… for Codecs & Media

Tip #954: VP9 Refresher

Larry Jordan – LarryJordan.com

YouTube uses the VP9 codec exclusively for 4K HDR media.

Apple introduced support for the VP9 codec in the fourth beta of macOS Big Sur, specifically for Safari. Here’s a quick refresher.

According to Wikipedia:

VP9 is an open and royalty-free video coding format developed by Google. It is supported in Windows, Android and Linux, but not Mac or iOS.

VP9 is the successor to VP8 and competes mainly with MPEG’s High Efficiency Video Coding (HEVC/H.265).

In contrast to HEVC, VP9 support is common among modern web browsers with the exception of Apple’s Safari (both desktop and mobile versions). Android has supported VP9 since version 4.4 KitKat.

An offline encoder comparison between libvpx, two HEVC encoders and x264 in May 2017 by Jan Ozer of Streaming Media Magazine, with encoding parameters supplied or reviewed by each encoder vendor (Google, MulticoreWare and MainConcept respectively), and using Netflix’s VMAF objective metric, concluded that “VP9 and both HEVC codecs produce very similar performance” and “Particularly at lower bitrates, both HEVC codecs and VP9 deliver substantially better performance than H.264”.

Here’s a link for more information.

… for Codecs & Media

Tip #958: How Do Audio Cables Prevent Hum?

Larry Jordan – LarryJordan.com

All cables pick up hum. Shielded, balanced cables cancel that hum in the mixer.

OK, so this is a bit off topic, but… I’ve known for years and years that audio cables with XLR connectors don’t have hum, while cables with RCA connector so. Today, I wondered: why?

Here’s what I learned at
ProSoundWeb.com.

“One cause of hum is audio cables picking up magnetic and electrostatic hum fields radiated by power wiring in the walls of a room. Magnetic hum fields can couple by magnetic induction to audio cables, and electrostatic hum fields can couple capacitively to audio cables. Magnetic hum fields are directional and electrostatic hum fields are not.

“Most audio cables are made of one or two insulated conductors (wires) surrounded by a fine-wire mesh shield that reduces electrostatically induced hum. The shield drains induced hum signals to ground when the cable is plugged in. Outside the shield is a plastic or rubber insulating jacket.

“Cables are either balanced or unbalanced. A balanced line is a cable that uses two conductors to carry the signal, surrounded by a shield (Figure 1). On each end of the cable is an XLR (3-pin pro audio) connector or TRS (tip-ring-sleeve) phone plug.

“Hum fields from power wiring radiate into each conductor equally, generating equal hum signals on the two conductors (more so if they are a twisted pair). Those two hum signals cancel out at the input of your mixer, because it senses the difference in voltage between those two conductors—which is zero volts if the two hum signals are equal. That’s why balanced cables tend to pick up little or no hum.”

XLR cables (called “balanced”) use two wires and shielding. RCA-type cables (called “unbalanced”) do not.

… for Codecs & Media

Tip #919: What is a Macroblock?

Larry Jordan – LarryJordan.com

H.264 compression tracks macroblocks rather than pixels to create smaller files.

Macroblocks are at the heart of MPEG and H.264 video compression. But, what is a Macroblock?

In order to get the smallest possible files when compressing into H.264, the image is divided into “macroblocks.” (HEVC uses something similar, called a “coding tree unit.”)

Wikipedia describes a macroblock as typically consisting of 16×16 pixels, which forms a processing unit in image and video compression formats based on linear block transforms, typically the discrete cosine transform (DCT). (DCT is used in JPEG, MPEG and H.264 compression.)

A macroblock is divided further into transform blocks. Transform blocks have a fixed size of 8×8 samples. In the YCbCr color space with 4:2:0 chroma subsampling, a 16×16 macroblock consists of 16×16 luma (Y) samples and 8×8 chroma (Cb and Cr) samples. These samples are split into four Y blocks, one Cb block and one Cr block.

The reason macroblocks are important is that when media is encoded, the compression tracks the location of each macroblock from one frame to the next, rather than the full pixel data. This reduces the size of the file significantly, but at the cost of a loss of color information.

… for Codecs & Media

Tip #904: Why You Should Avoid HDV

Larry Jordan – LarryJordan.com

Where possible, avoid using HDV to maximize image quality.

HDV was one of the first, if not THE first, consumer-grade HD video format. As such, it was an eye-opening opportunity to discover the benefits of HD.

However, compared to media formats today, HDV has three serious limitations:

• First, most HDV material is interlaced. While this plays acceptably on TV monitors, it looks awful on the web. This is because TVs are designed to display interlaced media, while the web is designed for progressive.
• Second, unlike all digital media today, HDV uses rectangular pixels which are stretched to fill the frame, rather than square pixels. This means that an HDV image won’t look as sharp as digital images today.
• Third, HDV records half the color information compared to most modern cameras. (And one-quarter the color of high-end cameras.)

NOTE: The only way to get rid of interlacing is to remove every other line of video, thus cutting vertical image resolution in half. Then, existing lines are either duplicated or guessed at using various methods of image interpolation.

So, if you are given the option to shoot or convert media into HDV, be very cautious before you agree. There are very few situations today where this makes sense.

EXTRA CREDIT

If you have existing HDV material, consider getting it transcoded to ProRes 422. While not required, you do need to start thinking about how to preserve and convert your older media assets, especially if you plan to edit them in the future.