Quote:
Originally Posted by Stedy
The first problem is with the use of an LM1881/sync strike type device. Continued use will eventually destroy the GBS-8220, it'll take 18 months to two years though. The LM1881 outputs 5V CMOS levels. The GBS-8220 accepts 3.3V LVTTL levels, when you exceed the device power supply (3.3V) with an input signal, you start damaging the microchip.
Reducing the Amiga Composite sync (5V CMOS amplitude) from 4.8V to <1V cured over 95% of problems with SCART TVs. A SCART TV expects a 1V signal.
The GBS-8220 (TVIA-5725) expects a TTL signal, logic 1 is >2.0V but less than 3.6V max, logic 0 is <0.8V and no less than -0.3V.
The black level offsets are important, from there all scaling takes place. I have not seen a gain control register yet but some chips can amplify or attenuate the incoming video to maintain a 100 IRE level. If it does not, we may need to play with external components or settings. I need play time 
Have you played with the decimation filters?
The RGB YUV block can affect the output video if the wrong polynomials are used. Are there any particular colours that look incorrect?
With most video issues, I need to see the issue, on a calibrated monitor, to comment. Photographing a monitor is not easy.
Sorry if some of my answers are a bit vague, I'm still studying the TVIA-5725 chip. |
Yep, I know exactly what your talking about. I'm no video engineer so it's good to. Have your input.
I'm wondering what to do about the 5v sync input. I'm thinking a revision of the sync strike will be needed. Composite and SOG use a negative 0.3V sync don't they? Is that what pure Csync should be?
I feed the 5V sync into my 15Khz BVM, would that cause damage as well?
The 5725 does have gain and bias registers for the ADC.
The Input and sync recovery is probably the most complex part of the chip. Your knowledge here will help greatly. The decimation filters are on in the OFW set for 4x oversampling and then averaging the result. I haven't tried disabling it yet.
I have enabled 4x oversampling on the output with interpolation, which is good.
There is a YUV conversion matrix for input and output, but I don't know if you can change them. Their is also Dynamic Range Expansion on the output.