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demoboard audio - video AN632/0892 summary page 1 summary of features .............................................. 1 2 introduction .................................... ................... 2 3 general description ............................................... 2 4 suggested application ............................................ 5 4.1 tv connections . ................................................... 5 4.1.a av board input signals from tv if stage. . . . . . . . . . . . . . . . .................... 5 4.1.b av board output signals to video/chroma processor .......................... 5 4.1.c av board outputs to audio processor . . . . . . . . . . . . . . . . . . . ................... 5 4.1.d av board picture in picture output . . . . . . . . . . . . . . . . . ................... 5 4.2 external inputs and outputs . . . . . . . . . . . . . . . ....................... 7 4.2.a scart . . . . . . . . . . . . . . . . . . . . . . . ....................................... 7 4.2.b svhs . . . . . . . . . . . . . . . ............................. ................... 7 4.3 mcu/pc interface. . . . . . . . . . . . . . . . . . . . . . ............................. 7 5 typical electrical characteristics ............................... 8 6 designing with av switches and matrices .......................... 10 6.1 performance optimization by pcb layout . . . . . . . . . . . . . . . . . . . ....... 11 6.1.a ground layout . ................ ....................................... 11 6.1.b signal paths . . . . . . . . . . . . . . . . . . . ....................................... 11 6.1.c component type and orientation . . . . . . . . . . . . . ............................. 12 6.2 performance optimization by circuit design . . . . . . . . . . . . . . . . . . . .... 12 1 - summary of features . 2 x scart connectors scart 1 : rgb + fb inputs, cvbs input, cvbs output (tv), stereo audio inputs, stereo audio outputs scart 2 : rgb + fb inputs, cvbs input, cvbs output, stereo audio inputs, stereo audio outputs . 2 x ext cvbs (cinch) connectors ext cvbs in, cvbs out (picture in picture) . 4 x stereo audio (cinch) connectors ext audio in 1, ext audio in 2, ext audio out tape out . tv av connections (internal connections) tv tuner : cvbs input, stereo audio inputs to video processor : rgb + fb, y/cvbs/sync, c/cvbs to audio processor : stereo audio outputs . mcu/pc interface 1/12
2 - introduction in recent years, the selection of hardware available for complete tv systems has grown considerably. peripherals such as vcrs, laser disc players, cam- corders, and home computers are now common- place on the consumer market. thus, modern tv receivers are required to control and route many signals between these external peripherals and various internal stages of the set. therefore it is becoming a standard practiseto feature many input and output connections at the rear of the unit (scart, svhs, rca, etc...). high-end tv sets have extra integrated features such as internal satellite decoders, 2 tuners, picture in picture, hifi sound, multistandards reception, etc. with menu control of all functionsvia the remote control. all these extra signal sources and functions must be handled at the audio/video matrix section of the tv set. the significance and complexity of this stage has been forced to grow in pace with these modern trends, and the use of a microprocessor for control is common place. in parallel with this, the physical size of the tv receiver chassis is required to be minimized. thus, modern tv receiver designs challenge traditional methods of switching these signals in terms of both economy and technical performance (since it is obvious that a greater number of uncorrelated signals present at this stage produce greater unwanted interaction ef- fects). the technical performance requirements are fur- ther stressed by the evolution of techniques that are intended to enhance both picture and sound quality (svhs and nicam for example), which re- quire wider bandwidth and lower distortion signal handling. conventional designsbased on discrete circuitry or low cost analog cmos gates are no longer feesi- ble. the market demand is thus for dedicated ics with optimised performance for handling all the audio and video interconnections under direct mi- croprocessor control. sgs-thomson has answered the demands of the tv industry with the introduction of optimized audio and video matrix products that can be directly controlled by the i 2 c bus. the high signal density, performance and flexibility of these components allow them to satisfy a wide range of applications. 