Adding NICAM Stereo Sound

-l Adding NICAM StereoSound Keith Cummins lnspired by Eugene Trundle's series of articles on Nicam green, the others being red. Apart from labelling there's stereo sound, and tempted by Maplin's advertisement, I nothing elsc - there are no controls. The rear of the unit decided to take the plunge and add Nicam to my own TV has a mains input lead, a five-pin DIN interfacing socket receiver. My current set is a Sony KVX2521U, which has for the audio and control functions. a BNC connector for dual audio channelsand spatial/pseudo-stereofacilities but the Nicam carrier input and two phono plugs that provide no Nicam. Before the addition of Nicam I had already left and right outputs for a separate hi-fi system. The connected to the set a pair of separate forward-facing Maplin decoder - I chose to buy mine assembled and speakerswhich sound much better than its internal ones. tested- is insidc the unit along with a small power supply to provide the l2Vl200mA required by the decoder. A control line from the TV set switchesthe power supply on Planning and off. The lot is assemblcd in an instrument case A considerableamount of thought and roughing out of measuring2fi)mm wide, 75mm high and 150mm deep. It ideas was undertaken before I even ordered the parts can sit inconspicuouslvnext to the TV set. required. Since I've been out of the TV trade for many Now for a bit about the decoder. Maplin provide years my TV activitiesare now restrictedto hobby interests comprehcnsive information with it - you'll get a good and fixing the odd set for friends and relations.As a result photocopy when you buy the decoder. When presented I was not in the position to have the set out of servicetor a with a Nicam sigrral the decoder produces left and right prolonged period and decided to install the Nicam decoder audio outputs. With no Nicam signal present the unit as a separate add-on unit which would be interfaced with revcrts to the normal f.m. sound condition: the audio the receiver during a "quick-hit" operation - it actually signal from the TV set's f.m. discriminator is routed took one hour from taking the back off to having the through the decoder, emerging as a dual mono signal.The pleasure of hearing Nicam sound. But I stress again that LEDs are provided with drive signals so that the unit's this was made possibleonly as a result of the lengthy initial statuscan be readily observed. deliberationsin my workshop. Once the decisionto have the decoder as a separateunit is made several potentially difficult problems go away. Power Arrangements First you don't have to find a location for the decoder You might wonder why I bothered to build a separate within the set, where it might cause or be subject to power supply when it might by possible to obtain the interferencc from other circuits such as the timebases. decoder's power from within the TV set. My reason for Secondly there's no need to drill holes in the front of the doing so was to ensure that the extra 200mA required by set to accommodate LEDs. Thirdly if you disposeof the the dccoder didn't overload a l2V supply within the set you can keep the deccldcr. Fourthly the decoder can receiver or perhaps damage a switch-mode power supply have an independent power supply (more on this later). within the set by imposing an cxtra load that it wasn't And fifthly the modifications to the receivercan bc kept to designedto accept. I did however feel that the risk when an absoluteminimum and are quick and easv. draining an extra 50 to 60mA to opcrate a control relay was minimal. So a l2V feed within the TV set energisesa relay which then turns on the separatepower supply. Warnings A warning is necessary at the start. The typc of modificationoutlined in this articlc is suitableonly when Interconnections the TV set involved has an isolated and earthcd chassis.ln Fig.l showsthe interconnectingcircuitry within the add- its literature on the decoder Maplin expressesparticular on unit. There arc two connectorsfor coupling to the TV concern about the possibility of people making direct set. Thesc are SKl, the Nicam carrier input, and SK2 conncctions, and recommendsthat qualified TV personnel which provides thc control and audio interface. We'll are consulted before doing scl.Maplin also makcs the point consider SK2 first. Pin 2 is the system earth (chassis that certain SAW filters may remove the Nicam carrier. I connection). When the TV set is switched on l2V is think one would havc to be particularly unlucky for this to happen however, especially as the decclder has an input sensitivityof lfi)pV r.m.s. At the output of a typical video dcmodulator chip thc signal amplitude can be 2V peak-to- peak, so the -2()dB Nicanr signal can be a further -56dB down and still be sufficient to drive the decoder. If this problem does arise however a separatedecodcr can still be used with another set, so you won't have wasted hours building one into an unsuitable recciver. This is another good reasonfor adopting the "add-cln" approach. Description The decoder unit is shown in the accompanying photographs. There are threc LEDs on its front face: f.m. The Nicam stereo decoder unit sits inconspicuously beside (i.e. non-Nicam), Nicanr and stereo. The stereo one is the main receiver. 