The introduction of the intercarrier sound (Parker System) of television reception, which was described briefly and illustrated by the block diagram of Figure 185, has resulted in a reduction of the number of tubes required and has furthered the design of table model TV sets.
In this system, the video and audio IF carriers are amplified together in the intermediate frequency amplifier. The gain versus frequency characteristic of the IF system differs from that of the dual IF system as shown in Figure 247. In this case, sound IF traps are not used and the end of the band is allowed to slope gradually as shown at B of Figure 247.
At the output of the second detector, a beat between the video IF and audio IF carriers (4.5.mhz) appears. This beat, which is frequency modulated by the sound signal and amplitude modulated (to some extent) by the video signal, constitutes a new intermediate frequency. The video amplifier which follows the second detector can be considered as an IF amplifier for this beat.
The intercarrier beat is separated from the combination signal at the picture tube grid and is passed through a frequency-modulation detector which is not sensitive to the amplitude (video) modulation of the beat . The sound modulation is thus sorted from the picture and subsequently amplified for operation of the loudspeaker.
Since the intercarrier sound system represents an economical method of producing a less complicated receiver , the television service technician should become familiar with the principles of operation and the requirements which it places on the characteristics of the composite television signal.
Fig. 247. IF Response Characteristics of Conventional Dual Channel and Intercarrier
Sound Systems.
TRANSMITTER PERFORMANCE CHARACTERISTICS REQUIRED FOR SUCCESSFUL INTERCHANNEL SOUND OPERATION: Since the intercarrier sound system is made possible by the fact that separate modulation methods are employed for the video and audio carriers, it follows that the performance of the system is dependent upon the exact composition of the transmitted television signal.
Characteristics of the signal which are of importance are:
1. The frequency stability of the video and audio carriers.
2. The accuracy of the difference-frequency between the video and audio carriers.
3. The depth of modulation of the video carrier allowed for maximum white signal.
4. The amount of phase or frequency modulation present in the video carrier.
Since the difference beat is produced between the video and audio carriers, any drift of frequency of either of them will cause an off-tune condition in the FM detection circuit at the receiver. This will result in either poor quality or loss of the sound portion of the transmission. Nothing can be done by the user to correct this since the alignment of the FM system is a factory or service technician operation. In the conventional dual channel system this condition can be corrected by adjustment of fine tuning. With crystal control at the transmitter, this requirement should present no difficulty.
The minimum modulation of the video carrier represents the white level. If the modulation of the sound carrier should be allowed to go to zero (100% video modulation) the 4.5 mhz beat upon which the operation of the system depends, would disappear. No sound output could occur during such periods of over-modulation. This would cause disagreeable sound interference in the form of a 60 cycle buzz (field repetition rate) or a 15,750 cycle whistle (horizontal scanning rate). The drop-out of audio IF would occur at field and line scanning rates and would appear as a modulation of the audio IF signal.
Any phase or frequency modulation of the picture carrier would result in a corresponding modulation of the 4.5mhz beat. The result would be the same type of 60 cycle or 15,750 cycle interference just described. Mis-adjustment of transmitter tuning or neutralization can cause such phase modulation.
Fig. 248. Typical Intercarrier Sound Circuits.
TYPICAL INTERCARRIER SOUND SYSTEMS: Figure 248 shows a typical intercarrier sound system, while Figures 249 and 250 illustrate variations of the method of take-off of the 4.5mhz audio modulated beat. In Figure 248, the circuit consisting of C2 and L3 acts as a series-tuned trap to remove the 4.5mhz beat from the picture tube input, and at the same time, develop a resonant voltage across the inductor to feed the modulated IF to the amplifier tube (T2) and the ratio detector (T3). The operating voltages of T3, as well as the grid circuit constants, are such that the stage acts as a limiter for the suppression of amplitude modulation.
Figure 249 shows an absorption trap L3-C4, coupled to the video output plate through capacitor C3 (2.2 mmf.). The operation of the circuit is similar to that of Figure 248.
The circuit of Figure 250 shows a variation in which the trap circuit is in the screen return. The screen acts as a triode plate for amplification of the IF beat, and the 4.5 mhz tuned circuit suppresses this frequency in the video output or plate circuit.
Fig. 249. Audio IF Take-off Employing a Shunt-Tuned Trap.
Fig. 250. Audio IF Take-off from the Screen Circuit of the Video Output Tube.
The constant daily analysis of Television equipment in the laboratories of the Howard W. Sams & Co., provides the continuing research and study necessary for the production of Photofact Folders covering such equipment. By-products, of almost equal importance, include the publication of practical educational information such as the Photofact Television Course.
To keep current on the latest developments in the TV art, and to become familiar with specific design and operational data, it is suggested that reference be made to the Photofact Folders issued for this equipment.