Composite

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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.

Composite
Video
Input
Compensating
Delay
Y
Lo-Pass Filter
Quadrature
Demodulator
(B-Y)
Compressed
Time Division
Multiplex
(R-Y)
(B-Y) and (R-Y)
Hi-Pass Filter
Component VTRComposite Recording
-No Comb-Filter Separation
-Less loss with Composite Codec Cycles
-Does not “cleanly” separate Luma/Chroma
½” Tape
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
Composite
NTSC
Input
(sampled
@ 4fsc
or 27MHz
depending on
manufacturer)
One Line
Delay
One Line
Delay
1/2
1/4
+
To Luma
Compensating
Delay
1/4
1/2
1/4
_
1/4
To Quadrature
Demodulator
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
Interface
BVW-2800
Composite
Input
A Typical Program
Supply Scenario
Digital Betacam
Producer’s Betacam:
Improperly decoded
composite video
Tape delivered
to PBS
Digital Betacam
PBS
Ingest
SDI
(component
video)
PBS MOC (Media Operations Center)
PBS
Transmission
Video
Server
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.
MPEG-2
Encoder
MPEG-2
Encoder
MPEG-2
Encoder
MPEG-2
Encoder
Statistical Multiplex
Flattened Files
with unwanted
“trapped Chroma
in Luma
41 Mb/s to
Satellite
transponder
PBS
MCPC
9 SD
programs
in mux
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
Note: Second ½ Hour Feeds Reversed
Trapped Composite
Artifacts
Component
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.
SDI Component
Interface
Method to
Reduce Artifacts
Digital Betacam
Producer’s Betacam
has cleaner
component video
Comb-Filter
Decoder
Analog
Composite
BVW-2800
Composite
Input
Tape delivered
to PBS
Digital Betacam
PBS
Ingest
SDI
(component
video)
PBS MOC (Media Operations Center)
Note using a separate comb filter decoder after
Betacam machine reduces encoder bit rate.
COMPARE
Comb Filter Decoder
Original Component Recording
Decoding through comb-filter decoder
made bit rates roughly equal
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 Harvey
[email protected]
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