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Effective June 1, 1961, all FM stations may employ stereophonic transmission - provided they give 10 days' notice (to the Commission and District Engineer), install "type-accepted" equipment and conform to the technical standards set forth in the new FCC ruling (see Appendix on Pg. 52).

• Anmerkung : Die FCC hat also mit Wirkung vom 1. Juni 1961 das amerikanische Stereo-Verfahren quasi per Gesetz (oder per Anordnung zwingend) für alle 50 amerikanische Staaten festgelegt. Ab dem 1. 6. 1961 darf demnach nur noch FM-Stereo mit dieser 19kc Pilotton-Technik ausgestrahlt werden. Alle anderen Stereo-Testsender "müssen" abgeschaltet werden. bzw. dürfen nur noch FM-Mono senden.

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In the approved system the main FM carrier is modulated by a combination of the left and right (L + R) stereo channels. A subcarrier at 38 kc is suppressed and amplitude modulated by the difference signal between left and right channels (L - R).

The stereo information is carried on the left-minus-right channel. If the left-plus-right and the left-minus-right signals are added at the receiver, a left channel signal is provided. If the left-plus-right and the left-minus-right are substracted, the right channel is obtained.

By modulating the main channel with the L + R signal, a listener not equipped for stereo reception will receive a full monophonic program.

Those equipped with stereo adapters will, of course, receive the full stereo effect. Another feature of this stereo system is that it permits use of one SCA multiplex subchannel in addition to the stereo subchannel.

• Anmerkung : Der sogenannte "SCA multiplex subchannel" ist ein dritter Kanal auf der gleichen UKW-Frequenz, eine kostenpflichtige US- spezifische Dauerberieselungstechnik für Kaufhäuser und Supermärkte - qualitativ auf Mittelwellen Niveau, im UKW Signal fast unsichtbar versteckt. Wir kennen das in Europa nicht.)

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Basically, this system is quite simple and most FM broadcasters will have no difficulty making the necessary modifications and additions to their current equipment layout. It is best to begin by getting a thorough understanding of how the system works, the reasons for the specified standards and the requirements for meeting them. The following material has been prepared to help FM operators get started.
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In stereophonic transmission two microphones are placed in front of the program source (the orchestra). These two microphones may be spaced from 10 to 50 feet apart depending on the size of the orchestra.

The one seen by the audience on the left is designated the left-channel microphone, and the one on the right side is the right-channel microphone. Left and right channels as shown in Fig. 1 are fed into a matrixing network.

The matrixing network has two outputs. One output is the left-plus-right where both channels are added in phase. The other output is the left-minus-right where the polarity of the right channel is reversed and then left and right channels are added together to produce a difference signal.

The left-plus-right channel, after a time delay, is fed into the main channel input of the FM exciter (see Fig. 2). The left-minus-right channel is first fed into a stereo sub-carrier generator such as the RCA BTS-1 (see Fig. 3).

A separate left-plus-right bridging output is provided to feed the AM transmitter with a monophonic signal to permit duplicate programming.
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The RCA BTS-1 Stereo Subcarrier Generator has two outputs: First, a double-sideband-suppressed-carrier signal centered on the carrier frequency of 38 kc; second, a carrier-pilot signal output at 19 kc, exactly half the carrier frequency.

The double-sideband-suppressed-carrier signal is fed directly into the main channel of the BTE-10B FM exciter. The 19 kc pilot signal is fed into one of the two subchannel inputs provided in the BTE-10B exciter.

The double-sideband signal and the 19 kc carrier-pilot signal then "frequency modulate" (das bedeutet FM) the main FM channel along with the normal L + R main channel modulation.
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If it is desired to transmit an SCA multiplex channel in addition to stereo, the audio input for this channel is fed to a standard RCA Subcarrier Generator adjusted to provide a 67 kc subcarrier. This subcarrier, modulated by the multiplex signal, is fed to the second subchannel input of the BTE-10B Exciter.
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The total modulating spectrum of the exciter is shown in Fig. 4. The modulating waveform consists of left-plus-right information shown under (A). (Das ist quasi das Mono-Signal für die alten mono Empfänger.)

