A DO328 CAB ALT warning alerted at 12;000 FT; so the crew completed the memory items; began a descent; got ATC clearance to 10;000 FT; and were able to regain cabin control using the Manual Mode as per the QRH.

Narrative

I was PIC and pilot flying on an IFR flight plan. We were level at 12;000 FT MSL when we got a 'CAB ALT' warning; which means that our cabin had not pressurized correctly. The emergency memory item for this is to descend immediately to 10;000 FT; or minimum safe altitude; and since we were only at 12;000 FT; I started a gradual descent and told my First Officer to request 10;000 FT. The frequency was busy and my First Officer asked for 10;000 FT and was told to standby. My First Officer then asked me if we were declaring an emergency and I said no; not at this time. I did not declare at that time because we were so close to the 'safe' altitude of 10;000 FT and because I thought it was possible that ice had accumulated in the main cabin door seal while we were on the ground previously in frigid temperatures. In any case; I felt that declaring an emergency was not necessary for this particular event. I had descended almost 400 FT from 12;000 FT and at approximately 11;600 FT ATC gave us the descent to 11;000 FT and then to 10;000 FT. During this time; my First Officer had used our QRH manual and had switched to manual pressurization control. At 10;000 FT we were able to pressurize and maintain a normal cabin pressure. I believe that I deviated approximately 400 FT between the time that we got the warning message and when ATC gave us the lower altitude.