US2270652A - Radio-receiving apparatus - Google Patents

Description

Jam 20, 1942- D. c. EsPLEY 2,270,652

RADIO-RECEIVING APPARATUS Filed July 22, 1939 Y 2 Sheets-Sheet 1 Jan. 2o, 1942.

D. c. ,EsPLEY RADIO-RECEIVING APPARATus Filed July 22, 1939 2 Sheets-Sheet 2 my z3.

, l by I E l Attorney aziz asc/Arab Patented Jan. 20, 1942 RADIO-RECEIVING APPARATUS Dennis Clark Espley, North Wembley, England, assignor to The General Electric Company Limited, London, England Application July z2, 1939, seria1N0.285,s32 In Great Britain July 27, 1938 (Cl. Z50-9) 4 Claims.

This invention relates to radio-receiving apparatus lof the type adapted to receive alternatively broadcast sound signals and television signals accompanied by sound, and comprising a frequency changer common to both the vision channel and the channel of the sound signals accompanying the television signals. In what follows, for b-revity, sound signals accompanying the television signals will be called simply sound, while the broadcastsound signals will be called broadcast. In apparatus of this type it is desirable on the grounds of economy to use the 'same ultimate receiver, usually a loudspeaker, for reproducing both the sound and the broadcast. It would be desirable on the same grounds to make other elements common to the lsoundand the broadcast-channels, if this could be done without loss in the quality of the reproduced signals. But there are certain difiiculties, about to'be discussed, in making many elements common. The object of this invention is to provide circuit arrangements, in which many elements are common to the sound and broadcast channels, giving high quality of the reproduced signals.

In order that the said diiculties, and the principles used in overcoming them, may be appreciated, it is necessary to consider the frequencies of the carriers involved. The values given are quite arbitrary, and it will be obvious to those skilled in the art that considerable departures from these values might occur without change in anything essential to the invention. The carrier frequencies of the vision, sound, short, medium, and long-broadcast are respectively 45, 41.5, 20-6, 1.5-0.55, and 0.330.15 mc. Broadcast carrier frequencies of neighbouring stations may diner by Aas little as 0.009 mc.; accordingly there must precede the final audio-frequency detector frequency selective means capable of distinguishing frequencies separated by such small dilferences. On the other hand, the width of the vision band of frequencies is so great that the intermediate frequency (I. F.) resulting from the frequency-changer common to both sound and vision must be at least several mc. But if f is the mean frequency transmitted preferentially by frequency-selective means of the ordinary kind, namely those tuned by inductance and capacity, and if f is the smallest difference in frequency that they are adapted to distinguish, f in general increases with f; if f is several mc., f is usually much greater than 0.009 mc. The frequency-selective means subsequent to the common frequencying carriers. That -is the difficulty attending all proposals to increase greatly, in apparatus of the type specified, the parts common to the sound and broadcast channels.

According to the invention the broadcast carrier, or a carrier resulting from the combination of that carrier with local oscillators of frequency fo, is applied to the input of the frequencychanger common to the vision and sound channels (whereb-y the desired economy of parts is achieved), and frequency-selective means, capable of distinguishing the frequencies of neighbouring broadcast stations, are introduced into the broadcast channel in such a way that they do not restrict the breadth of the band of frequencies that can pass along the common part of the sound and vision channels (whereby the necessary selectivity is achieved). It will be pointed out later that the said frequencychanger need not act as such when the said carrier is applied to it.

There are two places at which such frequencyselective means may lie.

(a) They may lie permanently .in that part of the broadcast-channel which is not common to the sound channel, that is to say between the source of the broadcast carrier and the input to the frequency-changer to which the broadcast carrier is applied.

