Intellivision composite video mod - Page 1
Forum section: electronics / projects

author: grips03
author_rank: Contributor
timestamp: September 07, 2014, 02:43:23 am
content: I'm trying to make a Intellivision composite video mod PCB. I used this 2N3904 based method and got poor results.http://solarfox.triluminary.net/hwh_intvav_p4.phpPoor results are video is slightly too darkjailbars - I think a low pass filter could correct thisghosting, i.e. you see the same image twiceThe native video signal going into the RF modulator is very weak and the signal swings below ground. I believe NTSC composite should be 1vpp and never go below 0v. So I want to shift the center point of the signal up to .5v and then make the gain of the amp so the signal is 1vpp. Not sure where to start, i.e. can a single opamp do this, do I need some other parts.Thanks,Dave

author: grips03
author_rank: Contributor
timestamp: September 07, 2014, 05:16:23 am
content: current NTSC composite video signal is ~50mVpp, but needs to be 1Vpp and level shifted to 1v I think for the TV to use.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 07, 2014, 05:42:15 am
content: Yes,Composite Video should be 1 volt p-p .The standard signal levelsare notsymmetrical between +1 volt & zero volts.If you Google for a Composite Video signal you will find something like this:-http://www.eetimes.com/document.asp?doc_id=1272387Looking at the waveform (Fig 1) on that site,you will see the original NTSC Black & White waveform.The Luminance levels are shown,including black level,as extending in the positive direction up to peak white at 100 units above the zero line,& the syncs extending in the negative direction to -40 units.This is the classic "IRE" graticule.Why 100:40 ?The story I heard ,(which may be an urban legend),is that the Committee originally wanted a 1.4 v Video signal,hence the graticule scale.When they went to 1 v p-p they just kept that scale.Looking further at Fig 1,you will notice that the zero line is labelled "blank level".When displaying a signal on a CRT,all the stuff like syncs,etc should be invisible,so the tube electron beam needs to be "cut off" when such signals are present.The standard allows for that by making a set signal level always represent cut off,or blanking level".Having a standard Composite Video signal,it is possible to change sources without having to "fiddle" with the TV each time.A TV or Picture monitor expects a signal having its zero volt level at blanking.These signal levels can be maintained by amplifiers with DC coupling,or more usually,by using AC coupling & DC Restoration,(usually a Clamp circuit).Depending on your TV or Pix Monitor,you may be able to get away with an AC coupled input signal,as sometimes the input circuits themselves include clamping circuits.

author: grips03
author_rank: Contributor
timestamp: September 07, 2014, 06:04:23 am
content: Given that post I might be able to use an opamp configured for gain (av) of 20. The current composite signal inside the console is .05Vpp, so .05Vpp X 20av = 1Vpp.Is there such a thing as video opamps with a gain of 20 and a band filter for NTSC video already built in? Do I need to worry about opamp bandwidth? I think NTSC video is 3.5Mhz.I think band filter will help clean up the verticals bars. I know when I use my Micomsoft Framemeiser with low pass filter feature it helps clean up the vertical bars on s-video signals from other consoles.thanks,Dave

author: vk6zgo
author_rank: Super Contributor
timestamp: September 07, 2014, 08:49:25 am
content: There is a possibility that your TV does have a clamp circuit,but your line syncs are non-standard.You can get that sort of problem,with the clamps happening at the wrong time.Otherwise,you would have to have an interfering signal which is,or close to a multiple of the line frequency.Are the bars stationary or drifting?Do you have an Oscilloscope?

author: grips03
author_rank: Contributor
timestamp: September 07, 2014, 11:53:46 am
content: Quote from: vk6zgo on September 07, 2014, 08:49:25 amyes

author: grips03
author_rank: Contributor
timestamp: September 07, 2014, 12:23:29 pm
content: http://console5.com/techwiki/images/2/21/Astec_UM-1285-8.pdf<--this is the RF modulator I removed.http://console5.com/techwiki/images/2/27/Intellivision-Service-Manual-Schematic.png<--this is the Intellivision schematic, RF section is in lower right. U10 is the color IC.Here is the data sheet for the color IChttp://console5.com/techwiki/images/7/74/AY-3-8915.pdfThanks for the help with this. I can post wave forms from the scope if that would helpThe vertical bars don't move