At a recent get-together of the Digital Cinema Society
, at post facility IVC in Burbank
, director/editor/inventor Philip Hodgetts
presented a session on Internet video and what everyone needs to
know to get the best quality video online. Many of the people in the audience were cinematographers and filmmakers eager to create Internet “reels” with video footage, and wanted to know how to create the best quality possible. Of course, all the same applies to anyone interested in creating an Internet production, series or project with video.
The first rule was–no surprise–start with the best quality source video, a version of the old garbage in-garbage out dictum. That means a steady camera and well lit scenes with good tonal range. It also means to avoid dissolves and fine detail. Hodgetts advises great lens on a so-so camera rather than a so-so lens on a great camera. “Lenses matter,” he said. “Lens, pixel density of imagers, pixel aspect ratio and resolution of the format and compression all contribute to the variables that govern perceived image quality.”
Hodgetts was also unequivocal about another criteria. “Progressive scanning of the image is good,” he said. “Interlaced scanning is bad. We should never have had interlace in HD. Progressive is superior for two reasons: it scales beautifully and it’s easier to encode.” Likewise, he warned against HDV’s MPEG-2 with long GoP encoding. “A long GoP master is going to be a very, very long encode,” he said. “And Long GoP was designed for transmission rather than editing, so it’s not easy to edit it frame-accurately.
The relative merits of 10-bit versus 8-bit for online video were also discussed. “Video with 8-bits per channel has 256 steps from black to white, and 10-bit video has 1024 steps,” he noted. “The extra steps in 10-bit video mean higher quality and more detail. However, compressed delivery formats like MPEG-2 and Windows Media use only 8-bits per channel. If your destination video is compressed, therefore, you don’t need to capture in 10-bits.” That said, continued Hodgetts, “even when delivery is 8-bit there are advantages for compositing quality and color correction quality that make 10-bits worth considering.”
Audio was also taken into consideration. Once again, starting with the best possible audio source is advised, which translates to a clean recording, good microphones and good quality mixer. To choose the proper microphone, he suggested a website that allow users to test out the choices
. Another website has 18 mic samples
with a bassy male voice, a female voice and another voice, at prices from $100 to $500 to over $1,000. “See if you can tell a big difference,” he said. “If not, go for one of the less expensive ones.”For audio compression, said Hodgetts, the ideal range is between 4:1 and 10:1, with the threshold of around -20dB. For recording, he said “it doesn’t matter which device, but uncompressed 16-bit is minimum.”
Pre-processing video is a very important step. “Getting them right makes the job of the encoding engine much simpler and therefore better results are obtained at reduced bandwidth,” he said. “We also use sharpening during the pre-processing on the submissions for re-encoding sites to compensate for losses that will happen during the encoding process.” He suggested de-interlacing the video (or reverse telecine); cropping out action safe if going smaller than 640×480; black restore/white restore; correcting the gamma; and formatting without letterbox.
“Computers, LCDs, plasma and projection TVs are progressive,” he said. “If you can’t shoot progressive and stay progressive, you have to de-interlace. The bonus is that it makes the material a lot easier to encode and scale.” With regard to black and white restore, Hodgetts observed that “content for television from PAL or NTSC can sometimes look washed out.” “The black appears dark gray and white appears as light gray,” he said. “The Black and White Restoration filter has the ability to correct this by setting a new Black and/or White level.”
Gamma is probably the most common filter to use–and perhaps the most important correction to make. Video gamma is usually 2.5 while computer gamma varies between 2.2 (PC) and 1.8 (Mac), said Hodgetts. “Gamma is a filter for compensating for the difference between various display technologies and devices, such as when encoding for hand-held devices and terminals,” he said. “Gamma is a non-linear filter and will only affect the mid-range tones but leaves the darkest and lightest parts unchanged. This is very helpful for darkening or lightening a picture without getting distortion in the white areas.”
Encoding usually reduces sharpness, added Hodgetts. “What became obvious during the experiments with uploading to YouTube and MySpace is that their encodes softened the results quite significantly. For the material we encode for re-encoding by YouTube, MySpace and others, we sharpened heavily–beyond what actually looks good in the encode before submission.”
* Use MPEG-4 H.264 video and AAC Audio in .m4 or .m4v containers.
* Stay 23.98 throughout and never
* De-interlace because all web video is progressive – and don’t blend to de-interlace
* Start with an Apple-compatible preset for iPod/iPhone/Apple TV
Looking at what formats work best for online distribution, Hodgetts noted that H.264/AAC MPEG-4 is he universal format, for sites including Joost, YouTube, Adobe Media Player, Apple TV, iPod and some online distribution. With regards to encoding H.264, Hodgetts said “the simplest way is to use Apple’s simple and easy presets for export for iPod and Export to AppleTV, with new presets for the iPhone.” Up to 640×480 native size and over 640-480, the frame rate to 30 fps is maintained with a data rate up to 1.5 mbits/sec. Up to 1280×720 native size and over 1280×720 (for 920×540), frame rate to 24 fps is maintained at a data rate up to 5 mbits/sec. (He notes that 720×480 will be encoded as 720×540). Depending on the image content, 720p24 can be “comfortably encoded” in 3-4 Mbits/sec. Compressor or Episode Pro give more control for Apple TV data rate. “This level of control is important when you’re paying for download,” said Hodgetts. “But these are comfortable data rates for uploaded delivery.”
Though he advised starting with “Apple Device” presets for iPod/iPhone compatible files that also work in Flash Player and Adobe Media Player, he also stressed that users should experiment with the advanced H.264 settings and with every encode. Hodgetts also got into the details of how to prepare material for the inevitable re-encoding done by MySpace and YouTube, as well as “getting the best results for direct use MP4.” “Video at H.264 at average 512 Kbits/sec,” he enumerated. “Resize to 640×480, black restore at 25 to kill noise in the blackest areas, don’t change gamma from the original source. The lesser amount of sharpening improves the appearnce after compression without appearing overly sharpened.”
Who knew that uploading a YouTube video could be so complicated? But for users concerned about image quality, there’s no free lunch. Just more encoding, preprocessing and compression.