(b) They may lie in the part common to the broadcast channel and the sound channel. There are then three main alternatives. (1)

The means may be introduced into this part` when the transference from television to broadcast reception takes place. Or (2) the part previously traversed by the sound signals may be modified, when the said transference takes place, so that this part becomes much more selective than before. Or (3) there may be in this part parallel paths, each containing a lter`,'of

which one transmits the intermediate frequency conveying the sound and Vision signals, and the other transmits a range narrow enough to distinguish neighbouring broadcast stations near a different 'intermediate frequency conveying the broadcast signals; then the former willtransmitnothing when the broadcast signals are being received and the latter will transmit nothing when the television is being received. No novelty is claimed for any of these alternatives.

In choosing between thesev ways, and minor l variants of them that will occur to those skilled in the art, it must be remembered that, in view of the relation aforesaid between 5f and f, if the frequency-selective means are to be of fixed character, it is desirable to place them in a part of the signal path where the carrier frequency is low rather than in one in which it is high. If they are introduced by modification of a part always present, the modification .may lconsist merely of a reduction in the same ratio of all the material frequencies of this part.

The invention will now be further explained with reference to some embodiments illustrated by conventional block diagrams in Figures 1, 1(a), 2, 3, 4 and 5, of the accompanying drawings; corresponding parts are denoted throughout by the same reference numeral. It will be assumed throughout that the intermediate frequency of the sound channel is 6.5 mc.; the intermediate frequency of the vision channel will then probably be 10 mc., but it will appear that the vision channel is hardly relevant to the invention.

In all the figures, two branches A and B proceed from a common aerial I. They are adapt'- ed to be rendered alternatively operative, so that A is operative when television is to be received and B when broadcast is to be received. The means by which they are rendered alternatively operative form no part of this invention, and many suitable means will occur to those skilled in the art. But it may be recorded that two means, or some combination of them, have been found especially convenient. One is to arrange the supplies to the various parts of the set so that, by a suitable switch, power can be supplied to the broadcast portion but not to the sound portion or vice versa. The other is to provide means for biasing valves, such as those in the amplifier 2 or the frequency change I2, so as to paralyse one or the other. In the figures of the accompanying drawings, the means is representedby a two-way switch I8.

Branch A leads to the R. F. amplifier 2; thence to the frequency changer 3, fed by the local oscillator 4 generating 35 mc.; andthence to the I. F. amplifier 5. From here the sound and vision channels diverge, being separated by frequency selective means. The sound channel passes through the I. F. amplifier' 6 and through detector and audio-frequency amplifier 'I to the loud-speaker 8; the vision channelthrough the I. F. ampliners 9, I and the second detector II to the cathode ray tube I4. For simplicity the elements I, 3, I0, II, I4 are shown only in Figure l. Branch B leads to the frequencychanger I2 fed by a local oscillator I3. The frequency-changer I2 may be preceded by a R. F. amplifier (not shown). Of course each frequency-changer and its local oscillator may be embodied in a single valve of known type, e. g. a triode-hexode. All this is standard practice. According to one feature of the invention as stated above, the output of frequency-changer I2 is applied to the input of frequency-changer 3. From this point the four arrangements diverge. In Figure l the frequency of the local oscillator I3 is so adjusted in known manner to the frequency of the broadcast carrier that the I. F. resulting from the frequency-changer I2 is 28.5 mc., (or even 41.5 mc.), so that an I. F. of 6.5 mc. isl produced by the frequency-changer 3. Further sharply-tuned frequency selective means I are either permanently at I5a in the lead between I, 2, and 3 (proposal (a) aforesaid), or introduced, when branch B is operative, at I5b in the sound channel anywhere between 3 and l, for example, between 5 and 6 as shownin Fig. (proposal (b) aforesaid); these means transmit a narrow frequency band, some 0.009 mc. wide, centred near 28.5 mc. (or 41.5 m0,),` if the means are at I5a, or near 6.5 mc., if the meansare at I5b. Thus the means I5 may be a crystal-tuned filter, which has the necessary selectively. Method (b3) aforesaid is not applicable, because both the broadcast and the sound are conveyed by the same carrier along the path common to them.

