Thursday, July 12, 2012

Camera targets and optical transfer function

Last week at the MET in Sydenham involved a lot of information relating to camera/recordings suitable for CCTV evidence in court. Part of this involves resolution and how that gets faithfully reproduced. 
The standard resolution chart pre-HD was the EIA 1956 chart;

Pointing a camera at this so that it fills the frame (yes, it's 4x3!) you then have graded scales of resolution. The idea is that if you had alternating white/black stripes how many could you have horizontally across the frame and still resolve the stripes? In the case of a good SD camera->monitor about 500 lines of TV resolution are possible. In HD it's 1,500 at best.

The security industry has its own testing standard. The Rotakin target was developed by HOSDB (Home Office Scientific Development Branch) as a means of auditing the efficiency of a CCTV system. It consists of a human silhouette target 1.6m in height. When the target fills the screen vertically it is said to be 100%R.
The target has various gratings for ensuring the modulation transfer function of the system allows footage to have the required resolution so that video material will stand up in court.
As well as lens, sensor, encoding/compression system the other factor that will affect how well each resolution grating is reproduced is lighting.
The requirement for identification is that the subject more than fills the frame (i.e. 120% R).At 100% R the A-grating should be discernible which implies the system has 500TVL resolution.

Wednesday, July 04, 2012

Electrical Network Frequency analysis

I've been running a training course at the Metropolitan Police's Video and Audio Forensic facility in Sydenham. I've been able to chat to some of the technical forensic guys and one thing has blown me away - Electrical Network Frequency analysis

The whole of the UK mainland is on an electrical power-grid; as generators run up and connect to the grid they have to be at the same frequency and phase-locked to the 50hz AC supply of the rest of the country (or there would be sparks!). Now although the mains supply is pretty accurate at 50hz load variations cause momentary changes that are reflected across the whole country; typically less than 0.2hz either way - mains can be (at any moment) 49.8 through to 50.2hz, but even over short integration periods it is darn close to fifty. I couldn't find any info for the UK, but here are some traces taken from the mains supply in Romania in 1998, three towns, separated by 800Km. 

Notice how the waveforms track each other precisely. It turns out that the pseudo-random sequence is very identifiable after the event. For this reason the MET have been sampling it for the last five years. They know exactly what the mains in mainland UK was doing at any point since 2007. It also turns out that induced mains hum is present in most audio recordings; either through pick-up in the power supply or from the hum of lightbulbs etc. You have to work hard to make an audio recording that doesn't have this watermark down there in the noise. 
They've even discovered they can extract it from the regular flicker of lights on video-recordings. This means that you have a forensic tool for checking the record date and time on evidence. 

There is a Wikipedia page on the subject and they've challenged me to send them some samples of speech recordings I've made over the years to see if they can time-match them.