ansi ieee 0206-1960

ANSI/IEEE Std 206-1960 (Reaffirmed 1978)

IEEE Standards on Television: Measurement of Differential Gain and Differential Phase

Published by The Institute of Electrical and Electronics Engineers, Inc 345 East 47th Street, New York, NY 10017, USA SHOl255

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ANSVIEEE Std 206-1960 (Redifinned 1978)

An American National Standard

IEEE Standards on Television: Measurement of

Differential Gain and Differential Phase COMMITTEE PERSONNEL

Subcommittee on Video Signal T r u u m i d o n Methods of Ywurement J . 11. BARSTOW. Chairman 1955-1959 J . L. JONES. Charman 1954-1955

J . 11. H;irstow 1954-1955 K. n. h n W n 1954-1957 R. I. Brown 1955-1957 R. D. Chipp l95Cl958 11. H. Diehl 1955-1958 S. Doh. Jr. 1954-1955 A. B. Ettlinger 1957-1959 hl . E. Claystein 1954-lW7

F. J . I k r r 1958-1959

II. Mate 1956-1959 L. R. Muffet1 1954-1955 R. 11. klnrris 19sC1959 R. E. ~IulleiiKer 1954-1955 R. S. O'Hricn 1954-1957 J . R. Popkin-('lurnian 1956-1959

t i . r. K ~ I I ~ 1955-1957 E. B. Pores 1955-1959 E. H. Schreibcr 1954-1959 I,. Starhover 1955-1959 D. Taylor 1958-1959 B. F. Tyro11 19sCl955 J. W. Wentwurth 1956-1959

J . F. Wiggins 1958-1059 W. B. whole 1957-1959

Committoo om VM.o Tubniqoa C. L. FRLDENDAI.~.. C h i r a a r 1959- S. DOEA. JR.. C k Y - 1 1957-1959

J. M. BARSTOW. V k - C h i n r o r 1959- G. L. FREDENDALL, V k - C h i m u r 1957-1959

I. (*. Abrahamr 1957-1959 S. W. Athey 1957-1959 A. J. Baracket 1957-195X

j . H. Battiron 1957-195a

E. E. hnhan i 1957-IPSR K. B. BCIIWII 1957-1959 E. Y. c ~ ~ l 1957-195a

S. Deut rh 1954-1959

V. J. Duke 1957-1959 J. R. H&k 1957-1959 J. L. Jm 1957-1959

W. J. Poeh 1957-1959 1. M. Barstow 1957-1959 L. B. b v i r 1957-1958

S. Dob2. Jr. 1959-

R. T. Petnutclli 1957-1958

Ad Hoc Sukoounittoo for tl10 R d o r of tho Ropua Stmadud 011 tho Mmmromont d DWorontid 0.in and DiUormtid phrr, 1-

W. T. WINTIINCHAM. Chimar W. F. Baiky J. M. Barstor

S. W. Jr. J. G. k. Jr.

c. H. Page

stmadud8corpitbn lWFlQd0

J. Avinr W. F. &de) M. W. Baldwin. J . T. Bmgert W. R. Bennett J. G. Brained D. R. Brown P. S. Carter A. G. Clavier

R. F. SHEA. C h i m e r J. G. KREER. Ja.. I'irr-Chiraan

S. Doh, Jr. P. Eliar R. f. lliivilaiid

R. J. Farbcr R. \V. Johnstnit D. G. Fink I. Kcrncy G. L. Fredcndrll A. E. Kerwieii E. A. G r C r c.. s. l r y A. B. Glenn Wayne >lawn V. M. Gr;ihaiii D. E. .\l;ixwell

R. .A. Hackhilad

Jr. G. A. F~pcrscii .A. C.. JCIIWII

C. H. PAGE. ViCr-ChaUr L. G. Cuwrlwc. I'iCe-Chairmar

P. l ler t t X i . L. Phillip H. I. Mctt R. L. Pritchrd H. R. \limn0 P. A. Redhead E. Mittclniaiin , R . S c m l l L. I f . Montgonicry. Jr. W. A. Shipnun

H. R. Terhune G. A. Morton R. C. Moyer E. W c k r J. H. \luIliKaii, Jr. R. B. Wikox A. A. Olincr W. T. Wintringhm

M.amr0-b- J. G. Kreer. Jr.

