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This photo shows the prototype ACS decoder installed in an old Harman Kardon AM/FM stereo receiver. Two aluminium brackets were used to suspend the decoder above the tuner board. A subcarrier decoder for FM receivers Many FM stations are now radiating piggyback signals with their normal stereo transmission. You can’t decipher these “hidden” signals using a standard FM receiver but you can by adding this low-cost ACS decoder. By JOHN CLARKE The jargon doesn’t sound very enlightening but ACS stands for Ancillary Communication Service. This is a technique whereby a normal FM broadcast transmitter carries one or two extra subcar­rier signals that ride “piggyback” along with the normal FM stereo transmission. These hidden transmissions have no affect on standard FM mono and 20  Silicon Chip stereo receivers. Only the main signal can be detected by such receivers, so most people are unaware that ACS signals are even being broadcast. To listen to these extra signals, you need to fit an ACS decoder such as the unit described here to your FM receiver. Despite this, you’ve probably already heard ACS broadcasts. Many department stores and shopping cen- tres now use this service to provide background music for their customers. And the program content is usually just straight music, with no voiceovers or advertising. Other ACS services include foreign language, news and spe­cial interest programs. Signal transmission Before we describe how our ACS decoder works, let’s take a look at how the ACS signals are added to the FM signal. A normal FM stereo transmission is made up of three compon­ents: (1) an L+R mono signal modulated from 0-15kHz; a stereo pilot tone at 19kHz; and a multiplexed L-R difference signal centred on 38kHz. These com- Fig.1: the ACS signals are produced by modulating subcarriers centred on 67kHz & 92kHz. These subcarriers are then mixed with the normal FM stereo compon­ents & used to modulate the main carrier. % MODULATION CARRIER STEREO CHANNEL STEREO PILOT 0 15 19 67kHz ACS CHANNEL 23 38 53 59 67 92kHz ACS CHANNEL 75 84 92 100 FREQUENCY (kHz) ponents are mixed together and used to modulate the main carrier out to 53kHz – see Fig.1. By contrast, the ACS signals are produced by modulating subcarriers centred on 67kHz and 92kHz. These two frequencies are well above the upper limit of the L-R difference signal to avoid interference. As a further precaution against interference, the ACS signal bandwidths are limited to just 6kHz. They are mixed at low level with the existing stereo components before being used to modulate the main carrier. ACS decoding At the receiving end, these ACS subcarrier signals are ignored by a standard FM receiver since they fall well outside the passband. In fact, the detected 67kHz and 92kHz subcarriers are effectively removed by the 50µs de-emphasis filtering. So, to detect ACS signals, we need to modify the receiver by fitting an ACS decoder immediately following the FM de­ modulator, before any filtering takes place. The ACS decoder described here can be switched to decode either ACS subcarrier (ie, either 67kHz or 92kHz). This is done using a single toggle switch; there are no other controls to worry about. The recovered audio 67kHz AND 92kHz INPUTS FROM FM DEMODULATOR decoder inside a separate case and run it from a suitable DC plugpack supply. We’ll have more to say about the installation later on. Block diagram Fig.2 shows the block diagram of the ACS Decoder. Its input signal is extracted from the FM demodulator in the receiver and is fed to two bandpass filter stages centred on the ACS subcarri­ er frequencies. These filters separate the ACS subcarriers from each other and from the other components of the normal FM stereo signal. S1a selects between the filter outputs, after which the selected sub­ carrier is boosted by amplifier stages IC2a-IC2c. The boosted signal is then fed into a phase lock loop (PLL) de­ modulator to recover the audio. Immediately following the PLL stage is a 150µs de-emphasis stage. This