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, Listen to hidden FM transmissions Subcarrier adaptor for FM tuners This simple adaptor circuit fits in your FM tuner and lets you tap into hidden FM transmissions .. By JOHN CLARKE Just recently the Department of Transport & Communications announced that it is ready to authorise subcarrier transmissions on FM broadcasts. Testing of these transmissions is going on right now and you can listen to them by building this simple adaptor circuit. In America, subcarrier transmissions on FM broadcasts have been used for years. The Americans refer to these services as Subsidiary Communications Authorisation or SCA. It is based on a 67kHz subcarrier which is placed on the main FM carrier. In Australia the same system is being used but it will be known as & LEO SIMPSON Supplementary Monophonic Transmission (SMT) which will be generally recognised as an Ancillary Communications Service (ACS). Not a very inspiring name, is it? Australian tests have been on single sub-carrier transmissions at 67kHz but developments in the USA provide for multiple sub-carriers, some carrying digital data and others carrying audio. Now 67kHz sub-carrier transmissions are about to be authorised as regular services in Australia. To coincide with this, we have designed a suitable adaptor which can be hooked into most FM tuners with a This the ACS adaptor board, shown about 30% larger than actual size. All the parts are readily available. 64 SILICON CHIP mm1mum of fuss. Low in cost, it uses just a few readily available integrated circuits. Before we describe the circuit of the adaptor, let's briefly talk about FM subcarrier transmissions. They will have no effect on standard FM mono and stereo radios. Also, they will be fully compatible with all existing FM radios, whether stereo or mono. In fact, unbeknown to the great mass of FM listeners, test transmissions have been going on for some time. But while all FM radios are presently unaffected they are able to pick up the sub-carrier transmissions and, with the addition of an adaptor such as the one we describe here, able to detect the audio signals which will generally be music. While we were developing this adaptor circuit, the ABC in Sydney was running ACS test transmissions on 2ABC-FM. The audio modulation was the program simultaneously being broadcast by AM station 2BL. In the near future, ACS broadcasts are likely to be background music suitable for offices and factories . SILICON CHIP'S ACS Adaptor is built on a compact printed circuit board (PCB) accommodating three low cost op amps - a phase lock loop IC, a 3-terminal regulator and a handful of resistors and capacitors. ,-----.. . -------~---~-----------.----A TWIN -TEE Fil TER 220pF 10k 1.1k 11k <>-11-.w.,,,.......>;,N,,~--w,;.,,---+--4-__.,.......- 4 _ - , DEMODULATED FM INPUT VR1 10k 10 >"......W,,,,......aj21N 5600 1.1k 3 ~W,,-..::jlN IC2 LM565 DEMO OUT VCO +6V MP 5 4.7 16VW 10k 't'--~-ll-"""""'~,-.'IN,f,r+-"M"'""'9--=j P, RE -+ 1 .001 +-__.....____,.__.____.,__-+_ __;r--.......,__--- G N D 0 - - - - - - - . _ _ . , _ __ _ _ _ _ __ .__--4_ _ .,. L------------------ll-------o PLL DEMODULATOR 67kHz BAND-PASS FILTER 18d8/0CTAVE LOW PASS FILTER GAIN= -1 IN r + 1 5 V-30V 1 + 35VWI .., 0. 22 AUDIO > " - - ~ ~ OUTPUT ---------------vGNO ACS ADAPTOR FOR FM TUNERS 61-1287 12dB/OCTAVE 6kHz LOW PASS FILTER GAIN = -10 Fig.2: the circuit for the ACS adaptor is essentially just a phase lock loop with input and output filtering stages. Note that TL081 op amps may be substituted for the TL071s. 67kHz INPUT FROM FM DEMODULATOR 67kHz BAND PASS FILTER PLL OEMOOULATOR 18d8/0CTAVE 6kHz LOW PASS FILTER GAIN = -1 12dB/OCTA VE 6kHz LOW PASS Fil TER GAIN = -10 AUDIO OUTPUT Fig. 1 Fig.1: this block diagram shows the four circuit functions of the ACS Adaptor. The corresponding functions are also marked on the circuit diagram (Fig.2). How it works Fig.1 shows a block diagram of our circuit. The 67kHz signal present at the output of the FM detector (in the radio to be modified) is first fed to a 67kHz bandpass filter and then to a phase-lock loop (PLL) which recovers the audio modulation on the 67kHz sub-carrier. The audio output of the PLL is then passed through a low pass filter which attenuates frequencies above 6kHz at the rate of 18dB/octave. Another 12dB/octave lowpass filter stage completes the conditioning of the signal before it is passed to an external audio amplifier. Fig.2 shows the complete circuit. Op amp !Cl and associated components provide the 6 7kHz bandpass filter. A twin-T network comprising four 1. lkO resistors and associated 0.0022µF capacitors is connected in the feedback network of the op amp. This gives some gain at 67kHz and heavy attenuation for frequencies above and below this frequency. An additional passive filter at the input to the twin-T network (220pF and 10k0) provides some additional rolloff for frequencies below 67kHz. In practice, the bandpass action covers a frequency range of about 10kHz above and below the 6 7kHz centre frequency. VRl sets the gain of the bandpass filter stage. IC2 is a Signetics NE565 phaselock loop which demodulates the 67kHz frequency