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 Harveytjharvey@ieee.org