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BT.656数据协议.pdf
®
BT.656VideoInterfaceforICs
ApplicationNote
July2002
AN 9728.2
Introduction
When ITU-R BT.601, formally CCIR-601, was developed
(which defined the YCbCr color space, the 4:2:2 YCbCr
sampling organization, and sampling resolutions), it was
soonfollowedbyITU-RBT.656(formallyCCIR-656).
BT.656definedtheparallelandserialinterfacesfortransmit-
ting4:2:2YCbCrdigitalvideobetweenequipmentinstudio
and pro-video applications. Active video resolutions are
either 720 x 486 (525/60 video systems) or 720 x 576
(625/50videosystems).
TheBT.656parallelinterfaceuses8or10bitsofmultiplexed
YCbCrdataanda27MHzclock(ECLlevelsareusedforthe
data and clock signals).Instead of the conventional video
timing signals (HSYNC, VSYNC, and BLANK) also being
transmitted, BT.656 uses unique timing codes embedded
withinthevideostream.Thisreducesthenumberofwires
(andICpins)requiredforaBT.656videointerface.
Ancillarydigitalinformation(suchasaudio,closedcaption-
ing,andteletext)mayalsobetransmittedduringtheblank-
ingintervals.Thiseliminatestheneedforaseparateaudio
interfaceandadditionalcontrolsignals.
Becauseofitscapabilities,aTTLcompatibleversionofthe
parallelinterfaceisnowbeingusedasachip-to-chipinter-
faceforvideoICs.
YCbCrVideoStream
The4:2:2YCbCrdataismultiplexedintoan8-bitor10-bit
stream:Cb 0Y0Cr0Y1Cb2Y2Cr2,etc. Figures 1and 2 illus-
trate the format for 525/60 and 625/50 video systems,
respectively,using8-bitYCbCrdata.
AftereachSAVcode,thestreamofactivedatawordsalways
beginswithaCbsample.Inthemultiplexedsequence,the
co-sitedsamples(thosethatcorrespondtothesamepoint
onthepicture)aregroupedasCb,Y,Cr.
Eachlineofvideoissampledat13.5MHz,generating720
activesamplesof24-bit4:4:4YCbCrdata,asshowninFig-
ures3and4.Thisisconvertedto16-bit4:2:2YCbCrdata,
resultingin720activesamplesofYperline,and360active
sampleseachofCbandCrperline.
The Y data and the CbCr data are multiplexed, and the
13.5MHz sample clock rate is increased by two times to
27MHz.
STARTOFDIGITALLINE
STARTOFDIGITALACTIVELINE
NEXTLINE
HCONTROLSIGNAL
EAVCODE
BLANKING
SAVCODE
CO-SITED
CO-SITED
F
F
0
0
0
0
X
Y
8
0
1
0
8
0
1
0
8
0
1
0
F
F
0
0
0
0
X
Y
C
B
Y C
R
Y C
B
Y C
R
Y
C
R
Y
F
F
4
268
4
1716
1440
DIGITAL
VIDEO
STREAM
FIGURE1. BT.6568-BITPARALLELINTERFACEDATAFORMATFOR525/60VIDEOSYSTEMS
1
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ApplicationNote9728
HCONTROLSIGNAL
STARTOFDIGITALLINE
STARTOFDIGITALACTIVELINE
NEXTLINE
EAVCODE
BLANKING
SAVCODE
CO-SITED
CO-SITED
F
F
0
0
0
0
X
Y
8
0
1
0
8
0
1
0
8
0
1
0
F
F
0
0
0
0
X
Y
C
B
Y C
R
Y C
B
Y C
R
Y
C
R
Y
F
F
4
280
4
1728
1440
DIGITAL
VIDEO
STREAM
FIGURE2. BT.6568-BITPARALLELINTERFACEDATAFORMATFOR625/50VIDEOSYSTEMS
T=1/13.5MHz
16T
50%SYNC
LEVEL
DIGITAL
BLANKING
138T
(720-857)
DIGITALACTIVELINE
720T
(0-719)
TOTALLINE
858T
(0-857)
T=1/13.5MHz
12T
50%SYNC
LEVEL
DIGITAL
BLANKING
144T
(720-863)
DIGITALACTIVELINE
720T
(0-719)
TOTALLINE
864T
(0-863)
FIGURE3. BT.656HORIZONTALTIMINGRELATIONSHIPFOR
FIGURE4. BT.656HORIZONTALTIMINGRELATIONSHIPFOR
525/60VIDEOSYSTEMS
625/50VIDEOSYSTEMS
SAVandEAVTimingCodes
SAV (start of active video) and EAV (end of active video)
codesareembeddedwithintheYCbCrvideostream.They
eliminatetheneedfortheHSYNC,VSYNC,andBLANKtim-
ingsignalsnormallyusedinavideosystem.TheEAVand
SAVsequencesareshowninTable1.