modulated (FM) signal from !Cl. The NE565 PLL consists of a voltage controlled oscillator (VCO) set to 67kHz and a comparator which compares the incoming frequency modulated 67kHz signal at pin 2 with the VCO signal fed into pin 5. The output of the comparator represents the phase difference between the incoming signal and the VCO signal and is therefore the audio modulation of the subcarrier. Treble de-emphasis (150µs) is provided by a 0.033µF capacitor (pin 7). The free-running VCO frequency is determined by the 0.00lµF capacitor at pin 9 and the resistance between the positive rail and pin 8 (lkO in series with VRZ). VRZ adjusts the oscillator frequency (also known as the "centre frequency") so that the incoming JANUARY 1988 65 signal is within the lock range of the PLL. To minimise noise in the demodulated output, it is important to reduce the lock range of the PLL to a minimum. This is achieved by shorting pins 6 and 7 together. To a lesser extent, the lock range and therefore the noise output becomes smaller for lower input signals so we keep the input signal reasonably low without prejudicing the PLL's operation. Following IC2 is the 18dB/octave filter employing IC3 which has a gain of one for wanted signal frequencies. This filter is followed by the final filter stage IC4 which has a gain of ten. The adaptor is ideally powered from the tuner or receiver it is built into so we had to make its input voltage requirements non-critical. The solution is to use a 12V 3-terminal regulator which enables the circuit to be powered from any unregulated DC rail from + 15 to + 30 volts. The three op amp ICs and the PLL ICIOI LA\231 16 FM IF' AMP, AF AMP FM OET I TPI FM OISCRI NULL CHECK Rll3 Rll2 18k 66 SILICON CHIP C!I ! 33p Cll3 0.01 are all biased to half the supply voltage by a voltage divider consisting of two 10k0 resistors which is decoupled by a 4.7µ,F capacitor. The centre-point of this voltage divider is connected to pin 3 of each op amp and the PLL. PCB assembly This photo shows the ACS adaptor installed in an older AM/FM stereo receiver, the Harman Kardon hk570i. We used two brackets to suspend the Adaptor above the tuner board of the receiver. 56k The PCB for this project measures just 5 7 x 89mm and is easily assembled. No special points need to be watched when installing the parts on the PCB except that component polarities must be correct. Note also that ICl has a different orientation to IC2, 3 and 4. When assembly and soldering are finished, check your work carefully and then connect a DC supply of between 15 and 30 volts. Now check the voltage at the output of the 3-terminal regulator, at pin 7 of the TL071 op amps, and at pin 10 of the PLL. In each case the reading should be close to 12V. The voltage at pin 3 of each IC should be close PARTS LIST IC201 LA3401 MPX 1 PCB, code SC061-1287, 57 x 89mm 3 TL071, LF351 FETinputop amps 1 NE565 phase lock loop 1 7812 3-terminal 12V regulator Capacitors R211 56k R213 47k + C209 22 16V R221 KJOk 1161 Fig.3: this is a portion of a typical FM/AM tuner (Sony ST-JX220A) showing where the ACS adaptor is tapped in, across C111, between the FM detector and the multiplex decoder. 1 1 1 1 2 1 1 1 1 4 3 1 1 4. 7 µF 1 6VW PC electrolytic 2.2µF 16VW PC electrolytic 1µF 35VW PC electrolytic 1 µF 1 6VW PC electrolytic 0.22µF metallised polyester .033µF metallised polyester .022µF metallised polyester .0068µF metallised polyester .0056µF metallised polyester .0022µF metallised polyester .001 µF metallised polyester 560pF polystyrene 220pF ceramic Resistors (0.25W, 5%) GND 1 X 20kf! 2%, 1 X 18kf!, 6 X 10kf!, 2 x 1.8kf!, 4 x 1.1kf! 2%, 1 x 1kf!, 2 X 56011, 1 .x 10kf! miniature vertical trimpot, 1 x 5kf! miniature vertical trimpot + 15V-30V GND INPUT Miscellaneous Hookup wire, audio leads, solder etc. Fig.4: take care when assembling the board. Note that IC1 is oriented differently from IC3 and IC4. to 6V and so should the voltage at pin 6 of each TL071. If everything is okey dokey, you are ready to install the adaptor in your FM tuner or stereo receiver. Finding the signal This is the tricky part. Ideally, you need access to the circuit diagram of your tuner or receiver. You need to identify a positive DC supply rail of between + 15 and + 30 volts. Then you need to find the output of the FM demodulator. In a stereo tuner this comes before the multiplex decoder and treble de-emphasis networks. In a mono tuner, you must identify the demodulator output before deemphasis. After de-emphasis, the 67kHz signal will be non-existent. We have shown part of a typical FM tuner circuit (Sony ST-JX220A) as an example of where the 67kHz signal must be picked off. As with most medium priced tuners, it uses two ICs to do most of its FM processing. These are the IF amp and detector IC and the following multiplex (MPX) decoder IC. The most convenient point to pick off the 67kHz signal is at the input to the MPX decoder. Setting up Having found the signal and made the necessary connections from the adaptor to your tuner, the continued on page 95 Full size PC artwork for the ACS Adaptor. Design by John Clarke. Luvverly, innit? JANUARY 1988 61 Ribbon or coax: which is best? All of the