with no limi ters.

For these tests programme was present and the interference hed e pulse recurrenoe frequency of 100 pis. The intorference was an artificially produced "un! t impulse" waveform. The avereges 01'

the results shown in Fig.4 are plotted in Fig. 5.

3.4.2.De-emphasis on both Systems.

These tests were then repeated using pre-emphasis and 50 ~sec de-emphasis on both systems. Twenty observers took part in the repeated tests. The average of the results shown in Fig.l3 are plotted in Fig.l4.

4. Discussion of Results.

4.1. Objeotive T0sts.

Figs. 1, 2 and 3 show the measured output noise for oonstant carrier input equal to 5qJ.v. through a 750, aerial; impedance. The eutput noise is plotted against ·varying impu~se amplitude. This impulse amplitude is expressed as a ratio of peak impulse in the IF amplifier divided by peak carrier (unmodulated) in the IF amplifier. It will be noted that as this ratiO increases through unity the FM system rapidly deteriorates.' This is the well-known improvement threshold of FM.~

a.mparisen of the three figures indicates that impulsive noise output increases with modulation on ~l with eith~r of the two limiters and also on FM when the peak impulse is . larger than the oarrier amplitude. The figures also show that the feed back limiter is better than the delay line type limit~r. These results are not a true indication of the subjective effe~t, however, and this point is discussed in the next paragraph. Oomp~rison of Figs. 1,. 2 and 3 with Figs. 11 and 12 shows that pre and de-emphasis do not greatly effect the AM results.

, 4.2. Subjective Tests

4.2.1. {Il.Jl Without De-emphasis

;Cn these measurements twenty-nine observers e soh Ina.de four tests in the comparison of FM with AM not pre-emphasised. The results are plotted in Fig.5. It will be seen that in ever! ceso the mJ system was at least two grades better than AM, while the two limiters came about midway between the ~r and AM SystoffiS. It is also apparent that there is no marked differenciJ between the twe limitors. The reason for this is that although the feed-back limiter produces a better signel to noise ratio than the other limiter, it possesses the disturbing characteristic that the inter­ference varies noticeably with changes of programme loudness,

~ B.B.O. Rosearch Report G.036 and G.036/2.

... 6 ...

This is apparently suffioiently annoying to compensate for the general reduotionin noise.

~ . . I

4.2.2. l?re-am12gasis and De';"em12hasis . on. bo_th Systems,

Twenty observers each made four tests and the results are. plotted in Fig.13 and averaged in F1g.l4. It will, b~ .. nO.ted that there is no marked difference between these results and those discussed. in the previous paragraph. It would appear that any improvement· caused by introduoing pre and de-emphasis into the AM system is counteracted by the 3 db. reductionofmoduletion et the transmitter.

Q2;nc,lusion.

When. the peak amplitude of the interferenoe in the IF amplifier is less than the peak amplitude of the oarrier, ]M mainteins a : oontinuous superiority of at least 2Q db. over.AM fitted with limiters. . If the impulse amplitude is greater then the oarrier, however, the FM system :LS at least one gr?de or 10 db.' better than.AM with either of thelimiters. This improvement is extremely important from El subjeotive view"'point. It is felt that the limiters must be considerably improved before .AM con be made comparable with FM for the reception of broadoast programmes in the presence of impulsive interference.

GFN/MP.

Report written by - G.F. Newell.

Work carried out by - G.F. Newell, J.G. Spencer, G.R. Frood.

1ft I~J":·I<Q'· (H. L. Kirke)

APIENDIX 1.

DESCRIl?TIOi~ OF FEE]) futCK TYPE IMPULSIVE NOIS~IMITER

The complete circuit diegrem of this limit er is shown in Fig.8 but the operation is more eE!sily explsined by first considering Fig.7A.. The diode hes en Budio frequency voltage applied between cathode end earth while its anode is connected to 8 positive DC supply. In the absence of the audio voltage on the cathodo the diode snode will attoin G voltage only slightly positive to eerth since the enode resistence of 4.7 W..n is so much higher than the diode resistance mid the cathode resistEmce. If the 8udio frequency waveform is epplied between the cethode and earth .the anode potenti81 will follow that of the cathode remaining slightly positive with respect to the instanteneous cathode potential so long AS the condenser C from anode to eerth cen charge quickly enough for the enode to rise with the cethode. Thus the anode pvtsntial will follow thet of the cathode so long as the cathode does not become more positive than the ~~ximum to which the enode cen rise due to the potential divider formed by R1 and R4, Gnd so long as the cathode potential does not rise too rapidly. A rise in the anode, potential necessitates chFrging the condenser C (from anode to esrth) from the DC supply which will have en impedance of 1 MU in parallel with 4.7 ~U1. A fall in potential can be followed more quickly because the condenser C discherges through the diode ond its cathode resistance. Now if the IF bendwidth is much higher than the highest audio frequency the impulsive interferenco will riso too rapidly for the anode to follow the cathode potentiE1l and thus the diode will become en open circuit for the duretion of the impulse. This circuit will therefore remove impulses which are positive in voltage with respect to the instanteneous8udio potentisl but will not affect negative impulses. Since negotive impulses ara limited in fln AM receiver to the peak cnrrier voltage they ere not so serious ss positive going ones. If it were considered necessary of course, a similer circuit with reversedpolari ty waveform food into it would remove the rOITlElining impulses. The values of components shewn in Fig. 7A will pass audio frequencies up to about 8 kC/s without undue distortion and above this frequency of course the distortion is only ~~udible 8S a reduction of amplitude since the harmonies necessary to hear the distortion are elmost inaudible even if reproduced by the loudspeoker end its tr811sfOI'IDer. The type of distortion cclUsed et 8 kc/sis shown in Fig. 7B where it will be seen the c8thode voltage is rising too fest for the ;wde to follow, but th~ falling voltage is eAsily followed. .Dso in Fig .. 7B is an illustration of an impulse produced by Q receiver hl:wing 8 bendwidth of Z 75 kC/s showing the limiting action. '.

