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Video Flash ADC 6 grey levels. — Parallax Forums

Video Flash ADC 6 grey levels.

tonyp12tonyp12 Posts: 1,951
edited 2013-02-24 13:13 in Propeller 1
I have not tested it,
but the idea is that the Prop pins hard switching at 1/2Vcc (1.65v) will act as fast comparators.

I terminated the incoming video at 150ohm instead of 75ohm, as to get a 1.5Vpp instead of 1Vpp
I put a 7k pull-up resistor to dc-bias the signal up by 1.5v.

2.2pf cap is to act as a 2.5mhz lowpass filter to filter out Chroma (if a color video).
The flash ADC will try to have all 6 pins at low if Hsync tip is active and each pin will asserted as voltage goes from 1.5 to 3.3v in six steps

Some values would need to be adjusted I'm sure, but could it work?
attachment.php?attachmentid=99509&d=1361661401
video_ADC.gif
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Comments

  • Bill HenningBill Henning Posts: 6,445
    edited 2013-02-23 15:27
    Interesting idea, but according to wikipedia, the above would not give a 6 bit result - see:

    http://en.wikipedia.org/wiki/Resistor_ladder
    http://en.wikipedia.org/wiki/Flash_ADC
  • tonyp12tonyp12 Posts: 1,951
    edited 2013-02-23 15:30
    Not 6bit (64 levels), but 6 levels the same number shades of grey the Prop video output.
    Should be able to sample video in real-time and add HID or time etc.
  • Bill HenningBill Henning Posts: 6,445
    edited 2013-02-23 15:44
    Oh!

    Sorry, I mis-read your original post. I think you are correct, it should work for six levels.

    Neat idea.
    tonyp12 wrote: »
    Not 6bit (64 levels), but 6 levels the same number shades of grey the Prop video output.
    Should be able to sample video in real-time and add HID or time etc.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2013-02-23 18:09
    Tony,

    I would recommend using a DC restorer circuit ahead of the ADC, instead of just capacitively coupling it. The problem with capacitive coupling is that the "zero" level will vary with the overall brightness of the subject, leaving no absolute reference for the sync tips or the black level.

    -Phil
  • tonyp12tonyp12 Posts: 1,951
    edited 2013-02-23 19:07
    Do you think a diode clamp will be enough?
    As a practical implementation of a dc restore cicuit uses two capacitors, two op amps, and an S/H that adds a lot of parts and cost.

    Maybe add 7th Prop pin for the ADC that will help tell the prop that the signal is to low or high.
    and then dc-offset on a line by line bases as to act as the b and c examples below.
    And attach a Prop pin in Duty mode (with added r/c) to the end of the ADC (lifting ground level) I could get variable offset on the fly.

    3303Fig04.gif
    Different types of video clamp
    a) Diode or sync tip clamp, b) Keyed clamp with reference voltage used as a sync tip clamp
    c) Keyed clamp used as a black level clamp, d) DC restore.
  • TubularTubular Posts: 4,703
    edited 2013-02-23 20:23
    Tony,

    Neat ideas, hope they work.

    Just wanted to point out the switching threshold is usually around 1.42v rather than Vdd/2, in case it affects your circuit calculations

    cheers
    tubular
  • tonyp12tonyp12 Posts: 1,951
    edited 2013-02-24 11:23
    A deluxe version with 3 video_in selector.
    Should I go with Sync clamp or Bias version?
    http://rohmfs.rohm.com/en/products/databook/datasheet/ic/audio_video/video_amplifier/bh76330fvm-e.pdf
    Being able to use 0.1 or 4.7uf input caps is a space saver.
    attachment.php?attachmentid=99518&d=1361733641
    3video.jpg
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  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2013-02-24 13:13
    If you can trust the video signal to be the correct 300mV/700mV (as in PAL, NTSC is much the same) then sync tip clamping would be ok, but otherwise the only rock solid way would be to get black level clamping.

    Given the low level of quantized levels sync tip should be petty good.
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