0699f(2).pdf

(328 KB) Pobierz

&/,33,1* 9,'(2 )$'(5

A new series of periodic articles in which we present

a variety of simple-to-build and use projects

especially suited for construction by those who are

comparatively new to electronics and wish to build

interesting, inexpensive, and practical circuits that

do not require a great deal of expertise or test

equipment, and which are likely to work first time.

Experienced constructors will find these designs

useful as well

so it’s something for everyone, we

believe!

Many camcorders include a

video fader facility, but this is of-

ten in the form of a pushbutton

control with a fixed rate of fade. A

fader of this type is certainly us-

able, but offers no creative con-

trol to the user. The extremely

simple video fader describe here

has a conventional control knob

that offers precise control over

the fade characteristic.

The circuit provides what is

virtually a conventional fade to

black action, but the fading is pro-

vided by asymmetric clipping of

the picture modulation rather than

by attenuating it. This method of

fading operates in dif-

ferent manner to a

conventional fader,

and it produces a dif-

ferent effect on-

screen when the pic-

ture is faded.

E\ 52%(57 3(1)2/'

both types of fade are perfectly

usable. This project offers an

interesting alternative to a con-

ventional video fade effect, and

it costs very little to build.

The fader circuit is designed

for use with a standard UK PAL

composite video signal. It is

normally connected between

the video output of the cam-

corder and the video input of a

VCR, and is used when produc-

ing a tape of the best bits from

the raw footage in the cam-

corder. However, it can also be

connected between two VCRs

or any items of video equipment

that use the right form of video

signal. It is not suitable for use

with equipment that uses any

form of RGB signal, digital

video signal, or separate syn-

chronization signals.

tally faded out, albeit in a very

weak form. As the picture is

faded you see a weaker and

weaker version of it, but the pic-

ture is otherwise undistorted.

With the clipping method,

the brightest parts of the picture

are faded first, then the mid-

tones, and finally the darker ar-

eas. The picture collapses

rather than fades from the

screen.

The fact that this is an un-

conventional form of fading

does not mean that it is an infe-

rior effect. This really boils

down to a matter of taste, and

SYSTEM OPERATION

The waveforms of Fig.1

show how the clipping process

provides a fading action. In

somewhat simplified form,

these waveforms each repre-

sent two line scans. Each scan

starts with a negative synchro-

nization pulse followed by a

high frequency color burst sig-

nal. The rest of each line scan

is the color modulation, which is

of the positive variety for the

UK PAL system. In other words,

the stronger the signal voltage,

the brighter the screen at that

point in the line.

Waveform (a) shows the un-

processed composite video input

signal. In waveform (b) the clip-

ping process has just begun, and

the asymmetric clipping process

FADER TYPES

With a conven-

tional fader, the com-

plete modulation sig-

nal is still present

even when the picture

has been almost to-

Fig.1. Example waveforms as the signal

is progressively clipped.

Maxfield & Montrose Interactive Inc

EPE Online, June 1999 - www.epemag.com - 574

&RQVWUXFWLRQDO 3URMHFW

has flattened the positive peaks in

the picture modulation. In the

waveform (c) the picture has

been almost completely faded to

black by the heavy positive

clipping, and in (d) the picture

modulation has been removed

completely.

! [

?>?66

6145B

23%%)

!"F

(h11

FB!

9>@ED

?ED@ED

C;"

"["

C;!

BE POSITIVE

Note that symmetrical clip-

ping would not give the desired

result because it would commit

both the picture modulation and

the synchronization pulses. This

would produce a grossly unsta-

ble picture well before the pic-

ture had been fully faded.

A fader of this type usually

brings in the odd reader’s letter

pointing out that it cannot work.

The point of contention is that

the color burst signal will start to

become clipped before the pic-

ture is fully faded to black. This

does indeed occur, because, as

will be apparent from the wave-

forms of Fig.1, the color burst

signal is positively building for

50 per cent of the time.

There is no danger of af-

fecting the synchronization

pulses as these are always neg-

ative and have no positive con-

tent. The distortion of the color

burst signal is unfortunate, but it

does not actually stop the sys-

tem from working properly.

Some color distortion will

occur, but only when the picture

has been virtually faded to

black. Some over-saturated col-

ors are likely to be produced,

but the picture will be so dark

that you will have to look very

close indeed to see them.

"['

Fig.2. Complete circuit diagram for the Clipping Video Fader.

one active component, and this

is transistor TR1. The circuit is

basically just a buffer amplifier,

which uses TR1 in the emitter

follower mode.

;

With the wiper (w) of poten-

tiometer VR1 at the bottom end

of its track, the circuit functions

as a straightforward emitter fol-

lower stage and no fading is pro-

?>?66

DB!

D? 21DD5BI 2!

C;!

?ED@ED

C;"

6145B

FB!

?ED@E D

;

- 2B51; 9>

3?@@5B DB13;

CIRCUIT OPERATION

As will be apparent from the

circuit diagram of Fig.2, the

Clipping Video Fader is very

simple indeed. There is only

Maxfield & Montrose Interactive Inc

Fig.3. Stripboard topside component layout, underside copper

track details and interwiring to off-board components. Note the

single break in the copper strips.

