Stereo Tv Decoder Circuit

  
A block diagram of the stereo-TV decoder is shown in A. It shows the overall relationships between the separate sections of the circuit; through show the details of each subsection. The decoder section centers around TCI, a standard 4.5-MHz audio demodulator. The output of IC1 is routed to SI, which allows you to choose between the internally demodulated signal and an externally demodulated one. Buffer amplifier IC2- a then provides a low-impedance source to drive IC3, an LM1800 stereo demodulator. When IC3 is locked on a stereo signal, the outputs presented at pins 4 and 5 are discrete left- and right-channel signals, respectively. In order to provide noise reduction to the L-R signal, you must recombine the discrete outputs into sum and difference signals. Op amp IC4-a is used to regenerate the L-R signal. It is wired as a difference amplifier, wherein the inputs are summed together (+L -R~).
Stereo Tv Decoder Circuit - schematic

Capacitor C18 bridges the left- and right-channel outputs of the demodulator. Although it decreases high-frequency separation slightly, it also reduces high-frequency distortion. The L + R signal is taken from the LM1800 at pin 2, where it appears at the output of an internal buffer amplifier. The rawL -R signal is applied to IC4-b, a 12-kHz lowpass filter. The L + R signal is also fed through a 12-kHz low pass filter in order to keep the phase sliift undergone by both signals equal. Next, the L -R signal is fed to Q2. It allows you to add a level control to the L-R signal.path; it provides a low source impedance for driving the following circuits, and it inverts the signal 180. Inversion is necessary to compensate for the 180 inversion in the compander. Next comes the expander stage. At the collector of Q2 is a 75-8 de-emphasis network (R27 and C29) that functions just like the network that is associated with Ql. Note that Q2 feeds both Q3 and IC5-a, a -12-dB per octave high-pass filter. The output, of that filter drives the rectifier input of IC6, an NE570. The 75-Hz high-pass filter at the rectifier input helps to prevent hum, 60-Hz sych buzz, and other low- frequency noise in the L-R signal from causing pumping or breathing. The NE570 contains an on-board op amp; its inverting input is available directly at pin 5 and via a 20-kQ series resistor at pin 6. The 18-kQ resistor (R30) combines with the internal resistor and C32 (0.01 ) to form a first-order filter with a 390- time constant. Because the internal op amp operates in the inverting mode, the -{L-R) signal is restored to the proper (L-R) form. The output of the expander drives another 75-Hz high-pass filter, but this one is a third-order type that provides -18 dB per octave rolloff. It is used to keep low-frequency noise from showing up at the output of the decoder. At this point, the (L - R) signal has been restored, more or less, to the condition it was in before it was dBx companded at, the transmitter. The L+R signal from IC3 is fed to a 12-kHz low-pass filter, 102-b, with a -12 dB per octave slope. The output of the high-pass filter is applied to a 75 de-emphasis network (R22 and C26). The L + R audio -signal is now restored properly. Ql is wired as an emitter follower to provide a high load impedance for the de-emphasis network and a low source impedance for level control R23. Next, the L + R signal is fed to the matrix decoder. Op amps IC7-a and IC7-b are used to recover the individual channels. First, IC7-b is configured as unity-gain difference amplifier. The (L + R) signal is applied to its inverting input, and the (L-R) signal is applied to the noninverting input. Therefore, the output of IC7-b can be expressed as ~(L + R) + (L-R) =-L+L-R-R = -2R. Similarly, .10 7-a is configured as a mixing inverting amplifier. Here, however, both sum and difference signals are applied to the inverting input. So, the output of IC7-a is {L + K) - (L - R) = -L - R - L + R = -2L. Because both channels have been inverted, the stereo relationship is preserved. The two op amps in 108 provide an additional stage of amplification to drive a pair of stereo headphones. If you don`t plan to use your headphones, or if you are content to use only your stereo`s headphone jack, all components to the right of line-output jacks J3 and J4 can be deleted. THE NOlSE*REDUCTION STAGE compands the L-R signal, and emulates dbx-style processing. As described elsewhere in this article (see box), true dbx processing is not currently possible in a home-built circuit due to the inavailability of the dbx IC`s. THE MATRIX STAGE separates 1 he L + R and L-R signals into the left- and right-channel components. Op-amp`lC8 and associated components provide an optional headphone output. If you do not wish to drive a pair of headphones, or plan to use your amplifier s headphone jack for that purpose, all components to the right of jacks J3 and J4 can be deleted.




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