RESEARCH DEPARTMENT

Final Report on Wrotham AM/FM Listening Tests

Repo rt No. A.032/2

·SerialN~. 19Sq/36

THE BRITISH BROADCASTING CORPORATION

ENGINEERING DIVISION

RESEARCH DEPARTMENT

FINAL REPORT ON WROTHAM AM/FM

LISTENING TESTS

Report No. A.032/2

Seri al No. !95lj./36

---(W. Proctor Wilson)

This Report is the property ot the British Broadcasting Corporation and may not be reproduced or disolosed to a third party in any torm without the written permission ot the Corporation.

Section

1

2

3

4

5

Report No. A.032/2

FINAL REPORT ON WROTffAM AM/FM

LISTENING TESTS

SUMMARY •••

INTRODUCTION

ANALYSIS OF LISTENERS' REPORTS

2.1. 2.2. 2.3. 2.4.

2.5.

2.6.

Receiver Locations Receiving Aerials •• Analysis of Listeners • Tuning of Receivers •• Comparison of Systems. Fading

Title

2.7. Summary of Listeners' Opinions

ASPECTS OF THE AM/FM COMPARISON NOT COVERED BY THE LISTENING TESTS

CONCLUSIONS • •

REFERENCES

APPENDIX

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4 7

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ID

CONFIDENTIAL

Report No. A.032/2

December 1954 Serial No. 1954/36

FINAL REPORT ON WROTHAM AM/YM

LISTENING TESTS

. SUMMARY.

.A Research Department Report 1 was issued in December 1950 as an interim statement on the results of the AM/FM listening tests from the VHF experimental station at Wrotharo. This report summarised the replies to the first 45 question­naires completed by those taking part in the tests. -

The AM transmissions terminated on 31st December, 1953 by which time a total number of 123 completed questionnaires had been received; all the replies are summarised in this final report. The main conclusions drawn from them, which are substantially the same as those given in the interim' report, are as follows:

i. Within a range of 30 miles from the transmitters the great majority of listeners considered either that.AM, AML and FM gave equally good results, or else expressed a-slight preference for FM.

ii. Beyond a range of 30 miles from the transmitters the majority of listeners expressed a marked preference for FM, mainly on account of the low levels of background receiver hiss and ignition interference.

iii. Many listeners did not notice any difference between AM and AML. A preference for AML over AM was expressed by some listeners living in locations 'subject to heavy motor-car ignition interference.

iv. Fading occurred on all systems at distances exceeding 40 miles from the transmitters and on the average appeared to be of equal intensity ori all systems. Between 40 and 70 miles, the fading was usually slight and due mainly to aircraft. At distances exceeding 70 miles, bad fading occurred at practically every listening·site.

In order that this report shall cover the 'who1e period of the listening tests, -some of the inform·ation contained in the interim report is repeated.

1. INTRODUCTION.

The number of medium- and long-wave channels available to the BBC has become insufficient for the adequate national distribution of three programmes.

2

The use of VHF transmissions in the 87"5 - 95 Mc/s band is a possible method of increasing the number of channels available for sound broadcasting. Practical considerations limit the choice of the modulation system to be used to the following three types:

a. AM (amplitude modulation)

b. .AML (amplitude modulation with a noise limite!' incorporated in a wideband receiver)

c. FM (frequency modulation)

(the transmitter is identical in the case of (a) and (b), the only difference being in the type of receiver used).

A number of BBC reports 2-

S have described the results of laboratory and field tests (carried out under controlled test conditions) to establish the relative merits of the three systems for broadcasting purposes.

Arising from these tests a decision was made to carry out a field trial of the three systems in order to obtain the opinions of a large number of listeners (technical and non-technical) in conditions of normal domestic reception. A trans­mitting station was built atWrotham (Kent) and two high-power transmitters were installed. FM was radiated on- 91'4 Mc/s, carrier power 25 kW, and AM on 93'8 Mc/s, carrier power 18 kW. Both AM and FM transmissions were radiated simultaneously with horizontal polarisation from a common aerial having an effective gain of 8 dB over a half-wave dipole, the mid-point of the aerial was 406 ft above ground level, and the site height 725 ft above sea level. The ambient field-strength contours of the FM transmission are shown in Fig. 1. The corresponding field strength for the AM transmission is less (on the average) in the ratio of square root of the carrier powers (1, 18 i. e. 1- 4 dB); it should, however, be remembered that field strengths on VHF vary widely between points a relatively short distance apart, particularly if the country is hilly or built up. The contours shown are average values over the p articular area concerned.

Pre-emphasis, with a time-constant of 50,usecs, was applied to the FH transmission, and a corresponding degree of de-emphasis was incorporated in the receiver. No pre- or de-emphasis was used in the case of the AM and AML trans­missions.