- 8 -

If we now consider the practicAl circuit of Fig.8 it will be seen that the battery of Fig.7A is replaced by a oondenser C3 which is kept charged by the diode D2 rectifying the anode waveform of the' output valve V2' By this means the steady DC potential of the battery is replaced by a smooth DC voltage proportIonal to t lle peak audio voltage. The time Mnstants of this circuit are such as to cause the potential of C~ to follow amplitude variations in the audio frequency waveform which are less than about 3 c/s. This means that the diode oannot pass impulses in silent passages of the audio input. .Uso since the speed with which the anode voltage oan rise is n funotion of the DC voltage charging the anode to earth capacity, impulses occurring in quiet pBsscges of the programme are more effectively de~lt with, than those occurring in loud passages.

Further examination of Fig.8 shows that the condenser from the diode anode to earth is not marked. This is formed by the diode wiring capacity to earth. One ether ohange 1s that the resistance Rl and R4 do not form a DC potential divider, but it will be seen that f9r changes of potential equivelent to frequencies higher than 30 c/s the condenser Cl does not alter the potential divider action of Rl and R4.

- 9' -

APPENDIXII

DESCRIPI'ION OF YELAY LINE '-TIVIPULSIVE NOISE LIMlTER

Fig.9n shows the simplified circuit of this type of limiter. The input audio waveform is fed through a high resistance RI to a diode anode in parallel wi~h the grid of a,cathode follower valve VI. The diode cathode is supplied with the audio waveform from the cathode of VI through a delay oircuit. The D.O. pO',entiel of the diode cathode is more positive than the diode anode so that the diode Dl is non-conducting in the absence of on 2udio waveform. The delay time must be os long as the duration of an impulse but very short compered to the half period of the highest cudio frequem:y tu be passed. This of course means that the overell bandwidth of the receiver must be very wide compered with the highest audio frequency.

In order to El void audio distortion the maximum rate of positive rise of the audio waveform must not be sufficiently great to oause the diode anode potential to rise above the diode cathode potential in the delay time.

Fig. 9b shows the relationship of anode and oathode waveforms when the delay time [md DC potentials are correct for the highest au.dio frequency. Fig. 8c shows the effect on an interference impulse of this limiter. The anode voltage of Dl rises too rapidly for the Dl cathode to follow and the diode becomes oonducting. The impulse is therefore reduced by. the potential divider set up by the resistance RI and t he impedance of the conduoting diode and Rc in series.

The practical circuit used in the tests is shown in Fig.lO, since this was an actual version of this limiter produced by a commercial radio receiver manufaoturer. The overall bandwidth of the receiver used was ± 80 kols which is approximately the same as that of the FM receiver tested for comparison •.

00 CJ ()

This drawing/specification is the property of the British Broadcasting Corporation and may not be reproduced or disclosed .to a third party in any form without the written per­mission of the Corporation.

z o ffi

'" o C -t 1J

~ Z Q. rr (JJ m r o ~

o o 7f-d 2 Jll

~ o o

~ <5

_.

I.F. PEAK

~. r

"l'r '~. j

:L i l'-.!

L _

-, .Jo. Q)

Vl -Vl

C m

:-~ :i ____ L-'.c;:J

~ rn .j>: Gl JJ ~ CII O· fll 0 I 1) I '" 0 fll t-.. l) ut ;-"" -I .~

I\J o m ~

z Q (JJ ITI

ISSUE I

41-11-48

e

o ~~ L E .- ..

>.. ..c: 0.. .. ... ~-g~i1 0..... ...

f c.S·~ 0.. .g"'O ~

-5~~~ Q..'V .... .. ... .. . - 0 .- ~ .

U "'O::IC coo o boO L.. .t::. .~ -;; c.: o.~ ~ ~ .;:; ~ 0 ~ ut '"'0 0-

.~ ~ e E 0 ~~~~u

........... L. 0 41

~£O ~~-= °i ~ ~ nS_

f'I:I .~ C,O '- .- u -- c '"OL....o 0 00 >.. ._

~QJo5·;! t--:ico..E

BBC

o o U)

NOISE .OUT-PUT IN db BELOW 100 % ,ONE MOD.

COMPARISON W\TH

OF F.rv," AND A.M. LIM \TERS.

;:; .• 0 [.:2;;;;; -!",,~- ~.~ .• ~-

6

G.044·3. '45He.ETS.

~~ J-::J Q..( ~U

;! 4: w 0.. lL

I • . ! I

BBC DS/44/P

t!i z a ~ (!J

lJ.J ~ .... (J UJ .., Cl J Cl)

SUBJ'ECT\VE RESULTS (AM. WITHOUT DE-EMPH~S\S)

COMPARISON WiTH

OF" FM AND A,M. L\M\TERS,

DR'N RESEA.RCH OfPT'

REPORT

G.044·4 14 SHEE.TS.

cfJ .J uJ to to w 0

~

e !uJ !(() ~o ~ ~;J ;z m ,

l~ ~ b

le( .

