GNDd vs GNDa

[mickcris]

Today I was RGB modding a PCE AV booster.  I have usually used GNDa, but when i was checking for ground on the AV booster, the one IC chip in it and also the grounds on the RCA jacks were connected to GNDd.
http://www.gamesx.com/misctech/pcebp.php

I am just wondering if one is better than the other to use and why there are 2 different grounds in the 1st place.  Most guides tell you to use GNDa like here:
http://www.mmmonkey.co.uk/pc-engine-rgb-mod/

[pnauts]

GNDa is for analog signals (as RGB), GNDd for digital signals (Data and adresses buses)

[mickcris]

thanks for the answer. i figured it was something like that, but i still find it odd that the GNDd is not connected to the ground plain so what is it actually grounded to? also still wondering why they chose to use GNDd on the AV booster.

[thesteve]

Both grounds are on the ground plane
The difference is what circuits the trace runs near

[mickcris]

Both grounds are on the ground plane
The difference is what circuits the trace runs near

Thanks.  I looked at my system again.  The 2 grounds are connected by the rf shield.  I was measuring with the rf shield removed which is what threw me off into thinking they were not connected.  This was in an original white pc engine system so it may be different on other ones.