3 - general description/architecture this demonstration board attempts to show the versatility, performance and simplicity of an audio/video matrix system that uses only three sgs-thomson ics. the circuit architecture is intended to reflect a typical high-end tv set av stage with scart connections (with rgb inputs), svhs inputs and outputs, internal and external cvbs and stereo audio connections. figure 2 shows a simplified circuit schematic of the demo board. two scart sockets are incorporated which de- liver two rgb and fast blanking signals which are switched by the tea5116. all audio connections are handled by the tea6420 i 2 c bus controlled audio matrix. any of the 5 stereo audio sources can be routed to one or more of 4 stereo outputs with bus programmable gain (from 0 to 6 db). all remaining video signal switching is performed by the tea6415b i 2 c bus controlled video matrix. two svhs inputs are featured with corresponding audio inputs. the y/c components connect to 4 of the tea6415b inputs, and an svhs output socket is included which can receive one of these y/c sources. an extra ext cvbs in (bnc connection) is featured to complete the video matrix inputs. this function is useful for y + c mix requirements (see applications section figure 4). the video matrix outputs connect to the tv output (luma + chroma), scart 2 cvbs out, svhs output (luma + chroma), and an external cvbs out (bnc connection), which can be employed as the pip output. the audio matrix (tea6420) handles all the periph- eral stereo audio inputs plus the tv if sound. two pairs of outputs are connected by cinch - one audio out which is to be used in conjunction with svhs out or cvbs out ; the other audio output pair is labelled tape out for direct sound record- ing or hifi connection. audio - video demoboard 2/12
21 22 23 24 4 20 5 19 6 18 7 17 8 r1 l1 1gnd 2 3 c v s 10 9 12 11 14 13 16 15 r2 l2 r3 l3 r4 l4 r5 l5 sda scl addr rout1 lout1 rout2 lout2 rout3 lout3 rout4 lout4 u3 t e a 6 4 2 0 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 u16 scart 1 6 5 4 3 2 1 jp1 r5 470 w r6 68 w c51 47 m f r50 20k w r7 10k w q1 - bc550 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 u17 scart 1 c b v cc c11 10 m f r15 27 w 1w r14 1k w 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 r1 r2 g1 g2 b1 b2 fb1 fb2 s1 s2 c r g b fb s gnd v cc u1 t e a 5 1 1 6 c9 100nf c1 100nf r3 75 w r4 75 w r1 75 w r2 75 w c10 100nf c2 100nf c3 100nf c5 100nf c6 100nf r12 75 w r13 75 w r10 75 w r11 75 w c7 100nf v cc r27 4x10k w r25 r28 r26 c12 22 m f c15 22 m f c20 220nf c19 220nf c18 22 m f c21 22 m f c13 22 m f c14 22 m f r24 75 w r29 75 w r42 68 w c16 220nf c17 100 m f r47 10 w v cc r16 150 w r17 150 w r18 150 w r19 150 w c250 22 m f r20 150 w r21 200 w r45 150 w r46 150 w nc rout gout bout f/bl out external rgb + fb output v cc c52 47 m f bc 550 r32 470 w r33 10k w v cc c52 47 m f bc 550 r32 470 w r33 10k w v cc c52 47 m f bc 550 r32 470 w r33 10k w v cc c52 47 m f bc 550 r32 470 w r33 10k w a 0 a 1 c26 4.7 m f a1 a0 r31 10k w r30 10k w 6 5 4 3 2 1 jp2 r52 r54 r53 r51 25 13 12 24 11 23 10 22 9 21 8 20 7 19 6 18 5 4 3 2 1 17 16 15 14 jp4 sw1 a1 a0 rgb sel scl y/c status sda ground scart 2 (8) scart 1 (8) cvbs from if/tuner stage y/c status y/cvbs/sync out cout to video/chroma processor from pc or mcu 25 pin d connector 6 5 4 3 2 1 jp3 c22 4.7 m f c23 4.7 m f rin lin rout lout to/from audio processor b c c32 4.7 m f c31 4.7 m f c30 22 m f jp6 jumper c40 4.7 m f c41 4.7 m f c27 4.7 m f c28 4.7 m f c29 4.7 m f 3 4 1 2 r9 75 w r8 75 w u20 svhs in1 3 4 1 2 u18 r41 68 w r44 68 w r40 68 w r43 75 w 3 4 1 2 r23 75 w r22 75 w u19 svhs in2 2 1 c39 4.7 m f u11 2 1 c38 4.7 m f u10 2 1 c25 4.7 m f u14 2 1 c24 4.7 m f u15 u13 bnc u12 bnc svhs out audio/video matrix demoboard cvbs out ext cvbs in l r audio in2 l r audio in1 c37 100nf 2 1 c34 4.7 m f u5 2 1 c33 4.7 m f u4 l r audio out 2 1 c36 4.7 m f u7 2 1 c35 4.7 m f u6 lr tape out u8 1 u9 1 v cc v cc + 10v v cc 19 20 1 3 5 6 8 10 11 18 17 16 15 14 13 12 9 742 inp inp inp inp inp inp inp inp out out out out out out out gn d v cc s d a s c l p r o g u2 t e a 6 4 1 5 b r53 +r51 = 5.6k w r54 + r52 = 1.8k w AN632-01.eps figure 1 : av demoboard detailed electric schematic audio - video demoboard 3/12