The 19 kc pilot carrier is shown under (B) and the double sideband information carrying the left-minus-right channel is shown under (C). A possible frequency- modulated SCA channel is shown under (D). (Das kennen wir in Europa nicht.)

One should remember that these four signals are only added, they are not mixed in the sense that is normally used with an r-f mixer. It is an additive process which could be accomplished by feeding the four signals into a common resistor. This waveform will then modulate the main carrier.
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One may ask why a double-sideband-suppressed-carrier system was chosen to carry the left-minus-right information. The great advantage of the double-sideband signal is the fact that with no left-minus-right information there is no signal to modulate the exciter, and for this reason the left-plus-right can modulate the exciter up to 90 per cent. Therefore, the loss of signal-to-noise in the main channel is only 1 db.

If a regular amplitude-modulated or frequency-modulated signal had been used, the unmodulated carrier would occupy space even if there were no left-minus-right information. This would have required greater sacrifice in main channel signal-to-noise.

Another advantage claimed for this system is that the type of adapter required for reception of the stereophonic information will not permit reception of an SCA channel by the general public. Thus the likelihood of pirating SCA background music is reduced.
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The composite signal previously described goes through the transmitter and antenna to the listener's (Stereo-) receiver. At the output of the discriminator in a conventional FM tuner three signals will be present :

1. First, the left-plus-right information, occupying a frequency space of from 30 to 15,000 cycles;
2. second, the left-minus-right information occupying a space from 23 to S3 kc; and
3. third, a pilot tone at 19 kc.

If the station is engaged in SCA service, there will be an additional signal occupying the space from approximately 59 to 75 kc (see Fig. 4).
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The signals from the FM tuner are processed in the following fashion: The left-plus-right channel, which is recovered directly in the receiver discriminator, is fed through a de-emphasis network to the matrix. The de-emphasis network will sufficiently attenuate the 19 kc pilot carrier and the double-sideband information and the possible SCA subcarrier to not overdrive any of the following amplifier stages.

The left-minus-right double-sideband information is extracted by a bandpass filter (23-53 kc) (see Fig. 5). The 19 kc pilot tone is extracted by another filter. After sufficient amplification this pilot signal is doubled in frequency and added to the double-sideband information.

The adding process has to be such that the amplitude of the derived 38 kc carrier is several times the maximum possible amplitude of the two sidebands. This signal is fed into an envelope detector.

The output from the detector is the left-minus-right signal. This signal is de-emphasized and in turn both left-plus-right and left-minus-right are fed into a matrix, which is the reverse of the matrix at the transmitter.

In the receiver matrix the left-plus-right and the left-minus-right signals are added to provide a left-channel signal. At the same time the left-minus-right signal is subtracted from the left-plus-right signal to obtain the right signal.

Thus the output of the matrix provides independent left- and right-channel information for dual audio amplifiers and speakers.

A possible receiver matrix is shown in Fig. 6. There are two adders consisting of tubes V1 and V2. The left-plus-right channel is fed into both adders in the same phase and amplitude. The left-minus-right channel, however, is fed with 0 degrees
phase to one and 180-degree phase to the other mixer. One tube adds the two signals while the other subtracts, and thus 2R and 2L are recovered, viz.:

picture here
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The discussion above indicates the means by which the separate left and right stereo signals are recovered from the receiver matrix.

At this point one may well ask why all this bother with matrixing. The main reason is compatibility (i.e., making the system such that the listener not interested or equipped for stereo still gets a usable signal).

This would not be the case if only left or right information were carried on the main channel. Due to the placement of the microphone a substantial amount of information would (in some cases) be lost to a listener receiving only the left, or only the right, channel. Matrixing, however, provides this listener with a left-plus-right information which contains all the essential components that are transmitted.

• Anmerkung : Auch das stimmt nur bedingt, denn es gibt bei der Addition der Signale L+R zu Mono die sogenannten Auslöschungen.

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Figure 7 shows the output of the transmitter matrix for different types of input.

