Processing AV video signal - Page 1
Forum section: electronics / projects

author: syntax333
author_rank: Regular Contributor
timestamp: December 22, 2018, 12:59:01 pm
content: Hi everyone i am new in this forum so i am sorry for any terminology mistakes.I am working with AV signal which is fed to Bias tee circuit. At output of bias tee circuit I got my AV signal Voltage shifted at a certain dc level (for example. form 0-1V to 4-5V, so 4V voltage shift).Now I am trying to get my original signal without voltage level shift (0-1V).Problem is that If I use any transistor level it will output me a digital signal. If I use High-Pass filter I afraid it will also eliminate DC levels in original signal as shown in attachment. (it is original signal not the shifted version (before bias tee).)So my question is that how can we shift the signal to its original position 0-1V?

author: Benta
author_rank: Super Contributor
timestamp: December 22, 2018, 01:37:31 pm
content: The usual way is to AC couple the signal and then clamp either black level or the sync pulses. For this you need to extract the sync information first, for example with an LM1881.

author: syntax333
author_rank: Regular Contributor
timestamp: December 22, 2018, 01:54:23 pm
content: I really appreciate your answer. I will definitely try to do that.

author: syntax333
author_rank: Regular Contributor
timestamp: December 22, 2018, 02:42:58 pm
content: I didn't understand one point, in order to reconstruct the original signal from shifted version I have to add the sync pulses from lm1881 and ac coupled signal right?

author: soldar
author_rank: Super Contributor
timestamp: December 22, 2018, 04:29:03 pm
content: I don't understand. Why do you want to reconstruct the original signal? Why don't you justusethe original signal?

author: Benta
author_rank: Super Contributor
timestamp: December 22, 2018, 04:45:48 pm
content: OK, to be more detailed:1: AC couple the video signal.2: place a quality analog switch (eg, DG419B) between the AC coupled CVBS signal and ground (or some reference voltage depending on the voltage shift you need).3: control the analog switch from the CSOUT output from the LM1881.This will clamp your sync pulses to ground/reference and thus DC shift your CVBS signal. You'll need a video buffer after this circuit, eg, LM6171.Also, you'll probably need to increase impedance before the switch, I don't know your video source.

author: syntax333
author_rank: Regular Contributor
timestamp: December 22, 2018, 04:49:49 pm
content: I tried to explain in figure. Basically,A signal obtained from HDMI to AV converter is fed to Bias tee circuit. In bias tee I am shifting the signal at a certain DC level. After that in order to send the AV signal to AV to HDMI converter I have to get the original signal without the DC shift. That is why I am trying to get the original signal because I will feed the signal to AV to HDMI converter.

author: Benta
author_rank: Super Contributor
timestamp: December 22, 2018, 04:53:17 pm
content: Quote from: soldar on December 22, 2018, 04:29:03 pmBias tee is a normal way of combining RF or video signals with DC, for instance to remotely supply an antenna amp with power through the coax.DC shifting CVBS signals is done all the time, in a VCR or TV perhaps 20 times in the complete circuit.Nothing special about this.

author: Benta
author_rank: Super Contributor
timestamp: December 22, 2018, 04:55:22 pm
content: Quote from: syntax333 on December 22, 2018, 04:49:49 pmSyntax333, are you sure that the CVBS-to-HDMI converter isn't AC coupled already? That would make this whole thing unnecessary.

author: syntax333
author_rank: Regular Contributor
timestamp: December 22, 2018, 04:57:12 pm
content: good point. I will check that first.