6 5 4 3 2 1 jp1 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 scart 1 scart 2 tv/au dio in sc1/au dio in sc2/au dio in audio in1 audio in2 tea6420 tea5116 r1 g1 b1 fb1 r2 g2 b2 fb2 tv/cv bs sc1/c vbs sc2/c vbs y1 c1 y2 c2 ext/cv bs in tea6415b 1 2 3 4 1 2 1 2 1 2 1 2 1 2 1 2 1 2 3 4 1 2 svhs in1 audio in1 svhs in2 audio in2 ext cvbs in tape out 1 2 1 2 scart 1 rgb +fb scart 2 rgb +fb ext audio out 3 4 1 2 svhs out 1 2 cvbs out 6 5 4 3 2 1 jp2 6 5 4 3 2 1 jp3 audio signal from tv to audio processor cvbs signal from tv to luma input of video processor to chroma input of video processor to rgb section of video processor AN632-02.eps figure 2 : simplified av board schematic audio - video demoboard 4/12
4 - application 4.1 - tv connections figure 3 shows the suggested method for connec- tion of the av demo board to the appropriate stages of a tv receiver. 4.1.a - av board input signals from tv if stage the cvbs signal from the tv receiver's if stage must be connected to the av board at jp2 pin 1. the corresponding stereo audio signals are to be connected to jp3 pins 1 (stereo right) and 2 (stereo left). 4.1.b - av board output signals to video/chroma processor the av board tv video output signals are con- nected on jp1 and jp2. jp1 has the rgb+fb component outputs on pins 1,2,3, and 4 respec- tively. usually, these signals connect directly to the video processor, since it has internal rgb switch- ing to accommodate the external rgb source and text rgb. however if this switching has to be performed externally to the video processor, an rgb+fb switch ic (tea5114a or tea5115) could be used. in this case, r21 should be increased to 300 w . jp2 connects the video matrix outputs to the tv. it must be noted that these signals are amplified (x2) at this point. pin 4 must be connected to the luma signal processing circuit, and pin 6 to the chroma processing circuit. the total load impedance on the video matrix outputs must not be less than 3k, (note that characterisation was performed with the 10k loads supplied on the pcb). when the av board is used in conjunction with the supplied pc software, a y/c status flag is output on pin 3. three cases must be considered as follows : (a) when an svhs source is selected in the tv display menu, the y signal is routed to jp2 pin 4 and the c signal to pin 6. the y/c status flag on pin 3 is a logic 0 in this case. the purpose of this function is to initiate control logic which can be used to bypass chroma traps (and consequently increase the luma bandwidth) when processing svhs signals. (b) when a cvbs source is selected, it appears at both outputs (pin 4 and pin 6). the y/c status is a logic 1 (+5v) in this case. (c) when an rgb source is selected, the corresponding sync signal appears at both outputs (pin 4 and pin 6). the y/c status is a logic 1 (+5v) in this case also. 4.1.c - av board outputs to audio processor jp3 connects the stereo audio outputs to the tv audio processor. pin 5 is stereo right and pin 6 is stereo left. 4.1.d - av board picture in picture output the cvbs out (bnc) can be employed for pip applications. the demonstration software has a dedicated menu to select different cvbs sources for the pip display. two scart sockets are incorporated which de- liver two rgb and fast blanking signals which are switched by the tea5116. all audio connections are handled by the tea6420 i 2 c bus controlled audio matrix. any of the 5 stereo audio sources can be routed to one or more of 4 stereo outputs with bus programmable gain from (0 to 6db). all remaining video signal switching is performed by the tea6415b i 2 c bus controlled video matrix. two svhs inputs are featured with corresponding audio inputs. the y/c components connect to 4 of the tea6415b inputs, and an svhs output socket is included which can receive one of these y/c sources. an extra ext cvbs in (bnc connection) is featured to complete the video matrix inputs. this function is useful for y + c mix requirements (see applications section figure 4). the video matrix outputs connect to the tv output (luma + chroma), scart 2 cvbs out, svhs output (luma + chroma), and an external cvbs out (bnc connections), which can be employed as the pip output. the audio matrix (tea6420) handles all the periph- eral stereo audio inputs plus the tv if sound. two pairs of outputs are connected by cinch - one audio out which is to be used in conjunction with svhs out or cvbs out ; the other audio output pair is labelled tape out for direct sound record- ing or hifi connection. audio - video demoboard 5/12
