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Surround Sound MIXER & DECODER PART 1 – By JOHN CLARKE Build this unit and add depth, realism and effects to your home videos. It provides realistic surround sound mixing, while an inbuilt decoder provides the rear channel signal during playback if a surround sound processor is unavailable. 22  Silicon Chip W HILE HOME VIDEOS usu- ally provide fairly bland viewing for all but a few doting grandparents and close relatives, this does not have to be so. Surround sound can capture the audience so that they become part of the action. Adding surround sound will add a new dimension to your video recordings. It may even stir you into creating bigger and better movie productions, as you experiment with surround mixing. As well as surround mixing, this Surround Sound Mixer & Decoder can also be used to mix normal stereo signals; ie, by using just the Left and Right channels. You can also mix in signals from two other sources via the A and B channels. By adding the Centre and Surround channels, you will have surround processing. Signals from the A and B inputs can be mixed into any of the Left, Centre, Right and Surround channels using the L-R and the C-S pan controls. The resulting surround sound signal is encoded into the Left and Right channels and is subsequently decoded on replay. To simplify the task of mixing, signal level meters are fitted to all four Main Features • • • Surround sound encoding and decoding. • • Compatible with normal stereo and mono outputs. • • • • • • A-channel panning between L-R and C-S. Encoding similar to 4-channel Dolby® surround format. Encoded signals can be decoded by Dolby Pro Logic® and passive surround sound units, or by using the internal decoder in the mixer. Separate Left, Centre, Right and Surround inputs, plus A and B channel inputs. B-channel panning between L-R and C-S. Separate level controls for all inputs. Balanced or unbalanced microphone and line input options. Single output level control. LED level meters for the L, C, R & S channels (-24dB to +3dB). output (L, C, R & S) channels. They comprise 10-LED displays with a -24dB to +3dB range in 3dB steps. In operation, they monitor the encoded Left and Right channel sign­als and the Centre and Surround channels. Surround sound playback The encoded signals can be played back in stereo or mono but in order to obtain surround sound, they must be re­ played through a stereo VCR and decoder. While the mixer does incorporate a simple decoder, its main purpose is to provide the meter signals. Ideally, for best sound effects, the L & R outputs from the VCR should be fed through a Dolby Pro Logic surround sound decod­ er. This could be a commercial unit or you could use either of the two units described in SILICON CHIP (see Dec.94-Jan.95 and Nov.95-Dec.95). Fig.1(a) shows the basic scheme. If you don’t have a Dolby Pro Logic decoder, the basic decoder built into the Surround Sound Mixer & Decoder can be used instead. In this case, the L & R outputs from the VCR connect to the Left Fig.1: the encoded signals on the video tape can either be decoded using a Dolby Pro Logic unit as shown at (a), or fed through an internal decoder in the mixer itself as shown at (b). In the case of (b), the Centre (C) channel is not normally used, while the Surround (S) channel should ideally pass through a 20ms delay before being fed to its power amplifier. January 1996  23 Fig.2: block diagram of the Surround Sound Mixer and Decoder. The various inputs are mixed in summing amplifier stages before being fed to the Left and Right outputs via level controls VR11a and VR11b. On playback, IC9a sums the Left and Right channels to provide the Centre output, while IC9b produces a difference output which is then filtered to provide the Surround output. and Right channel inputs of the unit in the line mode – see Fig.1(b). The overall volume can then be controlled by the Output Level control, while the balance is adjustable using the individual Left and Right level pots. Note that, ideally, the Surround channel output from the mixer unit should be passed through a 20ms delay (a suitable 20ms delay unit will be described in the February 1996 issue of SILI­CON CHIP). The Centre output is best left disconnected here, since it will have poor separation from the Left and Right chan­nels. Note also that the decoded sound will be nowhere near as realistic as from a Dolby Pro Logic unit. The decoder built into the mixer is very much a “poor man’s” approach to surround sound, although it can still give good effects. In either