Approved June 11,1959 Reaffirmed September 20,1972

Reaffirmed June 16,1978 IEEE skndudr Bwd

Approved December 4,1961 Reaffirmed November 10,1969

Reaffirmed July 18,1979 American National St.ndudr Institute

8 Copyright 1960 by The Inrtitute of Electrical and Electronia Engineem, Inc


1. INTRODUCTION 1.1 DeJinitions

HIS Standard describes a method for measuring differential gain and differential phase in equip- T ment transmitting either monochrome or color

television signals. The characteristics to be measured are defined as follows.

Differential Gain.' I n a video transniission system, the difference between (a) the ratio of the output amplitudes of a small, high-frequency sinewave signal a t two stated levels of a low-frequency signal on which it is superimposed, and (b) unity.

Note 1: Differential gain may be expressed in per cent by multiplying the above difference by 100.

Note 2: Differential gain may be expressed i n db by multiplying the common logarithm of the ratio described in (a) above by 20.

Nok 3: In this definition, level means a specified position on an amplitude scale applied to a signal waveform.

Note 4: The low- and high-frequency signals must be specified. For purposes of this standard these signals will be as they are afterwards described in this paper.

Differentid Phase.' In a video transmission system, the difference in output phase of a small, high-frequency sinewave signal a t two stated levels of a low-frequency signal on which it is superimposed.

Nok 1: Notes 3 and 4 applied to Differential Gain above, apply also to Differential Phase.

1.2 Scope of Afiplicdwn The primary application of this Standard is intended

to be in the field of routine operational and maintenance tests, where rapid interpretation and communication of test results is nicessary or desirable. The basic techniques described here are also applicable to laboratory measurements, proof-of-performance tests, and detailed maintenance procedures.

2. ANALYSIS OF THE MEASUREMENT PROBLEM

2.1 Characteristics of Television Signals Certain significant characteristics of both inono-

chrome and color television signals are illustrated b!. Fig. 1. Color television signals differ froin monochrome signals, shown in Fig. l (a) , priniarily by the addition oi two other signal components, which are shown sep;i- rately iri Fig. l(b). These are:

This is a revision of the tfefinition o f dilferciiti,il gain i i i " I ltl< Standards on Television: Detinitions of Television Signal 5lcusiire- iiietit Terms, 1955 (55 IRE 23. Sl)." PROC. IKE, ~-01. 4.3, pp (119- 622; May. 1955.

This is a revision of the definition of differential phase i l l "lltI< S:aiidards on Television: Definitions of Television SiKnal Xleasurc- inent Terms, 1955 (55 IRE 23. SI)," PI(OC. IRE, vol. 4.3, pp. 619- 622; May. 1955.

U (C )

Fig. 1-Waveforms of typical television signals. (a) Luminance or monochrome signal. (b) Chroniiiiaiice and color sync signals. (c) Composite-color signal.

2.2.1-A burst of about 9 cycles a t a subcarrier frequency of approximately 3.6 iiic transmitted during the blanking interval following each horizontal sync pulse except during the equalizing pulse and vertical sync pulse intervals. This serves as a phase refererice for subcarrier regenerators i n color nionitors, receivers aiid test equi ptnen t .

2.1.2-A 2hroniinance sigiial, consisting of the side- bands of the phase-and-a ni pl i t ude- mod ula ted su bcar- rier, transmitted during active scanning tinie. To a first degree of approxiniatioii, the phase of the chror:iinance sigiial controls doiiiiniint wavelength i n the reproduced picture, while the iiriiplitude controls purity.

