The scaler appears to be the most forgiving of the video devices on hand and a video signal was just possible with a clock of 29.4912MHz. This is a nice UART frequency but it is probably out of reach even when the circuit is on a real PCB. There is one frequency available that may be attainable and that is the other VGA clock: 28.322MHz.

The standard 25.175MHz VGA clock generates a horizontal resolution of 640 pixels, or 80 columns of 8-pixel wide glyphs. The 28.322MHz frequency is 9/8ths as fast and generates a horizontal resolution of 720 pixels, or 80 columns of 9-pixel wide glyphs. Of course, this is with the VGA horizontal frequency of 31.5kHz, where this design requires 38.4kHz.

The line length is shortened in order to get to the 38.4kHz. In this case the effective horizontal resolution would be reduced to 575 pixels, or around 71 columns of text. However, this shorter line is only required to support serial communications. This would be a requirement for something like a text based web browser, but non-network applications can use the original VGA timing. This would result a text mode with 90 columns using the font ROM's 8-pixel glyphs.

This change requires two different timing loops. The 38.4kHz loop would divide the line by 184 process cycles and the 31.5kHz loop would divide by 225.  The decision had already been made to handle either serial or audio, but not both concurrently. It now makes sense to split these features between these two video modes. The additional 41 process cycles in the 31.5kHz loop can then be used to process the audio. The rest of the loop would operate the same interpreter fetch and execute cycles as the shorter 38.4kHz loop. Therefore the only change between the loops would be a longer H-sync cycle to process the audio.

There would still be text and graphics modes with both loops and they can both support two video modes each depending on the field rate. The following examples show the supported video modes:

31.5kHz Horizontal, 60Hz Field (native 720x480 60Hz VGA)

• Hi-res Text: 90x60
• Lo-res Text: 90x30
• Hi-res Graphics: 360x240
• Lo-res Graphics: 180x120

31.5kHz Horizontal, 70Hz Field (native 720x400 70Hz VGA)

• Hi-res Text: 90x50
• Lo-res Text: 90x25
• Hi-res Graphics: 360x200
• Lo-res Graphics: 180x100

38.4kHz Horizontal Scan, 60Hz Field (**native 800x600 60Hz SVGA)

• Hi-res Text: 70x75
• Lo-res Text: 70x35
• Hi-res Graphics: 280x200
• Lo-res Graphics: 140x100

38.4kHz Horizontal Scan, 75Hz Field (**native 640x480 75Hz VGA)

• Hi-res Text: 70x60
• Lo-res Text: 70x30
• Hi-res Graphics: 280x240
• Lo-res Graphics: 140x120

** 38.4kHz modes are slightly modified versions of the VESA standards