rolls off frequencies above 1061Hz, thereby reducing noise in the audio signal and compensating for the 150µs boost (pre-emphasis) given to the audio signal before transmission. Finally, the recovered audio is fed to the output via a low pass filter which removes the original subcarrier plus any other un­ w anted components above 6kHz. In summary then, the 67kHz and 92kHz subcarriers are first separated S1b 67kHZ BANDPASS FILTER IC1a,IC1b output is fed into an auxiliary input of an amplifier. Once fitted, the unit is very easy to use. All you have to do is tune your receiver to an FM station and select the appro­priate auxiliary input on the amplifier. An ACS signal will now be heard (provided, of course, that the station is transmitting ACS signals). If the station is transmitting two ACS signals, the alternative signal can then be selected using the toggle switch. Provided you live in a good signal area and have a reason­able antenna, the ACS signal should be quite clean. But don’t expect it to sound as good as a regular FM stereo signal. That’s because of the restricted bandwidth (6kHz) and the fact that the signal is mono only. In addition, an ACS signal has only rela­tively low deviation, so you’ll need a strong signal to avoid hiss. It should be possible to fit the ACS Decoder to most FM tuners and receivers, and even to many portable FM receivers. Basically, there are a couple of ways you can go about this. First, if there is sufficient room, the unit can be fitted inside the receiver itself and powered from an existing supply rail. In fact, the prototype was fitted inside an old Harman Kardon re­ceiver – see photos. Alternatively, you could mount the S1a 6kHz 12dB/OCTAVE LOWPASS FILTER AMPLIFIERS IC2a-IC2c PHASE LOCK LOOP DEMODULATOR 150us DE-EMPHASIS ACS AUDIO OUTPUT 92kHz BANDPASS FILTER IC1c,IC1d Fig.2: block diagram of the ACS decoder. The 67kHz & 92kHz subcarriers are separated out using bandpass filters & the selected subcarrier then amplified & fed to a PLL demodulator to recover the audio. Finally, the recovered audio is filtered & fed to the output. March 1995  21 22  Silicon Chip DEMODULATED FM 560pF INPUT 10k .0033 .0015 1.1k 9 10 10k 10k 1k .0047 B C VIEWED FROM BELOW E 10k 1.1k 13 12 .0033 .0015 VCC/2 I GO 92kHz TWIN TEE FILTERS 560  .0015 1.1k 8 2 3 .0027 VCC/2 67kHz TWIN TEE FILTERS 430  1k .0047 IC1c 10k 7 .0027 1k 4 IC1a 6 TLO74 11 5 .0027 VCC/2 10k VCC/2 10 +12V 10k IC1d 10k IC1b 560  .0015 1.1k 14 430  .0027 1k 1 .01 10k S1a 2 4 12 5 0V +15-30V PHASE LOCK LOOP DEMODULATOR 22k COMP OUT VCO IN 9 VCO OUT IC3 4046 7 VCO 16 1 B 150us DE-EMPHASIS .015 10k GND 10 16VW 1k Q1 BC548 POWER SUPPLY VR1 10k 10k 11 IC2b AMPLIFIERS 5 6 100k 10pF REG1 IN 7812 OUT 10k 10k 10 10 10 35VW 8 DEMOD IC2a 3 TLO74 2 92kHz .0015 10k 6 VCO 14 INPUT 3 COMP IN 67kHz S1b .0027 VCC/2 220pF ACS DECODER 92kHz 67kHz 100k 10pF 4.7k 4.7k E 0.68 C 7 220pF 10 16VW 10 9 VCC/2 +12V 6kHz FILTER 12 13 IC2c 0.1 .0033 3k VCC/2 .012 6.2k 6.2k 10k 8 4 14 +12V .01 ACS AUDIO OUT 1 11 100  IC2d 100k 100k 10pF PARTS LIST 1 PC board, code 06303951, 137 x 80mm 1 DPDT toggle switch (S1) 11 PC stakes 1 10kΩ 5mm horizontal trimpot (VR1) Semiconductors 2 TL074 quad op amps (IC1,IC2) 1 4046 CMOS phase-lock loop (IC3) 1 7812 12V regulator (REG1) 1 BC548 NPN transistor (Q1) This close-up view shows the completed ACS decoder board. It should fit inside most FM tuners & receivers & can be powered from an existing 15-30V DC supply rail. Note that the decoder will not interfere with the reception of normal FM stereo transmissions. out using bandpass filters. The selected subcarrier is then amplified and fed to a PLL demodulator to recover the audio. Finally, the recovered audio is filtered and fed to the output. Circuit details Refer now to Fig.3 for the circuit details. This can be directly related back to the block diagram. IC1a & IC1b form the 67kHz bandpass filter, IC1c & IC1d form the 92kHz bandpass filt­er, IC2a-IC2c are the amplifier stages, IC3 is the PLL demodula­tor, and IC2d is the 6kHz low pass filter. In greater detail, the