6bits into 10? Video DAC - Page 1
Forum section: electronics / projects

author: NivagSwerdna
author_rank: Super Contributor
timestamp: November 08, 2020, 02:18:58 pm
content: I don't know much about video or DACs...I have an old RGB signal which is 6 bits in each channel... the obvious DAC is a ADV7123 but that is 10 bits?Should I feed the signal into the least significant 6 bits? (or the most significant)?Sounds like a stupid question but I'm not sure!

author: Ian.M
author_rank: Super Contributor
timestamp: November 08, 2020, 02:46:13 pm
content: MSBs if you want close to the full output range. For every unused high bit above the top data bit, you halve the output range.

author: BrianHG
author_rank: Super Contributor
timestamp: November 08, 2020, 02:47:09 pm
content: Quote from: NivagSwerdna on November 08, 2020, 02:18:58 pmYour bit -> DACs -> dacs -> dacs -> dacs -> dacs -> dacs -> dac------------------------------ Optional, improves output range so that s-0=full black, s-63=Authentic 100% white.s -> dacs -> dacs -> dacs -> dacIf you were to wire the dac's[3..0] to GND, the output will go from 0% black to only 98.5% white instead 100% white, but every single step would have exact equal voltage spacing. You can correct the 98.5% by altering the DAC's termination or current reference resistor. Though, the 10bit is so fine, using my above connections for the bottom bits will be outside your eye's ability to see or any 8bit ADC of an LCD to grab that missing s[1..0] error, or unless you use a 12bit dac.

author: BrianHG
author_rank: Super Contributor
timestamp: November 08, 2020, 03:03:46 pm
content: 1 last thing, you can also tie dac to GND and dac[2..0] to VCC, or vice-versa.This will produce a video output from ~0.75% to ~99.25%.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: November 08, 2020, 05:27:04 pm
content: Ah! Thanks for taking time to answer. I like your thinking!

author: Benta
author_rank: Super Contributor
timestamp: November 08, 2020, 06:07:06 pm
content: Actually, the ADV7125 would be the more obvious choice...

author: NivagSwerdna
author_rank: Super Contributor
timestamp: November 08, 2020, 07:14:34 pm
content: Quote from: Benta on November 08, 2020, 06:07:06 pmADV7125 is "No Longer Stocked" at Farnell

author: Benta
author_rank: Super Contributor
timestamp: November 08, 2020, 07:19:54 pm
content: Quote from: NivagSwerdna on November 08, 2020, 07:14:34 pmThat's a Farnell problem. The part is still active and in production.

author: james_s
author_rank: Super Contributor
timestamp: November 08, 2020, 07:20:11 pm
content: I have a FPGA dev board with a ADV7123 on it, grounding the LSBs works fine, I can't see any difference over using a DAC with fewer bits.

author: ghida
author_rank: Newbie
timestamp: November 09, 2020, 01:34:37 am
content: Quote from: james_s on November 08, 2020, 07:20:11 pmSee above. Your white is not 100% white.

author: BrianHG
author_rank: Super Contributor
timestamp: November 09, 2020, 02:51:19 am
content: Quote from: ghida on November 09, 2020, 01:34:37 am1. If you are using an LCD screen, during 'auto-adjusting', the ADC auto-gains to determine 100% white anyways since analog linear video is so touchy to begin with.2. Since our eyes perceive video levels logarithmicly yet we use linear DAC levels, the observer difference between 98% & 100% will never be noticed compared to a dark picture where you have 0% & 2% video levels. Only when using the DAC to drive component where the 3 levels are mixed to achieve RGB does this error get multiplied when rendering the 'Blue' color, hence this is why HDTV component has the 'tri-level' embedded sync which allows the video receiver to determine video amplitude and correct/auto gain for such video levels lost along long cables and non-exact termination resistors.

author: james_s
author_rank: Super Contributor
timestamp: November 09, 2020, 04:48:52 am
content: Quote from: ghida on November 09, 2020, 01:34:37 amIt's close enough to 100% white that I sure can't tell the difference. It's also exactly what the datasheet says to do if you are driving it with less than 10 bits.