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[I've been asked in many occasions, both by colleagues and clients, regarding the Motion/Sharpness preference in many of today's video conferencing endpoints. To clarify this topic, I asked Amnon Cohen-Tidhar, a video architect in the Networking Business Unit (NBU), to write this guest post as part of our ongoing "Ask The EXPERT" series]

Although not a “to be or not to be” question, many video communication experts debate about the issue of motion vs. sharpness. More specifically, the debate is all about our preferences: do we prefer motion or sharpness for our visual communications systems?

(CC) Fabrizio Lonzini

What’s more important – a faster, more fluid sense of motion or a higher level of sharpness?   It’s really in the eyes of the beholder (or should I say the be-watcher…). And so most video conferencing manufactures leave it for the customer to decide. Almost every endpoint on the market has this feature in the configuration menu. All you have to do is decide what your preference is.

That’s fair. But what does it actually mean?

Sharpness

Sharpness is, basically, the ability to separate between small objects or notice small details in the captured scene. In layman terms – is the image sharp enough? If your system works fine, you can use the resolution as a way to measure sharpness. After all, resolution means how many pixels (picture elements) you see. The more pixels, the smaller the objects you’ll be able to see.

But this is only if your system works fine, as pixels don’t tell the whole story. If the original image was captured poorly, due to a low quality system (optics, CCD sensor, etc.) more pixels won’t mean a better image. If your system runs over “noisy”, lossy network, or your bandwidth allocation is inadequate, a high resolution display won’t help.

In fact, if you’re watching SD broadcasts over fancy full HD display the important part is not the display’s resolution but the video scaler quality. You won’t get a bigger actual resolution than what the weakest “link” on your system can support. Poor scaling might even degrade the image quality, compared to the original low-res stream.

Hi res vs. Low res images at the same resolution.

Assuming the resolution is supported throughout the system, and sufficient bandwidth is allocated, higher resolution gives you sharper images, better details and a more realistic experience.

Motion

By “motion” we refer to the system’s ability to capture, process and display fast changes in a moving scene. This capability is measured in frames per second (FPS). The higher the FPS, the better the system handles fast action – you’ll be able to follow fast moving objects with less “blur” (as shown in the Fabrizio Lonzini picture at the top).

Motion is very important when you’re enjoying a fast action sporting event, but not that critical for video calling. In face to face communications, there’s usually minimal “action”. But the major benefit of a high FPS in video calling comes from a side effect – In many video communication systems the latency – the time it takes the system to process a frame from receiving an input to displaying an image – is a linear factor of the frame duration. By increasing the frame-rate you get shorter latency, which has huge impact on the quality of experience.

We’re all familiar with live TV interviews, where – due to high latency – the participants talk together or wait in awkward silence. Faster systems enable an uninterrupted communication flow.

Slow vs. fast capture

So – Motion or Sharpness?!

If motion and sharpness are so important, you probably wonder why you have to choose one over the other in the first place. After all, we want our system to have the best motion and the best sharpness we can get. We want to see all the tiny details even when the scene is dynamic, as we don’t want to miss a thing. Sadly however, as with most things in life, we can’t have it all.

When you buy a new car, you want it to be powerful yet economical. A huge engine gives it power but empties your pocket at the gas station; the economical ones don’t give you the thrill. What you usually do is compromise, and the same goes for your video conferencing system: Bigger resolution means more processing power needed, and a much harder task of keeping the motion; investing the processing power in motion means a smaller resolution supported. And as there’s no such thing as enough processing power, bandwidth and – of course – money, you choose your preferences and compromise.

The current high-end video conferencing systems let you choose between 1080p30 and 720p60 (first number is the resolution in pixel lines; second one is the FPS). While the first is 2.25 times bigger in pixels per frame, the latter has twice the number of frames per second. IMHO both are a great experience for video conferencing (at least for some).

If you’re in a small room, using a medium size display, the 720p60 is probably a better choice, not because of the better “motion”, as your peers probably don’t play tennis on the other side, but for the major improvement in round-trip delay. I’d consider the 1080p30 for large conferencing rooms, with big displays and many participants, where a more detailed image makes a lot of difference.

Bottom line – setting motion/sharpness preferences is easy, and available with most end-points. Now you just have to try it, and choose your side.