input signal is picked off from the FM demodulator via a 560pF capacitor and coupled to pin 6 of IC1a via a 10kΩ resistor. IC1a and IC1b together function as cascaded twin-T filter stages centred on 67kHz. In the case of IC1a, the two 1kΩ feedback resistors and the .0047µF Fig.3 (left): the final circuit is based on two quad op amps (IC1 & IC2) & a 4046 PLL (IC3). Twin-T filter stages IC1a & IC1b form the 67kHz bandpass filter, while IC1c & IC1d form the 92kHz bandpass filt­er. The selected signal is then amplified by IC2a-IC2c & demodulated by the PLL. Q1 buffers the demodulated signal, while IC2d rolls off the response above 6kHz to reduce noise. capacitor to ground form one half of the twin-T filter, while the two .0027µF capaci­tors and the 430Ω resistor form the second half of the filter. Because the twin-T filter network has a high impedance at 67kHz, IC1a essentially functions with a gain of one at this frequency due to the 10kΩ feedback resistor. At the same time, frequencies on either side of the 67kHz centre frequency are heavily attenuated by the filter action. So IC1a allows the 67kHz subcarrier to pass through while drastically curtailing frequen­cies that are outside the pass­band. The output of IC1a appears at pin 7 and is fed to a second twin-T filter stage based on IC1b. Note that cascaded filter stages have been used here to ensure adequate attenuation of the adjacent stereo signals and the ACS subcarrier at 92kHz. Filter stages IC1c & IC1d operate in identical fashion to IC1a & IC1b, except that their passband is centred on 92kHz. Switch S1a selects between the two subcarrier frequencies and feeds the resulting signal to IC2a via a 220pF capacitor and a 10kΩ input resistor. IC2a, IC2b and IC2c each function as inverting amplifier stages with a gain of 10 and thus provide an overall gain of 1000. The 220pF capacitors at the inputs of IC2a & IC2c roll off the response below 67kHz, while the three 10pF feedback capacitors limit Capacitors 1 10µF 35VW PC electrolytic 4 10µF 16VW PC electrolytic 1 1µF 16VW PC electrolytic 1 0.68µF MKT polyester 1 0.1µF MKT polyester 1 .012µF MKT polyester 2 .01µF MKT polyester 2 .0047µF MKT polyester 3 .0033µF MKT polyester 5 .0027µF MKT polyester 5 .0015µF MKT polyester 1 560pF ceramic or MKT polyester 2 220pF ceramic 3 10pF ceramic Resistors (0.25W, 1%) 4 100kΩ 4 1.1kΩ 1 22kΩ 5 1kΩ 14 10kΩ 2 560Ω 2 6.2kΩ 2 470Ω 2 4.7kΩ 1 100Ω 1 3kΩ Miscellaneous Hook-up wire, solder, mounting brackets, screws, nuts, etc. the high frequency response to reduce noise in the signal. Demodulation IC3, a 4046 phase lock loop IC, has everything we need to decode the FM signal. It contains two phase comparators, a vol­tage-controlled oscillator (VCO) and a source follower. The signal from IC2c is AC-coupled to pin 14, after which it is buffered and fed to a phase comparator. This compares the incoming frequency with the VCO frequency at pin 4 and produces an output at pin 2. This output is then filtered and applied to pin 9. It controls the VCO so that it March 1995  23 6 1 2 3 Fig.4: install the parts on the PC board as shown here, taking care to ensure that all polarised parts are correctly oriented. It is a good idea to use PC stakes at all external wiring points. 1uF 2 3 10k 10k 0.68 .012 Q1 VR1 10k .01 1k 6.2k 3k 100k 220pF 1.1k 560 1.1k 1.1k 1.1k 560  1 10uF 22k 10uF remains in lock with the input signal. The filtered VCO control voltage represents the phase dif­ ference between the incoming signal and the VCO signal and thus represents the audio modulation on the subcarrier. However, rather than extracting the demodulated audio directly from pin 9, it is taken from the output of the internal source follower at pin 10 instead. This ensures that we don’t load down the VCO control signal and create further distortion. 