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What are the differences between the 1080p version and the 720p version of the same media?

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Differences in video probably won’t be noticeable for untrained eye. 1080p video would have to be downscaled anyway.

Let’s assume the original video was 1080p. In this case the 720p video was first scaled, then compressed. On the other hand, 1080p clip was first compressed server-side, then scaled on your machine. 1080p file will obviously be bigger. (otherwise it would offer higher resolution, but at lower quality, ruining the visual experience and invalidating the point of using higher resolution1)

Lossy compression usually causes visual artifacts that appear as square blocks with noticeable edges when video is paused, but aren’t visible when you play it with normal framerate. 1080p file will contain more square blocks (caused by compression) than 720p video, but those blocks will be of approximately the same size in both videos.

Doing simple math we can calculate that 1080p video will contain 2,25 times more such blocks, so after scaling it down to 720p those blocks will be 2.25 times smaller than in actual 720p video. The smaller those blocks are, the better quality of the final video is, so 1080p video will look better than 720p video, even on 720p screen. Resized 1080p video will appear slightly sharper than actual 720 clip.

Things get a bit more complicated if source material was bigger than 1080p. The 1080p clip is first scaled to 1080p and compressed before you play it and then scaled once again while playing. The 720p clip is scaled only once and then compressed. The intermediate scaling step which is present in 1080p video case will make its quality slightly worse. The compression will make 720p even worse, though, so 1080p wins anyway.

Audio

It’s not only video that is compressed, but audio too. When people decide to use higher bitrate for video compression, they often do the same with audio. 1080p version of the same video may offer better sound quality than 720p video.

Do you have questions about video and recording? Ask the Doctor! We will answer all your questions on our blog. Submit your questions today…

HD-SDI Cabling and Loss

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HD-SDI is regularly transmitted over low-loss digital video grade RG6-style coaxial cable up to a nominal maximum distance of about 100 meters. However, when a lesser diameter cable like the RG179 is used, higher loss is encountered. This is primarily because of the size (cross-section) of the center conductor. Electrical signals travel through a wire by an effect known as the ’skin effect’ and the lesser the area available, the more the attenuation. Using RG179 for transmitting HD-SDI signals is good to about 100 feet. Another important factor is to use cable sweep tested for the highest possible frequency -bandwidth. Although the Mil-SPEC RG179 cable has a very strong outer jacket, the RG179 MIL-C-17/94F is rated to carry a maximum of 3GHz, with “attenuation rated at 21dB per 100 feet, maximum, at 0.4 GHz”. In comparison the Belden RG179DT (Digital Truck) cable carries signals to 4.5GHz. The ultimate distance limitation occurs for any digital signal when the perceived signal jitter component seen by the receiver impairs its ability to recognize and reconstruct bit transitions.

Range

The range of operation for an HD-SDI receiver is specified in SMPTE 292M to at least -20 dB at one-half the data clock rate, or about 743 MHz. Therefore, a standard level 0.800 volt peak-to-peak digital transmission may be attenuated to as low as 0.080 volt, or 80 millivolts, while performing reliably. A very high-grade receiver may recover the HD-SDI signal at a level as low as -30 dB, or 70 mV.

To perform a cable loss calculation, the designer should look for the attenuation in dB at 743 MHz, or a frequency very close to that value, on the cable specification loss chart. Cable loss is based on a 100 foot length or a 100 meter length depending on the chart column used. Divide the cable run distance by 100 and then multiply by the dB value to attain the total attenuation in dB for that run. Refer to manufacturer tables. The SMPTE recommends the designer factor in about 10% less cable than the calculated run length so as to build in a safety margin for reliable operation.

Coax is not the only medium available. Serial digital video may be routed through fiber optic cable for essentially unlimited distances depending on the system configuration. A convertor is used to convert the electrical signals to light signal, and then sent along the fiber optic cable whereas the other end would have the decoder to convert the light signals back to electrical signals. If your application requires long runs of cable, do not hesitate to contact us for suggestions and solutions.