In Fig. 7A with a sine wave input into the left channel, there is no input into the right channel. The output is a sine wave half the input magnitude from the left-plus-right and the left-minus-right channels, all three signals in phase.

If a sine wave is fed into the right channel only, the output again is a sine wave of one-half the input magnitude from the left-plus-right channel in phase with the primary signal and a sine wave of one-half magnitude from the left-minus-right channel but with opposite phase (polarity) relative to the input signal (see Fig 7B).

If both left and right are fed with a sine wave in phase, there will only be an output from the left-plus-right channel with two times the magnitude of the input signal. There will be no output at the left-minus-right channel (see Fig. 7C).

Finally, Fig. 7D shows an input into the left and right with the same frequency, same amplitude but opposite polarity. This provides an output signal from the left-minus-right channel, which is in phase with the left channel. There will be no output from the left-plus-right channel.

The conditions shown under Figs. 7A and 7B are not possible under a practical operating condition. They can, however, be obtained by using an audio generator feeding left or right.

However, conditions in Figs. 7C and 7D are possible. Conditions
shown in Fig. 7C would arise where a musical instrument is placed at an equal distance from both microphones. Being an equal distance away from the left and the right microphones, the sound emanating from the instrument would arrive at the two microphones in phase with equal magnitude.

This will only cause an output from the left-plus-right channel. There will be no output from the left-minus-right channel. If the instrument is placed sideways (assuming the distance between the microphones and the instrument is large relative to the distance between the two microphones), the voltages out of both microphones can be of essentially the same magnitude and opposite phase. This corresponds to the condition shown in Fig. 7D.
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It will be helpful in understanding the stereo system to consider how the several types of signals shown in Fig. 7 will modulate the transmitter.

As previously noted, the output of the matrix is a left-plus-right signal (which is slightly delayed) and a left-minus-right signal.

The left-plus-right signal is fed directly into the audio input of the exciter. The left-minus-right signal is fed to the stereo subcarrier generator. In the stereo subcarrier generator a double-sideband-suppressed-carrier AM signal will be produced.

If the left-minus-right channel consists of a sine wave only (a condition shown in Fig. 7A), the output of the BTS-1 stereo subcarrier generator will consist of two sidebands of a certain magnitude being spaced from the 38 kc carrier by the modulating frequency.

The amplitude of the two sidebands will vary in direct proportion to the magnitude of the left-minus-right signal. If there is no left-minus-right signal, there are no sidebands. The 38 kc carrier has to be suppressed sufficiently to not modulate the main carrier more than 1%.

This means that if there is no left-minus-right modulation, there is no output from the BTS-1 subcarrier generator.

Under the conditions shown in Fig. 7A. therefore, there will be an equal modulation percentage caused by the left-plus-right channel and by the left-minus-right channel. The left-plus-right and left-minus-right DSB signals will each modulate the main carrier 45%, so that the sum of the two signals will modulate the main carrier 90%. The 10 per cent remaining is reserved for the 19 kc pilot carrier.

Condition 7B will cause the same modulation percentages as 7A. Left-minus-right polarity is reversed. In 7C the left-plus-right channel will modulate the main carrier 90%. Since there is no output from the left-minus-right, there is no modulation due to the double sideband signal. The only signals present under this condition are a 10% 19 kc pilot carrier and 90% left-plus-right channel signal.

Under the conditions shown in Fig. 7D no modulation is present due to the left-plus-right signal but the left-minus-right signal will now have two sidebands of sufficient magnitude to modulate the main carrier 90%. There will also be 10% modulation by the pilot carrier.

The peak deviation of the main carrier by the left-plus-right and by the left-minus-right is 90%. However, the 90% peak deviation is never caused by left-plus-right or left-minus-right at the same time. The sum of the two will always be not more than 90%.

If it is desired to use an SCA channel in addition to stereophonic transmission, all modulation percentages must be deceased by 10%. This means the maximum left-plus-right or left-minus-right nodulation may be 81%.

The pilot carrier is 9% and the SCA subchannel is 10%. An additional requirement is imposed upon the SCA subchannel.