6 .0015 10k .0033 .0033 IC3 4046 10k 10pF 6.2k 4.7k 4.7k 10k 10k 2x.0015 0.1 100 IC2 TLO74 10uF 10k 5 1 1 IC1 TLO74 10k REG1 .0027 4 10k 560pF .0033 10uF 100k 1 10k 2x.0015 10uF 35VW 100k 10k 1k 100k 10pF 2x.0027 10k DEMODULATED FM INPUT 10pF .01 .0027 220pF 10k 10k .0027 430  1k .0047 1k 1k 430  .0047 ACS AUDIO OUT GND S1 +15-30V INPUT 5 GND 4 .015 S1b selects the free-running VCO frequency by switching in the appropriate capacitor value between pins 6 & 7. When the .0027µF capacitor is selected, the VCO free-runs at 67kHz. Alternatively, when the .0015µF capacitor is selected, the VCO free-runs at 92kHz. VR1 sets the centre frequency and the locking range. Immediately following the PLL is the 150µs de-emphasis network. This network is simply a low-pass filter and consists of a 10kΩ resistor and a .015µF capacitor. The filtered signal is then buffered by emitter-follower stage Q1 and fed to the 6kHz lowpass filter stage (IC2d). Two 6.2kΩ resistors, a .0033µF capacitor and a .012µF capacitor make up the filter components. This stage operates with a gain of -1 for frequencies below 6kHz and rolls off the response at 12dB per octave for higher frequencies. Its output appears at pin 14 and is coupled to the output terminals via a 100Ω resistor and a RESISTOR COLOUR CODES ❏ No. ❏   4 ❏   1 ❏ 14 ❏   2 ❏   2 ❏   1 ❏   4 ❏   5 ❏   2 ❏   2 ❏   1 24  Silicon Chip Value 100kΩ 22kΩ 10kΩ 6.2kΩ 4.7kΩ 3kΩ 1.1kΩ 1kΩ 560Ω 470Ω 100Ω 4-Band Code (1%) brown black yellow brown red red orange brown brown black orange brown blue red red brown yellow violet red brown orange black red brown brown brown red brown brown black red brown green blue brown brown yellow violet brown brown brown black brown brown 5-Band Code (1%) brown black black orange brown red red black red brown brown black black red brown blue red black brown brown yellow violet black brown brown orange black black brown brown brown brown black brown brown brown black black brown brown green blue black black brown yellow violet black black brown brown black black black brown ACS SUBCARRIER SIGNALS PICKED OFF HERE Fig.5: as with most FM tuners, the Sony ST-JX220A uses two ICs to do most of its FM processing. These are: (1) an IF amplifier & demodulator IC; & (2) a following multiplex (MPX) stereo decoder IC. The most convenient point to pick off the subcarrier signals is at the output of the demodulator (detector) IC. 1µF capacitor. The associated 100kΩ resistor prevents large offset voltages from appearing at the output. Power for the circuit can be derived from just about any +15-30V rail (normally from inside the receiver). This is fed to 3-terminal regulator REG1 to derive a +12V supply rail. In addi­tion, a half-supply rail (Vcc/2) is derived via a voltage divider consisting of two 4.7kΩ resistors and this biases all the non-inverting inputs of the various op amp stages. Construction All of the parts for the ACS Decoder except switch S1 are installed on a PC board coded 06303951. Fig.4 shows the assembly details. No particular order of assembly need be followed but we suggest that you start by installing PC stakes at the 11 external wiring points. The two wire links can then be installed, followed by the resistors, capacitors and ICs. Make sure that the ICs are correctly oriented and use your multimeter to check each resistor value before installing it, as some of the colours can be diffi­cult to decipher. Finally, complete the board assembly by installing VR1, transistor Q1 and REG1. Note that REG1 is mounted flat against the PC board with its leads bent at right angles and is secured using a screw and nut. Don’t bother wiring up the switch at this stage; that step comes later, when the unit is installed inside a receiver. (check the ICs and the regulator). Assuming all is well, check that the regulator output is at +12V. You should also find this voltage on pin 4 of IC1, pin 4 of IC2 and pin 16 of IC3. Finally, check that +6V is present on pins 3, 5, 10 & 12 of both IC1 and IC2. Initial tests Installation Once the board assembly has been completed, connect your multimeter in series with the +15V supply input and apply power. A 12V DC plugpack will make suitable temporary power supply, as it will have a no-load output of about 17V DC and will only be lightly loaded. Check that the quiescent current is no more