High-grade DTV promises to deliver a home theatre experience and might impact movie theatre attendance. Large home screens and surround audio can create the theatre experience right at home. Why go to the movies and pay a lot of money, when you can stay home and enjoy content from a DVD or VoD just a few months after its release?
TV has always been a threat to the movie industry. In 1950, movie theatre attendance was more than 3 billion yearly (80 million weekly). Heavily affected by TV, the number dropped to 1 billion (20 million weekly) by 1968. Can the movie production industry find a way to exploit the limit of DTV compression and transmission?
Only the most extravagant home theatres, can match the ambiance and massive sensual stimulus, as well as the group experience, of a small multiplex theatre. And there is something to be said for the sensation of a group scream or sigh that cannot be duplicated at home.
HDTV broadcasting was launched (in part) with the goal of delivering a unique viewing experience, over the air, to homes for free. Cable operators showed little interest in HD, so terrestrial broadcasters envisioned HD as a means to gain an advantage and reclaim audience.
With the proliferation of new 1080 60p native displays, an opportunity may exist for theatrical content producers to deliver a differentiating experience and regain some of the revenue lost with declines in movie audiences.
Broadcasters will be hard pressed to deliver a true 1080 60p video format as it requires twice the bandwidth of 1080 30i or 720 60p. Source material does not exist in a native 1080 60p format. DVD’s store HD content as 1080 30i or 1080 24p. Scaling up to 1080 60p is done either at the source DVD player or at the display.
By the numbers
If the numbers are examined, use of the 1080 60p video format is not beyond the reach of broadcasters constrained to a 6MHz delivery channel. First, compare the data rate for uncompressed 1080 formats:
1080 60p, 4:2:2 => 1920*1080*60*2*8= 1,990,656,000; 102:1 compression ratio 1080 30i, 4:2:2 => 1920*1080*30*2*8 = 995,328,000; 50:1
As can be seen, 1080 60p doesn’t fit in SMPTE 292 SDI channel. Even with the adoption of the 3Gb/s serial interface, SMPTE 424, the 1080 60p bandwidth requirement will present content distribution challenges in a production infrastructure.
However, 1080 24p isn’t as bad as one might think. 1080 24p, 4:2:2 => 1920*1080*24*2*8 = 796,262,400 which does fit in an HD-SDI stream and is already being delivered by some broadcasters. 3:2 pulldown and scaling converts the 24fps format to the display’s native resolution.
To further reduce the data rate, conversion from RGB to YUV color space is required. Because of the limited ability of the human visual system to perceive color detail, this conversion may be unperceivable. Taking this concept one step further, because ATSC broadcasts use a 4:2:0 color sampling structure, the data rate can be reduced even more.
1080 24p, 4:2:0 => 1920*1080*24*1.5*8= 597,196,800
Which is less than
1080 30i, 4:2:0 => 1920*1080*30*1.5*8= 746,496,000
So, 1080 24p actually has less data than 1080 30i. Subsequently, an argument can be made that there is a lot of free space in a transport stream that can be exploited.
Beyond 1080 60p
Some professional and consumer equipment manufacturers believe the ultimate resolution of home DTV displays in 10 years will be 2K. This is the 2K, 2048x1536 format now being used for digital cinema.
Comparing the number of display pixels:
1920 x 1080 => 2,073,600
2048 x 1536 => 3,145,728
This shows that 2K has about 50 percent more pixels than a 1080 display. Let’s look at the number of pixels for a 2K image at 24p using three different color sampling structures:
4:4:4 => 1,811,939,328 pixels
4:2:2 => 1,207,859,552 pixels
4:2:0 => 905,969,664 pixels
So, 2K at 24p 4:2:0 is about 21 percent larger than 1080i. Therefore, it may just be possible to compress 2K, 24p, 4:2:0, while maintaining acceptable image quality so it can be delivered in a 6MHz channel. This becomes even more realistic as advanced codecs (AVC & VC-1) are used in the broadcast chain.
2K by the numbers
Looking at the numbers in a little more detail, the 2K display format results in:
2048 x 1536 = 3,145,728 pixels per “frame.”
Comparing the data rates for 24p, 30p and 60p:
Pix/sec RGB Pel/sec 8 bit RGB/sec 24 HZ P => 75,497,472 226,492,416 1,811,939,328 30 Hz P => 94,371,840 283,115,520 2,264,924,160 60 Hz P => 188,743,680 566,231,040 4,529,848,320
An advantage of 24p is that if a 48Hz or 72Hz refresh rate is used, mapping onto the display is simplified, 2:1 or 3:1. This minimizes temporal artifacts that may be produced by frame interpolation. It’s possible that the loss of detail in a rapidly changing scene with current frame rates might be eliminated.
What about the audio?
If all this engineering energy has been successfully expended to deliver a much higher video resolution to consumers, will AC-3 audio suffice? Data rates for compressed Dolby Digital multichannel sound top out at 640kb/s and present virtually no additional challenge for the transport stream.
E-AC-3, Dolby Digital Plus, extends audio channel numbers up to 13.1 at data rates up to 6.144Mb/s. Dolby E can handle eight audio channels as an AES-3 pair with a data rate of 1.536Mb/s at 48KHz and 16 bits. This would allow up to 7.1 channels of surround sound.
A hybrid solution
Although it appears quite possible to deliver 2K format video over a 19.39Mb/s MPEG-2 transport stream, the stumbling block is finding an adequate audio format. Or is it? Dolby Digital (AC-3) is still in its authoring infancy while speaker technology continues to improve. Unless you have a gigantic home theatre, 5.1 audio is completely adequate for a 2K video experience. The difference between uncompressed and Dolby Digital barely is perceptible, even to golden ears.
Granted, moving up to large 2K displays will take some time. Today, QXGA computer monitors operate at 2K resolution with a 72Hz refresh rate. IBM offers a 22.2in 3840 x 2400 pixel model, so availability of 2K displays is really not an issue. And who knows what will be possible 10 years from now in display technology?
Content production is the issue for broadcasters. Most are still struggling with conversion to HD-capable infrastructures and workflows. Only the movie industry is working in 2K and 4K Digital Intermediary (DI). The DI process occurs after film acquisition and digitalization. Content is edited, composited, conformed and effects added in the digital domain. The finished piece is then used to produce release prints on film, or can be presented in digital cinemas. This may represent an opportunity for Hollywood to attain parity with the TV industry by virtue of being the sole content creator capable of providing 2K material for transmission.