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1 Vp-p constant video amplifier???

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Full Member level 2
Nov 24, 2001
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video source can be various value
how can i realize constant 1Vp-p video out whatever input value.
Best regards.


One way is to build a peak(level) detector that can give you the signal
type level and sw the signal to the wanted amplification to 1vp-p.
Best regards. 8)

solution: AGC

Hi H_u_n_t_e_r,

What you are looking for is an AGC (automatic gain control). Since you are talking about an analog signal, there is a very easy way to do automatic gain control: look at the sync tip.

In NTSC, the sync tip should 'always' be -286 mV while the active signal on the line is 714mV (for the 100% white level). But when a video signal is attenuated, the sync tip is generally attenuated as well. Hence, if the sync tip is 10% higher than normal, there is good chances that the active portion and the burst are 10% higher as well. So don't worry about the signal amplitude, just look at the sync.

How to find the sync tip? Use a regular sync extractor like the inexpensive GS1881 family (SOIC-8). It will give you the sync position and the back porch position (0V level). If you do some 'sample and hold' at both positions and substract the 2 levels, you have the sync amplitude.

Then, using an amplifier which gain is controlled by a voltage, you can increase or decrease the gain of the overall signal when the sync amplitude is lower or greater than 286mV. Just remember this: you don't always want you AGC to change gain every lines. An AGC often have a specs for slow or fast response time.

Many decoders (analog to digital) like the Phillips SAA7114 have an automatic AGC that will do the job. Look closely on the internet and you may find something that will do the job for you.

Hope this helps. TurboPC

Thanks your answers. Also There is a important thing whish its board area saving. So i need monolitic IC for this task.
I ve a video FM transmitter and it accepts 1Vp-p input level otherwise modulation problem occuring .
I ve different video source which each source has different video level.
Please your ic suggestions.

what is the your comments about
GB4550 it may be suite?
best regards.

Dear H_u_n_t_e_r,

The GB4550 is a DC restorer used to bring back the black level to 0 volt. I usually use the the GD457x for DC restorer in conjunction with the GS1881 (or GS188x and GS498x) which have a nicer SO8 package.

The EL4452 from Elantec can be used to control the gain of the amplifier with a DC voltage. If you have restored the DC level with the GS4570 (using the GS1881 clamp signal), the black level will be 0V. If you can sample and hold the bottom sync level, you have the sync amplitude which you can use to control the EL4452 gain.

Sorry! I have not used any single chip solution. Look at TDA8708A from Philips. The analog part has an AGC and the restored signal goes on pin 19. At first glance, this is what you want.

The NTE1264 and BS7180AS are AGCs for VCR. I did not look much at them.

Again, search the Web. I found these chips easily. You may find other like these.

Have fun, TurboPC.

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