I guess this depends on the dvr model you have and its specification. With mine, I can set the recording rate individually for manual record mode and motion detect mode. I set both manual record mode and motion detect mode. With manual record mode...I set to the lowest frame rate but with motion detect I set to the maximum frame rate. So when I view the video including the pre-alarm recording...the pre-alarm recording shows the low frame-rate but then when actual event comes, the the frame rate increases to the maximux set.
But I switched off manual record mode since it is such a hassle to sip through all the uninteresting recording whereas motion detect shows you exactly what you need. It also has the pre-alarm recording meaning it is always recording into memory all the time and only records to disk when the event happens including about 1 minute of video before the motion detect is triggered. The post detection after all motion has stopped is maximu set to 40 secs. I have the avtech761.