I now did something like a brute-force-test with the "--monitor cga"-option.
First I ran a double-FOR-loop to get a long list of modelines with 60hz:
for y in {104..800..8}; do for x in {104..800..8}; do ./switchres $x $y 60 --emulator groovymame --monitor cga ;done;done > /tmp/output_50hz.txt
Then I did the same with 50hz instead with output to a different- file, grepped for modelines, sorted and removed double-entries:
grep -h ModeLine output_60hz.txt output_50hz.txt | sort -ru > output_all.txt
After that i added all modelines below 640x480 of output_all.txt to the crtTV-"Monitor"-section of my xorg.conf and restarted X.
"DISPLAY=:0.1 xrandr" now showed many modes, but obviously lot's of them have not been accepted at all. I commented out these unaccepted modes (as an example I show the modes for horizontal-616-resolution):
# ModeLine "616x584x50.00" 12.625000 616 640 704 808 584 586 592 625 -HSync -VSync interlace
ModeLine "616x576x50.00" 12.216600 616 640 696 792 576 578 584 617 -HSync -VSync interlace
# ModeLine "616x568x50.00" 12.097800 616 640 696 792 568 571 577 611 -HSync -VSync interlace
# ModeLine "616x560x50.00" 12.097800 616 640 696 792 560 567 573 611 -HSync -VSync interlace
# ModeLine "616x552x50.00" 12.097800 616 640 696 792 552 563 569 611 -HSync -VSync interlace
# ModeLine "616x544x50.00" 12.097800 616 640 696 792 544 559 565 611 -HSync -VSync interlace
# ModeLine "616x536x50.00" 12.097800 616 640 696 792 536 555 561 611 -HSync -VSync interlace
# ModeLine "616x528x50.00" 12.097800 616 640 696 792 528 551 557 611 -HSync -VSync interlace
# ModeLine "616x520x50.00" 12.097800 616 640 696 792 520 547 553 611 -HSync -VSync interlace
# ModeLine "616x512x50.00" 12.097800 616 640 696 792 512 543 549 611 -HSync -VSync interlace
# ModeLine "616x504x50.00" 12.097800 616 640 696 792 504 539 545 611 -HSync -VSync interlace
# ModeLine "616x496x50.00" 12.097800 616 640 696 792 496 535 541 611 -HSync -VSync interlace
# ModeLine "616x488x50.00" 12.097800 616 640 696 792 488 531 537 611 -HSync -VSync interlace
ModeLine "616x480x60.00" 12.629040 616 640 704 808 480 482 488 521 -HSync -VSync interlace
ModeLine "616x480x50.00" 12.097800 616 640 696 792 480 527 533 611 -HSync -VSync interlace
ModeLine "616x472x60.00" 12.188880 616 640 696 792 472 474 480 513 -HSync -VSync interlace
ModeLine "616x472x50.00" 12.097800 616 640 696 792 472 523 529 611 -HSync -VSync interlace
# ModeLine "616x464x60.00" 12.093840 616 640 696 792 464 468 474 509 -HSync -VSync interlace
# ModeLine "616x464x50.00" 12.097800 616 640 696 792 464 519 525 611 -HSync -VSync interlace
# ModeLine "616x456x60.00" 12.093840 616 640 696 792 456 464 470 509 -HSync -VSync interlace
# ModeLine "616x456x50.00" 12.097800 616 640 696 792 456 515 521 611 -HSync -VSync interlace
ModeLine "616x288x50.97" 12.685600 616 640 704 808 288 289 292 308 -HSync -VSync
ModeLine "616x288x50.00" 12.196800 616 640 696 792 288 289 292 308 -HSync -VSync
ModeLine "616x280x52.33" 12.685600 616 640 704 808 280 281 284 300 -HSync -VSync
ModeLine "616x280x50.00" 12.078000 616 640 696 792 280 283 286 305 -HSync -VSync
ModeLine "616x272x53.77" 12.685599 616 640 704 808 272 273 276 292 -HSync -VSync
ModeLine "616x272x50.00" 12.078000 616 640 696 792 272 279 282 305 -HSync -VSync
ModeLine "616x264x55.28" 12.685600 616 640 704 808 264 265 268 284 -HSync -VSync
ModeLine "616x264x50.00" 12.078000 616 640 696 792 264 275 278 305 -HSync -VSync
ModeLine "616x256x56.88" 12.685599 616 640 704 808 256 257 260 276 -HSync -VSync
ModeLine "616x256x50.00" 12.078000 616 640 696 792 256 271 274 305 -HSync -VSync
ModeLine "616x248x58.58" 12.685599 616 640 704 808 248 249 252 268 -HSync -VSync
ModeLine "616x248x50.00" 12.078000 616 640 696 792 248 267 270 305 -HSync -VSync
ModeLine "616x240x60.00" 12.604800 616 640 704 808 240 241 244 260 -HSync -VSync