BBC OS/44/P

uJ J Ul ~. n. UJ

~ n..

_ COMPARISON O~ FM. AND A.M. W\TH LIM\TERS.

DR'N M,~,

CH'O ~ AP'D Irf].)I

l:tJ M

+

RESa~RCH DEPT

I ,: j'

I11

III

I ~

--------~ -, REPORT

'G.04A-·5 14,51-1EEiS.

I~ g i

ro 3:" -f:1l m~

i ~Z 0 l> ~ -

I~ 12 I~ !o

~ f~ If i~ ~

f.G) ~ (J) :Ill I- rn Q (Tl 0 il, m 0

~~~ ~

•

I <n I§

Th!~ drawing; specification is the property of tne British Broadcasting Corpe,ration and m~y ~.e not be reproduced or disclosed to a third t------.------------j plny in any form without the written .,er-mis~ion oi the Corporation.

SOURCE OF MOCU\..~'T\ON

PE.A.K AMPLIFIE.R r- PROGRAMME.

Me.'TER

ATTENU~TOR

1000f A.. M.

L...-___ ---'-V\N'l\vj~ v,.......~ ........ ~ RECEIVEI<

IOO.ll.. !'OOo..

PRE-EMPH. AMPLIFIER A..M.

NETWOR'!(. --- S.S.G

I

'" 1 AMPLIFIER. .1"\--0

~

" IOO.n.

F.M. RECE.IVER

AMPLIFIER r- F.M S.S.G.

S.FO. I ~ I PU"SE • GENE.RA.TOR I

FIG.G. SC~EMATIC DIAGRAM OF APPARATUS FOR L.IWlITER

COMPARISON OF ~.M. AGAINST A.M. WITH E1THER OF' TWO LlMITERS WITH

IMPULSIVE. INTERFERENCE..

( ~,...,.

(- I FMI

I

I I I

~~. I . I

--=--,-,: '-'" I F.M·

I ~t<'\·1

SQUA.RE '-A..W

METE\<

C.C.I.F. NETWORK

HIGH PA.SS FILTER

fo :. '2S0rv

ATTENUAiOR

_ i - (/;1

d: -c 1'

I m ~ I (JI) ,

OUTPuT L..r--r STA.Ge.. ~

1...S.

cc CD ()

!0~ ram rn \I

rn~o -f JO (JJ -i .~ . -J

This drawing'specification is the property of the British Broadcasting Corporation and may not be reproduced or disclosed to a third p.lrty in any form without the written ,.er­mission of the Corporation.

AUDIO INPUT.

I

F'G.7(0.)

AUDIO OUTPUT. ">

4·Th\.1l.

_L..+ ... 9vD.C . -I I

-'-

VOL.TS

+

ANODE. WA.VE.FORM.

SLOW RISE. QUE. TO

CHARGING C THROUGH \ IVltt AND 4-. "7 Mn..I=AST

DISCHARGE iHROUGH OIOOE.

CA.THOOe. WAVE~OR!y\.

TIME --....

lMPUL.SE ON C.ATHODE.

IMPULSE ON ANOOE.

FIG.7(b)

FEED BACK LlM1TER (THEOREJ\c;AL)

-, lA _ v;

C m

~ G) 'l (Jl- III

;i08 m ~7J -t ~ III

~ 01

This drawing/specification is the property of the British Broadcasting Corporation and may not be reproduced or disclosed to a third party in any form without the written i,er­mission of the Corporation.

INPUT

SOI<..t'l..

-

0, 0041

I 4 '"1IV1.Q. c.:=':3 1<.4-C::':::I RI I ,

1 I

C,3

'Olf'-I=

FEED- BACK

IM.n..

D2 0041

FIG.S.

LIMITER

+2G5v

Vc.. "--'1 PEN.45

(ACTUAL)

:: I V') --"" - c

t.. m C»

L.S.

C2.

410 R~ KA

!o on ...... -o

'(')

, This drawing! specification is the property 0.

the British Broadcasting Corporation and may not be reproduced or disclosed to a third party in any form without the written per­mission of the Corporation.

01.

F\G. 9 Ca.)

DELAY-LINE LIMITER.

ANODE WAVEFORM ---,;c..--. __ "­

" " '\ W~VE!=ORM.

~ I VI _ -VI

-- C I m ~ (lQ

'\ CA'THOOE D.c:._J.~ _______ ~~~\:-__________ ~

A.NOOE Q.C.

DIODE. CONDUCTS