The composite color sigiiiil, coiisistiiig of the sum of the various compoiieiits, ii1)I)eiit-S as shown i i i Fig. l(c). Sote that the effective iisis ot the c,hroniiii,iiice sigiial iiiiiy vary through the luiiiiii.iiice riiiige since th i s irxis cwiiicitles \v i th the level oi the iiioiiwliroliie sigrial C O I I I -

~'ollcl1 t .


Differential gain other than zero i n a video transmission system may cause undesirable variations in the purity of reproduced colors as a function of luminance level. Siinilarly, differential phase other than zero i n a video transmission system may cause undesirable variations i n dominant wavelength a s a function of luminance level.

2.2.1 Differential gain other than zero in a niono- chrome transmission system produces compression or expansion. The effects of differential phase in a monochrome system are minor and may usually be disregarded.

2.3 Significant Variations in Operating Conditions The average picture level (APL)' of a television signal

depends upon the average luminance of the. televised scene. For faithful reproduction, the system as a whole must transmit low video signal frequencies extending to zero or dc. I t is not necessary to transmit the so- called dc component through all parts of the system, however, since this component can be restored a t any desired point by dc restorers or clampers. There are, therefore, two significantly different sets of Operating conditions in television systems, depending on whether the dc component is present or absent. These conditions are illustrated in Fig. 2.

2.3.1 When the dc component is present, as shown in Fig. 2(a), the amplitude range required for the monochrome component of a television signal is fixed at 140 IRE scale units.'

2.3.2 When the dc component is absent, as shown in Fig. 2(b), the signal for a given luminance varies with the apl. For practical purposes, it is sufficient to consider signal conditions corresponding to variations in the average picture level from 10 per cent to 90 per cent. Under these conditions, the total amplitude range required for the monochrome component of it television signal in ac-coupled equipment is equivalent to 105+96 =201 IRE scale units.

3. REQUIREMENTS FOR STANDARD MEASUREMENTS

3.1 Apparatus Required

The apparatus required to measure differential gain and phase in accordance with the method'described in this Standard is shown in block diagram form in Fig. 3. It consists of a test signal generator, an output signal analyzer, and means for displaying or indicating the test results.

' XPL, or average picture level, is defined as the average signal level, with respect to blanking level. during active picture scanning time (integrated over a frame period, excluding blankin intervals), expressed as a percentage of the difference between t i e blanking and reference white levels. (Cj. Fig. 2).

For a description of the IRE scale for measuring television signal levels see "IRE Standards on Television: Measurements of Lumi- nance Signal Levels, 1958 (58 I R E 23. Sl) ." PR&. IRE, vol. 46, pp. 482-486; February, 1958.

14% APL

n IO0

n

U -/os

Fig. 2-Variation of signal excursions with APL. (a) DC component present. (b) DC component absent (includca effects d standard sync and blanking 3gnab.)

(b)

Fig. &Apparatus for the meayrtment d differential gain pad differential phaae.

3.2 Rcquirmnts for the Test Signal 3.2.1 A m p l i l d e Range. The low-frequency compo-

nent of the test signal should be capable of exploring the amplitude range corresponding to the blanking-to- reference-white range of a normal composite picture signal for each of the following average picture level conditions: 10 per cent, 50 per cent, and 90 per cent.

3.2.2 Continuity. If the low-frequency signal explorea the amplitude range in discrete steps, the separation between steps should not exceed 12.5 IRE scale units, where 100 IRE scale units equals the blanking-to- reference-white range.

3.2.3 Frcquenc+s. A high-frequency sinewave of 20 IRE scale units peak-to-peak amplitude and of a frequency approximately equal to the color subcarrier frequency (3.579545 mc) should be added to the low- frequency signal (on- the order of 15 kc). (See Section

3.2.4 Additwnal General Requirements. The test signal should contain such elements of a composite signal (sync pulses, color sync bursts, etc.) as may be required for proper operation of clampers or other control devices included in the specific equipment or circuit under test. The test signal should not alter the normal operating characteristics of the specific equipment or circuit under test.

5.1.)

