Archiving Legacy Composite Recordings Made On Component VTRs
Terry HarveySouthern 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
QuadratureDemodulator
CompressedTime Division
Multiplex (B-Y) and (R-Y)
(B-Y)
(R-Y)
Compensating Delay
Y
½” Tape
CompositeVideoInput
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, 20dB @ 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 QuadratureDemodulator
To Luma CompensatingDelay
CompositeNTSCInput(sampled@ 4fscor 27MHzdepending on manufacturer)
1/2
1/2
1/4
1/4
1/4
1/4
Non- Adaptive Decoder Comb Filter ResponseThe 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!
CompositeInput
CompositeInterface
PBS MOC (Media Operations Center)
PBS Ingest
SDI(component
video)
A Typical Program Supply Scenario
Tape delivered to PBS
BVW-2800 Digital Betacam
Digital Betacam
Producer’s Betacam: Improperly decoded composite video
PBSTrans-missionVideoServer
Flattened Fileswith unwanted“trapped Chroma in Luma
MPEG-2Encoder
MPEG-2Encoder
MPEG-2Encoder
MPEG-2Encoder
S
tatis
tical
Mul
tiple
x
PBS MCPC9 SD programs in mux
41 Mb/s to Satellitetransponder
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.
ComponentTrapped CompositeArtifacts
Trapped CompositeArtifacts
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.
CompositeInput
PBS MOC (Media Operations Center)
PBS Ingest
SDI(component
video)
Tape delivered to PBS
BVW-2800
Digital Betacam
Digital Betacam
Producer’s Betacam has cleaner component video
Comb-FilterDecoder
AnalogComposite
SDI ComponentInterface
Method toReduce Artifacts
Note using a separate comb filter decoder after Betacam machine reduces encoder bit rate.
Decoding through comb-filter decodermade 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: http://www.extron.com/company/article.aspx?id=ntscdb4
Sony BVW 75 Maintenance Manual
Terry [email protected]