information I have read about UHF antennas in Australia recommends the use of 75-ohm coax cable and most of the installations I have seen in this country have used coax. But I have been overseas to the US and Japan on a number of occasions and there they seem to use ribbon cable frequently. Why is tha~? Why is there such a preference in Australia for coax? I can't believe that our reception conditions for UHF would be any more difficult than in cities in Japan and the US. • We doubt whether UHF reception conditions are any more difficult in Australian cities than overseas. Based on typical attenuations figures at UHF for ribbon cable and coax cable, you might think that ribbon would be the more desirable. After all, most coax cables have losses of 10dB/30m or more at UHF which is considerably more than the nominal losses of la.dder-type ribbon cable but we would not recommend ribbon cable on that basis. It is possible that in some situations, 300-ohm ribbon cable might give comparable reception and three 7-segment displays), you will need to run 12 separate data lines plus the supply lines. That makes it messy. We don't think UARTs (Universal Asynchronous Receiver/Transmitter) would be practical or cheap since you would need two 8-bit UARTs to send and another two to receive, plus all the timing circuitry required. Nor would A-D and D-A conversion be practical or cheap because that also implies quite a lot of supporting circuitry. The only practical way seems to be to send all 12 data lines via RS232 receivers and transmitters. We suggest you use the Motorola MC1488 quad line driver and MC1489 quad line receivers, both of which are quite cheap. You would need three to transmit and quality to 75-ohm coax but you could only justify its use in areas away from the sea which don't have high rainfall. Once ribbon cable becomes wet or coated with a salt film, it starts to produce much higher losses. It deteriorates quickly too and is subject to signal pickup on the cable itself, producing leading ghosts and smeary pictures. As far as we are aware, coax cable is universally used throughout Europe for UHF reception and the same can now be said for the USA and Japan. It is also true that many if not most installations in apartment buildings overeases would not feed UHF signals via distribution system to each tenant. The signal losses, particularly in older installations using inferior cables, would make it impractical. Rather, it is standard practice to feed the UHF signals to a down-converter so that the signals are distributed at VHF. The message is: forget ribbon for UHF. If coax cable losses are .likely to be a problem, you should use a masthead amplifier to boost the signal before it is distributed. three to receive. You will also need ± 12V supplies for the transmitters in addition to 5V for the remote display. We admit that our suggested solution does not look simple, but it is probably the cheapest way. Corrections Digital Fundamentals, Dec. 1987: Fig.6 on page 92 has been reproduced incorrectly. The type down the left-hand edge of the diagram should read INPUT A, INPUT B, INPUT C and OUTPUT D. In addition, the second last paragraph on page 92 should read as follows: At times t 1 through ta the three inputs are never high at the same time. However, beginning at time ta and ending at time t 9 the three inputs are all high so that output D goes high. Subcarrier Adaptor continued from page 67 setting up procedure is relatively simple. First, make sure that VR1 is set so that its wiper is turned toward the LM565. This will provide maximum signal level. Now adjust VR2 so that there is audio signal. Find the extreme settings of VR2 where the audio signal drops out, then set VR2 halfway between the two extremes. VR1 is used to minimise noise from the audio signal when the FM signal level is poor. Adjust the trimmer until the sound becomes distorted and then back off the adjustment until distortion is no longer audible. If you have a strong FM signal, adjustment of VR1 will have no effect on the noise level and so it should be left at its maximum resistance setting. UHF Antenna continued from page 21 enable you to terminate cables from your VHF and UHF antennas. A single cable goes from the diplexer to your TV set. Alternatively, the diplexer output may be fed to a splitter and then to various TV wall plates around your home. Tune your TV to the local UHF station(s) and then orient the antenna for best reception. Secure the cable to the mast with plastic cable ties to prevent the cable from flapping in the wind. Seal the balun box with silicone sealant to weatherproof it. Painting Depending on where you live, painting the antenna cart be worthwhile, particularly in seaside areas or near industrial areas where there may be a lot of fallout. In these cases, we suggest painting the antenna with an etch primer and then finishing with an aluminium loaded paint such as British Paints "Silvar". As a final comment, if you are building the antenna to receive stations right at the top of band V, say between channels 59 and 69, a small improvement can be gained by shortening the dipole elements by 5%. JANUARY 1988 95