3.3 Requirements for the Test Signal Analyzer The test signal analyzer should provide means for

measuring the amplitude and phase of the fundamental component of the high-frequency signal as functions of the level of the low-frequency signal.


3.4 Presentation of Test Results 3.4.1. Voltage. The voltage corresponding to 100 IRE

scale units (blanking-to-reference-whi te) should be stated.

3.4.2 Average Picture Level Conditions. Results should be presented for 10 per cent, 50 per cent, and 90 percent APL conditions separately or for that single condition yielding the largest value of differential gain (or phase). (See Section 5.3.)

3.4.3 Diferential Gain Data. Differential-gain data may be expressed as:

a) A function of one of the stated low-frequency levels with the other stated level arbitrarily fixed, or

b) The extreme values of differential gain with respect to that portion of the differential gain function judged to be most nearly constant (plus implies expansion, minus implies compression), or

c) The maximum range of the differential gain (difference of extreme values).

3.4.4 Diferenfial Phase Dafa . Differeritial phase data be expressed as: A function of one of the stated low-trequency levels with the other stated level a t blanking level, or The extreme values of differential phase with respect to the value a t blanking level (plus implies leading phase; minus implies lagging phase), or The maximum range of the differential phase (difference of extreme values).

3.4.5 Supplementary Information. The general portion of the amplitude range associated with a numerical specification should be designated black, center and white.

3.4.6 Typual Examples of Test Results. The same data for a television transmission circuit may be presented as follows: (Di8erentirlGrin and Phase.) (Low-frequency signal 1.0 volt, blanking to reference white)

A . See Figs. 7-10, for presentations as for a ) in both sections 3.4.3 and 3.4.4. Also, reier to Sections 4.3 and 4.4 below.

B. Using methods found i n b) of sections 3.4.3 and 3.4.4:

Differential Gain AInp!itude Differential Region Phase in Percent db Degrees

A PL -

lopercent +2 +0 .2 black 0

- 7 -0 .6 white - 3

Sopercent +2 +0 .2 black 0

0 0 .0 center +0.5

2; 0 0 . 0 center - 2 -0 .2 wh(te

Wpercent +5 +0.4 hhck n 0 0 .0 ce!i ter + 1

-2 -0.2 white - 1

C. tTsiiig methods found in c) of sections 3.4.3 a i d 3.4.4:

D(ffcrenliu1 Ga in Differential Phase Per Cent db in Degrees

+9 0 . 8 3 . 5

4. METHODS OF MEASUREMENT5 4.1 Low- Frequency Component of the Test Signul

4 .1 .1 Basic U'aveforms. 'The low-frequency cornpoileiit of the test signd niay have any waveform consistent with the requirements of Section 3.2, but simple wave- fornis such its the siriewave, staircase, and sawtooth, i n which the fuiidaiiiental is on the order of 15 kc, itre usually most convenient i i i practice. Siiice it is desirable to use a test sigiid containing horizontal sync pulses, it is convenient, though not essential, to use ;L IOW- frequency waveform that is fully contained withiii a line period. Examples of suitable staircase, sawtooth, and sinewave signals are shown i n Figs. 4 and 5.

1.1.2 Provision for Yarying the .4verage Picture Level. Any simple waveforni wi th a i ! iiiherent duty cycle of 50 per cent during actual presentatioii time may be used directly its the low-frequency cornporient of the test sigtial for differential Kairi measurements uiider 50 per cent apl conditioiis. Its peak-to-peak itiiiplitude, exclusive of sync pulses, should be set a t 100 I R E scale uiiits. The required variation i n the average picture level of the complete signal C;LII be obtaiiied by presenting this 50 per cent APL test information for only one-fifth of the total active scaiiiiiiig tinie i n each field period. The re- niainiiig four-fifths of the active scanning tinie should be used for the transmission of a coiistant low-frequency level, which should be set a t blaiikirig to provide 10 per cent APL conditions, and at reference-white to provide 90 per cent APL conditions. Practical exaiiiples of such time-shared signals are showii i i i Fig. 4.