than about 25mA (no input signal). If it is much more than this, switch off immediately and locate the source of the problem before proceeding The ACS Decoder can be mounted inside the receiver using suitable brackets and the toggle switch mount­ ed on the rear panel. This done, the switch can be wired to the PC board using rainbow cable – see Fig.4. The power supply connections (+15-30V & ground) can be run using hook-up wire. Ideally, you should have a circuit diagram of your receiver so that you can find a suitable supply rail. Important: make sure that the ACS Decoder and all connecting leads are kept well away from any mains wiring inside the receiver. In addition, you should run a separate earth lead between the switch body and the metal chassis if the switch is not earthed via the rear panel (eg, if the rear panel is plastic). If you are installing the decoder inside a receiver, the audio output lead can be internally connected to a spare pair of line input sockets (eg, aux). This lead can be run using light-duty hook-up wire. Note that you will have to connect the two sockets in parallel, since the decoder only has a single mono output. Alternatively, if the board is mount­ ed inside an FM tuner, the decoder’s output can be run to an additional RCA socket installed on the rear CAPACITOR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ Value IEC EIA 0.68µF 680n 684 0.1µF 100n 104 0.012µF 12n 123 .01µF 10n 103 .0047µF 4n7 472 .0033µF 3n3 332 .0027µF 2n7 272 .0015µF 1n5 152 560pF 560p 561 220pF 220p 221 10pF   10p   10 March 1995  25 Fig.6: this is the full-size etching pattern for the PC board. Check your board carefully for possible defects before installing any of the parts. panel. This audio output can then be con­nected via a Y-adapter shielded cable to the line inputs on your stereo amplifier. You now have to find the signal at the output of the demod­ulator. In a stereo tuner, this comes before the multiplex decod­er and treble de-emphasis networks. In a mono tuner, you must tap into the demodulated output before de-emphasis has taken place. After de-emphasis, the ACS subcarriers will be non-existent as we’ve already pointed out. Fig.5 shows a typical FM tuner circuit (Sony ST-JX220A) as an example. As with most such tuners, it uses two ICs to do most of its FM processing. These are: (1) an IF amplifier & detector IC; and (2) a following multiplex (MPX) stereo decoder IC. The most convenient point to pick off the sub­ carrier signals is at the output (in this case, pin 6) of the detector IC. Alternatively, the signal can be picked up at the input to the multiplex decoder IC. A suitable power supply rail for the decoder can usually be picked up from the regulator board inside the receiver. Testing The ACS Decoder should initially be tested with S1 set to 67kHz and VR1 at mid-position. Apply power and tune in one of your regular FM stations. This done, select the ACS decoder (using the selector switch on the amplifier) and check for the presence of an ACS signal. If no signal is heard, try adjusting VR1 until a signal is heard. Failing this, retune to another station and try again. When an ACS station comes up, adjust VR1 for best signal, then switch to the 92kHz position and adjust VR1 again so that both ACS signals can be heard. If no signal is present on 92kHz, try other stations in turn until you find one that’s broadcasting ACS signals on both frequencies. Copyright The signal for the prototype ACS decoder was derived by soldering the input lead directly to the output pin of the demodulator IC in the Harman Kardon receiver. If you don't have a circuit diagram of your receiver, use a CRO to determine which pin is the demodulated output. Alternatively, you may have to test each pin of the demodulator IC on a trial & error basis until an ACS signal is heard. 26  Silicon Chip Finally, readers are warned that recording or broadcasting received ACS programs without proper authorisation may breach copyright. If you have any doubts about your obligations, check with the copyright holder. SC