Of these proposals a has the advantage that the means I5 is permanently in place, but the disadvantage that f/f aforesaid is large. Proposal b has lthe advantage that f/f is less, but arrangements (switch 2'I in Figure 1a) for introducing filters I5 or modifying existing filters have to be ganged with the switch (or equivalent) selecting A or B by any suitable means well known to the art, as for example the ganging bar 28 schematically illustrated in Fig. 1a. All forms of this embodiment have the drawback that the frequencies of the local generators 4 and 3 have to be very accurately constant. Further a frequency-changer generating an I. F. as high as 28.5 mc., though possible is outside normal prac'- tice and adds to the expense.

In the embodiment ofV Figure 2 the frequency changer l2 produces an I. F. such as is used in ordinary broadcasting sets, namely about 0.5 mc. The alternative usual broadcasting I. F., namely about 0.1 mc., is possible, though less suitable. Then the frequency selective means I5 according to the invention can be I. F. lters such as are used in such sets, permanently situated at I5a; crystal filters are not required. (Of course a crystal filter could be introduced as aforesaid at I5b in Figure la; and method (b2), but not (b3), could be used; but these alternatives are not generally desirable.) Now means 30, well known to the art'and including for example a condensor, is provided, which means may be introduced into the circuit by a switch 3I. Said switch is ganged by suitable means, such as a ganging bar 32 schematically indicated in Fig. 2, with the switch I8 (-or its equivalent) selecting A or B. Said means, when B is selected, changes the frequency of the local oscillator 4 from 35 to 6 (or 7 mc., so that the I. F. produced by frequency changer 3 is again 6.5.

vThe main drawback of this embodiment, if Shortwave broadcast carriers are to be received, is that fo, or some harmonic of it, is likely to lie in the band of these carriers; this is known to be undesirable.

It is to be observed that it is not absolutely necessary that the broadcast carrier should be changed in frequency at all, if the disadvantages of a Variable intermediate frequency are tolerated or overcome by making the frequency of the oscillator 4 variable. But this alternative (which applies also to the embodiment of Figure 4) is seldom likely to be advantageous.

In the embodiment of Figure 3, when the selecting switch I8 (or its equivalent) is changed from A to B, the local oscillator 4 is suppressed by manipulation of a switch 33. The frequency changer I2 must now give an I. F. 6.5 mc.; the stage 3, no longer a frequency changer, can be used as an amplifier. The modification of the stage 3 from a frequency changer to an amplifier can be accomplished by any means well Aknown to the art, as for example changing the bias of the stage by manipulation of a switch 34 adapted to contact either of the points 35 or 36, said points having different potentials impressed thereon." The switches I8, 33 and 34 are all ganged vfor simultaneous operation, as schematically illustrated by the gauging bar 31 in Fig. 3. But the frequency-selective means I5a must be highly selective; for f/f is large, though not so condensers I6, I1.

large as in Figure 1. It is now immaterial whether they are permanently at Ia or introduced as aforesaid at |512 in Figure la; but method (b3) is again not -applicable because there is no difference between the sound and broadcast I. F. carriers. It is to be understood again that lh need not lie between 5 and 6, as

shown in Figure la; it may lie anywhere between the input to 3 and the output from 7. The drawback characteristic of the embodiment of Figure 2 is not present.

'I'he embodiment of Figure 4 may be regarded as a combination of those of Figures 2 and 3. Frequency changer I2 produces an I. F. about 0.5 mc.; when B is selected, the local oscillator 4 is suppressed; and the frequency selective means are introduced according to alternative b aforesaid.

