What is the "4:2:2" Profile in MPEG-2 Compression Heirarchy?
By Bob Paulson (SMPTE Life Fellow and Long-time Technology Committees member)
As the wide-ranging technopolitical marketing presentations drew to an end at the January Meeting on New Digital Tape Formats, one statement in the Sony presentation prompted Wilson Chao (owner of Cambridge TV Productions) to ask (approximately), "Do I understand that Sony has a proprietary MPEG compression scheme?" Sony's Nick DiLello's answer was essentially "No, but yes."
I stood to explain the "No, but -- " part of that answer, from the perspective of a member of the SMPTE Technology Committee which has created the 4:2:2 profile recommended standard document. But Section Chair Phil Ozek closed the lengthy meeting before I could utter Word One. Here is my answer, more detailed than I was going to give at the meeting. It has been hastily constructed from memory and submitted immediately via e-mail, without taking time to check spelling or details and times against SMPTE Committee meeting records. Dont pillory me, therefore, if some of the recollections can be contradicted later by fact!
Sony has developed a compression scheme for its Betacam SX digital signal processing systems which uses an I-B-I-B-I---- sequence of compressed frames. It does not use P frames. Per the emerging MPEG standard, editing can be done on I (Intra-coded) frames; B Bidirectionally coded) frames remove temporal redundancy in the compressed frame sequence between I frames; P (Predictivly coded) frames. B and P frames are not required by MPEG rules to be editable. Sony has developed a proprietary technique which allows editing on both I and B frames (ie, each frame in the 10:1 compressed video sequence).
Ergo, "No." -- editing on I frames in their 4:2:2 Profile compression scheme is not proprietary. "Yes," the ability to edit on ANY frame of an SX recording on Sony Betacam SX decks is proprietary. That tape can be edited on other future competitive equipment manufactured to Sony's drawings, but presumably only on I frames, unless the manufacturer licenses Sony's proprietary technology.
Those of you that are already MPEG compression experts understand what I have just written. If you don't, perhaps the following tutorial will help. In any event, petition the NE Section Managers to schedule a Spring session on digital video and audio compression.
MPEG (Moving Picture Experts Group, separate from "JPEG" - Joint Photographic (still picture) Experts Group) is an international standards setting organization. Its mission is to establish a group of global public domain standards for compressing "television" (video plus multiple audio channels) signals in a very sophisticated and expensive encoder. The input signals accommodated range from miniature images in a voice line teleconferencing system with a single voice channel, to wide-screen HD TV with five-channel surround sound, with various computer resolution, NTSC and PAL images and stereo sound in between.
There are four MPEG-2 ranges, called "Levels," with related Input and Output levels and constraints. Input image resolutions, expressed in pixels, are High (1920 x 1152 or less), High-1440 (1440 x 1152 or less), Main (720 x 576 or less), Low (352 x 288 or less). Output data rates are up to 80, 60, 15 and 4 Mbits/sec, respectively. NTSC and PAL (SD) video are Main Level; HD video could be either High or High-1440.
MPEG-2 also specifies five ranges of video signal processing sophistication, call "Profiles" -- Simple, Main, SNR, Spatial and High. Their features can be readily explained in another several thousand words! Briefly, for transmission (home delivery) of standard 525/59.94 and 625/50 video signals, a Main Profile/Main Level (MPML) specification has been frozen. This supports digital video transmission at bit rates from approximately 2 through 15 Mbits/sec, compatible with capabilities of existing terrestrial broadcasting, cable and satellite channels.
Remember, MPML specifies a range of techniques for compressing these signals in an expensive transmitter encoder, and transmitting them embedded with appropriate technical information to allow their decoding for picture display and sound reproduction in an inexpensive receiver decoder. This is known as an assymmetrical compression scheme.
(Those of you at the meeting will recall that Panasonic's Steve Mahrer called the DVCPRO system's compression scheme "Symmetrical" - the capabilities and costs of the encoder and decoder are the same.)
MPML compression reduces a full bandwidth digital component video signal (270 Mbits/sec) to a nominal 1.5 to 15 Mbits/sec. Uncompressing this signal at a receiver, editing it or inserting local video or audio, and then recompressing the resulting signal for retransmission, results in a picture and sound quality disaster. Therefore, MPEG asked SMPTE about two years ago to recommend a "Professional Profile" for compressing digital component Main Level pictures, so that they could be edited after first compression without objectionable quality degradation.
The result was a "4:2:2 Profile" recommendation, with a maximum data rate of 50 Mbits/sec. "Professional" was rejected as a descriptor by SMPTE, because it has as many definitions as "beauty" (it's in the eyes of the beholders).
Tektronix took the lead in setting up a variable compression test system and making evaluation tapes. The tapes were played for attendees at the February 1995 SMPTE Advanced Imaging Conference in San Francisco, in short sessions that were repeated regularly during the two days of papers sessions. They included picture sequences with compressions ranging downward from none to 50 to 30 to 20 to MPML Mbits/sec. Both first and eighth generation recordings were included.
The viewers' subjective opinions were evaluated by the test systems committee, and forwarded to MPEG after the meeting as a recommendation to add the "4:2:2 Profile/Main Level" standard to the MPEG document.
Ratification of this recommendation has been underway ever since, evidently without technical or political challenge. At the recently concluded December series of SMPTE Technical Committees meetings at Sony San Jose, further controlled viewing and balloting sessions were held, in conformance to ITU requirements for generating data on viewers' judgments of picture quality.
All SMPTE members should be aware of and proud of the Society's internationally supported and accepted role in creating this new MPEG Compression option. Its existence means that production houses and freelancers have a standard for producing compressed video edited master tapes, on modestly prioced equipment, which can be edited further without quality degradation.
For answers to questions, call, fax or e-mail Consultant Bob Paulson at OmniMedia Communication, Phone/Fax 508-366-4694, firstname.lastname@example.org.