The Recorder That Only Kept Four Days
Why this matters
A homeowner's car was scraped in the driveway on a Tuesday night. The following Wednesday she went to the recorder for the footage and the earliest video on the system was from the previous Saturday evening. The proposal had said 30 days of recording. The recorder had never kept more than about four, and nobody had looked since the install. By the time the dealer was on the phone the footage was gone for good, so the first hour went to containing the damage rather than to diagnosis, and the diagnosis turned out to be simple arithmetic that should have been done before the system was sold.
First, stop losing more
The office call came in at 15:40 on Wednesday, October 8. The service manager did three things before anyone worked out why.
Check every place a copy might exist. Some IP cameras record to a memory card in the camera as well as to the recorder. A tech logged in remotely and checked all eight cameras: none had a card fitted. There was no cloud backup on the account. The recorder was the only copy, and its oldest recording was Saturday, October 4, 19:12. The scrape had happened on September 30. There was nothing to recover.
Protect what was still on the disk. The recorder overwrites its oldest footage continuously, so anything the customer might still need from the last four days was itself about to disappear. The tech exported the last four days of the driveway and front door cameras, about 1 TB at their bitrate, to a USB drive the customer plugged into the recorder at her request; it ran overnight. Footage leaves the system only to the account holder and only on request, and the export went on the ticket by camera, date range and recipient. See related: The Camera System Handover and Privacy SOP.
Tell the customer the truth the same day. The manager called her back:
OFFICE: The video from the 30th is not on the
recorder. It was set up in a way that only
keeps about four days, not the 30 we told
you. That is our mistake. We have saved the
last four days for you, and a tech will be
out tomorrow to fix the settings and prove
the 30 days.
CUSTOMER: So it was never keeping 30 days?
OFFICE: No. It filled up in about four and has
been recording over itself since.
There was no point softening it. She had already looked at the recorder and would have known.
What the storage page showed
The storage page as the tech first saw it, logged in remotely that afternoon:
RECORDER STORAGE, AS FOUND
disk: 4,000 GB, status normal, overwrite ON
oldest recording: Oct 04 19:12
now: Oct 08 16:05 (at first check)
CH CAMERA MODE STREAM BITRATE
1 front door continuous main 12 Mbps
2 driveway continuous main 12 Mbps
3 street view continuous main 12 Mbps
4 side yard N continuous main 12 Mbps
5 side yard S continuous main 12 Mbps
6 back yard continuous main 12 Mbps
7 garage inside continuous main 12 Mbps
8 basement door continuous main 12 Mbps
Every camera recording 24 hours a day at its maximum bitrate. That is what many recorders do when cameras are added and nothing else is changed, and the installer had not changed anything else.
The arithmetic nobody did
A bitrate is a rate of data. To turn it into disk used per day, convert bits to bytes and seconds to days. See related: Recorder Storage Math: Bitrate, Days and Cameras.
- One camera at 12 Mbps: 12 megabits per second divided by 8 bits per byte is 1.5 megabytes per second.
- Per day: 1.5 MB/s times 86,400 seconds in a day is 129,600 MB, which is 129.6 GB per day.
- Eight cameras: 129.6 times 8 is 1,036.8 GB per day.
- Retention: 4,000 GB divided by 1,036.8 GB per day is 3.86 days.
Check it from the other direction: 3.86 days times 1,036.8 GB per day is 4,002 GB, which is the disk. And the storage page agrees: from Saturday 19:12 to Wednesday 16:05 is 3 days and about 21 hours, or about 3.87 days.
For 30 days on the same 4,000 GB, the whole system can average no more than 4,000 divided by 30, or about 133 GB per day. The installed configuration used 1,036.8 GB per day, nearly eight times that. No drive in this recorder's bays would have closed that gap by itself, and a proposal promising 30 days should have shown this calculation with the camera count and bitrates in front of the customer.