Forty receivers manufactured to a BBC specification were distributed to technical and non-technical listeners living at distances up to 135 miles from the transmitters. They were designed to receive either AM, AML or FM, automatic frequency control was incorporated to overcome the effect of oscillator drift. The loudspeaker and audio stages of the receiver which were common to all three systems, were typical of those used in moderately priced domestic medium-wave receivers.

Those taking part in the tests were asked to complete a questionnaire, a copy of which is shown in the Appendix. The transmissions were commenced on 17th July, 1950 and the AM transmission was terminated on 31st December, 1953, by which time a

I. • "'1"*"b I. ,.

GROlNO c.oN1OURS

0' '2OOFT

o 400" r" GOO Ft.

2. 3.

~o Miles

40 Kilometres

N

t

Fig. I Measured field str

af f.m. transmitte:n1t~ cantours Power a rotham.

A.erial height 25 kW Slt~ height 406 ft A.G.L Aerial gCln ~~~ ft A.S.L

Frequency 91·4 Mc/s

4

total of 123 completed questionn~ires had been received, it is these replieswilich are summarised in the following sections.

2. ANALYSIS OF LISTENERS9 REPORTS.

2.1. Receiver locations.

The locations in which the receivers were installed are sho'WD. in Fig. 2, from which it is seen that a fairly large proportion of the sites were within ao miles of the transmitters. The object of this distribution was to obtain as much information as possible regarding the widely differing reception. condi iiiona obtaining in the London area.

2.2. Receiving aerials.

Outdoor half-wavelength dipoles were installed for a few of the listen~, but generally speaking, in order to obtain a quick turn-round of receivers, indoor aerials were used. In all cases the indoor aerial was a half~wavelengt.h dipole made of twin flex: m01,lD.ted on the picture rail of a ground- or· first-'-floor room, and in a few cases in the loft of a house.

2.a. .Analysis of listeners.

Number of technical listeners Number of non-technical listeners

2.4. Tuni~g of rec.eivers.

72

51

Only three listeners reported any difficulty in tuning the receiver, but this did not appear to be associated with one particular modulation system.

20 5. Comparison of systems.

Wi thin 30 miles of Wrotham, reports were received from 56 listeners.

13 expressed no preference, all systems giving excellent reception. 31 expressed slight preference for FM. 9 expressed a marked preference for FM. ~ expressed a slight preference for AM or AML.

SUMMARISED GRADINGS*

Receiver Hiss Impulsive Interference

IP JP P SD D IP JP P SD D

FM 44 8 4 0 0 42 10 3 1 0

AMI. 18 18 13 4 3 32 10 13 1 0

AM 18 14 16 5 3 31 5 13 4 3

* A nota on the &~Ad!n& of interferen0e ia &iYBn at the end Of the Ap pendlz.

,. . l \j1ti ili! ,. . ,.

GROUND cc:»nOURS 130

20 30 Miles

ao 30 40 Kllomt!trcs

r NU"'bvr~ ;n$'de imall c.relu d_"Oh number 01 rccc'~Cf' ,nstalled

~ The 'ar .. c Clrcl.' CrI ,,_nUed 00 W'oll'om ot ,nUnolS 01 10 mitu

N

t

Fig. 2 Receiver installations for

a.m./f.m. tests from Wrotham

6

Between ro and 50 miles from Wrotham, reports were received from 32 listeners •

. 5 expressed no preference, all systems giving excellent reception. 11 expressed a slight preference for FM. 16 expressed a marked preference for FM.

SUMMARISED GRADINGS

Receiver Hiss Impulsive Interference

IP JP P SD D IP JP P SD D

FM 21 7 3 1 0 il.9 8 3 2 0 AML 10 8 5 5 4 17 5 5 2 3 AM 9 8 6 6 3 15 5 5 4 3

Between 50 and 70 miles from Wrotham, reports were received from 15 listeners.

3 expressed a slight preference for FM. 11 expressed a marked preference for FM. 1 expressed a slight preference for AM or .AML.

SUMMARISED GRADINGS

Receiver Hiss Impulsive

IP JP P SD D IP JP

FM 7 6 2 0 0 9 3 AML 1 1 4 3 4 5 2 AB 1 1 2 5 4 5 2

Interference

P SD D

3 0 0 2 1 3 1 1 4

Between 70 and 90 miles from Wrotham, reports were received from 12 listeners.

4 expressed a slight preference for FM. 8 expressed a marked preference for FM.

