Recorder storage: buy retention days, not terabytes

A recorder is specified by how many cameras it accepts and how many terabytes it holds. Neither is the number that matters. The number that matters is how many days back the footage goes, because that is the question that gets asked — usually on a Monday, about the previous Wednesday.

The arithmetic

Each camera writes a stream at some bitrate. A 4MP camera at a sensible frame rate and compression writes roughly 4 Mbps. That is 0.5 MB a second, 43 GB a day. Sixteen of them is about 690 GB a day. A 4TB recorder therefore keeps a little under six days; the 2TB unit that was quoted keeps under three.

Run the figure for the system being proposed before it is ordered. Every recorder manufacturer publishes a calculator, and every camera's bitrate is on its datasheet.

What retention should be

Thirty days is the usual floor for a business. Not because anything reviews a month of footage, but because that is how long it takes for an incident to be noticed, reported, and asked about — an invoice queried, a missing item counted, a complaint made. Fourteen days is a reasonable minimum where budget is tight; below that the system is recording for the sake of recording.

Ways to keep more days without buying more disk

• Motion-triggered recording on cameras watching a corridor at night. Continuous recording of an empty corridor is what fills most recorders.
• Two streams per camera: a lower-bitrate continuous stream, and full quality only on motion or alarm.
• Frame rate. Twelve to fifteen frames a second is fine for identification; thirty is for sport. Halving the frame rate very nearly halves the storage.
• H.265 over H.264, where every camera and the recorder support it. Between a third and a half less data for the same picture.

Surveillance-rated disks

A recorder writes sixteen streams continuously for years. A desktop hard disk is designed for bursts and idles, and it will fail in a recorder well inside its warranty — usually without the recorder saying so. Use disks rated for surveillance; the premium is small and the alternative is discovering, on the day the footage is needed, that the recorder has been silently writing to nothing for a month.

Check it monthly

Open the recorder and look at the oldest recording. If it is not at least as old as the retention that was agreed, something has changed — a camera's bitrate was raised, a disk has failed, motion recording was switched to continuous by somebody looking for a clip. It is a one-minute check and it belongs on the AMC visit sheet.