Method (b2) or (b3) is preferably adopted. In the former the frequency-selective means I5 are provided by changing the frequency to which the frequency-selective means already present in the sound path are tuned. These means usually consist, at least in part, of couplings between the frequency changer 3, and the I. F. ampliers 5 and S. When branch A is selected, they must transmit preferentially.frequencies near 6.5 mc. (Those, if any, that are common to the sound and vision channels must also transmit the vision I. F. preferentially.) When branch B is selected, they must transmit preferentially a range of some 0.009 mc. near 0.5 mc. The range of their transmission can be lowered by the introduction of the Said condensors IS and I'I may be introduced into the circuit when the branch B is selected by switches 38 and 39, also operated by the ganging bar 31. I have found that, if the couplings are of the kind usual between the I. F. stages of a television set, the increase of selectivity associated with the decrease in the frequency of the transmission range may be so great that no other frequency-selective means are necessary in order to secure the selectivity required in broadcast reception.

In the embodiment of Fig. 5 (Fig. 4 with method (b3)) the couplings provide two paths in parallel, one, I9, transmitting only a range of 0.009 mc. near 0.5 mc., and the other, 20, transmitting only a band near 6.5 mc. vThe former will not transmit anything when channel A is open, the latter will not transmit anything when channel B is open.

The embodiments of Figs. 4 and 5 are the preferred embodiments of the invention. For in order to add broadcast reception to television reception al1 that has to be added is the frequency changer I2 with its local oscillator I3 and the condensers I6, I1 or the parallel paths. Moreover the frequency-changer I2 produces the conventional I. F. and can be of a conventional type; and the frequency-changer, when it is no longer required, can be used as an amplifier. Thus, when the frequency changer is a hexode, the signal input will usually be applied to the rst control grid and the local oscillations to the second control grid; when it is to be used as an amplifier the second control grid will be held at a fixed potential or used for gain control.

It is to be understood that the means for distinguishing between neighbouring broadcast stations need not be wholly as described above. In accordance with usual practice there will usually be frequency selective means in the path B prior to the frequency I2, which aid the means described in securing the necessary selectivity.

Having now described my invention, what I claim as new is:

1. A heterodyne radio receiver adapted to receive alternately broadcast sound signals and sound accompanied television signals, comprising means for receiving the television and accompanying sound signals, said means comprising a first frequency-changer common to both the vision and sound channels, means for receiving the broadcast sound signals,said means comprising a second frequency-changer adapted to produce the broadcast sound intermediate frequency, means connecting the input of said second frequency changer to a broadcast sound receiving aerial and means connecting the output of said second frequency-changer to the input of said first frequency-changer, means for rendering operative either the said television and accompanying sound receiving means or the said broadcast sound receiving means, and means for converting said first frequency-changer into an amplifier when the said broadcast sound receiving means is rendered operative.

2. A heterodyne radio receiver as in claim 1 wherein the said second frequency-changer is adapted to produce the same intermediate` frequency as the said first frequency-changer and those parts of the sound receiving means of the said television and accompanying sound receiving means which lie subsequent to the said iirst frequency changer in the path of the signals form part also -of the said broadcast sound receiving means.

3. A heterodyne radio receiver as in claim 1 wherein the said second frequency-changer is adapted to produce an intermediate frequency considerably less than that produced by the said rst frequency-changer, wherein those parts of the sound receiving means of the said television and accompanying sound receiving means which lie subsequent to the said first frequency-changer in the path of the signals form part also of the said broadcast sound receiving means and wherein the said broadcast sound receiving means comprises further elements which are connected to elements in the said subsequent parts when the broadcast sound receiving means is rendered operative to form means capable of distinguishing between the frequencies of neighbouring broadcast stations.

4. A heterodyne radio receiver as in claim 1 wherein the said second frequency-changer is adapted to produce an intermediate frequency considerably less than that produced by the said rst frequency-changer, wherein those parts of the sound receiving means of the said television and accompanying sound receiving means which lie subsequent to the said first frequency-changer in the path of the signals form part also of the said broadcast sound receiving means, and wherein at least one frequency selective coupling between two stages in the said sound receiving means comprises two branches in parallel, one forming part of the said broadcast sound receiving means and adapted to transmit only broadcast sound signals and to distinguish between the frequencies of neighbouring broadcast stations, and the other adapted t-o transmit only the television and accompanying sound signals.

DENNIS CLARK ESPLEY.

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