If you’ve been working with video over the last decade, you’ve probably heard the myth that MPEG type compression has been rejected by courts because it records only the changes in a scene. This simply isn’t true. It’s one of the strangest myths, and I’m surprised it is still alive, but I just heard it again a few weeks ago.
I apologize for the long post here, but I hope to bury this bogus idea once and for all.
I first ran across this issue about 10 years ago when some of the rapid transit authorities who were interested in moving from analog to digital video systems in their buses were told by some competitors that the video would not stand up in court. Their legal departments were concerned.
So, I researched the matter to see what, if any truth there was to it. What I discovered was exactly the opposite. Courts had already dealt with these questions and ruled the video was admissable.
One of the most notable opinions came from a high level judge in the UK, who wrote her opinion as a summary for how all UK courts should treat digital video evidence. It came after years of working with cases and seeing the need for a conclusive opinion. Her comment was that even if the video was not watermarked, it should still be accepted as evidence. However, she highly recommended that all manufacturers watermark their recorded video. Watermarking adds a great deal of authority to the video evidence by verifying that it has not been tampered with.
Her other emphasis was on how the recorded video should be handled when used as evidence in court. Having a well documented chain of custody and clear processes and procedures for how to handle such evidence until the moment it ends up in court, was the most important thing of all. This was how all evidence should be handled to provide credibility that it is what it claims to be.
There were a number of other published papers by expert lawyers who had handled cases involving digital video. They made the matter even clearer. Courts regularly allow faxes, emails, photographs and audio recordings as evidence. All of these can be tampered with. Most are, or can be, modified in some way electronically to produce copies for use in court. The only real and valid concern is whether there is any indication that the evidence was in fact altered, or if anyone who handled it had a history of modifying evidence. Otherwise, most courts won’t even allow lawyers to suggest that the evidence might have been tampered with.
One lawyer, as an example, went on to explain how easy it is to fake photographs. People have done this for years. But simply the possibility that a picture could be faked is not enough to reject it as evidence. If it were, no photos or any other electronic documents such as emails or faxes would ever been usable in courts.
So, there is no truth to the myth.
However, besides the legal side, the whole technical reason for rejecting MPEG type video compression makes no sense.
One of the arguments I’ve heard was that if only the changes in the scene were recorded, then those changes could be overlayed onto a different background to make it look as if the person had been somewhere else. True, but you can just as easily do the same thing with JPEG video or even lossless compressed video. It is easy to extract moving people from a static background. Good watermarking, however, easily establishes whether this kind of alteration has taken place.
Besides, it isn’t true that only the changes are stored with MPEG type compression. Usually, once every second or every few seconds, a full copy of the scene is captured and recorded. These are called “I-frames”.
Secondly, each frame is re-assembled during playback. You can stop on any frame and see the whole picture. You don’t see only the changes, but the full scene.
I’ve had some odd discussions with the FBI about this matter. They seem to be one of the last strongholds for this myth.
You would think that the FBI would know. And I’ve seen a lot of people accept these concerns as valid simply because the FBI says it is a real issue. So, I’ve tried my best to speak to the experts at the FBI to find out their reasons. Whoever I have spoken to, however, has not convinced me they understand any better than anyone else, and its quite clear that a number of people in the FBI are quite ignorant about how compression really works. So, this is actually a technology problem.
What the FBI states that they want utlimately is lossless compression, so that no data is lost. They seem to think that would be the best, but even this is the wrong thing to be focusing on. On top of that, it is completely impractical. It could take over half a terrabyte of storage for a single day of lossless compressed video. No one is going to pay for something like that, or the bandwidth to transmit it, especially when there is no good reason to do so.
What makes the request for lossless compression the wrong goal is that they are willing to settle for fewer frames per second and lower resolution video as long as they could get the lossless compression. This is exactly the wrong compromise to make when you want video to be used as evidence.
I’ve explained to them and others that the last thing you want to do is throw away frames or resolution. This is value image data. You can’t identify people without enough resolution. You can’t see critical events without a fast enough frame rate. Why would you ever want to trade this data for lossless compression?
More importantly, compression technologies have been specifically designed to eliminate the data from video that is the least important. It is data that they eye can’t see or doesn’t notice, even if you study the video carefully. Good compression can only be recognized by experts, and for most people it often looks identical to the original. Of course, this is good compression I’m talking about, and many times we see recorded video that has seriously compromised the quality. But it doesn’t take lossless compression to get good video, which is why MPEG-2 has been used for years in DVDs.
The whole idea that MPEG type video is somehow modified while JPEG is truer is another weird idea that simply isn’t true. The people who start these kinds of rumors just don’t know how compression works.
If a system could accurately extract only the changes in a scene and record all those changes, you would have lossless compression. In fact, lossless compression uses this exact technique to shrink the size of the data. You don’t lose any information if you don’t send the same information over and over again with every frame. There is no need to send it more than once, except for the possibility of data corruption. Sending the whole frame over and over does make the data more immune to corruption, which is exactly why MPEG video sends I-frames so often, but that is the only reason to send it more than once.
JPEG, on the other hand, makes lots of compromises with the video data. The number of colors are limited, the edge information and detail is simplified, and contrast data is reduced. Look at it this way: A good JPEG image might be one-tenth the size of the original image. That means that JPEG is throwing away 90% of the data. But the end result with a good compressed image is that it looks almost identical to the original, to our eyes.
Since MPEG type compression, called temporal compression because it is compressing across time, only differs from JPEG because it throws away data that is being repeated, this actually throws away a lot less information than the spacial compression of JPEG. So, if anything, temporal compression thows away less information. In actual fact, the I-frames that MPEG sends use the exact same method of compression that JPEG does, but the added temporal compression focuses on reducing the same data that was already sent or recorded, since it adds no more information.
So, the whole idea that MPEG is throwing away valuable data while JPEG is not is simply wrong.
I’ve heard other arguments and concerns. The best one is that a lawyer might cast doubt on the validity of the video when they tell a jury that the whole frame isn’t being recorded. This concern is easily overcome by simply explaining as I did above: No information is lost when you can recognize which pixels have changed and only send the data on the changes. There is no reason to repeat the data that didn’t change. It gives you no more information. Nothing is lost by this. Lossless compression uses the exact same approach.
More importantly, MPEG type compression, which includes the latest H.264 technology, allows you to record 4X – 10X as many frames per second in the same storage space and with the same bandwidth – or you can record 4X – 10X higher resolution video. Using MPEG type compression, therefore, means you can include far more valuable and important data that can help you identify people and what has happened at the scene of the crime.
Clear pictures with higher resolution and close to full motion video is the most valuable recording you can provide to court for evidence. The video should be watermarked. And the video evidence should be handled using well established chain of custody procedures. All the rest is just a modern day myth based on misunderstandings of technology.
It is time to put this one to rest.