EPE Online, June 1999 - www.epemag.com - 575

&RQVWUXFWLRQDO 3URMHFW

COMPONENTS

Resistors

R1 100 ohms

R2 120 ohms

R3 2k7

R4 10k

R5 560 ohms

R6 2k2

All 0.25W 5% carbon film or better

Potentiometer

VR1 1k rotary carbon, linear

Capacitors

C1 47u axial electrolytic, 16V

C2 10u radial electrolytic, 25V

C3 100u radial electrolytic, 16V

Only two components are mounted on the front panel of the

finished Video Fader.

towards the top of the VR1’s

track, and the positive part of the

output waveform is totally re-

moved when the wiper is at the

top of the track.

The positive picture modula-

tion is removed by this clipping,

but the negative synchronization

pulses are unaffected. In fact, the

off-center biasing provides a

massive amount of “headroom”

for negative-going signals, and

ensures that no distortion of the

synchronization pulses occurs.

The current consumption of

the circuit is largely dependent on

the setting of fader control VR1. It

is only about 3mA or so with the

picture fully faded, but rises to

about 9mA or 10mA when no fad-

ing is used.

devices, is more vulnerable to

heat damage than most other

types of component. Conse-

quently, this should be left until all

of the other components and the

solder pins have been fitted.

The solder pins are used at

the points where connections

will be made to the controls,

sockets, and battery. Single-

sided pins 1mm in diameter are

required. ”Tin” the tops of the

pins with solder and it should

then be easy to make reliable

connections to them.

The unit should fit into virtu-

ally any small to medium size

case, but make sure that you

select a case that can accom-

modate the rather bulky battery

pack. In theory there is some

C;#

C;$

16 ?ED

16 9>

Semiconductors

TR1 BC559

pnp

transistor

Miscellaneous

S1 s.p.s.t. miniature toggle switch

B1 12V (8xAA cells in plastic holder)

SK1, SK2 phono socket (2 off)

Metal or plastic case to suit (see

text); 0.1-inch pitch stripboard,

22 holes x 18 copper strips; control

knob; battery connector (PP3 type);

connecting wire, solder, etc.

See also the

SHOP TALK Page!

Approx. Cost

Guidance Only

(Excluding Batts & Case)

$10

vided. As the wiper is moved

further up VR1’s track, the bias

voltage fed to TR1 becomes

higher, and it gets ever closer to

the positive supply potential.

The output voltage at the emit-

ter (e) terminal of TR1 is about

0 65V higher than the input volt-

age at its base (b). This factor,

together with the high input bias

level, makes it impossible for

the output voltage from TR1

to go more than about one

volt positive.

CONSTRUCTION

The component layout and

wiring for the Clipping Video

Fader project are shown in Fig.4,

along with the trackside view of

the stripboard. The board mea-

sures 22 holes by 18 copper

strips and there is just a single

break in the copper strips.

Construction of this board is

extremely simple, and complete

beginners should have no problems

in building it. Transistor TR1, in

common with other semiconductor

FB!

! [

<?7

Fig.4. This simple circuit pro-

vides audio fading, but it is

better to use an audio mixer

to handle the audio side of

things.

Therefore, severe clipping

of the picture modulation oc-

curs. The clipping becomes

heavier as the wiper is moved

Maxfield & Montrose Interactive Inc

EPE Online, June 1999 - www.epemag.com - 576

&RQVWUXFWLRQDO 3URMHFW

advantage in using a metal

case that will screen the circuit

from any sources of electrical

interference, but in practice an

inexpensive plastic box is per-

fectly satisfactory for this type

of project.

Potentiometer VR1 and

switch S1 are mounted on the

front panel, but it is probably

best to fit the two phono sockets

(SK1 and SK2) on the rear

panel, otherwise the connecting

cables tend to get in the way.

Bolt the stripboard panel on the

base panel of the case using

metric M2 5 or 6BA screws, and

include spacers about 6mm

long to hold the board clear of

the case.

Ordinary phono leads are

used to provide the interconnec-

tions, although a phono-to-

SCART lead may be needed to

make the connections to the

television set or monitor. These

days most camcorders are sup-

plied with a selection of leads

and adapters, and the leads

supplied with your camcorder

will probably be sufficient to

wire everything together.

With the output of the fader

connected to a television or

monitor the effect provided by

adjusting VR1 can be seen im-

mediately, and it will be obvious

if the unit is not functioning

properly. As with any project, if

there is any sign of a malfunc-

tion switch off at once and re-

check all the wiring.

It is normal practice for the

audio side of things to be han-

dled by a separate audio mixer.

If you do not wish to do things

this way and would prefer to

TESTING, TESTING

Once completed, this unit

requires no setting up and it is

ready for immediate use. For

initial testing it is advisable to

This is comprised of just a

logarithmic potentiometer con-

nected in standard volume con-

trol fashion. For stereo opera-

tion two of these circuits are re-

quired, one for each stereo

channel. A dual-gang poten-

tiometer should then be used so

that the two channels can be

adjusted in unison.

Final positioning and wiring of components inside the com-

pleted model. Note how the phono sockets (mounted on the

rear panel) and circuit board are located towards one side of

the metal case. This is to allow sufficient space for the 8-cell

battery pack.

use a VCR or camcorder as the

signal source, and to monitor

the output signal using a televi-

sion or monitor.

Maxfield & Montrose Interactive Inc

have an audio fader incorpo-

rated in this unit it is merely

necessary to add the ultra-

simple circuit of Fig.4.

EPE Online, June 1999 - www.epemag.com - 577

Plik z chomika:

fred1144

Inne pliki z tego folderu:

0699i(2).pdf (461 KB)
0699h(2).pdf (269 KB)
0699l(2).pdf (482 KB)
0699o(2).pdf (220 KB)
0699f(2).pdf (328 KB)

0699f(2).pdf

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: