Add-on decoder for AM stereo AM stereo is now broadcast in Australia on an What is C-QUAM experimental basis. This add-on decoder works C-QUAM is an acronym for Compatible QUadrature Amplitude with the Motorola C-QUAM system. We show how Modulation. That's certainly a mouthful — let's see what it means. to add it to two recent Playmaster tuners. First, the system is compatible with existing tuners. That means that any by JOHN CLARKE & GREG SWAIN ordinary (mono) AM tuner can receive a stereo broadcast and produce the same There is growing pressure in Australia automatically decoding all four systems result as if it received a mono signal. The for the official introduction of AM will be developed. AM stereo system does not make stereo. As we go to press, the existing tuners obsolete. Department of Communications has yet With the situation unlikely to be to give its decision as to whether all four resolved in the near future, we have Second, C-QUAM is a quadrature decided to present this decoder based on competing AM stereo systems will go system which means that it uses the the Motorola C-QUAM system. There ahead or whether one system, such as relationship between two carrier signals are currently five stations using the the Motorola, will be preferred. The that are 90° out of phase to encode the Motorola system in Australia: 2WS in left minus right (L — R) information. At other three systems are Harris, Kahn and Sydney; 3UZ, 3KZ and 3AK in Magnavox. the same time, the mono signal (L + R) is Melbourne; and 5KA in Adelaide. In the United States, the Motorola the conventional amplitude modulated system is currently the front runner. It Let us state right from the beginning, signal. has been chosen by more radio stations though, that many AM tuners are not There's a problem with quadrature than any other and has been adopted by capable of being converted to stereo. modulated signals though. They produce several major car manufacturers, We'll explain the reasons for that later distortion in the envelope detectors including General Motors, Ford, and give you some pointers on choosing (usually a diode) of normal AM tuners. Chrysler and Nissan. Many Japanese hifi a good candidate for conversion. In That's because the envelope detector sees companies have also adopted the addition, we'll show you how to add the the sum of the two carriers. As shown in Motorola system. That's not to say that decoder to the Playmaster AM/FM Fig. la, the magnitude of the sum of the other systems will disappear — it's Stereo Tuner (Nov '78) and • the those two vectors — which the tuner's possible that tuners capable of Playmaster Hifi AM Tuner (Dec '82). envelope detector would see — is: L +A ENVELOPE ERROR a DETECTOR DETECTOR DETECTOR DETECTOR LOCK +vCC 0 I+L+R 1 EXVELOPE ERROR LEVEL DETECTOR DETECTOR DETECTOR L-A 1icos1/ i+1.+R Pa a L-R IF IN VARIABLE GAIN DETECTOR (a) ows.110 O. or.. PHASE PHASE ADC FORCED DETECTOR DETECTOR DETECTOR MORO 400 INTERNA PART OF BAND PASS FILTER (25Hz) VCO +4 L 116 X17 11 13 OSCILLATOR OSCILLATOR ACC'0 PILOT FEEDBACK IN GOUT FILTER Fig. 1: To avoid distortion in normal AM IN tuners, quadrature signals must be con- verted to a compatible form. Fig. 2: Mock diagram of the MC13020 AM-stereo decoder IC. ELECTRONICS Australia, October, 1984 The assembled stereo decoder. There is also a capacitor under the PCB. could eliminate the problem by multiplying each carrier axis by the cosine of the angle that resulted from the addition of the L + R and L — R signals. Fig. lb shows that when this is done, the result is the 1 + L + R signal that we want — the standard AM tuner sees this signal as the same signal received from a monaural AM broadcast. Thus we have complete compatibility. The C-QUAM system also adds a 25-Hz pilot tone to the L — R information at 4% modulation. This serves several purposes: it signifies that a stereo transmission is present; it permits decoding of the L — R signal, and it aids in the control of mono-stereo switching. The MC13020P At the heart of the decoder is a single integrated circuit, the Motorola MC13020P. This is housed in a standard 20-pin dual-in-line package. A block „s• diagram of the IC is shown in Fig. 2 while Fig. 3 is a schematic of the complete decoder circuit. Taking an overall look at the block ,f(1 + L + R)2 + (L R)2. audio, or 1 + L + R. It is the difference diagram of the decoder IC (Fig. 2), we see However, the envelope detector in a between the two expressions that is the that the decoder takes the output of the standard AM tuner expects to see simply cause of the distortion problem. AM IF amplifier, decodes the C-QUAM the carrier and the left and right channel Motorola found, however, that they signal, and provides left- and right- .8101011.1.011101111MOVW .111111111110111.1111111111MMINM IN7OUT 8.3V + 35V 0 LM317 MAX. STEREO AD.( 12011 LED 0.1 .0033 .0033 100k 100 g 35VW1_ AUTO 10-a .0033 1 6VWI_ 68011 10011% 10011 22011 Si MONO I 20 1 12 15 0 ENVELOPE STEREO FORCED .01 DET DET DETECTOR LAMP MONO 3 455kHz IF LEFT LEFT IF INPUT IN UT OUTPUT AUDIO 8 J OSC IC1 OUTPUT L1 A FEEDBACK MCI 3020P RIGHT 8 OUTPUT RIGHT 14, c> 180pF EI 39pF 17 OSC LEVEL 4 10p INPUT PILOT PILOT DETECTOR 0 OUTPUT ERROR PHASE CO-CHANNEL DETECTOR FILTER. AOC' d 220pF1 LOCK AMP DETECTOR INPUT INPUT INPUT 0 OUPUT + 4.341:0 16 19 2 2.2 33 100k 16VWI 16VW 5600 .0033 I 2.7k 4.7 16VW ADJ IN g Cl 4 OUT 2.2 16VW MOTOROLA STEREO AM DECODER 2/TU/- Fig. 3: Stereo decoder circuit. Left and right audio signals are produced from an encoded 455kHz IF. ELECTRONICS Australia, October, 1984 e, The decoder PCB fits easily inside the Hifi AM Tuner cabinet. Add-on decoder for AM stereo channel audio outputs. In the absence of signal. This can be synchronously VCO frequency of 3.6MHz. a good stereo signal, it will produce an detected by conventional means. Coil L 1 and the capacitor network on undegraded mono output from both The process to detect or demodulate pins 17 and 18 (Fig. 3) set the VCO channels. the conventional quadrature signal frequency. This network is easily The first step in decoding the stereo involves first deriving a reference phase adjusted to suit either IF. information is to convert C-QUAM to from the transmitted signal. That's the The level detector senses carrier level QUAM. That conversion is purpose of the phase-locked-loop (PLL) and operates on the Q AGC (automatic accomplished by comparing the outputs that we'll now describe. The phase gain control) block to provide a constant of the envelope detector and the I (L + detector is a product detector — its amplitude 25Hz pilot signal at pin 11. It R) detector in the error detector. output is equal to the product of the two also sends information on signal strength Let's say, for example, that the input signal voltages (in this case, a to the pilot decoder. incoming signal is monaural. Then it reference carrier from the VCO and the The Q AGC (pin 11) output drives a consists only of L + R information, and QUAM signal from the variable-gain low-pass filter, made up of a 4000 the envelope detector and I detector see block). If the two signals are of the same internal resistor, a 7.5k0 resistor, a 4300 the same signal. Therefore the error frequency and 900 out of phase, the resistor and a 4.7p,F capacitor. filtered DC output of the detector will be detector does not produce an error From that point, an active filter (made zero. This DC output is fed back to the signal. up of both internal and external However, when the incoming signal is VCO as an error signal. Thus, the VCO components) is coupled to the pilot locks onto the input carrier frequency stereo, there will be an error signal decoder, pin 14, and another low-pass and we have our phase reference to the I produced. The envelope detector sees the filter is connected to the co-channel same signal as it did before (1 + L + R) and Q demodulators. input, pin 12. because it is not sensitive to the phase The RC network on pin 19 (see Fig. 3) modulation. But the I detector is filters the output of the phase detector to sensitive to phase modulation and sees ensure a steady DC error signal to the Stereolmono switching only the (1 + L + R) Cos° information. VCO. Automatic stereo/mono switching is When both signals are sent to the error The internal VCO operates at eight performed by the pilot decode block. detector, a 1 /cos0 correction factor is times the IF input, thus ensuring that its This has two modes of operation which produced. frequency is well outside the broadcast we will now describe. In the variable-gain block, the band. Typically, the IF will be 455kHz On a strong signal, the decoder will incoming C-QUAM signal is multiplied so the VCO runs at 3..64MHz. Note, switch to stereo after it sees seven by the 1 /cost9 factor to derive a however, that many synthesised tuners consecutive cycles of the 25Hz pilot conventional quadrature or QUAM use a 450kHz IF and this requires a waveform.