Troubleshooting

Noisy / flickering pixels

When setup correctly, the output of RGBtoHDMI should be completely noise free and any flickering pixels usually means that an adjustment in the sampling menu is required. This will generally only need to be done once for any particular machine. If you ars regularly switching between machines that shared the sample profile but require different settings then it would be better to create a separate profile for each machine by copying and renaming the existing profile .txt file or folder in the Profiles folder on the SD card.

For both digital (TTL) and analog sources, the first thing to try is an Auto calibration of the sampling phase.
This can be done using the option on the main menu or holding SW3 (hold until it starts).

You can also manually adjust the sampling phase in the sampling menu and that should be adjusted to eliminate or minimise noise.

If you are using the analog interface the next option to try is manually adjusting the DAC settings in the sampling menu which set the level slice values for the 1 bit A to D converters (these are not altered by the auto calibration). Try adjusting each of the YUV or RGB levels as appropriate a few values in either direction to see if it improves things. (leave each one at it's original value if it doesn't)

After making any changes don't forget to use the Save Configuration option in the main menu to keep those changes

Teletext Mode 7 on the BBC Micro

This is the most difficult source to sample as the 6 Mhz teletext pixel clock is generated in an unusual way from the 16 Mhz master crystal which involves resistor/capacitor filters and this results in the pixel clock being very asymmetric with the amount of asymmetry varying from machine to machine.
If you only get problems with certain combinations of foreground & background colours you should always try running the Auto calibration with those being displayed. Again manual adjustment should also be tried and there are additional adjustable phases A-F to cope with the asymmetry. Finally try manually setting the clock multiplier to x12 or x16 before calibration or manual adjustment but x16 might not work as it overclocks the CPLD so you could end up with a blank screen, streaking or a very noisy image with that setting.

If you are unable to completely resolve any mode 7 noise issues by making the above adjustments then you can try the following hardware changes:

1. Reduce the length of the BBC video cable to 30cm if it is significantly longer than that.

2. If you are connecting to a BBC micro or Master 128 via the analog board, try a direct digital connection instead as some of the very narrow pixels resulting from an asymmetric clock can exceed the input bandwidth of the analog board.

3. Adjust the values of the resistors/capacitors in the 6Mhz clock circuit on the BBC to make the clock more symmetric.
   For further details of this modification, see this page: Mode-7-Clock-Modification.

Screen flashing on and off

This indicates that the source input timing doesn't match the profile and you will often get this when the wrong profile is selected but it can also happen with some machines that have a programmable CRT controller which some software (e.g. demos) has reprogrammed to output a non standard signal. You will need to create a new profile to display such a source (see page on creating a new profile). If you end up with multiple profiles for a computer you can replace the original single profile with a folder instead and put the multiple profiles in that folder. The software should automatically select the correct profile corresponding to the source timing as long as there is sufficent difference in the hsync frequency, number of lines or sync polarity. (see examples in the profiles folder on the SD card)