case, separate amplifiers are required for the Left, Right, Surround and Centre channels in order to drive the loudspeakers. The Left 24  Silicon Chip and Right channels are normally fed to an existing stereo amplifier, while a second stereo amplifier can be used for the Surround and Centre channels. Alternatively, some Dolby Pro Logic decoders have several audio amplifiers built in. Inputs & outputs As shown on the main circuit diagram (Fig.3), each input has a stereo jack socket which can accept either a microphone or a line level signal, as selected by a toggle switch. Either a balanced or an unbalanced source can be used for the microphone input, while line level inputs must be unbalanced. If necessary, unbalancing can be achieved by using either a mono plug or a stereo plug wired with the ring connection to ground. At the other end, the outputs are run to RCA sockets to provide the Left, Centre, Right and Surround (L, C, R & S) signals. For recording purposes, the Left and Right channels only connect to the tape recorder (or VCR). Although making a stereo recording is fairly straightfor­ ward, 4-channel recordings will require a fair degree of prac­ tice. Fairly obviously, you will need four microphones – one for each channel. For a concert, the Left, Centre and Right micro­phones should be spread across the stage. The rear channel micro­phone can either be placed behind the stage or within the audi­ence, depending on the effect you want. The A and B inputs can be used to add background sounds or music to one or more channels. And, if desired, you can produce the effect of movement between one channel and another by pan­ ning. There are four panning controls in all (two for the A input and two for the B input) and these provide panning between the Left and Right channels (Pan L-R) and between the Centre and Surround channels (Pan C-S). Block diagram Fig.2 shows the block diagram of the unit. Starting at the left, there are six amplifiers for the Left, Centre, Right, Surround, A and B inputs. The output Output level control The SUM3 and SUM4 outputs are now fed to output level con­trols VR11a and VR11b, respectively. These are sections of a dual-ganged pot and are used to adjust the encoded Left and Right channel output levels. From there, the encoded signals are fed to the Left and Right channel output sockets. They are also used to drive the Left and Right signal strength meters. In addition, the encoded Left and Right channel outputs drive summing circuit SUM6 and difference circuit DIFFERENCE 1. The SUM 6 output provides the Centre channel and is inverted (IC10a) before being fed to the output socket and to the Centre meter. PARTS LIST 1 sloping front console cabinet, 170 x 213 x 31 x 82mm 1 PC board, code 02302961, 144 x 194mm 1 PC board, code 02302962, 76 x 105mm 1 PC board, code 02302963, 72 x 82mm 1 self-adhesive front panel label, 166 x 215mm 1 self-adhesive rear panel label, 165 x 78mm 6 10kΩ log pots (VR1-VR6) 4 10kΩ linear pots (VR7-VR10) 1 10kΩ dual ganged pot (VR11) 1 1kΩ horizontal trimpot (VR12) 7 SPDT toggle switches (S1-S7) 6 6.35mm stereo PC board mount switched sockets 1 2 x 2-way PC-mount RCA panel socket (Altronics P0211) 1 DC panel socket (to suit plugpack) 1 12VAC 300mA plugpack 4 knobs with blue insets 2 knobs with red insets 2 knobs with purple insets 3 knobs with black insets 15 cable ties 1 15m length of single shielded cable 1 1.5m length of yellow hook-up wire 1 500mm length of red hook-up wire 1 500mm length of green hookup wire 1 800mm length of blue hook-up wire 4 9mm tapped spacers 4 6mm untapped spacers 4 3mm dia. x 12mm screws 5 3mm dia. x 6mm screws The DIFFERENCE1 output provides the Surround signal. This is rolled off above 7kHz by low pass filter stage IC10b before being applied to the output socket and metering circuitry. Circuit Refer now to Fig.3 for the complete circuit details. Although it may appear quite complicated at first glance, there is in fact a considerable amount of duplication for the various inputs. Let’s begin by taking a look at the 1 3mm nut 74 PC stakes 4 11-way pin headers (13mm long pins) Semiconductors 10 LM833 dual op amps (IC1IC10) 1 TL071, LF351 single op amp (IC11) 4 LM3915 log. display drivers (IC12-IC15) 1 7812T 3-terminal regulator (REG1) 1 B104 1A bridge rectifier (BR1) 4 BC328 PNP transistors (Q1Q4) 4 1N914 signal diodes (D1-D4) 40 3mm red LEDs (LED1-40) Capacitors 1 2200µF 25VW PC electrolytic 1 100µF 16VW PC electrolytic 12 47µF 16VW PC electrolytic 6 10µF 16VW PC electrolytic 20 2.2µF 16VW PC electrolytic 19 0.1µF MKT polyester 2 .0027µF MKT