Home Video Distribution Center

8/11/2019 Home Video Distribution Center

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AUDIO VIDEO SENDER CENTER

The audio-video sender center is a miniaturized broadcast TV transmitter that can beused for transmitting programs from an Audio-Video USB Interface Player at a distance up to 20meters. With this system, several TV sets can tune and monitor the same program from oneplayer at the same time. This can also be used for a computer with a TV output jack so thatseveral TV monitors can view the computer output at the same time without any physicalconnections.

PRINCIPLE OF OPERATION

Audio-Video USB Interface Player consists internally of two major functional blocks.One is the stored digital data and Universal Power DVD module which processes the signal

picked up from a flash drive into a composite video signal.

The other is the Modulator which transforms the composite video signal into an RFsignal suitable for transmission in a way thats no different from an ordina ry TV broadcast, sothat the A/V USB Interface Player output can simply be directly fed to the antenna and withoutany modification needed.

The built-in modulator is actually a complete-in-itself mini-TV transmitter. It is,therefore, theoretically possible for a VCR output to be directly hooked up to a radiatingantenna and without need for a A/V Sender Center to effect wireless transmission.

Unfortunately, the RF signal of the built-in modulator is so weak that the TV receiver must havean extremely low noise and highly sensitive front end (tuner) circuitry to be able to receive afairly good signal at a reasonable distance. Commercially, manufactured TV sets, incidentally,are not made that good. Such a TV set can be designed and assemble but it would come outvery expensive to the point that it is already commercially viable. Furthermore, the weak signalof a A/V USB Interface Player can easily be masked by external electrical noise and watching theTV go haywire every time your sister uses the hair dryer or someone turns on the light in theadjacent room is definitely very disgusting.

Thus this Audio-Video Sender Center is the media for transmitting the programs from asingle Audio-Video USB Interface Player to several TV sets. It is not only very convenient butalso relatively an economical and practical means for the home, office, or schools.

The Audio Video Sender Center is illustrated in figure 1. It is actually a low power TV transmitteror in other words, a high power modulator, that radiates sufficiently strong RF signal for good


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reception within the 20-meter radius range (indoor) for multiple or remote A/V USB InterfacePlayer program viewing. This range would sufficiently be enough to reach every TV set in mostFilipino homes without necessarily disturbing the peaceful coexistence of the people in theneighborhood who may want to watch their own choice programs.

The A/V Sender Center modulator circuit mixes the composite video and audio signalsfrom the VCR with an internally generates RF carrier to form a correspondingly stronger RFsignal. This is then fed to a two-stage RF amplifier (Q1 and Q2) for amplification before finallybeing fed into a radiating antenna for multidirectional wireless transmission.

CIRCUIT DESCRIPTION

The A/V Sender Center is very similar to that of the RF modulator circuit featured in

EE24. In fact, both use essentially the same modulator circuit based on Motorola MC1374 IC.

The main critical component of special concern here are coils L1 and L2. Coil L1determines the RF frequency of the modulator, while coil L2 sets the sound subcarrierfrequency of the modulator. Video and Audio signals from player are fed separately to pins 11and 14 of the MC1374. The Video signal modulates directly the RF generator, while the audiosignal frequency modulates the sound subcarrier, which is subsequently mixed to the videosignal prior to the RF modulation. The modulated RF signal appearing at pin 9 of MC1374 IC isthen further amplified by a couple of RF amplifiers formed by transistors Q1 and Q2. Signal takeoff points for the antenna is through the antenna matching coil L5.

Briefly, the functions of each component may be described as follow:

Resistors R1, R2, and R3 set the DC bias of the sound subcarrier input (pin 1).

Coil L2 with capacitor C2 are resonant elements of the sound subcarrier oscillator while both C1and C3 are just coupling capacitors.


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AUDIO VIDEO SENDER CENTER

Parts List

Semiconductors:

IC1 -MC1374, TV Modulator Chip

Q1 -2SC1923, NPN RF Transistor

Q2 -MPS6507, NPN RF Transistor

Q3 -PN100 NPN Transistor

D1 -1N4148 Silicon Diode

D2 -1N5240/(7910), Zener Diode 10V W

D3-D6 - 1N4001, Rectifier Diode 1 Amp.

LED -RED Jumbo, Light Emitting Diode

Resistors: (All are - Watt, 5% tolerancefixed carbon composition typeunless otherwise specified.)

R1 -6.8 Kilohms

R2 -3.3 Kilohms

R3, R7, R21, R23 -2.2 Kilohms

R4-R6, R22 -470 Ohms

R8 -30 Kilohms

R9 -180 Kilohms

R10 -56 Kilohms

R11, R17 -220 Ohms

R12 -560 Ohms

R13, R20 -75 Ohms

Electrolytic Capacitors:

C8 -100 F/16V

C9 -1 F/16V

C10 -10F/16V

C17 -2200 F/25V

Special Type Capacitors: (All are rated at50V.)

C1 -47 pF or 50 pF Ceramic, NP0type

C2 -150 pF Polystyrene

C6 -70 pF Ceramic, NP0 type



CV1, CV2 -5-25 pF Trimmer cap.

Coils: (All are no.22 AWG magnet wireunless otherwise specified. See text fordetails.)