rgb out audi o in1 cvbs out av demoboard svhs in1 1 2 1 2 2 3 4 1 1 2 6 5 4321 jp1 65432 1 jp2 6 5 4 3 2 1 jp3 luma chroma ext rgb int rgb tea5640c + tea5040 video processor trap in/out rgb out pip processor teletext processor internal satelite decoder (if fitted) video if tda4433/tda8120 + tda8124 vhf/huf tu ner tv sou nd out ext sound in tv sound in audio processor sound if + stereo decoder power tv AN632-03.eps figure 3 : av demoboard connections to tv receiver svhs + audio source 1 svhs + audio source 2 svhs in1 + audio in1 svhs in2 + audio in2 ext cvbs in y+c svhs out cy av demoboard y + c mix circuit AN632-04.eps figure 4 : suggested method for y + c mixing function audio - video demoboard 6/12
4.2 - external inputs and outputs 4.2.a - scart scart 1 obeys the norms concerning its cvbs output (pin 19) and the stereo audio outputs (pins 1 and 3), i.e. the tv tuner/if cvbs signal and corresponding audio signals are sent directly to the scart 1 outputs. this facility allows connection to a vcr or paytv decoder for example. the cvbs and audio input signals are directed to the matrix inputs. both scart connections have their pin 8 signals connected to the mcu connector (jp4) to allow the micro to initiate the automatic switching function (see below). also, the rgb switching can be initi- ated automatically from the scart 1 pin 8 signal by positioningsw1 to the junction of r51 and r52. 4.2.b - svhs two external svhs sources can be connected to the av board. alternatively, svhs in1 (and audio in1) can be employed to connect a satelite decoder if fitted. quite often there is a requirement to mix the y and c components of the connected svhs devices. this function is almost invariably required in appli- cations that feature the pip facility. figure 4 illus- trates how the av demo board can fulfill this requirement by using svhs out and ext cvbs in to interface to an external y + c mixing circuit. this is an economical way to perform this function since only one mixing circuit is required to serve two separate svhs inputs. the video matrix (tea6415b) under i2c control selects the svhs source to be mixed and at the same time routes this (y+c) signal to the required output (or outputs) i.e. scart 2 and/or cvbs out (pip). note that the demonstration software does not perform this function automatically, but can simu- late this by appropriate selection within the pip display select or scart 2 output select menu (ie. ext cvbs in selected) and then the appropiate selection of input source within the svhs out select menu. 4.3 - mcu/pc interface the av demo board has a 25 pin odo connector to provide connection to a pc or microprocessor for control. the mcu controls all switching functions by deliv- ering serial i 2 c bus signals (sda + scl), with the exception of the rgb source selection which is controlled by a single input (jp4 pin 1) or scart 1 pin 8. both scart pin 8 signals are sent to the mcu connector (jp4) for status or detection functions. when used with the demonstration software, this information is used to generate the automatic switching function. note that if both scarts are activated, scart 1 is given priority. more detailed information on the operation of the supplied con- trolling software can be seen in the av demo board software user guide. a detailed description of the demonstration soft- ware is available on request. it provides both : -useful information for software development when using a microprocessor (pc software - user guide) - essential information for modification of the exist- ing program (in c language) (pc software - analysis) after power-up, the av board configuration is inde- terminate. therefore, an initialisation routine must be performed