polyester 2 680pF ceramic 6 220pF ceramic 1 180pF ceramic 1 100pF ceramic Resistors (0.25W, 1%) 4 1MΩ 8 4.7kΩ 4 100kΩ 12 2.2kΩ 19 22kΩ 4 1.2kΩ 4 16kΩ 8 1kΩ 2 20kΩ 4 680Ω 4 13kΩ 6 220Ω 1 12kΩ 4 150Ω 30 10kΩ 5 100Ω 2 8.2kΩ input circuitry for the Left signal. This circuit is based on op amp IC1a which is wired in the balanced configuration. Fig.3 (following pages): the input and summing circuitry is based on LM833 dual op amps (IC1-8), and these are also used in the decoding circuitry (IC9-10). IC11 is used to derive the split supply, while the four signal level meters are based on LM3915 display driver ICs. January 1996  25 ▼ levels from these stages are set by potentiometers VR1-VR6 re­spect­ively. The Left amplifier output connects to summing junction SUM1 which comprises IC4a. This mixes in the Centre amplifier output after it has been attenuated by 3dB. Similarly, the Right ampli­fier output connects to summing junction SUM2 (formed by IC5a) and this also mixes in a -3dB Centre signal. The A and B amplifier outputs are each amplified by two, using IC7a and IC7b respectively. This is done to compensate for losses in the following L-R pan circuit stages. The resulting L-R pan signals are then mixed into the SUM1 and SUM2 junctions. Similarly, the Surround amplifier output is summed at SUM5 with the C-S (Centre to Surround) pan control outputs. The summed output is then filtered using low-pass filter stage IC6a, so that only signals below about 7kHz are fed to the following stages. Following IC6a, the Surround signal is fed in two different directions. In one direction, it is first phase shifted by 180° (ie, inverted), then attenuated by 3dB and mixed at SUM3 with the signal from SUM1. In the other direction, it is fed straight to a 3dB attenuator (ie, no phase shifting) and then mixed at SUM4 with the signal from SUM2. The process so far is similar to the encoding process used for Dolby Surround Sound recording, except that no noise reduc­tion is used in the Surround signal path. This lack of noise reduction encoding circuitry is not important in this applica­tion, particularly as we wanted to keep costs down. 26  Silicon Chip January 1996  27 Assuming that S1 is closed (LINE), the input signal is attenuated by the 220Ω resistor and the overall stage gain is +1. The output from IC1a appears at pin 1 and is fed to level control VR1. IC1b, IC2a, IC2b, IC3a & IC3b are the input amplifiers for the Centre, Right, Surround, A and B channel inputs respectively. These stages are all identical to IC1a and their outputs feed level controls VR2-VR6. Following VR1, the Left signal is fed to summing amplifier IC4a via a 10kΩ resistor. Similarly, the Right signal is fed via a 10kΩ resistor to summing amplifier IC5a. The Centre channel output at the wiper of VR2 is buffered using IC4b before being applied to each of these summing junctions via a 14kΩ resistance (made up of 13kΩ and 1kΩ resistors in series). This arrangement effectively attenuates the Centre channel signal by 3dB with respect to the Left and Right signals. That’s because IC4a & IC5a operate with a gain of -1 for the Left and Right signals, and a gain of -0.714 for the Centre signal. Moving now to the Surround channel, the signal on the wiper of VR4 is coupled to pin 6 of IC5b, where it is summed with the Centre-Surround (CS) pan signals (more on these shortly). The output of IC5b then drives IC6a. This op amp is wired as a 2-pole lowpass filter stage and rolls off frequencies above 7kHz. Performance of Prototype Signal-To-Noise Ratio Better than 84dB with respect to 1V output Frequency Response: L, C, & R Channels: -1dB at 10Hz & 40kHz A & B Channels: -3dB at 40Hz & -1dB at 40kHz S Channel: -3dB at 7kHz Total Harmonic Distortion 0.01% at 1kHz and 300mV input Decoder Separation Surround to Centre Channels: 42dB minimum at 1kHz Left to Right Channels: 76dB at 1kHz Left & Right to Centre Channel: 12dB Left & Right to Surround Channel: 15dB Signal Handling 2V RMS maximum for line input Sensitivity: Mic Input: 30mV for 300mV out. Line Input: 300mV for 300mV out. Assuming that S1 is in the MIC position, it has a gain of -10 for signals fed to its inverting input and +11 for signals fed to its non-inverting input (as set by the 22kΩ feedback resistor and the 2.2kΩ input resistors). However, signals applied to the non-inverting input are first attenuated by 0.909 using a resistive divider (2.2kΩ & 22kΩ) before being amplified. As a result, the overall stage gain for signals applied to the non-inverting input is +10, which matches the gain for the inverting input. This gives good