CATHOOE . D.C.

A.NODE D.e.

n : : IN1PU'-SES~~"1 I; /"'" i I I~ I I I I I I I

I t.-DOT,..e:D PORTION

REMOVEO BY \..IMITER

DIODE CONDUCTS.

r=IG.9(c)

..-....-OJ--...-Th-i$-d~-aW-i-ng-!~;;;fl~at~=i~-;,;--P~-~-pe-r-ty-o-f...,.l-----------~-----,.....------------------~ ~ the British 5f~,,(il:"1tif1g Co'p(.Jratic!1 and may· re a ..!. v; ~ CJ' not b6 reproduc~rl or dise:losed to a .third .. .. '-·C o () ! p~rty in any form w,rl-,out tb~ written j>er-. -4>- m

~r_)~ ____ 4_n-l1S~~!-O-n~o"-f-t-he---co-rp~o~-ti~O~~ ,~ ____ ' __ ~! __________________________________ ~__________________ , _________ ~ __ s~

o m -0 -I

~---------------1------~\1~5IAKA~~--------------------~----------~--------~------~-a-GSv. I

150.n.

I

L.IMITER IN/OUT

FIG.IO,

PRACTICAL CIRCUIT OF' DELAY •

LINE

. I

i

I,u.F

OUiPUT

'OKA

LIMITER.

-----i

This drawing. specification is the property of the British Broadcasting CorporAtion and may i a not be reproduced or disclosed to a third r-w".----------------f p3'rt)' in any form without the written per-mission of the Corporation.

100 0·\

I MPUI.-SE I.F. PEAK CARRIER

Vl I~Vl

~ C (J) m

n o . ~ , :u ~~ -(/I ~o

z ro ~TJ -t m:rl XJ~ en-

» z o » ~

This drawing specification is the proper-Iv of the British Broadcasting Corporation and may not be reproduced or disclosed to a third party in any form without the written per­mission of the Corporation.

z o m

Z a.. eT

~ 0 ~.

(5 0 -,.2 0

a ~

~ ~ o o o o m-

~o

4-0

50

60

'70

80;:0

90

tOO 0-1

IMPU,-SE. \.1=: P5AK CA.RRI ER

-~ (f) - -(J)

I C t" m III

----- - ---------~---------------'------------------------'

ISSUE I,

11-11-48

I 1--......---,)

! ~I

BBC DS/44/P

ILl > ~ u UJ h Ol ::> t/)

Tf: o,Tf ,Ht fh1 'iiLr:,d1f, ,!:;ilt!. tt:; ~o 2,0 10 0 ~O 20 10 ,0 30 20 10 0 30 20 10

, N2 CF= OBSERVEI<S.

I=IG.13. SU8J"ECT\YE RESULTS '( DE-EMPHASISTH~OUGHOUT)

j

o

-f rn (JI

-1

RESEARCH DEPT COMPAI=<ISON

WITH OF F.M. AND A.M. LIM\TERS.

DR'N CH'D'

Re:~ORT

G.044·13. 14- SHeETS.

co CD ()

n 0 ~

~~ -AI 4-I~

Z ro s1l --LTJ (1l~ ~.

cn~ 0

1> ~

l>()O ,,:r;;:o dd"i.

~ r

~Al

Ci1 Ig _ I ;JJ

~O~.Q ~~gl m ..... " fl1~ .... .... . C/I_

~

This drawing specification' Is the property of ,-.---- ---.,'--'.-'->'~---~- "--'--"'--' -··T---,.-------'-----' - ,_,0 ._--:--- '-l the BritIsh Broadcasting CorporatIon and may 1 A ~ Vi I not be reproduced or disclosed to a third .A--"._.",,-- -_.' --~~~,--~ _. __ ., ., --. ..--. - _ .. -:- VI I party In any form withol!t the Written per· '. "'" ~

O~~I Ut .~ .+ 1+ ~-I-l-J I' (Jl {Jl 01 /JJ X' X X

('10 0 ~jjl"u;-fj;-

~ (]I UI Ul

f f 0 I/) 1/1 ]

~.» :1l ~ ~ ~

:E ~ :::j =1 I I

o TI JT1 m r '" ~ 0 -< 1 • 01

[: ~ Z A JT1 r r i ~ - =t ~ /11 1J ~

.. J

~ 0 ~ (j) -n JT1 f11 1J ;>\) I\) J> < lj) IiJ m {}l (j)

~~ -i° rn I rr1 AI ~..}.l> o 3! G) "T1 Cllm -(j) IG) I»J\J­o (j) J1l -C(j)m~ -t c ......... !4

(j)

AVERAGE.

I\)

o

,DISTURBING SLIGHTL.Y OlSTUR81NGi ~ERCEPiIBLE. i ,i( ~ Ii I

~ 0 g ~ c3

t;'P n~ ;t:t:t::;:: i!.:.'J.i:rd -:-;EHt, 4;11:iH[+~.Lttf.:jt~~~ .:i.:tI~;l :~ 'M.l!'W-'ml .fC::I;iU ~.:t r;iim~~~E[:I:;';j;jl~ri~f~::E~:):~~~ft ,,':'l'::j:i~[;'r::T~~ :!>L::~:::;I ;'.'."~ H .. ,..rt::!~-r 1t,+".Oi fitH'~i " . ,';"4= '~F~J:fH '.'''',E'' t , . , .• ,1., .. ,.", ~:"·.t.·,,·, :n: 'I" "dtt:.:'L~'lr~F'. '.'-"'J2.1'1"""~' .,.! ". -' ... ' .C'-q ,'../'.;. " ., _ ... , ..... J

;+,. ~\+ .",,,.J.,,. '1'" ',lHi+ J,-,+"O_" ~p., .•• :rt.J '1, ", "',+ ""1'" "if' ,,.. "-,-, "'1"'''' '·il.lj~"i"""U:··'·I" ',,, ... "; .',,_ .. I ," ·"'tJ";~~,·,,,-',l·t·,j ,,+ l·ft.... ~~~~..... ·!-+~t ...1+--,-- .~-•. +- fl-, . j-lt"r+ ,"~' -, - '~-f! ·'+4. +t -I -.,..- ........ ,-> .'~. -t~ .. ~. -~.!...j. ...... I...., .... -t ·~'-:·t-[··\ - .-.....•... ' . ,- ,.-.. -'-'~~' --,+'1- 'J.+" ... -, . F¥. '" f'~ -rHr m~f +t+lffi;il::l §ffiW':-l; ~:-t±±ttt::::;t+;8: 1jt;~w~~ .. ;; ~:r:, : Br 6L;t:Ir#.H..J =rt ':::11" :tr::'c.;Ttt: i'drtj: :":~ tt:: i~:ccn~}'lli: " :;1:::~':r~:-:r'1;:::::T';: :i;::::;p-::t::;;::-::-:1

"vi "flIffi" .• " "., ~ , ""W"" , .. "'-":re "~It:·"''"' ,'" ~ ~,! "1""1"'", """ '~4'c'S:r"~l··L,.j m1j'nt~~~lrt;:· r~1'1#f.:: t ~tH111cr: :~:~~l~~+' .~~ ~~t;;i~tH~{#.~tili ~:E BN~1ili;~$j~tH~~J&~ ~Htt~~ ~~/'f:ffi{:~i~ ;~J~~rHtJ~{q~~~l ,·tt<i",g,·'tl i:;fll~,tf'W +!1;... ~'tt" t,+il~~t'~ ",~"J.t:'"+'+i~+"." jt~ri:t)·!l.'l.·~·"· \;,\.q,l::i-::'i""'· ,., .. ,.,..~lJK'I.· .! .. _,t .. _" •• "",,,.J,l .. ,,.\ .. , ""'J

". ., ~.,.:"~. , .. t ,-c' .. + t' c,~, ... +" ... H" T~ijl~" '.4 ... " .,t:.;:"'" ',H •• ,.~J. ",' -"-.h::i;:J:!.:J' .• h.'I'·' .t·'+·'+:r·' • ""';:"::";':'" ;..;.;..::i';:~.~;.j:;::.:.:.L..:.:.i.:";' :hi±1'-'-' 'mttiE fJ;fWH '+l~.j,·:t+:,· ~!tf:fHl1:!-r.~ .. · Ht '.;--l!:',-IH'-; ,,,, • .-J;g '-"'~"~t'"''';''''-HJ~''' '!~"'"""l=:" """" i"" .. --,.', ., .. , .. t",c.r;·"~r" j 3?IFlli ~ [J ~t#(lf~; h~~htlf'r~r;~l :~ ~ tl#t#g~ ~t~; ~ 8ff#f~~:; ;~?;;J _;~~ ;H~f.Ar#:+~ ill~~'1~~~~t:~H~f;1!:~.t~~Jfh~$tf:;:i rr.d"i~ :t!:t:~,::.d1Lu+~ !If;.t. mi'Jif +.+~ ·+H11-'1'H""ff.+ 'N 'inl"X"! 1":&'" 'h ti~~~Efif'r" ,-·li:' -r,.:J,,:f,~\.d:::Tft', ,- .!. t_.+,J,,' ..... ~ ,t' ., , t ~f ~it~tffi+ :;~;; ;1;t 0tO~ ~ . ;;t;t ]t~~;~·j:t;~~~i ~.U ~~, i~i~i~j:8:ml~i;; ~111' .~. ~gl; ;1~~4~:~~ _~ s:~la}~ ~"'> ~ '~-. ;; ~-~ (_~ ·~~tl~-:~: ~iijt ~~-~ .~i~ ~ ~.7J ~:.:it.~~ ~~t;!ti:f~hFl.1= Net.1:1 ., . , tttm·::r-rffrftf!f+ "I';: t"T:-i .. '11f"1" ., .. t!:S,. • .. EJ"·h:;:; """'''"''"''1+4,,,:I:::C\. ,j ... 1,.d ... ,g,t- .. , ..... [ ... ¥m.L

-4+ :tt-. '!"tiJ. :qJl 1E'~ .' !P=?-'- +r'~'r: In: .~~t~l:!t.~trL~ ~T!: :t-':~+i:..'":!:"t! .?!!JTI!! -~;;rrtilLi :i~.E~·~-!·,·p!~t~:::~t.:-.:·~~-~n·:: :t:E~t~ ~~ ~;-t.J-:r .. ~.~~F:·L ~·;~~r~-S·~::.~f~ -.::1. ...... ;ii::t..tiJ~J::·.:~J·;l ""t eLk ,. '- "At.::r:: ,,", .• ~;1.tl· .. ~' .. ·,·! •. ,~r'-'-'".t'-,H·'.',· I:::;' ,. ,., .. ""., -,,_. " .. ,4',.·,· ''';' "., .. ;,.t;i.f\..' '.'-+'" f ... , .. ::~~, .. , •. " .. ", "'~".' ~r-~' ;":rJ~ ~:;.._..,.. __ -1-4- ..... , ,f+-- .--<'_ .... -.- ~-,..,.·~t .... . .. 1 - .•. ,.,~..;.,...-.- .Wo- --!- •• ~~ ~,.~ .... ," w,:-~ ~-'''++·t'· ,~,!-J ...... "t- 1+n-li·-'J.-t'k:, ........ • ·,--_·t-· ·,t'·! .. -,--l"'·' .-t--r-~-······- ·-··-.-t ........ 1t~+ ctt: .,.,,' ,~-'- ...... '-t+t"'- j ;:i11:-+ H~ .. :lI,,'---;-L'" .. , .. ' .... ~ •. ,.1",'" l~' .. , ':1' ·'t'· '1"" I"" ,,, I'<;;~ ..... J , .. ,,, ,t, '~"±± .. ~ -t r;;r "L~f.1c;.1 mf f~F rt;r ILl'''~;i- O::-iiGtH :.'~;~~~::::.i1.p;: :'!1~ .~: l.!1;:rUF'!:;~I~: :.': 'K' ,.=: :,':': ; "et,;:1 iF"1 ,". :~::" :1, :t·LI-.. ~ t4:::. ;..; .. :1' '::,':':1::':· :::.-;:.1::.1 ~ ,., ~H ~t : I..~ ~ .. ~_~:~: ~~"\' ->-:,. :::+1.=~: ~:~ :-~.: ~~ ::t.:t:,-~": .. *.- .~: ~~. -: ;t8~.:i::7:fr~b.~r7~ ~'-~~7:_ '~7; l:~-:7::·:+:~q:+: ~~ ):;::: lt~,~ ,j:".i:ji "+[1+-,4' ,t;''''1ttHt ++trtmT·±!!..t+·tt .)fu·'~·~·l ~ .. , ... , +, .. ,§L'~'l" ~ .. ' .. +H , .... ,.=T:=:!w.:j::: '- ·, .. ·t· .. 1 J ,,' I:i:_ .. , , .. , ,~.~.L .. ·r-a thtfrh'. ,0.1'1: ++,,·14 ,::-;:z. ........ w_' '"' ., ,~ . ., '" 4-'-' • ".".t~., .... ". , .. , ffi' ,,;;: .. -'~.-. "J. r" ..••. "Tt;·, ~t;: .. + ' "" ,,' .~."" f ..... ', t1+t+r.n-1"~".J. t-j-tt f.-oh\ • .!.. t~ ... t-o-... -- +-l.::!...:i1-'" ,-+ ....... t-+T:..:::. ~t'.,.· r, .. ·t-·- ........... '----J ... ~._ -"- .- .......... ':"_~:::_.-.-.;:;..-+T +~;- - ......... ~.~. -±-.L:. .. ·r ~ \ .:.:.1.:-=--.::.'-' -11.>---~t-r_t:~~~~-----.1

,,, . '" ,m ., m±' ,",," 'C'l"" "'l' ""r", ",'" '"' ,~, " "" ,~ cc 'C, '., ,," ,u, d,,* ,,' ~ "'C;',,",:'l,:;:J., ... , ',,,I :f1+~tf~ 00 i#EtITJ!:tm ,:tttrj:mT rl .. f-t .. , ,~:ml-::fl'"' "t~ .. !o., +'fii::J+ ';:1+1~~ N ... 1 .. ~"'f,'- ,·-llt~'E.'-<! "'J ""L ' ~ t ., .. t'" .,- .. I." I ..... ~+.!. ~ 4- ~,; :.~£4E~::~: :±4r;.~l··:~ ~~l'---:~:: +: .. ~, ~f;~;: ;: .. ~ ~:. __ .. ~t< _:.1. ~ :..~:1==7!~:: ~:::,~~:.r~l"~7";t: ... ~-ti-·;~ ;--:4 .. :'~'::: -:~':f~:<-?::~'::::; ~t:t$W ~~t~fi t:~., --+-, _11=\ ,,4, ,:.\1 .. _.t.+ ~~!, ... L'"'' ",' t' .. n:ttt.Mm'--t.i~ '\'::R ........ ;!-t-,~ -'1""-'. t. T ... ~ .... ·t\, . I ... , ..... " ... h ".,.!" ,

*~ ~ ~: -r: ~r~~ :tt~ ~t~f:~~T:'~~-J~{;-if.:~~~.r:, ~~;~: ':,:~; .. ; ~~~: ~: ;~:t-:-~~I~; ::~ .~~ ~ .. :~~ ~r:~'!':~ ,::~ ?:~ ~!~:>:~~. ~:'A~ ~~-=-;.~;r~~~~;~~;;< :g~ .,t:tJ ... :t1 t+1::t'rtlkU.J_, r.t!'" .jr~:~:tL.Lt.:..·.ll~L .. ,..f .... +r·ftt' ' .. "d" 'I "1'- ,J . .:-li .•. ,. -;---' '1 .'. ".' .'·" .... l"~' ,."1~"~.",. ~ -- ts::~ .'''''' .,'H.!,l·,t· 'f""·"· S¥fMlli'h' .,. -Ii-'- ~" r~"""" +. '..-'"," ,. ''- • "... 1" _ ... ' , "1' .. ··_·- + ~ -I ~ ":{"~ "" -""',H-' ,+' ,.I~·"I' ,,~, -" L " . .,_ .. - . ·.'0"·". ~-!- .. +-I--'~· ..... r"t·~t .... +it- ~I ... __ ~--L- -I ~ '~1Ji: ... ,. t~:~:L '. -...... ~~,:-~~. --t ~ -..:;' , •• ---'.1 ---I ... ~! .... -I-.; ..... ·-1- 'l!"1-' t·, - •. '~ .)~- .' ·-'"~J:.1.:·~1 '1

