The Effects of Compressed Acquisition on Downstream Video Quality Discovery Communications Discovery Development and Technology Group

Overview File based acquisition Acquisition file formats and compression The post and distribution chain Test procedures Results Conclusions

The HD Acquisition Ecosystem Tape Based SystemsFile Based Systems

Commonalities of file based cameras Use flash memory or hard drives –Flash based media increasing in capacity, decreasing in cost Often use new codec technology –Compression efficiency increasing, new codecs Enable tapeless workflows in post –Support for new workflows and codecs continues to grow

Today’s Compression Schemes in Post and Acquisition Bit Rate (mb/s) Inter /Intra GOP Length Down SamplingChroma Sampling HDCAM SR480IntraINone4:2:2 HDCam140IntraI1440x10803:1:1 DVC Pro100IntraI 1280x :1.3:1.3 J2K (Thomson) 100,75, 50IntraI Variable AVC Intra100, 50IntraI 1440 by 1080 on 50 mbps 4:2:2, 4:2:0 at 50 mbps XDCAM HD 35, 25, 18LGOP x 10804:2:0 HDV25LGOP x 10804:2:0 VC-3 (DnX) 36, 145, 220 IntraI None 4:2:2 Apple Pro Res 145, 220Intra INone 4:2:2

A long chain to the consumer Acquisition Compression NLE / Post Compression Tape Delivery / Compression Network Post Compression Air Master Compression Playout Server Compression Broadcast Compression MSO/ Distributor Compression HDCAM XDCAM DVCPRO AVC Intra JPEG 2000 HDV MPEG Intra DnX Pro Res AVC Intra DVCPRO HDCAM SR HDCAM D5 DVCPRO DnX Pro Res AVC Intra DVCPRO HDCAM SR HDCAM D5 DVCPRO MPEG-2 AVC Intra JPEG 2000 MPEG-2 AVC MPEG-2 AVC

Test Scope Objective: –Discover the effects of different codecs and bit rates in the acquisition, post production, and transmission processes on video quality delivered to the home consumer. Test Devices –Sony F23: Digital Cinema camera, records to HDCAM SR –Panasonic HPX-3000: Broadcast camera, records to P2 cards using AVC Intra or DVCPRO codecs –Thomson Infinity: Broadcast camcorder, records to Rev Pro discs using JPEG 2000 or MPEG-2 I frame compression –P2 Mobile: Panasonic field recorder for P2 Test results are not the final word on these codecs, but a reflection of their implementation in these devices.

Test Procedures – Camera Tests Discovery has a policy of testing cameras before they are allowed to be used on our productions –Studio Tests Standard studio battery with sets designed to challenge cameras –Field Tests Location shoots, often outdoors, with optically challenging material All footage used in these tests was gathered during our regularly scheduled camera testing.

Test Procedures – Post Tests Tests designed to assess the effect of mezzanine level post production compression on various codecs –Materials created in the camera codecs were encoded to the following mezzanine post production codecs: Avid DnX (VC-3) at 145 mb/s Avid DnX (VC-3) at 220 mb/s –Materials were output to both HDCAM and HDCAM SR for transmission testing to determine the impact of additional tape format compression. HDCAM: 145 mb/s, DCT-based compression HDCAM SR: 480 mb/s, MPEG-4 Intra Frame compression

Test overview – Transmission Tests Transmission tests were performed using backup playout and air chains. Four different transmission scenarios were tested: –MPEG-2 transmission with air chain devices –MPEG-2 transmission without air chain –MPEG-4 transmission with air chain devices –MPEG-4 transmission without air chain

MPEG-2 Air Chain

MPEG-4 Air Chain

Reviewing the results Test results were reviewed using three different methods: –Double blind viewer testing using ITU 500 criteria for expert viewing. –PQA Analysis using the Tektronix PQA 500. Tests were run modeling both expert viewers and typical viewers. Tests included PSNR, PQR, and DMOS models. –Split screen comparison of test results and codecs.

Results – Visual Analysis Acquisition Codecs The visual effects of the different acquisition compression formats were apparent at the pre- transmission stage to our expert viewers. –AVC Intra at 100 mb/s had the least noticeable impairments, performing well across all the image tests. –JPEG 2000 at 100 mb/s had similar performance as AVC in low complexity images, but there were noticeable increase in visual impairments in more complex images. –AVC Intra at 50 mb/s performed well in some tests, albeit with a predictable impairment due to reduced bit rate. –JPEG 2000 at 50 mb/s had a high level of visible artifacts with most of our test images.

Results – Visual Analysis Post Production Path The different compression schemes typically made minor differences in the post production chain. - Expert viewer panels could see concatenation effects when applied to the uncompressed source. - Effects were less obvious with AVC and J2K at 100 mb/s. - As bit rates were reduced the effects became more pronounced and easily identified by the test panel.

Results – Visual Analysis Transmission Path MPEG-2 with Air Chain –AVC scored consistently, with the 100 mb/s clips occasionally being confused with the master uncompressed clip by our expert viewers –JPEG 2000 scored far less consistently with both 100 and 50 mb/s clips being scored randomly throughout the range, particularly by the typical viewer panel. The expert viewer panel tended to always score J2K 50 mb/s as having the highest level of visual impairment.

Results – Visual Analysis Transmission Path MPEG-2 without Air Chain –The master clip was clearly recognized by our expert viewers in this test, scoring the highest –AVC Intra 100 and 50 scored comparatively well, and had results slightly below the master clip’s. –JPEG 2000 scored lower, with 100 megabits typically in the middle of the results and 50 megabits almost always at the bottom of the rankings

Results – Visual Analysis Transmission Path MPEG-4 AVC Tests –Both viewer panels had a much harder time judging the MPEG-4 transmission tests, both with and without the air chain. The typical viewer panels were usually able to distinguish differences, but frequently unable to determine what constituted a higher quality image. The expert viewer panel consistently rated the uncompressed master at the top with both AVC and the J2K 100 clips following closely. J2K at 50 mb/s was rated at the bottom of both MPEG-4 tests. –The level of total picture degradation and the softening of the image made it harder to see differences in the picture. The picture softness may be caused by our current AVC decoder.

Results - PQA PQA results were less conclusive. –The PQA typical viewer models produced a variety differences in the results. In simpler sequences, JPEG2000 was often the leader In more complex sequences AVC Intra was often the forerunner. –Expert viewing models demonstrated more predictable results.

Results - PQA For the MPEG-2 and MPEG-4 transmission paths the PQA tended to correlate with the viewer test results. –MPEG-4 results showed little difference between the codec concatenations with or without the air chain devices. –They also showed much lower scores for the sequences overall as compared to the MPEG-2 results. –As previously mentioned, the MPEG-4 decoder may have contributed to these results.

Conclusions Acquisition compression formats are not necessarily damaging to the final image quality when used at higher bit rates. –At the higher bit rates, expert viewers sometimes scored the acquisition codecs equal to or higher than the uncompressed master after MPEG-2 transmission. –Lower bit rate codecs were clearly distinguishable at any stage of codec concatenation throughout the acquisition, post, and transmission paths.

Conclusions Compression applied in the transmission paths can be a “blender” tending to mask initial visual differences. –There were minimal noticeable differences with the air chain devices. –Clear differences between the MPEG-2 and MPEG-4 compression processes in all tests. Visual impairments induced by codec concatenation in the production and transmission process are generally not discernable to the average viewer, and in many cases are statistically irrelevant even to expert viewers at critical viewing distances.

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