SUMMARISED GRADINGS

Receiver Hiss Impulsive

IP JP P SD D IP JP

FM 6 2 1 3 0 5 2 .AMI, 0 2 3 3 4 4 1

AM 0 2 3 2 5 4 1

Interference

P SD D

1 4 0 1 4 2 1 2 4

Between 90 and 135 miles from Wrotham, reports were received from 8 listeners.

il. expressed no preference, no 'system giving a satisfactory service. 1 expressed a slight preference for FM. 6 expressed a marked preference for FM.

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SUMMARISED GRADINGS

Recei ver Hi ss Impulsive Interference

IP JP P SD D IP JP P SD D

FM 1 3 1 1 2 3 1 2 1 1 AML 0 0 1 2 5 2 0 3 1 2 AM 0 0 1 1 6 2 0 2 1 3

2.6. B'ading.

Fading was reported by many listeners at distances exceeding 40 miles from Wrotham. Opinions varied regarding the relative severity of the fading, but it appeared that on the average all three systems were equally affected.

The fading became more severe with increasing distance from the transmitters; a brief analysis of the reports is as follows:

Distance from Wrotham 40 - 70 miles

Total number of reports received Number of reports of fading Number of listeners reporting fading Number of listeners reporting fading Number of listeners reporting fading

as "slight" as "marked" as "severe"

Number of listeners attributing fading to aircraft

Distance from Wrotham over 70 miles

Total number of reports received Number of reports of fading Number of listeners reporting fading as "slight" Number of listeners reporting fading as "marked" Number of listeners reporting fading as "severe" Number of listeners attributing fading to aircraft

2.7. Summary of li steners I opinions.

23 16

6 6 4

11

20 17

5

6 6 7

The results of the listening tests may be summarised briefly as follows:

i. Within a range of approximately 30 miles from the transmitter no marked preference was expressed for any system, reception conditions were excellent on AM, AML, and FM. Where any preference was stated it was for FM, because of the absence of receiver hiss. Listeners were impressed by the comparatively crude aerial systems which gave satisfactory reception on FM.

ii. Beyond a range of 30 miles the majority of listeners expressed a distinct preference for FM.

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Hi. Many listeners did not notice any marked difference between AML and AM.

FM

A preference for AM1 was expressed by some listeners living in locations subject to heavy impulsive interference.

iv. Fading at times was reported on AM, AM1 and FM at distances in excess of 40 miles. Many of the cases of fading reported from points between 40 and 70 miles from Wrotham occurred only in the presence of aircraft. Beyond 70 miles tropospheric fading occurred and limited the value of the service in many instances. The effect of the fading when present was approximately equal on all systems.

v. Owing to the different characteristics of the three modulation systems in the suppression of receiver hiss and ignition interference, it is difficult to decide how to estimate the field strengths required for comparable grades of service on the three systems. This is particularly the case for.AML which, compared with AM, gives no suppression of receiver hiss but a substantial reduction in impulsive interference. The follo~ ing table, based on both the present tests and previous tests carried out by Research Department in a wide variety of conditions, is thought to represent a reasonable compromise between the conflicting considerations:

First Class Service Area Second Class Service .Area

Field Strength .Approximate Range Field Strength Approximate Range (mV/m) (mV/m)

1 45 miles (70 km) 0"25 60 miles (95 km)

Al1L* 3 35 miles (55 km) 1 50 miles (80 km)

AM 10 25 miles (4,0 km) 3 35 miles (55 km)

* Assuming receiver I.F. pass band of' ±75 kc/so

3. ASPECTS OF THE AM/FM COMPARISON NOT COVERED BY THE LISTENING TESTS.

The field trials described in this report deal with an important, but nevertheless a single aspect of the problem associated with the establishment of a VHF broadcasting service, namely the relative performance of three different modulation systems, on the basis of a single transmitter for each. FM unquestionably gives a superior performance on this basis. It does so, however, only by occupying a greater radio-frequency bandwidth than either of the other two systems under review.

There are two important associated considerations which arise, but which are outside the scope of this report. One is the relative performance of the systems under conditions of interference from stations operating on the same or adjacent channels. The other is the relative complication of receivers, and the difficulty listeners may experience in tuning them.

4. CONCLUSIONS.

The listening tests described show that within a distance of 30 miles from

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the transmitter the three systems, AM, AML, and FM, ~ive equally ~ood performances. If crude indoor aerials are used a satisfactory service will still obtain but AM and AML may be subject to a sli~ht amount of back~round receiver hiss. At distances exceedin~ 30 miles from the transmitters FM ~ives a performance superior to that of the other two systems on account of the low level of both back~round receiver hiss and motor-car i~nition interference. AML is superior to AM only in areas subject to heavy impulsive interference.