412 TELEVISIONAPRIL 1991 equipment loading the decoder's outputs. Note that the decoder's outputs are at a low impedance, terminated by R3 and R4 - this, alongwith the 1V r.m.s. output, helpsto keep the level of hum and other extraneousnoise low. The 210V decoder's output operational amplifiers are used whether a,c, the unit is decoding Nicam or has reverlcd to the f.m. 1".'mono mode, so the unit must be powered whether or not Nrcam{1 Nicam is beingreccived. ca(fier(r 3l input ))t letz The indicators operate on the basisof a simple logic. If 7 it's not Nicam, it must be f.m., i.c.Allicam : f.m. Thus with the power relay energisedbut no other input the f.m. LED lights. The Nicam carrier is fed directly to the decoder PCB via the BNC connectorSK1. Power Supply Circuit The 24OV a.c. mains input is connected via a conventional mains lead. The live side of the supply is taken to fuse Fl which is live whether or not the unit is switchedon. When relay RLA is energisedthe. mains input is connectedto the primary winding of transformer T1. Its secondarywinding supplies l5-0-l5V at 250mA to the full- wave rectitier diodes Dl/2 which charge the reservoir capacitorC2. The output from this simple rectifier circuit is fed to pin 1 of the 7812 l2V regulator chip. Cl is included Fig. 1: Nicam decoder unit, wiring of instrument case to provide protection againsttransients and r.f. noise. peripherals. Maplin decoder connections on the right: pins not shown aren't connected. Construction Construction of the unit is straightforward. Fig. 2 shows the layout adopted. All I did was to position thc major componentson thc chassisof the instrument caseand mark clut thc hole positions using the transformer, decoder PCB etc. as their own templatcs. The relay is mounted on its head, securedto the chassisby double-sidedtape. A hole is drilled at either sidc to enable a cable tie tcl be passed thrclugh the chassisand around the rclay. A right-angled screen is mounted bctween the power supply and the decoder PCB, serving also as a heatsink fbr ICl. The power supply componentson the secondaryside of Tl are rnounted on a piece of Veroboard which is bolted to, but PL'I PL5 PL6 PL3 spacedaway from, thc chirssisplirte. The layout shown gives, I believe, the best arrangcment of the connectors on the decoder PCB relative to the peripheral connections, LEDs etc. so that the wiring is Fig. 2: Basic layout in the instrument case, wiring detail not neat and straightforward- follow Fig.l. As I consider that shown. LEDsare fitted to the front where appropriate. this project is one for experienced people only it's not Chassis connection necessaryto provide details of cvery last nut and bolt. It pointed comply IEC Mono audio trom f.m. demod should however be out that to with regulationsa clearanceof 3mm must be provided between 12Vrelay supply mains connectionsand other metal parts. R channet Beforc starting work on the TV set it's a good idea to To aldio amps L channet make up the leads that connect to SKI and SI(2. Coaxial cable is used for the former and, as shown in Fig.3, OIN p(ug screenedfour-core lead for the latter. Colour coding is a (ro(to sK2jSK2) 1667Zl matter of choice - that shown is in accordance with the Fig. 3: Details of the S-pin inrcrtacing DINplug and lead. cable I happenedto have. present at pin -5. Thus currcnt flows through the relay's winding and its contacts close. switching on the separate ReceiverModifications power supply. Mono audio from the TV set's f.m. The following points should be borne in mind when discriminator is fed tcl the decoder via pin 3. Outputs from considering the modifications to be made to the TV the decoder to drive left and right audio channelswithin recelver. the TV set are fed via pins 1 and 4. These outputs are also The original mono feed to the left and right channels fed to sockets SK3 and SK4 which provide left and right (assumingthat the set has left and right channels with a signalsfor a hi-fi system,tape recorder etc.

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