The MPEG standard

By definition, MPEG samples have no more and no less than 8-bits uniform sample precision (256 quantization levels). For luminance (which is unsigned) data, black corresponds to level 0, white is level 255. However, in CCIR-601 recommendation chromaticy, levels 0 through 14 and 236 through 255 are reserved for blanking signal excursions. MPEG currently has no such clipped excursion restrictions.

What is the best compression ratio for MPEG ?

The MPEG sweet spot is about 1.2 bits/pel Intra and .35 bits/pel inter. Experimentation has shown that intra frame coding with the familiar DCT-Quantization-Entropy hybrid algorithm achieves optimal performance at about an average of 1.2 bits/sample or about 6:1 compression ratio. Below this point, artifacts become noticeable.

What about MPEG artifacts ?

If the encoder did its job properly, and the user specified a proper balance between sample rate and bitrate, there shouldn't be any visible artifacts. However, in sub-optimal systems, you can look for:

• Gibbs phenomenon/Ringing/Aliasing : Too few AC bits, not enough pre-filtering
• Blockiness : Not considering your neighbors before quantizing
• Posterization : Too few DC bits
• Checkerboards : DCT eigenimages as a result of too few AC coefficients
• Color bleeding : Not considering color in encoder cost model

Are there any real time MPEG encoders ?

• 3DO announced its real-time MPEG encoding system which requires an Apple(R) Macintosh Quadra(R) running Apple Macintosh System 7.1 or later, with at least three unused NuBus expansion slots and a minimum of 8MB of RAM; an RGB monitor; and a 3DO International Station for MPEG playback.

• The C-Cube CL-4000 is a single-chip, real-time encoder that can process true MPEG-1 SIF rate video.
  A single chip is needed for +/- 15 pel motion estimation at SIF rates (352x240x30 Hz), two chips for +/- 32 pel at SIF rates (hierarchical), 5 or 6 chips for MPEG-2 at CCIR-601 rates (704 x 480 x 30 Hz). Highly microcoded architecture. Can code both H.261 and JPEG. Implements high picture quality microcode programs. (more details from CICC'93 and HotChips '93 conference)

• DiviCom announced that it has received a multi-million dollar order from Bell Atlantic for MPEG-2 encoders. DiviCom had the first public demo of their real-time MPEG-2 encoder, system multiplexer, and fully MPEG-2 compliant transport stream to ATM interfaces at the World Media Expo'94 in LA. DiviCom's DMC-2 program encoders compress a single video channel and one or more audio channels, multiplexing them into a fully MPEG-2 compliant transport bit stream.

• IBM and SGS-Thomson plan to introduce more hard-wired, multi-chip solutions in 1994. Boards based on a variation of SGS-Thomson 3400 MPEG decoder are being used with the DirectTV (150 channel) MPEG+ decoders, which are running at CCIR resolution (704x240x60 fields/sec).

• AT&T has had a real time MPEG-2 broadcast quality encoder in production since December 1993. It produces MPEG-2 compliant video bit streams (unlike others who claim MPEG-2 but are actually MPEG-1 or non-standard 1+).
  The video is multiplexed together with compressed audio into a MPEG-2 compliant transport stream. It's features include:
  • resolutions up to 704x480
  • adaptive field frame motion estimation
  • adaptive field frame DCT
  • film is coded at 24 fps using automatic 3:2 pulldown detection
  It was shown in demonstrations of real time encoding and decoding at the Western Cable Show at Anaheim 1n 1993, at NAB in Las Vegas in March 1994, and at NCTA in New Orleans in May 1994.
  The encoder handles bit rates up to 15 Mbps. The demonstration at NCTA was showing broadcast quality at 3 and 5 Mbps. The decoder demonstrated at the same shows used the AT&T MPEG-2 decoder chip which has also been available since December 1993. The chip is available in a PC board product.

• FutureTel manufactures and sells a real-time MPEG encoder listed at about $15,000. It is a high quality unit called PrimeView which handles 30 frames/sec with NTSC, PAL , or SVideo inputs. It has been used successfully with several other manufacturers decoders. FutureTel also makes a T1/BRI/PRI (ISDN) communications card to allow you to transport the data in real time across the public (digital) phone network.

• The Image Computing Systems Laboratory of the University of Washington has a real-time MPEG-1 decoder and encoder, both video and audio (layer 2), based on a single Texas Instruments DSP (MVP TMS320C80).
  It is described in :

  	Lee, W., Golston, J., Gove, R. J., and Kim, Y.,
          "Real-time MPEG video codec on a single-chip multiprocessor,"
          Digital Video Compression on Personal Computers: Algorithms and
          Technologies,
          Vol. 2187, pp. 32-42, 1994.
  
  	Lee, W., Gove, R. J., Read, C. J., and Kim, Y.,
          "MediaStation 5000: Integrating video and audio,"
          IEEE Multimedia,
          Vol. 1, No. 2, pp. 50-61, 1994.
  

  The MPEG-2 codec is being worked on and they are licensing the technology (source code) to those interested.

• Other companies producing encoders are :

  Compression Labs (San Jose, CA)
  Minerva Systems (Santa Clara, CA)
  National Transcom (England)
  Optibase (Dallas, TX)
  Optivision (Davis, CA)
  Scientific-Atlanta (Norcross, GA)
  The Network Connection (Atlanta, GA)
  

  Costs will range from $10,000 up through six figures. These companies' current products are MPEG-1 and MPEG-1+ real-time encoders (with National Transcom possibly also having a real-time MPEG-2 encoder). Many of the products should be readily upgradeable to MPEG-2 when the chips become available.

  Whether a given product yields broadcast-quality video depends on (among other things) the bit rate of the compressed video, the quality and nature of the source material, pre- and post-processing, and, ultimately, the eye of the beholder.