common mode rejection for balanced signals (eg, from a microphone). For unbalanced signals, the inverting socket connection must be ground­ ed externally by a mono plug (or by earthing the ring terminal of a stereo plug). This means that only signals at the socket tip will be amplified, with IC1a now operating as a non-inverting amplifier. TABLE 1: RESISTOR COLOUR CODES ❏ No. ❏   4 ❏   4 ❏ 19 ❏   4 ❏   2 ❏   4 ❏   1 ❏ 30 ❏   2 ❏   8 ❏ 12 ❏   4 ❏   8 ❏   4 ❏   6 ❏   4 ❏   5 28  Silicon Chip Value 1MΩ 100kΩ 22kΩ 16kΩ 20kΩ 13kΩ 12kΩ 10kΩ 8.2kΩ 4.7kΩ 2.2kΩ 1.2kΩ 1kΩ 680Ω 220Ω 150Ω 100Ω 4-Band Code (1%) brown black green brown brown black yellow brown red red orange brown brown blue orange brown red black orange brown brown orange orange brown brown red orange brown brown black orange brown grey red red brown yellow violet red brown red red red brown brown red red brown brown black red brown blue grey brown brown red red brown brown brown green brown brown brown black brown brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown red red black red brown brown blue black red brown red black black red brown brown orange black red brown brown red black red brown brown black black red brown grey red black brown brown yellow violet black brown brown red red black brown brown brown red black brown brown brown black black brown brown blue grey black black brown red red black black brown brown green black black brown brown black black black brown Fig.4: install the parts on the main PC board as shown here. Note particularly that IC11 is a TL071. The filtered output from IC6a is summed in IC8b with the signal from IC5a. It is also inverted by IC6b (ie, phase shifted by 180°) and summed in IC8a with the signal from IC4a. Note that, in both cases, the filtered Surround signal is attenu­ated by 3dB in the summing amplifiers due to the 14kΩ input resistances (again made up of 13kΩ and 1kΩ resistors). Following these two summing amplifiers, the signals are fed to output January 1996  29 The Surround Sound Mixer and Decoder is built into a compact console case with a sloping front panel. Note that there is a fair amount of internal wiring to be run, most of it between the main board and the front panel controls. level controls VR11a and VR11b. The encoded Left and Right signals are then coupled to their respective output sockets via 2.2µF capacitors. Panning Now let’s take a look at how the pan signals are derived. In the case of the A input, the signal at the wiper of VR5 is first buffered and amplified by IC7a. This stage functions as a non-inverting amplifier with a gain of two. The output from IC7a is then applied to pan control VR8 via a 4.7kΩ resistor and to pan control VR7 via a second 4.7kΩ resistor and two 10kΩ isolating resistors. VR7 is used to pan the “A” signals between the Left and Right channel summing amplifiers (IC4a and IC5a), while VR9 does the same for the “B” signals. Similarly, VR8 and VR10 (Pan C-S) pan the “A” and “B” signals between the Pan L-R controls and the input to IC5b. In theory, VR7 and VR9 pan between the Left and Right chan­nels, while VR8 and VR10 pan between the Centre and Surround channels. In practice, however, there is some interaction between these controls. Surround sound decoding The internal decoding circuitry is 30  Silicon Chip based on IC9a, IC9b, IC10a & IC10b and is normally only used on playback – see Fig.1(b). IC9a and IC10a are used to derive the Centre channel. This is achieved by first adding the Left and Right channel outputs together in summing amplifier IC9a. The output of IC9a is then buffered by unity gain inverter IC10a and coupled to the Centre output socket. A different technique is used to derive the Surround out­put. In this case, the encoded Left and Right channel outputs are fed to IC9b which is configured as a difference amplifier. This configuration is arrived at by feeding the Left channel to the inverting (pin 6) input and the Right channel to the non-inverting (pin 5) input. The output from IC9b is simply the difference between the two input TABLE 2: CAPACITOR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ Value IEC Code EIA Code 0.1µF   100n   104 .0027µF   2n7   272 680pF   680p   681 220pF   220p   221 180pF   180p   181 100pF   100p   101 signals. This signal is filtered and invert­ed by low-pass filter stage IC10b and fed to the Surround output sock­et. Signal meters As mentioned previously, the circuit contains four signal level meters which monitor the Left, Right, Centre and Surround outputs. These four meters are all identical, so we’ll just look at the