Wednesday, June 4, 2008

Leveling the bitrate playing field

There’s a nasty game being played, and the end user always loses. Only last night I came across a project where a reputable competitor went in with a storage quote that was half their original quote. I first raised the issue in 2006 and I have since seen wiser specs written that protect the end user. But it is not yet common-practice and so I am reiterating it here.

End users demand that vendors bid comparable technologies against the written specifications for framerate and image size – and the vendors comply. But, what if one vendor gains a price advantage by tweaking an important factor that the end user did not know should be specified? All the security director has is two bids with two different prices that look equivalent on paper. How can they be sure to get what they need? What is the important factor used by all the vendors but not always explained? The simple answer: the bitrate.

Bitrate measures data transfer rate between the imaging source (camera) and the storage device for the video stream, expressed as bits per second (bps). Correct bitrates matter so much in surveillance systems because (i) bitrate caps the quality of the video stream with lower bitrates yielding lower quality video streams and (ii) appropriately compressed CIF images look consistently better than over-compressed 4CIF images even though it is common sense to think of 4CIF as being superior; (iii) bitrate affects storage needs: unreasonably low bitrates pack more hours of video onto a storage device but uses excessive image compression to get there, again losing video quality.

Imagine a competitive bid situation. Two vendors both recommend systems that meet the specs required, say 30FPS at 4CIF. However, one vendor may adjust the bitrate to create an advantageous price by reducing the amount of storage required to lower the overall bid (remember the framerate and image size still meet spec).

Adjusting the bitrate does not breach the customer’s spec and it does not mean that a vendor is unscrupulous or underhanded. Tweaking (lowering) the bitrate permanently removes image detail from the video stream, making the images look fuzzy or pixelated (blocky), depending on the type and degree of compression. It's a bit like you asking 2 artists to paint a mural on your 6x4 yard wall. That is the 'image size', that is exactly analogous to something like 4CIF. The first artist quotes $1M, the other $2M, because the first artist takes less than half the time because his brush is twice as thick. The paintings are the same size, 6x4, but the detail from one artist is much finer than the broader stroke work of art. Bitrate affects the amount of fine detail that is preserved in the final image. And the problem is that it only gets slightly worse as you use slightly lower bitrates. 10% lower bitrate means a marginally inferior image, which is subtle to the eye but a resolution chart will give you an objective assessment of the true resolution. Unfortunately in the move to video over IP many people are confusing 4CIF image sizes with high TVL lines of resolution, when in fact there is only a distant relationship. Bitrate has the final say.

By the way, I do admit that in the absence of objective measurements the acceptability of video is a subjective opinion, however objective measurements have existed since the dawn of time – we just seem to be ignoring them now. Frequently the video quality of the newly installed system is lower than expected but the end user has nothing else installed to compare it against and doesn’t know why what they asked for isn’t what they received. The problem becomes acute when using PTZs, busy scenes and low-light situation when automatic gain control kicks in.

This is where the end user suffers: they got what they specified but didn’t get what they wanted. They got an inferior 4CIF image where the fine detail they expected is lost.

End users deserve to make a fair comparison of every bid proposal they receive. They are entitled to full disclosure of all factors affecting the system’s price and most importantly they should simply demand to see a demo of the system using the bitrates assumed in the quotes. Otherwise they are being misled, with all the consequences that entails.

Click here for a podcast on this topic.

Monday, June 2, 2008

Back to basics

Converting our video from a fluidly fluctuating voltage analog signal to billions of digital ones and zeros cannot hide the fact that what we all really care about is how it looks. And it all starts with light. Especially for those of us that see IP cameras and encoders as merely network devices, it might be helpful to understand what's really going on.

Should I choose a fixed or varifocal lens? Do I need a day/night camera or IR illumination or both? Fixed, manual or auto iris? At night time what difference does deep twilight, a full moon or a quarter moon make to the lighting level and which type camera will cope?

I found this link to be helpful as both a refresher for video and imaging basics, or if you are new to security, then a library of the few basic building blocks everyone should know about video.

Click here to visit the Camera Learning Center.