4.1.3 Special Factors Pertaining to Circuits in Which Al l Stages are D C Coupled. When all stages iiicludiiig the output of the test signal generator are effectively DC coupled, there is no need to vary the APL of the test signal, because the amplitude range occupied by the siKnal is the same for all APL conditions i n each stage.

4.1.4 Special Factors Pertaining to Circuits in Which -411 Stages are . i C Coupled. LVheii all stages are ac coupled, and are able to pass a signal of somewhat greater than normal amplitude without overload effects, stitlid- ard measurenieiits can he niade with a test signal whose low-frequency coinponeiit has a duty cycle of 50 per cent. provided the ami)litude of the low-frequency signal is increased to cover the full r'iiige occupied by norniitl picture signals uiider 10 per ceiit to 90 per cent :\PI, coii- ditions. .Assuming the preseiice of sync pulses with pe,ik amplitudes of 40 I RE scale uiiits, the low-frequency signal should have ;t peak-to-peak ainplitude of 178 I RE

The methods discussed iii Section 4 are for illustration only. They are not intended to preclude other methods now in use. or which may be devised in the future. provided that such methods meet the requirements of cectioii .<.

4


SEE NOT€ I

(a)

- /oo

- 0

- -40

-100

- 0

- -40

(C)

- 100

- 0

40 --

NOTES: 1 ) Front Porch is optional. 2 ) Horizontal blanking 1 7 per cent i f 7 per cent vertical blanking

is used: 25 per cent if vertical blanking is not used. 3) High frequency Component may be transmitted on any portion

of the signal provided circuit under test is not adversely affected.

4 ) 50 per cent APL conditions ran also be provided by presenting the test waveform on every line.

Fig. 4-Examples of test signals.

Fig. 5-Examples of a test signal employing a sinusoidal waveform.

scale units when a sawtooth type of signal is used, and horizontal plus vertical blanking is taken into account. When the sinewave type of low-frequency signal is used without additional blanking, the corresponding amplitude is 184 IRE scale units as illustrated in Fig. 6. \$‘hen these expanded signals are used, the data should be processed so that the results reported for each APL

5

Fig. &Example of a test signal of e x r d e d amplitude satisfactory for measurements in circuits in whic all staged are AC coupled.

condition correspond to the following ranges relative to the blanking level of the signal.

Significant Ranges

APL Sawtooth waves Sine wave and sync pulse, no blanking with sync pulse

and blanking included

10 per cent 50 per cent 90 per Cent

61-161 30-130 0-100

84-184 53153 23123

4.2 High- Frequency Component of the Test Signal 4.2.1 Frequency. As stated in Section 3.2.3, the fre-

quency of the high-frequency component of the test signal should be approximately equal to the color subcarrier frequency (3.579545 mc). Unless the equipment under test employs special circuits which require precise control of the subcarrier frequency, deviations of the order of + 1 per cent from the subcarrier frequency should not appreciably affect the results of tests made in accordance with this standard.

4.2.2 Amplitude During A c t d Test Inktval. To satis- fy the requirements of this standard, the high-frequency component of the test signal should have a peak-to-peak amplitude of 20 IRE scale units during the actual test interval. If a low-frequency signal of greater than normal amplitude is employed for the special caae described in Section 4.1.4, i t is important that the high-frequency component not be expanded in proportion to the low- frequency signal but remain at the normal of 20 IRE scale units peak-to-peak amplitude, as illustrated in Fig. 6 .

4.2.3 Ampl i td t? During Other I n k r d s . The high- frequency component may be transmitted at any reasonable amplitude (including zero) during other intervals of the test signal, provided the equipment under test is not adversely affected. In the event that the equipment under test requires standard color sync bursts for proper operation, these must be added to the test signal. Fig. 4 illustrates several possible test signals with and without separate color sync bursts.