What each camera actually needed
The tech walked the property with the homeowner and asked one question per camera: what is this camera for, and how often does anything happen in its view?
| CH | Camera | Job | Activity |
|---|---|---|---|
| 1 | front door | identify people and packages | moderate |
| 2 | driveway | identify people and vehicles | moderate |
| 3 | street view | overview, plates not expected | constant traffic |
| 4 | side yard N | overview, a gate | rare |
| 5 | side yard S | overview, trash cans | rare |
| 6 | back yard | overview | rare |
| 7 | garage inside | overview | rare |
| 8 | basement door | overview, a door | rare |
Detail where it is needed, and only there. The front door and driveway cameras are the two that have to show a face or a car clearly. They kept their full 12 Mbps main stream, because cutting the bitrate on a camera whose job is identification throws away the job.
Motion recording on the two detail cameras. Recording only when something moves, with a pre-record buffer of a few seconds before the motion and a post-record period after it, keeps the event and drops the empty hours. The tech drew detection areas that covered the walkway and driveway and excluded the street, a hedge and a flag that moved in the wind.
The street camera continuous, at a low bitrate. Its job was an always-on overview: what came and went along the street at any time, including movement too slow or too far away to trigger motion on the driveway camera. Fine detail was not its job. It went to 4 Mbps continuous.
The five quiet cameras on motion at a lower bitrate. Side yards, back yard, garage and basement door saw activity a few minutes a day. They went to motion recording at 4 Mbps.
The new estimate, worked the same way
Motion recording needs one more input: duty, the share of the day a camera actually records. The tech estimated it from what the cameras had captured over the previous four days, scrubbing the timeline for activity.
- Front door and driveway: 12 Mbps is 129.6 GB per day at full duty; at an estimated 15 percent duty, 19.4 GB per day each, 38.9 GB for the pair.
- Street view: 4 Mbps is 0.5 MB/s, or 43.2 GB per day, continuous.
- Five quiet cameras: 43.2 GB per day at full duty; at an estimated 5 percent, 2.2 GB per day each, 10.8 GB for the five.
- Total: 38.9 plus 43.2 plus 10.8 is 92.9 GB per day.
- Projected retention: 4,000 divided by 92.9 is 43 days.
Forty-three days against a 30-day promise left about 13 days of margin, which mattered because duty is an estimate. A windy week, a new neighbor's dog or a tree whose shadow crosses a detection area can double a quiet camera's duty.
The measured check
An estimate is not retention. The tech came back seven days later and measured. The recorder's file search lists each recorded segment with its size; summing one full day per channel gave the actual daily total.
- Measured daily total: 104 GB per day, higher than the 92.9 estimate. The front door and driveway had run at about 18 percent duty rather than 15, and the two side yard cameras at about 9 percent rather than 5, the second because of a swaying shrub at the edge of one detection area, which the tech then trimmed out of the area.
- Measured projected retention: 4,000 divided by 104 is 38.5 days, still above 30.
He also booked a check for 45 days after the change, because the disk would only reach its new steady state once the old continuous footage had been overwritten. That footage filled the disk, and at about 104 GB per day it would take about 38.5 days to replace it all. At the 45-day check the oldest recording on the storage page was 38 days old. That number, read off the recorder, went on the ticket as the retention the customer actually has.
What changes the answer
The customer wants the driveway continuous at full detail. That is a reasonable request after a driveway incident. At 12 Mbps continuous the driveway alone uses 129.6 GB per day, or 3,888 GB over 30 days, nearly the whole disk. It means a second drive of about the same size, and the conversation is about storage rather than settings.
Higher-resolution cameras. The math scales with bitrate, not with the number of cameras alone. Replacing two cameras with higher-resolution models at a higher bitrate changes every figure above.
Motion detection that fails to trigger. Motion recording trades storage for the risk that a slow or partial event never starts a recording. The pre-record buffer, careful detection areas and the continuous street overview were how this system handled that risk, and the customer was told the tradeoff in plain terms before choosing it.
The fix the dealer made to itself
The install checklist now carries a storage line: camera count, mode, bitrate and estimated duty per camera, the calculated retention, and the retention measured from the storage page at a follow-up visit. A proposal may not state a retention figure that the calculation on the checklist does not support. See related: How to Set Up a Network Video Recorder and Confirm Retention.
References
- See related: Recorder Storage Math: Bitrate, Days and Cameras
- See related: How to Set Up a Network Video Recorder and Confirm Retention
- See related: The Camera System Handover and Privacy SOP
- Recorder manufacturer documentation for recording modes, pre-record buffer and file search