Video amplifier gain...

YILMAZ KIRÇİÇEK

Hi All,

I will design my first video circuitry and need some information about video systems.

Main issue is video signal peak to peak amplitude. I have checked some of the reference design and see that composite video output varies between 0.7V to 1.3V after on board 75 Ohm termination.

It is confused me, because all documents states that the peak amplitude as is 1Vpp for 140 IRE.

Is there an Automatic Gain Control circuitry on receivers like TV sets? How the receivers restores the DC levels?

I will design a video amplifier but not sure about gain requirements. Could you please share your experience or reference a document or an application note ?

Best Regards.

over 13 years ago

0 Hooman Hashemi over 13 years ago

TI__Mastermind 22825 points

Hello,

I believe the 0.7V is the black level to white level difference after termination in 75ohm (from a 75ohm source). 1.4V is the unterminated voltage.

If you include the sync tip (~ -0.3V), then the terminated voltage would be around 1V, which is what your diagram is showing. Without sync, you are talking 0.7Vpp, terminated.

DC restoration works on the basis that the back-porch (shown in your diagram) is considered black. Thus, the DC restoration circuitry "restores" that level to 0V or whatever the rest of the circuitry interprets as black level. There may be AGC circuits in receivers which are either use custom voltage levels or where the attenuation from the video source to the receiver is variable or unknown. If the levels are known and will not vary, AGC is not necessarily required but can be present.

Regards,

Hooman

0 Hooman Hashemi over 12 years ago in reply to Hooman Hashemi

TI__Mastermind 22825 points

Hi,

It just occurred to me that the 1.3V level might be the voltage level quoted with the luma and chroma signals combined. The 1Vpp (including sync) is with luma only and not including chroma on top of it.

This is the E2E post that got me thinking that might be the case:

http://e2e.ti.com/support/amplifiers/high_speed_amplifiers/f/10/p/225901/807345.aspx#807345 (See Randy S.'s reply on Nov. 28, 9:40AM)

Regards,

Hooman

0 YILMAZ KIRÇİÇEK over 12 years ago in reply to Hooman Hashemi

Intellectual 905 points

Hi Hooman,

Thank you for your time and answers.

Best Regards.

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Video amplifier gain...