Fadin~ occurs at distances exceedin~ 40 miles from the transmitters, at distances exceedin~ 70 miles the fadin~ on all systems is such that even a second­class service is unlikely,

In ~eneral, the listener reports differ from results of tests described in previous Research Department reports in that the ~rade of service for a ~iven field stren~th is rather better. This is because earlier tests were usually carried out under conditions of severe i~nition interference, whereas the level of interference avera~ed over all the listenin~ sites covered by this report was probably low.

These conclusions apply to AM, AML and FM usin~ sin~le transmitters of equal

power. On this basis FM ~ives a superior performance but at the expense of occupyin~ a ~reater radicr-frequency bandwidth than that used by the other two systems.

An important factor in the choice of a system for a National VHF broad­castin~ system which is not covered by the tests is the relative performance of the systems under conditions of common channel and adjacent channel interference.

5. REFERENCES.

1. BBC Research Department Report No •. A.032, Serial No. 1950/30, "Interim Report on Wrotham AM/FM Listenin~ Tests".

2. BBC Research Department Report No. A.017, Serial No. 1946/4, "Frequency Modulation".

3. KIRKE, H.L.: "Frequency Modulation, BBC Field Trials", BBC Quarterly, Vol. I, No. 2, Summer, 1946.

4. BBC Research Department Report No. G.036, Serial No. 1947/12, "Impulsive Interference in AM and FM".

5. BBC Research Department Report No. G.036/2, Serial No. 1948/39, "FM/AM Improvement Above and Below the Improvement Threshold or Noise Barrier".

6. BBC Research Department Report No. G.044, Serial No. 1949/10, "Impulsive Interference: Comparison of FM Reception with AM Reception usin~ Noi se Limi ters" •

7. BBC Research Department Report No. A.028, Serial No. 1949/34, "An Experimental Comparison of Amplitude and Frequency Modulation at a Frequency of 90 Mc/s usin~ Indoor Aerials".

8. "The BBC Scheme for VHF Broadcastin~", BBC Quarterly, Vol. VI, No. 3, Autumn, 1951.

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APPENDIX

WROTHAM EXPERIMENTAL TRANSMISSIONS QUESTIONNAIRE

1. SITE

1.1. Na;me

1.2. Address

1.3. How far is the site from motor traffic?

1.4. Is the traffic continuous or occasional?

1.5. On what floor is receiver installed?

2. AERIALS

2.1. Give details of aerial used. (type and position)

3. RECEIVER

3.1. Has any difficulty been found in tuning-in?

3.2. Do you notice any difference in quality between FM, AM, AML?

3.3. How does the quality c0m.P are with your local medium wave station?

4. RECEPTION CONDITIONS

4.1. Receiver Hiss (a steady background llrushing" noise). Do you notice receiver hiss on any of the transmissions? If so give the grade for all three systems and for your local medium wave service (See Note at end of questionnaire). If more than one type of aerial has been used, give the grade for each.

4.2. Interference from motor cars (sharp crackling interference as car passes). Do you notice car interference on any of the transmissions? If so give the grade for all three systems and for your local medium wave service.

4.3. Other electrical interference (from domestic appliances such as vacuum cleaners refrigerators). Do you notice interference from these appliances on any of the trans­missions? If so give the grade for all three systems and for your local medium wave service.

4.4. Fading or distortion. Has any fading or bad distortion been observed? If so on Which system was it most noticeable? Did you associate any such fading with the proximity of aircraft?

4.5. If you have a television receiver, does car interference affect eitlier the sound or vision signal? Give grade of interference in each case.

Systems: FM 91'4 Mc/s AM 93'S Mc/s AML 93'S Mc/s

I 1 j

'\ t

j

5. PREFERENCE FOR· SYSTEM

5.l. On which system would you prefer to listen for a broadcasting service? Give reasons.

5.2. Do ¥ou prefer the chosen system to your local med~um wave service? Give reasons.

6. OTHER OBSERVATIONS

Have you any other observations to make on the tests?

(If there is insufficient space here, use sep ar ate sheet).

NOTE ON GRADING OF INTERFERENCE

11

In order to facilitate analysis of the information received, interference should be graded according to the following scale:

IP (Imperceptible)

JP (Just perceptible)

P (Perceptible)

SD (Slightly disturbing)

D (Disturbing)

No interference heard.

Interference just audible in gaps in the programme when listening with attention.

Interference audible but not annoying.

Interference rather annoying but not so bad as seriously to degrade the programme value.

Interference very annoying and'such as seriously to degrade the programme value.

EXAMPLE OF ANSWER TO SECTION 4.2. (This does not represent in any way the interference expected in practice)

FM SD

.AM

SD

.AML

P (occasionally)

MW

IP

Q2/4/51