The sound IF system of the television receiver is similar to the IF system of an FM radio receiver. Two major differences from the standard FM receiver should be noted:
1. The intermediate frequency for FM receivers has been standardized at 10.7 MHz, whereas the sound IF for inter carrier television receivers has been standardized at 4.5 MHz. (This is the frequency difference between the video and sound carriers, and it is held constant at the transmitter.)
2. The deviation of the television sound carrier for maximum modulation has been established at 25 kHz (a total sweep of 50 kHz), whereas the maximum deviation for standard FM broadcasting has been set at 75 kHz (a maximum sweep of 150 kHz). Because a lower deviation is employed for television sound, a narrower passband can be used for the television sound IF system, and a shorter linear region can be used for the detector. The passband of a typical intercarrier sound IF amplifier is about 150 kHz, and the linear region of the detector usually does not exceed 100 kHz. A high quality FM receiver might have an IF passband of 300 kHz and a linear range in the detector of as much as 2 MHz.
A typical intercarrier sound IF stage has a voltage gain of 100 times. If the stage is operated as a limiter, its gain is approximately five times. Note that the overall gain from the RF tuner to the FM sound detector is approximately the same as the overall gain from the RF tuner to the picture detector. In other words, the sound IF amplifier compensates for the reduced amplification of the sound signal through the video IF amplifier.
Unlike the video IF amplifier, automatic gain control is not used in the intercarrier sound IF amplifier; that is, the sound IF section operates at maximum gain regardless of the incoming signal strength. A limiter stage operates as a peak clipper, and, in turn, little change in sound volume is normally observed from strong- to weak-station reception. However, the sound output may become slightly noisy, or "buzzy," if the incoming signal is very weak. Adequate sound output is normally obtained although the picture signal is so weak that the image is barely visible in the snow interference. Of course, in case of sound IF or FM detector malfunction, the situation may be reversed-little or no sound output may be present although the picture display is normal.
SOUND IF TAKEOFF
The takeoff point in an intercarrier receiver must follow the video detector. A sound detector requires a definite amount of signal to provide a noise-free signal. The amplification this signal receives is provided by the sound IF amplifier, the video amplifier, or combinations of both. The video IF amplifiers do not amplify the sound signal very much because the signal is attenuated in these stages.
The takeoff point for most modern receivers is at the video- detector output (Fig. 13-1). The 4.5-MHz signal is coupled through a small capacitor from the video-detector output to the sound IF amplifier input. In other receivers, the sound IF takeoff point is in the output circuit of the video amplifier to take advantage of the gain supplied by the video amplifier. Typical circuits are shown in Fig. 13-2.
Fig. 13-1. Sound takeoff point after picture detector.
Fig. 13-2. Sound IF takeoff points in video amplifier.
Semiconductor intercarrier sound circuitry can be classified into transistor and integrated types. Figure 13-3 shows a typical transistor intercarrier sound-system configuration. In most transistor TV receivers, the 4.5-MHz intercarrier sound signal is taken from the video-driver stage and it is amplified in two sound IF stages. The 4.5-MHz sound IF amplifiers can be compared with video IF amplifiers, except that the former have narrower pass bands. Triode transistors that require neutralization are commonly employed; for example, C64 and C66 are neutralizing capacitors.
Some transistor TV receivers utilize integrated sound circuitry, as shown in Fig. 13-4. An equivalent circuit of the IC unit is seen in Fig. 13-5. This unit performs- the functions of 26 conventional components. As is customary in most designs, the tuned circuitry is external to the IC unit. The tuned input circuit in Fig. 13-4 applies the 4.5-MHz signal to a three-stage, direct-coupled amplifier in the IC unit. Each differential stage is coupled by an emitter-follower to the next stage, as seen in Fig. 13-5.
Q1 and Q2 in Fig. 13-5 form the first differential-amplifier stage, coupled in turn by Q3 to Q4 and Q5, the second differential stage. Output from the second stage is coupled by Q6 to the third differential stage comprising Q7 and Q8. As shown in the block diagram, the intercarrier sound signal is transformer coupled to the base of Q1; Q9 and Q10 operate in a voltage regulator configuration. We recognize that although fewer tuned circuits are employed with an IC unit, more semiconductor devices are included.
Fig. 13-3. Typical transistor sound IF configuration.
Fig. 13-4. Intercarrier sound system that uses an integrated circuit.
Fig. 13-5. Equivalent circuit of an integrated unit with a block diagram showing
stages contained.
Fig. 13-6. Frequency modulator of a carrier wave.
Fig. 13-7. Balanced phase-shift (Foster-Seeley) discriminator configuration.