4.3 Measurement of Diferential Gain

4.3.1 A Method Suitable for Test Signals with Stuircase or Sawtooth Waveforms. Fig. 7(a) is a simplified block diagram illustrating the measurement of differential gain by means of test signals similar to thoae in Fig. 4. In this method, the output signal from the equipment


UAArAm LEVEL

(c) Differential gain in per cent at level 11 - lOO(b./u- 1) Differential gain in db at level 11 =20 log b/u

Fw. 'I-Diagram illuatmting the measurement of differential gain when using the test signal d Fig. 4(a).

under test is pasaed through a band-pass filter (order of 1 rnc bandwidth centered a t the high frequency), which rejects the low-frequency components. A filter suitable for this purpose is shown in Fig. 7(b), in which R is the nominal impedance of the circuits between which the filter is intended to operate, 4 2 1 is the center of the pass band, and w1/2r and w t / 2 r are the frequencies a t the nominal limits of the pass band. The high-frequency component is then directly displayed on an oscilloscope. Fig. 7(c) illustrates such a display for the test signals of Fig. 4(a). Differential gain appears as a variation in the envelope of the high-frequency signal. Transients occur at the riser positions of the stairsteps and should be disregarded.

4.3.2. A Method Suitable for Test Signals with Sinus- oidal Waveforms. Fig. 8(a) is a simplified block diagram of equipment' which mayhe used to measure differential gain by means of a test signal like that shown in Fig. 5 . In this method, the high-frequency component of the output signal received from the equipment under test is separated from the complete signal, and is applied to an envelope detector,' the output of which is displayed

haw and gain measurements in color television systems," IRE &us. ON BROADCAST AND TELE- VISION RECEIVERS, vol. BTR-1. pp. 14-17; July, 1955.

'H. P. Kelly, 'Differential

7 Ibid.. see Fig. 11.

- 1

+ 1-0.4 WHITE

LE V€L &LICK

(h) Fig. SSimplified block diagram and waveform illustrating the

measurement of diflerential gain using a test signal of the type shown in Figs. 5 or 6.

on an oscilloscope. As shown in Fig. 8(b) the vertical deflection of the oscilloscope trace is proportional to the differential gain. Horizontal deflection for the oscilloscope may be provided by the use of a low-pass filter to recover the low-frequency sinewave from the test signal. A phase shifter in the horizontal deflection circuit com- pensates for the delay difference between the horizontal and vertical circuits.

4.4 Measurement of Diferential Phase 4.4.1 A Method for Test Signals with Staircase or Saw-

tooth Waveforms. Fig. 9(a) is a simplified block diagram of equipment which may be used to measure differential phase with test signals similar to those in Fig. 4. The high-frequency component of the test signal, separated from the coniplete signal by a suitable band-pass filter (see Fig. 7(b)] (order of 1 mc bandwidth, centered a t the high frequency). may be compared with a reference signal of the Same frequency in a phase detector.' The reference signal may be regenerated from the color sync burst or derived from the high-frequency component of the test signal. A phase shifter may be used to obtain a zero indication on the part of the oscilloscope trace corresponding to blanking level.

As shown in Fig. 9(b) the vertical deflection of the oscilloscope trace is very nearly proportional to differential phase. Differential phase can also be measured by introducing a known phase shift (by means of a calibrated phase shifter) to bring any particular portion of the trace to the signal zero reference.

4.4.2 A Suitable Method for Test Signals with Sinus-

' Ibid., see Fig. 9.

6


$-#L+- .. - t-- 8LA#a/M nwrc

LCVCL

(b) Fig. 9-Simplified block diakram and waveform illustrating the

measurement of differential phase using the test signals d Fig. 4(a).

Fig. I0-Simplified block diagram and waveform illustrating the measurement of differential phase using a test signal of the type shown i n Figs. 5 or 6.

ozdal Waveforms. Fig. 10(a) is a simplified block diagram of equipment6 which may be used to measure differential phase wi th a test signal such as that shown in Fig. 5. This apparatus is similar to that shown in Fig. 8(a) except that the high-frequency component of the

differential phase, and the oscilloscope can be calibrated to be direct reading as shown in Fig. lo@).