when developing the controlling soft- ware. this requires a total of 10 16-bit words. the first byte of each word consists of the unique chip address for the tea6415bor tea6420, the second byte being the necessary data for the required connection. a suggested initialisation routine is shown in ta- ble 1. table 1 : av board initialisation routine word 1st byte (chip address) 2nd byte (data) 1 1000 0110 0000 0011 tea6415b initialisation 2 o o 0000 1011 3 o o 0001 0011 4 o o 0001 1011 5 o o 0010 1100 6 o o 0010 0001 7 1001 1000 0001 1000 tea6420 initialisation 8 o o 0011 1000 9 o o 0101 1011 10 o o 0111 1000 the initialisation procedure consists of 6 words for setting up the video matrix (address = 86 (hex)), and a further 4 words for setting up the audio matrix (address = 98 (hex)). the set up configuration programmed is as follows: (1) the tv cvbs input signal is sent to all outputs except svhs out, which receives svhs in 1 y and c connections. (2) the tv audioinput is routed to all audio outputs except ext audio out which is connected to audio in 1. a gain of 0 db is set for all audio outputs. note that this configuration is programmed by the supplied software when first installed and also when the function is selected. audio - video demoboard 7/12
all subsequent configurations are made by issuing 16 bit words for video matrix setup, and for the appropriate corresponding audio matrix setup. table 2 shows the required 2nd byte of transmission for the various av board configurations. table 2 : av board configuration table (2nd byte of transmission) output selection video matrix audio matrix 00001xxx y/cvbs/sync (out) 011g1g0xxx audio out (tv) 00010xxx c(out) (tv) 00000xxx scart2 cvbs (out) 000g1g0xxx scart2 audio out 00100xxx y svhs out 010g1g0xxx ext. audio out 00101xxx c svhs out 00011xxx cvbs (out) 001g1g0xxx tape out input selection 00xxx011 tv cvbs in 0xxg1go000 tv audio in 00xxx010 scart1 cvbs in 0xxg1go001 scart1 audio in 00xxx000 scart2 cvbs in 0xxg1go010 scart 2 audio in 00xxx001 y 1 svhs in 1 0xxg1go011 audio in 1 00xxx100 c 1 svhs in 1 00xxx101 y 2 svhs in 2 0xxg1go100 audio in 2 00xxx111 y 2 svhs in 2 00xxx110 ext cvbs in 0xxg1go101 audio mute note : g1 and go set the audio gain as shown below : g1 go gain 006db 014db 102db 110db 5 - typical electrical characteristics symbol parameter value unit supply v cc supply voltage 10 v i cc supply current (all outputs driven) 260 ma rgb section bandwidth (v rgb = 0.7v pp , see figure 5) 29 mhz crosstalk f rgb = 1mhz f rgb = 5mhz -61 -51 db db output d.c. level 0.9 v video section (cvbs) bandwidth (v in =1v pp , see figure 6) 15 mhz crosstalk (f in = 5mhz, see figure 7) -56 db output gain tv video outputs other video outputs - when terminated 6.5 0.5 db db output d.c. level tv video outputs other video outputs - when terminated 3.2 1.25 v v video section (svhs) bandwidth (v in =1v pp , see figure 6) 15 mhz crosstalk (f in = 5mhz) -52 db output gain (when terminated) +0.5 db output d.c. level (when terminated) 1.25 v audio - video demoboard 8/12
5 - typical electrical characteristics (continued) symbol parameter value unit audio section bandwidth (v in =1v rms , gain = 0db, see figure 8) 630 khz crosstalk (v in =1v rms ,f in = 1khz) -96 db supply rejection (f = 1khz, v = 0.5v pp ) -80 db signal to noise ratio 100 db thd v in =1v rms ,f in = 1khz, gain = 0db v in =1v rms ,f in = 20hz to 20khz, gain = 0db v in = 0.5v rms ,f in = 1khz, gain = 6db 0.005 0.008 0.04 % % miscellaneous crosstalk between rgb and video sections (f in = 5mhz) -65 db crosstalk between cvbs and svhs sections (f in = 5mhz) -51 db AN632-05.tif figure 5 : rgb -3db bandwidth typical characteristics AN632-06.tif figure 6 : video -3db bandwidth typical characteristics AN632-07.tif figure 7 : video crosstalk typical characteristic AN632-08.tif figure 8 : audio -3db bnadwidth typical characteristic audio - video demoboard 9/12
