To MPEG-2, or not too..............................................
The Great Compression Debate
By David Fox
Broadcasting is becoming "an MPEG world," says Al Kovalick, principal architect, video communications division, Hewlett-Packard. MPEG-2 has been accepted as the digital transmission system. But why stop at distribution, when you could also use it for production?
Some manufacturers seem to have accepted that this will happen, although it may take some time for broadcasters to fall in behind, as it will mean replacing all their equipment. But is it the best way forward?
There are various MPEG-2 formats. "It is widely recognised that MPEG-2 4:2:0 is NOT suitable for production due to its lower chroma resolution. However, 4:2:2, in particular the I-frame only structure, may find some limited applications if minimal picture manipulation is required (wipes and dissolves are OK, DVE movements are not)," says Paul Dubery, product marketing manager, Tektronix. MPEG's disadvantages also include "cost (the asymmetric nature of MPEG means a more expensive coding process and some system design constraints), as well as less acceptable jog/shuttle performance and less efficient editing operations."
But, this doesn't stop Kovalick, and many others, predicting that there will be just two formats in future: DV and MPEG.
"MPEG has been accepted for acquisition (SX), contribution (EBU network, ATM, satellite), distribution (EBU, telco, satellite) and transmission (terrestrial, satellite etc.). MPEG-2 4:2:2P@ML was designed for programme production applications with high bit rates and short GOPs [Groups Of Pictures -- how many pictures per MPEG sequence]," says John Ive, director strategic planning, Sony.
Because of family similarities, he believes quality degradation of decoding and encoding can be avoided by staying within the MPEG family throughout the chain. While SX is already MPEG compliant, Sony has also announced plans to develop a half inch MPEG VTR and disk-based storage, and is working on VTRs, hybrid and nonlinear systems, transmission systems and coding/transcoding solutions for MPEG.
"Acquisition needs to record a lot of material to small and reliable tapes (or other media) at excellent quality," says Hannes Heckel, product marketing manager, FAST Multimedia. He believes 4:2:2P, with its long GOPs and IBP structure is perfect for this. Post production needs "high data rates and frame accurate random access without sacrificing the input quality, so 4:2:2P (GOP=1, I-frame only) fits these demands. Transcoding from long GOP 4:2:2P is not too crucial. For distribution, a high quality at very low data rates is required, colour reduction is fine for viewing. Thus, MP (long GOPs, IBP, 4-8 Mbps, 4:2:0) is the perfect format." Even MPEG-2 4:2:0 can be cut, although "that still requires careful handling," says Claude Guillaume, managing director, Thomson Broadcast UK. "Such limitation would imply decoding before processing then recording. Besides the cost, multiple stages of processing would rapidly degrade the picture quality. Of course new technology such as the 'Mole' will allow the cascading of a number of processes but the likely cost would be prohibitive in the short to medium term." However, MPEG-2 4:2:2P allows several processes to be cascaded without serious picture degradation (as shown in RAI tests).
Much more....................................
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