---.. ... -"'1'4~ ~j - _<0- ~,~~-;;:1-r .. :. :t'":-;~:-:I~:--:-J-'-"-t--:+:.~ r:';'''1 t:·~ :'t..,.!"~, -.::-:~::::::-r-~" ~" ,-:. 1" .:'-.-rr'~., ~t--:-::tT"'" '_1-" ,-:;-:---- )'.'" rt~ :.-,~""-:::r:' ~--:-r"--;" ,j:*, ... ·m,-Hr:·~,, __ l' ,H"~~+".· .-£_o"t-" j ,,, .. ,, ...... 1 fr' 1,. ]' ,,11.,.~L. ,."-.,, ~' .', .... B··~·,'··l.· .. t" .. •• .. ), .. ft§ .-, 1 .. , .... 1. .:L,·r .... ,... .. ·f~·' "-''''st' ~"4!Eif" ~'" ..... ,. -- •• ~ .... , .. ,,,,, .. 1 ... , •. , .. "::J_El-"" J- ,,, ' BB" ., ,., " ..... ' " .... ,1. -, .... ,. "'" +. \... .., ti1'-:-+"1~+ .... t .. • .. ~~I~~T ,......-t- , .. PI 4--•• ,-' ~,l ~1.~~""t".:..Llr I1 f ••• ..,,<.~ ~ ... t!:!.l..:r;':.:.t!.l,.tL~' I.- ~~:-:.~~_ - • ~,. -,...-.-, - ''''-''- - t~ I' --t - T- - ~ ..... l.~ • .. +l..... ... ..... r.~ ,~_ ... --' ,<-~.. -: .. It _~ ~-~:·;-i·~~..-,...+~·t~:: .... ~ -::::r:-l< .:.-t-.. _r r~-Y-: ,~- :-.~ -;-r' 4 ••• :--'" ~l~.r J.I-~ .-~jj.~. I .,..·-:,-'"":t:-:'-:-;--;-I -:t-.... --.:

....... ".11: .-.,t..", '"-It· ... ~· "·1_1·~tt1·~· 'I"' ., .. t.,ClI L "," ,~-. It ........ "":K ,t,. , ." 1'" -m' , .. ~:t:L I'" .• 1 ,I~"'" . .-, .,. ,.. " ",.",.,-.. 4·""'·~'1+·'·~'"'' '_·· .... · .. 4;·'··"1',,·,,'.,'·1 .. "1 -' •. ,,,, .... , t '1 ""1 '~"~':':I:':'""""l ~;''''I ., ... , ..... ,...., ....... .,...,-t' +--'.j.,~", .... '"11 -..- ..1-" .....,.;.. .. J"'-·~i'\-· '~-t· ·t.*'·~--i-l~,t .~, , t ~, •• \ ,., - ~ -t~ ~ • :""=1 ~ , 1 '1 ~r" , --I-L~. ~ ••• -_., l- - -,., -'1 " ~j-~-: -::+:-!-;: "-:;":, .: ..... ¥.~::: 4--:rJ~~~' ~:::~"'~"';1:-. ...t...I.:-:'4" .~.j.---t-;-;- ';/1 -:';f ....... ,..-~_~ .... :~ ...... ~..-:, ::+.~ i--:r:.::C :::-n;-: .... ·-r.-.. -t~- :-::-':-:"'- ," -1.:::::-:1'.-- :-.' "<"~1" ;H:',I". ".·r - ""'71:'" ':1t' ['i' ,~: ... ~:-i~ ,1. ,.A.i· )' ·:cc.:i. I ,·T~7 .. " '1'"1''' "-'~'" .+ .. ~jt~,.; .. ~ , .:! ,'" 1" < t ..... -:;-i;~:: :tf~r~-·~!:1.:"'~""~1- :.tt~·ht: ~r:~' !:'_!I;;l:-:r.:.:t'~_:.~" , ~ ~:--:l~;: ,;;;:·:·d~ .... !" T+·r .. ~ ... "t : :~,t~ :'\ :1' ~', <~.~ >~.:i~L.:~r:--:-::f'~;:t:·: .. '."-'" i· .: -' t-:': .. '·:r. t T .--,

ri:.~E!£i ~~~~i~::ki:}(:~l~Hiill.i1}FhL·L]u~q~'1;L~djt1=~lr·l;;it~kt,jU"t;Et fJ1Cj; jJ;i'-JSL.;.T' .!' I2jL~: ···±td·' ",. ~;" '1-'-;jt;"--t:,:, Wif,:!'.[[, ... "(.,,, ! .... " ,,1-- .. ··- '1' .+J.i. .. 1 . ,,'" '." t.']' 'E'''' --'c 1. 't- . r . t. '" ..... j, [l": '1' •. " .... ! .. ' --l" , . . ,. t .. i ~~. .. ... \.,. i m';:r;cc :t~~J·T:":··f~3.:f-:::·;' .;~-r~tti-tr ~~:~··::!,tt~~~·l.~: :r:,J,-:' ;A'~:-!";-f'Jl ::~-:~:-:-:·:t··i_~rrTi! .!·jtll:_~~·:r:L' J ~ .1: ':, ·~-:J:"'KJ: :.: ,,'::'-)' t-:;:-'t.:·I~.j: ',·::; ... :r. :1,;: :, ':··~4~j-i