Fig. 13-8. Effect of limiting action on incoming intercarrier sound signal--an
overdriven amplifier clips off the positive and negative peaks from the incoming
signal.
Frequency modulation of a carrier wave is depicted in Fig. 13-6. In turn, a detector is required to discriminate between the deviations above and below the center frequency and convert these deviations into a signal voltage having an amplitude that varies at audio frequencies. Figure 13-7 shows an FM detector configuration that is called a balanced phase-shift discriminator.
The mutually coupled tuned circuits in the primary and secondary windings of transformer T are tuned to the center frequency of the intercarrier sound signal. Because a discriminator has very little ability to reject amplitude modulation (such as noise pulses), the last IF stage is operated as a limiter.
The effect of limiting action on the incoming intercarrier sound signal is depicted in Fig. 13-8. Because a discriminator circuit uses a push-pull rectifier arrangement, the diodes conduct on alternate half-cycles of the signal waveform and produce a plus-or minus output signal. The phase characteristics of the tuned circuits are such that a negative-output voltage is produced at frequencies below 4.5 MHz and a positive-output voltage is produced at frequencies above 4.5 MHz.
Next, consider the ratio-detector circuit shown in Fig. 13-9.
This configuration has a general resemblance to a discriminator circuit, with an important difference: The rectifier diodes are polarized oppositely. As will be explained, this feature provides considerable self-limiting action in the FM detection process, and therefore receivers that use ratio detectors often do not operate the last IF stage as a limiter.
Fig. 13-9. Ratio-detector configuration.
In a ratio detector, the two rectifier voltages are added algebraically and the sum is held constant by a large electrolytic capacitor C3, which has a value of 5 µF in this example. It is this effectively constant charge on the electrolytic capacitor that rejects amplitude variations in the signal-the large capacitor tends to absorb any peaks or valleys that may be present in the intercarrier sound waveform.
Note that at the center frequency of 4.5 MHz, the voltages across C1 and C2 in Fig. 13-9 are equal and opposite, so that there is no output voltage. On the other hand, when the incoming signal swings below the center frequency, D2 conducts more than D1 and the output voltage swings negative; conversely, when the incoming signal swings above the center frequency, D1 conducts more than D2 and the output voltage swings positive.
However, the voltage across C3 remains practically constant. It is helpful to note that the voltage across C3 tends to change some what from time to time, inasmuch as R1 and R2 function as bleeder resistors as well as load resistors. In practice, the change of voltage across C3 is more rapid for downward modulation of the incoming signal than for upward modulation.
Since sync-buzz pulses produce downward modulation of the intercarrier signal (in most cases), the ratio detector may not be able to completely reject high percentages of sync-buzz noise. Therefore, when this is a problem in receiver operation, the last IF stage is operated as a partial or complete limiter.
TROUBLESHOOTING
At this point, it is helpful to consider and summarize some basic troubleshooting principles. A simplified block diagram for a TV receiver is shown in Fig. 13-10, with basic oscilloscope test points indicated. If the sound IF amplifier and discriminator is checked with a sweep-frequency signal having a center frequency of 4.5 MHz, the output waveform from the discriminator normally appears as shown in Fig. 13-11. Note that this is an S curve, and that it is being "marked" at its center frequency of 4.5 MHz with an AM beat-marker signal. When alignment of any tuned circuits is required, the technician should consult the receiver service data and proceed accordingly.
Fig. 13-10. Simplified block diagram of a TV receiver showing basic oscilloscope
test points.
An RF signal generator and a VTVM are also basic trouble shooting instruments for sound IF and detector circuitry. Basic troubleshooting tests for sound IF and detector stages are depicted in Fig. 13-12.
Fig. 13-11. Normal S curve for a discriminator with an AM center frequency
beat marker.
Fig. 13-12. Basic troubleshooting tests for sound IF and detector stages.
SUMMARY
The sound takeoff for modern intercarrier TV receivers must follow the video detector. The sound detector requires a definite amount of signal to provide a noise-free signal. The 4.5-MHz signal is coupled through a small capacitor from the video detector output to the sound IF amplifier input. Sound takeoff point is generally in the output circuit of the video amplifier to take advantage of the gain supplied by the video-amplifier stage.
The sound IF system of the TV receiver is similar to the IF system of an FM radio receiver except the IF frequency for FM is at 10.7 MHz, whereas the sound IF for television receivers has been standardized at 4.5 MHz. This is the frequency difference between the video and sound carriers and is held constant at the transmitter.
QUIZ
1. What is the IF frequency for FM receivers? For television receivers?
2. What are the main differences between FM and television sound IF systems?
3. What part of the video IF amplifier is the sound takeoff section?
4. Name the basic demodulation circuits used in television receivers.