L1 -3 T x 3.5 mm diameter

L2 -Sound Subcarrier Coil, 21806/01-8

L3 -3 T x 4 mm Diameter

L4 -3 T x 4 mm Diameter primary,

-3 T x 4 mm Diameter secondary

L5 - Antenna Matching Coil


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CONSTRUCTION

The Home Video Distribution Center, being a very high frequency device, is very

sensitive to pc board component layout. Hence, proper care must be observed in constructingthis project to achieve the desired results. The PCB pattern shown in figure 3 has been carefullylaid out to minimize undesirable components interaction. Make sure that the PCB being used isa truly faithful reproduction.

Here are the important points to consider when constructing . Dont ever miss any step.

1. Capacitors C2, C6, C13, and C16 are special type temperature compensated capacitors.Do not substitute other types. These capacitors cost by as much as three times theirordinary type counterparts but they can save you later on from troubles arising from

frequent realignment due to the influence of changing environment conditions on thecomponents.

2. Do not use a socket for MC1374. It is not advisable here because interactive capacitanceof the socket leads could influence the device frequency. Be very extra careful insoldering the leads of the IC.

3. Coils L1, L2, L3, L4, and L5 should be wound in a manner shown in figure 4. These coilsshould be positioned in a manner that they can be automatically aligned in the positionshown in the photo once soldered into the PCB. (Observe that adjacent coils areoriented 90 degrees or at right angles with each other.) Note that coil L4 consists of two

coils mounted very close together such that the pair appears as if they are a singlecontinuous coil.

4. The antenna matching coils is soldered directly to the matching base connector (solderlug) of the antenna, and is oriented in standing position.

5. Keep all the components lead lengths to this barest minimum.Needless to say, it is always worth observing the photos shown before starting

with constructing the project to avoid getting any discouraging results. The project maybe housed in an HC851 all-purpose housing. Thus, component mounting has beentailored to suit this space.

The antenna is securely mounted to the enclosure with a screw in its base. Theantenna connection is made through a short bare wire soldered to one end of thematching coil and the other end to the PCB point.

The audio input jacks should be mounted close as possible to their connectionpoints in the PCB.


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A hole is bored through the battery compartment to fit the transformer in itsplace. The power supply filter capacitor, owing to its size, is soldered onto the foil sideof the PCB and lying on its side to fit the available enclosure space.

ALIGNMENT PROCEDURE

After completing the project, thoroughly recheck all the soldered componentsand other connections. All components must be in their proper locations and correctlyoriented according to their polarities or lead designations, as the case maybe.

Countercheck using the schematic diagram and parts list as guide. If everything iscorrect, proceed with the alignment. You need a TV set to this; but not the so calledrechanneled types, a video tape recorder/player, a video tape, a plastic alignmentscrew driver, and a small flat screwdriver.

Follow this Procedure:

1. Tune the TV sets to either channel 3 or 4 (whichever is free in your locality).Turn RV1 fully clockwise, i,e., at its maximum setting.

2. Connect the Home Video Distribution Center to the VCR as shown in the illustration.Both the VCR and the HVDC must be located at least 10 feet from the TV set duringthe alignment.

3. Pull the TELESCOPIC antennas to their entire lengths of both the HVDC and the TVset. Disconnect the lead wires of the TV aerial antenna outside the house.

4. Turn on the TV set, the HVDC, and the VCR, simultaneously. Feed the video tape andhave it played.

5. Slowly expand the coil L1 along its axis using the plastic alignment driver until theVCR program appears on TV. Expand the coil until the best possible picture quality isobtained. Adjust the TV fine tuning also for best results.

6. If the picture appears distorted, e.g., the picture is bending or silvery, slowly rotateRV1 counterclockwise until the picture defect disappears.

7. Slowly adjust coil L2 for the best possible sound reception on TV.8. If the picture and sound seems not tuning together, slightly expand or compress L1 a

little bit more and/or adjust the TV fine tuning control. Repeat step 6 until both thesound and picture tune in.


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9. Move the TV set antenna to any position until the TV pictures becomes snowy.You could retract or make the antenna shorter if necessary or move the HVDC alittle farther from the TV.

10. Adjust CV1 for best TV picture quality.

11. Repeat step 8 if the picture is still unsatisfactory.12. Adjust CV2 for best TV picture quality.

APPLICATION HINTS

The A/V Sender Center can be used with any type of video tape recorder equippedwith standard RCA jack for video and sound output. The video signal level output ofthe player is factory adjusted at 1 volt peak-to-peak, which is the optimum signal theHome Video Distribution Center can handle. Too much or too little would result to apoor picture quality. Consequently, players that have been repaired must bechecked if the standard signal output level is still at the correct standard level.

The A/V Sender Center trims down excessive video signal through RV1 but wontfunction if the output signal is already very weak.

The best location is the center most spot of the house. This would allow an effectivecoverage span of 40 meters. Outside this range, reception is still possible but pictureand/or sound quality would deteriorate progressive with the distance.

One important thing to bear in mind: The A/V Sender Center WILL WORK ONLYWITH TVs EQUIPPED WITH BUILT-IN RABBIT EAR ANTENNA. Disconnect theexternal or the aerial antenna whenever would be used. Position as far as possiblefrom the TV set. Two meters would already be enough as the minimum distance toavoid jamming other TV station reception.


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Home Video Distribution Center

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