Record resolution and recording frame rate are the most important specification for any DVR. Both of them determined the quality of video.
For real-time recording, the frame rate is 30fps, it's fixed for every DVR. the highest recording speed for a 4 channel DVR is 120fps, 8 channel DVR is 240fps, 16 channel is 480fps, 24 channel DVR is 720fps, 32 channel DVR is 960fps.
(Note: At 3-5 frames per second recording, you will not omit any activity, there is no real need to record in real time unless there are regulatory reason.)
Now the processors in most of DVR can not recording all the channels in realtime.
Typically most of DVR provides recorded resolution as QCIF, CIF, 2CIF, and D1, 960H
QCIF: 176 X 120 Pixels
CIF: 352 X 240 Pixels
2CIF: 720 X 240 Pixels
4CIF: 704 X 480 Pixels
D1: 720 X 480 Pixels
960H: 976 x 582 Pixels
HD: 1280 x 720 Pixels ( for IP camera, 1080P)
Why D1 recorded resolution is so important ?
For some surveillance projects like casino, it demands D1 display resolution and recorded resolution, that's a legal requirements for each channel to be originally recordded in D1 resolution.
D1 resolution is the highest resolution that DVR can record in. Typically, the higher the resolution you choose to record in the lower the frame rate you have to select. DVR with D1 recording solution with powerful processor. In most case, the higher of the record resolution, the more data that DVR need to process.
CIF Reslution is an image comprised of 352 X 240 Pixels, then for each channel, a 1 CIF real time (30fps) DVR needs to process 20,275,200 bits of data. (Most pixels displayed on today's systems are 8 bits of data per pixel).
D1 Resolution is an image comprised of 720 X 480 Pixels, then for each channel, a 1 D1 real time (30fps) DVR need to process (720 X 480 X 8 X30)= 82,944,000 bits of data.
960H represents the number of effective pixels in the horizontal direction of the CCD image sensor, it can provides 976 X 582 Pixels resolution, it's the highest resolution in analog cctv system.