6 - designing with audio/video switches & matrices the true matrix nature of the tea6415b and tea6420 allow greater design flexibility. since tv receiver av stage architectures vary from design to design, a specific dedicated ic approach can often restrict the designers capacity. therefore the flexi- bility of these components allow them to be consid- ered as building blocks for av stages, whilst having optimised performance and signal processing. both devices have the option of two different i 2 c addresses which can be programmed by a voltage on the addr pin. this facilitates employment of more than one device on the same bus to provide more inputs and/or outputs if required. to further add to its versatility, the tea6415b can switch composite (cvbs), luma (y), or sync sig- nals, with aligment of the bottom of the sync pulse to a fixed clamping level, or can also be used to switch other signals such as chroma (of an svhs source) and mac signals. for the latter cases, it is necessary to remove the internal clamping ar- rangement to avoid signal clipping. this can be accomplished by biasing, the signal to a sufficient d.c. level with a simple resistor divider. for opti- mum dynamic range, it is recommended to pull-up to around v cc /2. figure 9 shows the range of unclamped signal measured on a typical device, and illustrates the optimum biasing point. note that the tea6415b adds 6.5db gain and should thus be taken into account for unclamped input signal range to avoid output signal clipping. neither device is intended to drive low impedances. therefore, in situations where 75 w ha s to be driven, for example, an external buffer is required. this need only be a simple and inexpensive tran- sistor follower. the nominal load on the video ma- trix outputs should be 10k w , and not less than around 3k w . for the audio matrix, the minimum output load resistance is 2k w . considering crosstalk performance, the imped- ance of the signal sources must be sufficiently low. therefore, when an input is not used in the appli- cation, it must be bypassed to ground with a 220nf capacitor in the case of video switches and matri- ces, and 4.7 m f in the case of the audio matrix to ensure good crosstalk rejection performance. 12 10 8 6 4 2 0 2 4 6 8 101214 dc output level dc input level v cc =8v v cc = 10v v cc = 12v AN632-09.eps figure 9 : typical unclamped signal range audio - video demoboard 10/12
ground traces tea6415b pin1 AN632-10.eps figure 10 : tea6415b ic footprint ground layout psu ground rl1 i1 s1 rl2 vs1= rgnd (rl1 + rgnd) s1 t2 = 20log vs1 s1 (db) rgnd AN632-11.eps figure 11 : ground path induced signal crosstalk psu ground rl1 s1 rl2 rgnd2 s2 rgnd1 AN632-12.eps figure 12 : starconnected ground layout method 6.1 - performance optimisation by pcb layout the application of integrated circuits designed for video signal manipulation does not necessarily en- sure adequate circuit performance. although, care is taken to optimise the performance of the ic by design, when implemented on a badly configured circuit board, the performance can be less than satisfactory. at higher frequencies, parasitic elements on the pcb cause signals to radiate to other parts of the circuits. the control of this signal crosstalk effect is of paramount importance in the design of audio/video stage circuits, since a concentration of uncorrelated signals occurs mainly at this region of the chassis. good hf layout techniques must be used when designing the pcb. some general rules for guidance are as follows : 6.1.a - ground layout extensive and appropriate grounding is of singular importance. ground layout can effictively reduce capacitive signal crosstalk by providing a preferen- tial low impedance path for radiated signals. on the other hand, the ground traces themselves can become the source of the signal crosstalk in certain situations. we can consider two cases of crosstalk by ground layout: (a) at the switch or matrix ic connection; the signal crosstalk can be considered as mainly due to capacitance, and thus extensive ground layout should be employed to shield adjacent inputs or outputs using a comb-like configuration as shown in the tea6415b ic footprint layout example of figure 10. the interpin ground traces should extend out from the ic footprint to maintain the shielding right to the signal source connection to the pcb. this method should be adopted for any regions of concentrations of signals (such as scart connectors for example). (b) at the signal (input or output) terminations, or where low impedances are driven (in the case of the rgb outputs of rgb switching ics - tea5114a, tea5115, and tea5116 for example) ; the signal crosstalk is