ModeLine "616x240x50.00" 12.078000 616 640 696 792 240 263 266 305 -HSync -VSync
# ModeLine "616x232x60.00" 12.117600 616 640 696 792 232 234 237 255 -HSync -VSync
# ModeLine "616x232x50.00" 12.078000 616 640 696 792 232 259 262 305 -HSync -VSync
# ModeLine "616x224x60.00" 12.117600 616 640 696 792 224 230 233 255 -HSync -VSync
# ModeLine "616x224x50.00" 12.078000 616 640 696 792 224 255 258 305 -HSync -VSync
# ModeLine "616x216x60.00" 12.117600 616 640 696 792 216 226 229 255 -HSync -VSync
# ModeLine "616x216x50.00" 12.078000 616 640 696 792 216 251 254 305 -HSync -VSync
# ModeLine "616x208x60.00" 12.117600 616 640 696 792 208 222 225 255 -HSync -VSync
# ModeLine "616x208x50.00" 12.078000 616 640 696 792 208 247 250 305 -HSync -VSync
# ModeLine "616x200x60.00" 12.117600 616 640 696 792 200 218 221 255 -HSync -VSync
# ModeLine "616x200x50.00" 12.078000 616 640 696 792 200 243 246 305 -HSync -VSync
# ModeLine "616x192x60.00" 12.117600 616 640 696 792 192 214 217 255 -HSync -VSync
# ModeLine "616x192x50.00" 12.078000 616 640 696 792 192 239 242 305 -HSync -VSync
# ModeLine "616x184x60.00" 12.117600 616 640 696 792 184 210 213 255 -HSync -VSync
# ModeLine "616x184x50.00" 12.078000 616 640 696 792 184 235 238 305 -HSync -VSync
# ModeLine "616x176x60.00" 12.117600 616 640 696 792 176 206 209 255 -HSync -VSync
# ModeLine "616x176x50.00" 12.078000 616 640 696 792 176 231 234 305 -HSync -VSync
# ModeLine "616x168x60.00" 12.117600 616 640 696 792 168 202 205 255 -HSync -VSync
# ModeLine "616x168x50.00" 12.078000 616 640 696 792 168 227 230 305 -HSync -VSync
# ModeLine "616x160x60.00" 12.117600 616 640 696 792 160 198 201 255 -HSync -VSync
# ModeLine "616x160x50.00" 12.078000 616 640 696 792 160 223 226 305 -HSync -VSync
# ModeLine "616x152x60.00" 12.117600 616 640 696 792 152 194 197 255 -HSync -VSync
# ModeLine "616x152x50.00" 12.078000 616 640 696 792 152 219 222 305 -HSync -VSync
# ModeLine "616x144x60.00" 12.117600 616 640 696 792 144 190 193 255 -HSync -VSync
# ModeLine "616x144x50.00" 12.078000 616 640 696 792 144 215 218 305 -HSync -VSync
# ModeLine "616x136x60.00" 12.117600 616 640 696 792 136 186 189 255 -HSync -VSync
# ModeLine "616x136x50.00" 12.078000 616 640 696 792 136 211 214 305 -HSync -VSync
# ModeLine "616x128x60.00" 12.117600 616 640 696 792 128 182 185 255 -HSync -VSync
# ModeLine "616x128x50.00" 12.078000 616 640 696 792 128 207 210 305 -HSync -VSync
# ModeLine "616x120x60.00" 12.117600 616 640 696 792 120 178 181 255 -HSync -VSync
# ModeLine "616x120x50.00" 12.078000 616 640 696 792 120 203 206 305 -HSync -VSync
# ModeLine "616x112x60.00" 12.117600 616 640 696 792 112 174 177 255 -HSync -VSync
# ModeLine "616x112x50.00" 12.078000 616 640 696 792 112 199 202 305 -HSync -VSync
ModeLine "616x104x60.00" 12.165120 616 640 696 792 104 111 112 128 -HSync -VSync doublescan
ModeLine "616x104x50.00" 12.117600 616 640 696 792 104 124 125 153 -HSync -VSync doublescan
I tested which of the remaining modelines produced a solid, working picture. Here they are:
ModeLine "616x576x50.00" 12.216600 616 640 696 792 576 578 584 617 -HSync -VSync interlace
ModeLine "616x288x50.97" 12.685600 616 640 704 808 288 289 292 308 -HSync -VSync
ModeLine "616x288x50.00" 12.196800 616 640 696 792 288 289 292 308 -HSync -VSync
ModeLine "616x280x52.33" 12.685600 616 640 704 808 280 281 284 300 -HSync -VSync
ModeLine "616x280x50.00" 12.078000 616 640 696 792 280 283 286 305 -HSync -VSync
ModeLine "616x272x53.77" 12.685599 616 640 704 808 272 273 276 292 -HSync -VSync
ModeLine "616x272x50.00" 12.078000 616 640 696 792 272 279 282 305 -HSync -VSync
The others of them only brought up a garbled picture. I also tried variations with +HSync and +VSync or both, but that didn't bring a change.
This is almost always the same pattern for all horizontal resolutions at least somewhere down to 400 (in steps of 8 ). Although I didn't test them all.
I will try around some more.