Previously any SCA component appearing in a frequency band from 30 to 15,000 cycles had to be attenuated at least 60 db. Now, when a station is engaged in stereophonic broadcasting, components from the SCA subchannel should not exceed a limit of -60db, over a frequency range from 30 cycles to 53 kc. This will require additional filtering in the SCA subcarrier generator output.

The permissible SCA channel will be at a center frequency of 67 kc. The maximum deviation possible is ±7.5 kc with a maximum modulating frequency of approximately 8 kc. Higher modulating frequencies will be cut off by the high pass filter (required to keep the components to a level of -60 db, from 50 to 53,000 cycles to comply with FCC rules) in the output of the SCA subcarrier generator.
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The FCC's ruling on stereo transmission is reproduced in the Appendix on Pg. 52. The statements under paragraph 1 and 2 are self-explanatory. Paragraph 3 sets forth the requirements for stereo-plus-SCA. These have been discussed above.

Paragraph 4(c) may need some explanation. If the requirement under 4(c) should not be met, in other words, if the second harmonic should cross the time axis with a negative slope, the left-minus-right information would be reversed in polarity.

This would essentially mean that the left and right channel information would be reversed. The left information would be coming out of the right side speaker and vice versa. The same reasoning applies to the requirement under 4(k)- namely, that the relationship of the left-plus-right relative to the left-minus-right must be maintained. If the deviation would be upward in some transmitters and downward in others, it would mean that with the same receiver sometimes the left and right channels would be reversed.

These requirements show that one has to be careful to maintain the proper polarities in the left and the right and the left-plus-right and the left-minus-right channels as well as in the interconnection between the stereo subcarrier generator and the exciter. For this reason polarized plugs should be used everywhere in the system.

Paragraphs 4 (1) and 4(m) spell out the technical requirements for channel separation.

When the left-plus-right channel and the left-minus-right channels are added in the receiver matrix, complete separation of the left and the right channels will be obtained only if the left-plus-right and the left-minus-right channels have the same frequency response and all frequencies contained in both these channels arrive at the receiver matrix at the same time (with proper phase).

If there are departures from the equal frequency response curve or from the equal phase curves, it will not be possible to completely compensate left-plus-right and left-minus-right. Channel separation will suffer.

The new rules specify that the frequency response of the left-plus-right and the left-minus-right channels shall be within ±3.5 % (±0.35 db) relative to unity. If there is a steady state signal into the left channel only, there will be a signal of the same frequency one-half the amplitude in the left-plus-right and left-minus-right channels.

When these two components arrive at the receiving matrix, the phase difference between the left-plus-right and the left-minus-right should not exceed ±3 degrees for any frequency between 50 and 15,000 cycles.

This requirement necessitates a delay network that is adjustable within certain limits. One should remember that these requirements for frequency response and time delay in the left-plus-right and the left-minus-right channels are to insure proper separation between the left and the right channels.
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The "note" following 4(m) indicates a method of checking channel separation. It says, in effect, that if there is a certain signal in the left channel only, the signal recovered at the receiver in the right channel should be attenuated by at least 29.7 db over a frequency range of from 30 to 15,000 cycles.

If this separation is obtained, no further measurements are required. Otherwise, it must first be determined whether the lack of separation is due to insufficient frequency response or excessive time difference.

Paragraphs 4(n) and 4(o) indicate an additional requirement as far as the transmitter is concerned: Crosstalk from the left-plus-right into the left-minus-right. This figure should be better than the requirement for the separation. The requirement for crosstalk from the left-plus-right into the left-minus-right or vice versa is 40 db relative to 90 per cent modulation of the other channel.
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FIG. 1. Block diagram shows essential elements ol new FM stereo system.
FIG. 2. RCA Type BTE-10B FM Exciter.
FIG. 3. RCA Type BTS-1 Stereo Subcarrier Generator.
FIG. 4. Total modulating spectrum covered by exciter.
FIG. 5. Block diagram showing how FM stereo is received.
FIG. 6. Typical receiver matrix for FM stereo.
FIG. 7. Output oi the transmitter matrix for different types of input.