5. LIMITATIONS AND COMMENTS 5.1 Hagh- Frequency Signal

The primary objective of this standard is the measurement of differential gain or phase in the chrominance frequency region. I t should be noted that measurements of differential gain or phase with a specified high-frequency signal such a s the cotor subcamu do not necessarily indicate the performance of the ayatem at other frequencies. This is particularly true in the caat of circuits which employ spectrum separation, pn-unpbasis or de-emphasis techniques. However, it should be noted that when the frequency separation between the high- and low-frequency components is reduced. the difficulty of making a measurement may be increased and meab

brements are practically impossible when the l o r a d high frequencies are barely separable by filters.

5.1.1 Frequency. A frequency other than 3.579545 mc may be used for special purposes. When such a frequency is used, it should be stated when presenting the results of measurements.

5.1.2 Amplitude. A high-frequency component amplitude of other than 20 IRE scale units peak-to-peak may be used for special purposes, but should be specified when presenting test results. For example, greater r~#)- lution in measuring differential gain or phase characteristics in low-noise transmission circuits may be achieved by decreasing the high-frequency component amplitude-

5.2 W w n to FCC Rules* To provide data with reasonable correlation to FCC

transmission requirements, i t is recommended that the low-frequency exploratory signal, exclusive of sync pulses, be adjusted to 80 IRE scale units, peak-to-peak, and that the superimposed high-frequency signal have a peak-to-peak amplitude of 40 IRE scale units. Thia test signal then sirnulater conditions which are mmo- what more severe than FCC requirements. Measure- ments should be made at 10 pa cent, SO per cent and 90percent APL. ~

5.3 Signa$cunce of 50 per cart APL Coditions I t is recognized that the amplitude ranges occupied by

normal picture signals under 10 per cent and 90 per cent APL conditions overlap each other, and that tests at 50 per cent APL do not provide information beyond that contained in the results of tests made at 10 per cent and 90 per cent APL. Tests at 50 per cent APL are significant, however, because this condition comes closest to

output signd is cornpiired with n reference high-fre- Section 3.682, paragraph 20-vII 3, in aFCC queticy signal ir i such a way that the trace on the oscil- Rules Covering Radio Broadcast Services." The FCC standanla fa

the phase shifts and 2) the amplitudes of aubcarriera for saturated This is itccomplished by using a 90" phase shifter and a primariesand their com Iementsat 75 percent d full amplitude, with p h ;ise de t ec tor. Over it reiison ii ble opera t i n g range, the ~ ~ ~ e ~ ~ ~ r ~ h ~ ~ ~ ~ c ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ e s ~ ~ ~ 2; o u t p u t of the detector is very nearly proportional to the suck.

loscope represents the differential phase characteristic. Color television broadcasts Specify tokranw Which aPdY to 1)

7


simulating average transmission conditions. Where time or facilities permit tests under only one APL condition, the results are most significant if 50 per cent APL is selected.

5.4 Noncomposite Signals

The standard tests do not fully cover the case of noncomposite signals, since the maximum excursion of the subcarrier signal in the black direction (for the 90 per cent APL condition) is increased by about 4 IRE scale units when the sync pulses are absent. If there is any reason to suspect difficulty with noncomposite signals, a further test may be made either by removing the sync pulses from the test signal or by slightly increasing the amplitude of the low-frequency signal.

5.5 Sync Compression or Expansion The standard tests do not directly provide for the

measurement of sync coinpression or expansioii, although this characteristic may be readily measured by direct observation of the test signal on an oscilloscope using the I R E scale.

5.6 Color Sync Burst Distortion Distortion of the amplitude or phase of the color sync

burst relative to the chrominance signal may be intro- duced by such factors as poorly adjusted clampers or burst regenerators. Therefore, such equipment should be adjusted properly before measurements of differential gain and phase are made.

8


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ansi ieee 0206-1960

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