resistive and is normally coupled via the ground traces themselves. briefly, this effect is caused by the relatively large signal currents flowing to ground at low inpedance terminations in the circuit. the small amounts of ground path resistance are sufficient to superim- pose this signal on the grounded side of another signal's termination resistor as illustrated in fig- ure 11. this type of crosstalk effect can be minimised by using a different type of ground layout at these areas of the pcb known as ostar connectiono, and is shown in figure 12. this method involves employing unique ground traces for low impedance terminations which are joined together only at the psu ground terminal. 6.1.b - signal paths signal traces should be kept as short as practically possible, ie. the switching ic should be placed as close as possible to the signal input connectors (scart, rca external inputs, svhs inputs, connec- tion of signal from the tuner/if stage, etc.). audio - video demoboard 11/12
information furnished is believed to be accurate and reliable. however, sgs-thomson microelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no licence is granted by implication or otherwise under any patent or patent rights of sgs-thomson microelectronics. specifications mentioned in this publication are subject to change without noti ce. this publication supersedes and replaces all information previously supplied. sgs-thomson microelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of sgs-thomson microelectronics. ? 1994 sgs-thomson microelectronics - all rights reserved purchase of i 2 c components of sgs-thomson microelectronics, conveys a license under the philips i 2 c patent. rights to use these components in a i 2 c system, is granted provided that the system confo rms to the i 2 c standard specifications as defined by philips. sgs-thomson microelectronics group of companies australia - brazil - china - france - germany - hong kong - italy - japan - korea - malaysia - malta - morocco the netherlands - singapore - spain - sweden - switzerland - taiwan - thailand - united kingdom - u.s.a. 6.1.c - component type and orientation for a.c. coupling video signals, it is important to select an appropriate type and value capacitor. for example, for chroma signals (that do not have lower frequency synchronisation components) a 220nf polyester or multilayer ceramic capacitor is best, whilst a cvbs (or y) signal would usually require a much higher value (10 to 22 m f) electrolitic capacitor. a low profile circuit design is better than vertical component orientation. the use of ic sockets (al- though offering easy circuit maintenance) de- grades the performance and should thus also be avoided. a surface mounted component layout gives the best performance of all. 6.2 - performance optimisation by circuit design it has already been stated that source and load impedances play a significant role in the circuit performance. the switch and matrix devices have been designed to operate sufficiently in normal circuit architures without the need to modify the defined impedances. it must be reminded that psu decoupling is essen- tial at any node where signal related supply cur- rents will flow such as v cc pins of ics, and the collectors of transistor followers used for driving loads. again the type and value of capacitor used is important. best video performance is achieved when power supplies are decoupled with a high value electrolytic or tantalum bead capacitor in the range 10 to 47 m f, connectedin parallel with a much lower value polyester or multilayer ceramic type in the range of 100 to 220nf. further psu decoupling improvment can be achieved by connecting a se- ries resistance in the psu connection just beforethe decoupling capacitors (which must be mounted as close to the devices v cc pin as possible) of a suitably low value so as not cause any significant voltage drop (in the region of 10 to 50 w for exam- ple). the implementation of a series resistor in the input signal path (after the coupling capacitor) to the tea6415b can occasionaly offer improved cros- stalk rejection. the optimum value of resistor will be largely dependant on the pcb parasitic capaci- tances. audio - video demoboard 12/12

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