While modules based in the circuit design by zx.zigg.net are okay, their performance very much depends on the NE555 meeting the datasheet specification. Some “NE555” chips from the far east, don’t. I’ve investigated this on a different module and concluded that the 7555 chip is better in this application.redgatemodels wrote: ↑Wed Nov 29, 2023 11:51 pm How about this? https://www.ebay.co.uk/itm/225407247107 ... media=COPY
I'd rather get something from the UK
But I have not tried the module from this seller.
It’s part of the history of the analogue TV signal. A video signal has to contain the following elements:redgatemodels wrote: ↑Thu Nov 30, 2023 2:32 pm As an aside, why "back Porch" what are the origins of the phrase? I know its a timing signal etc, just curious as to the name.
- The signal containing the picture information,
- A horizontal pulse to synchronise the horizontal position of the electron beam so that lines of the picture align correctly on the screen,
- A vertical pulse to synchronise the vertical position of the image.
But with colour images, you need more control signal information.
Either side of the sync pulses, there is a fixed level signal at the voltage for the ‘black’ video level. Note that the bottom of the sync pulses is at a lower (more negative) level. The level before the sync pulse is called the front porch. The one after is the back porch. It’s believed the name comes from the shape of this part of the waveform when viewed on an oscilloscope.
With colour TVs, the circuitry samples the voltage of the signal after the sync pulse, where the back porch is. This helps it automatically determine the black video level of the video signal. This means the TV can cope far better with variations in the received signal level and produce a better picture.
However, the first two versions of the ULA chips for the ZX81 go straight from the sync pulse to the border area “colour”, which is white.
This confuses most colour TVs, hence they then display a very dark picture (dark grey to black).
Mark