.... 1+1·,·. h' ·,,"cF'u·"'":-:t'-- .,. ,. ":L:.;::.:;r'· .1....;.;.J .~r.l t~" ''''i ""+" ',"'c. 'T'r:f" "",' ... , . ":;.:j . lip ... ,,1,.,., ... :, ..:+- ,,' ..... , '1. ~' .-t .. i ""1

lEt~; . ·t+i?:'h: ;-j1 ;-~(-r::12;:j i;:';t'E::l~:·'L~~~t:~i~: ::i)il~Ef]S; {PG:; y~; iHi:?liFI T :7<:: r N.. l ~·;:l0I~ . .r :TF':T~;j~ ,( C:> rs "><7! ~,..."..i!', ~+.f. .,.. r' .. ··-"'·r'·'~ .... ~"'"T""+-- \ ... '1""-4" ;'T·lr~·---t--·~ ~"·r·-· ...... , ........ _" ...... , ~ ~ · .. · .. ·-·· .. _ .. --1"-·' .. ··, .. ,'-><· "c. t .... -. ~ .. -t .... ..,

~~;~~~ ~!t~~t:t);i; :~~~~t*~j~;:Xli;~· ~r;:t~i~~~~~~l~!;l~i:~F+, ~1 ;i~W; ~( :;+~:{4f~i-t;I]±fl~~-il:e-f:~#t~~J ~ ~+J.j!;) _ .. " CL... =rt .. 'lit-I' ";L .~., ""ij'" L" --' .. ,,+. ". ,-+, "j'1. "q:.; .. ,~ .. ~ ... , !~"+L' H·' tb'}' "H .. ·' t. "-"'. ". [".,i . ,f: 1-'''1'''tft" .", , .. !':'- .. ,.! .... I'~ -' :;g ..... ~,:J +~,::'~,~ -: ::~-~ ;~ .. r;L: ~::.: ~~~t~i- :~-nt+~f !r::it~~" ;;i~1:- +1it ~: .... +iT~~~-:f~-<\; ~-i- ~~~~lf' ~ 8:: ~;t~ +1 ~!~ .... :~~~:?! :fi:.q.'::i;~ ~~8% :5rr~:::i': :~:I.:-t : ;~ ... ~i::::71

I'H-"~t' ±£h'tE~f.:ffi1 '1'" r'" 'rrlT:"'§"tt!., .rf,,~W~'::ti~"·11 .,.::;: +1~j:t;q:t7tU.tt't .. ~.t. ' ell "".J'k;' '''rL''''''\'r-'·''l''';+'.c.~.;ijf~"j~ '-1"1 :1::~1:,~ ~ ~~i-~ ~ :r~~t~ ;~1-ry~::~. ~~.t~I::. 1'-:~ - .. : .22>,:~:+f:~ ~~ . +; ~~:T~~ :p:t:.: ~~;'::1~: ~ ~~~:. ~.:~~ .: .;.~ .:~:; .. ~~~.:~: !:i:+t::~ ::f: ~j,~_-~~~~~ ~ rf~~tL~ ~

'. ·,r ,2,..1 ,;:r:.:llt-,;..'t;;';+··J:::rrl:.t;·...;-l .• .j;,,..,. 1.1.::!.,~, -r"~R'+"WsC"+-1.t+ .+t:;:h:r~tt:tf7[i'" '::-<Ifi·;· t1j~ijl, ... ~ .... ,,~ti·'" '['is' •. i14+'""'C"'ill+ ·i.~'r .~'":t",. l*' "['+i"m" ~ r ... +.·I;.+o •. l.;'; ...... ;.,J"1:,~ I.J..;-fm'-'-'H-1'J +Cl 1"+. i~- +h ... [':1''';+ rrc •. ,", ,r", .... ..' c,.: " .. ,,' r ·'·,·,··t '~H-"W ,r';- +r - .,H ... ·, ... t " •• ~r-, T~-t :-!d:±:..:.·..+I·1- .'+-l-t-rt++.t--, .;:.;:ct.-+~LJ: "':rrt· r+-i-- ..... * ..... hriH·t+,.;;- ....;.r·~;+. ~ ~:' ;f L.;. .,. ''''fHTt-" q¥- t-i--t· 'rh i+ ·'·Hit--:j·· ~~~ "_1,1.,

.. ;!: ::::t.J.:;";~"'·r~' .. -j...·~-t~!-+ .... ' ... •· .... ~·',' -r--i- ~ _ .. _'-4 --H- 1-' ~+",~., +- n:;._ .... ;.,-1--- ... .l..r-t-+-+ .,., .... +:L., ~,,~ ... + k"".~+_ •. -+-t-~+ t .. - +~""·fr-±.j: '1'~ it' 'rH ~J:g~ :;:tL •• "':ii ~filfEi7' .. iif.trttt :1+j'- ttiTi-+t+ h~' tt...:-l=t;, t;':~r'" t . ''t'~ ,.Jl1li:+r l.ij:5. 31m:.~" rgtf~·~ ~ . ,~ . -;~.~i::t:r +!= .1, ..... ~~ ... ,..:.- r;t; J.~, t~~)i.~ :t1~ ·ts-t-~ :tt± m .~I-"-illiJ· ..0- • • -H-' tt-rh:t:4:i-~:' tt::'~lt':r ~ ·~·H;.t=-- -."4~-;', ...... ~., ".':;~-+ '-!T' :,,~.~:I, "'ri :' .U-

10 lm 1::Q

.~""'c.l ._~.=>" __ •. ____ ,. ---- .,-_ .. _-_ .. _ .. - - .. _-_.