meter that monitors the Left output. The circuit is based on IC12 which is a 10-LED display driver wired in dot mode. In operation, the incoming signal is first buffered by emitter follower stage Q1. It is then recti­fied by D1, filtered and applied to pin 5 of IC12. The filter components on pin 5 consist of a 0.1µF capacitor and a 1MΩ resistor, connected in parallel. These give the meter a fast attack time and a slow decay response, so the meter effectively displays the peak average value. As well as acting as a buffer, Q1 also compensates for the voltage drop across D1, since its emitter is always approximately 0.6V above its base. While this compensates fairly well, the balance is not perfect since there is more current through Q1’s base-emitter junction than through D1. This slight imbalance is taken care of by using VR12 to set an offset voltage on pin 3 (RLO) of IC12. This jacks the pin 3 voltage up so that it equals the voltage at pin 5 when the input signal is tied to ground. The full scale deflection value for the meter depends on the voltage on pin 7 and is set by the 4.7kΩ and 680Ω resistors. In this case, the voltage on pin 7 is set to 1.64V, which corre­ sponds to a peak value of 3dB above 774mV RMS (ie, LEDs 1-10 lit). As a result, the meter is calibrated for 0dBm, which corre­ sponds to 1mW into 600Ω. Power supply Power for the circuit is derived from a 12VAC plugpack. This is fullwave rectified using BR1, filtered by a 2200µF ca­pacitor and applied to REG1 to derive a regulated +12V output. IC11 is used to provide the circuit ground, so that the op amps are effectively fed from split supply rails. It does this by buffering the 5.45V output from a voltage divider (12kΩ & 10kΩ) wired across the regulator output. The 100Ω resistor at IC11’s output isolates the op amp from the following 100µF capacitive load and prevents oscillation. As a result, the +12V rail is 6.55V above ground, while the 0V rail is 5.45V below ground; ie we effectively have split supply rails of +6.55V and -5.45V. Construction Despite the circuit complexity, building this unit is quite straightforward. Most of the circuitry is contained on three PC boards: (1) a main board coded 02302961 (144 x 194mm); (2) a display driver board coded 02302962 (76 x 105mm); and (3) a LED display board coded 02302963 (72 x 82mm). Begin the construction by checking the PC boards. In particular, check for any breaks in the tracks and for shorts between adjacent tracks. The board mounting holes should all be drilled to 3mm, while a 3mm hole is also required on the main board for the regulator (REG1) mounting screw. Fig.4 shows the parts layout on the main PC board. Start by installing PC stakes at all external wiring points, then install the wire links (using tinned copper wire). The next step is to install the ICs. Note that these must all be oriented in the same direction. Note too that IC11 is a TL071 while the rest are all Make sure that all polarised parts are correctly oriented when building the main PC board. The 2200µF capacitor (bottom, right) is installed on its side and is secured to the board using silicone sealant to prevent lead breakage. LM833s, so don’t get them mixed up. The bridge rectifier (BR1) can also now be installed (orient it as shown), followed by 3-terminal regulator REG1. Secure REG1’s metal tab to the PC board using a screw and nut. The resistors and capacitors can now be mounted. Table 1 lists the resistor colour codes but it is also a good idea to check them with a multimeter, as some colours can be difficult to decipher. Table 2 lists the capacitor codes. Make sure that the electrolytic capacitors are all correctly oriented and note that the 2200µF capacitor is mounted on its side. Use silicone sealant to secure the body of the 2200µF ca­pacitor to the board, to prevent its leads from flexing and eventually breaking. As shown on Fig.4, three of the 6.35mm stereo sockets are mounted directly on the main board. Install these now, along with the 2 x 2 RCA socket package. The mounting clips on the underside of RCA socket package will have to be removed using side cutters before it is installed on the board. That's all we have space for this month. Next month, we will resume with the parts layout diagrams for the display driver and display boards and give the complete wiring and testing details. We will also publish the full-size PC board patterns and the front-panel layout. Note: “Dolby”, “Pro Logic” and the Double-D symbol are trademarks of Dolby Laboratories Licensing Corporation, San Francisco, CA 94103-4813 USA. January 1996  31