Archiving Legacy Composite Recordings Made On Component VTRs Terry Harvey Southern Illinois University WSIU-TV (PBS)

The Problem Most broadcasters assume that the best way to archive component VTR recordings is to use component video interface. Older recordings were generally made via composite input. Hence, component interface playback will contain many composite artifacts.

Component VTR in a Composite Facility Component VTR originally designed as a “drop in” replacement for composite VTR. Designed for minimum quality loss in multi-generation composite record/playback. To ensure low composite video tape generation loss, the NTSC decoder in Component VTRs does not fully separate luma and chroma.

Lo-Pass Filter Hi-Pass Filter Quadrature Demodulator Compressed Time Division Multiplex (B-Y) and (R-Y) (B-Y) (R-Y) Compensating Delay Y ½” Tape Composite Video Input Component VTR- Composite Recording -No Comb-Filter Separation -Less loss with Composite Codec Cycles -Does not “cleanly” separate Luma/Chroma

Remember the Capstan Switch? 2F/4F (color field line up switch) NTSC has 4 field video sequence. When recording via composite input, 4F position ensures that upon playback, trapped chroma in luma track will recombine properly with trapped luma in chroma track. “Russian Roulette Effect” Composite recording with capstan 2F setting: if recombination is 2 fields out, 3 MHz notch on composite output, results in resolution loss. When both recording and playback of component VTR is component only, 2F position is used as there is no NTSC color frame.

2 Line Digital Comb Filter One Line Delay One Line Delay + _ To Quadrature Demodulator To Luma Compensating Delay Composite NTSC Input 4fsc or 27MHz depending on manufacturer) 1/2 1/4

Non- Adaptive Decoder Comb Filter Response The Benefit Interleaved Chroma is “more” fully removed from Luma. Luma has less Chroma artifacts. The Downside Vertical and diagonal resolution is lost. Vertical color transitions have “averaged” color. Each codec cycle, picture drops one line.

Interfacing Betacam Up to and thru 1990’s, studio interfacing to component VTRs was via composite interface Later dubs to Digital Betacam or to non-linear editor systems via component interface PBS ingested submitted Digital Betacam via SDI (component) interface: Chroma artifacts on tape were not removed from Luma! Trapped composite artifacts reduce MPEG2 encoding efficiency!

Composite Input Composite Interface PBS MOC (Media Operations Center) PBS Ingest SDI (component video ) A Typical Program Supply Scenario Tape delivered to PBS BVW-2800 Digital Betacam Producer’s Betacam: Improperly decoded composite video

PBS Trans- mission Video Server Flattened Files with unwanted “trapped Chroma in Luma MPEG-2 Encoder MPEG-2 Encoder MPEG-2 Encoder MPEG-2 Encoder Statistical Multiplex PBS MCPC 9 SD programs in mux 41 Mb/s to Satellite transponder A test using PBS NOC Playout in 2007 Video ingested with trapped luma/chroma artifacts will affect MPEG-2 encoding efficiency: video with higher level of artifacts will be encode with more difficulty. Use of PBS NOC statistical multiplex to test and compare relative compression efficiency.

Test: Comparison of Composite vs Component Recording with Composite Playback Two component VTR recordings: one composite interfaced second component interfaced sent by PBS thru satellite statistical multiplex. Stat. Mux. To demonstrate reduced efficiency of MPEG 2 encoding: composite artifacts will raise video entropy hence require more bandwidth to encode.

Test Methodology One hour test over PBS MCPC MPEG-2 Transport Stream. First ½ hour:  Program 14 carried Component Video and Program 15 carried Composite Video. Second ½ hour:  Program 15 carried Component Video and Program 14 carried Composite Video Note disparity between bitrates.

Component Trapped Composite Artifacts

Trapped Composite Artifacts Component Note: Second ½ Hour Feeds Reversed

How to Remedy Composite Artifact Problem Legacy composite interfaced recordings can be made good:  Use composite output with Capstan Switch set to 4F for playback.  Use good quality comb-filter NTSC decoder. Comb-Filter decoder will filter composite artifacts and ensure high frequency luma for picture fine detail is restored.

Composite Input PBS MOC (Media Operations Center) PBS Ingest SDI (component video ) Tape delivered to PBS BVW-2800 Digital Betacam Producer’s Betacam has cleaner component video Comb-Filter Decoder Analog Composite SDI Component Interface Method to Reduce Artifacts

Note using a separate comb filter decoder after Betacam machine reduces encoder bit rate.

Decoding through comb-filter decoder made bit rates roughly equal Comb Filter Decoder Original Component Recording COMPARE

Summary-Conclusion Component VTRs do not employ comb-filter decoders. Component VTR decoders are designed to minimize composite cascading losses. Component interfaced playback of composite interfaced recording will lead to composite artifact noise which reduces subsequent MPEG encoding efficiency and loss of luma high freq. above 3 MHz. Inserting a comb-filter decoder between machines reduces artifacts recovers lost luma frequencies resulting in a sharper picture.

References Digital Television: Edited by C.P. Sandbank. John Wiley and Sons 1990 The Art of Digital Video: John Watkinson. Focal Press 1990 Extron Electronics website: Sony BVW 75 Maintenance Manual

Terry Harvey