TheXYstatusword,whichalsoindicateswhetheritisan
SAVorEAVsequence,isdefinedas:
F=0forfield1;F=1forfield2
V=1duringverticalblanking
H=0atSAV,H=1atEAV
P3-P0=protectionbits
P3=V ⊕H
P2=F ⊕H
P1=F ⊕V
P0=F ⊕V ⊕H
where ⊕representstheexclusive-ORfunction.Thesepro-
tectionbitsenablesingle-biterrorstobedetectedandcor-
2
rected (and some multiple bit errors detected) at the
receiver.Althoughtheyshouldalwaysbegeneratedinvideo
ICinterconnectapplications,theyareusuallyignoredbythe
receivingIC.
BT.656usestheBT.601-definedverticalblankingintervals,
asshowninFigures5and6.However,notethatactivereso-
lutionsotherthan720x486and720x576maybesup-
ported(effectivelycroppingtheimage)byadjustingwhere
the EAV and SAV codes and vertical blanking intervals
occur.
SquarePixelVariation
A variation using square pixels has been developed,
althoughnotendorsedbytheBT.656standard.Insteadofa
27MHz clock, a 24.54MHz clock is usedfor 525/60 video
systems(640x480activeresolution),anda29.5MHzclockis
usedfor625/50videosystems(768x576activeresolution).
Figures7and8illustratethedataformat,Figures9and10
illustratethehorizontaltimingrelationships,andFigures11
and12showtheverticalblankingintervals.
ApplicationNote9728
TABLE1. BT.656EAVANDSAVSEQUENCE
D9
(MSB)
1
0
0
1
D8
1
0
0
F
Preamble
StatusWord
8-BITDATA
10-BITDATA
D7
1
0
0
V
D6
1
0
0
H
D5
D4
D3
D2
D1
D0
1
0
0
1
0
0
1
0
0
1
0
0
P3
P2
P1
P0
1
0
0
0
1
0
0
0
BLANKING
FIELD1
ACTIVEVIDEO
BLANKING
FIELD2
ACTIVEVIDEO
LINE1(V=1)
LINE21(V=0)
LINE264(V=1)
LINE283(V=0)
LINE525(V=0)
LINE
NUMBER
1-3
4-20
21-263
264-265
266-282
283-525
F
1
0
0
0
1
1
V
1
1
0
1
1
0
H
H
(EAV)
(SAV)
1
1
1
1
1
1
0
0
0
0
0
0
H=1
EAV
H=0
SAV
FIGURE5. TYPICALBT.656VERTICALBLANKINGINTERVALSFOR525/60VIDEOSYSTEMS
LINE4
FIELD1
(F=0)
ODD
LINE266
FIELD2
(F=1)
EVEN
LINE3
LINE1
FIELD1
(F=0)
ODD
BLANKING
FIELD1
ACTIVEVIDEO
LINE313
BLANKING
FIELD2
ACTIVEVIDEO
BLANKING
FIELD2
(F=1)
EVEN
LINE625
H=1
EAV
H=0
SAV
LINE1(V=1)
LINE23(V=0)
LINE311(V=1)
LINE336(V=0)
LINE624(V=1)
LINE625(V=1)
LINE
NUMBER
1-22
23-310
311-312
313-335
336-623
624-625
F
0
0
0
1
1
1
V
1
0
1
1
0
1
H
H
(EAV)
(SAV)
1
1
1
1
1
1
0
0
0
0
0
0
FIGURE6. TYPICALBT.656VERTICALBLANKINGINTERVALSFOR625/50VIDEOSYSTEMS
3
ApplicationNote9728
HCONTROLSIGNAL
STARTOFDIGITALLINE
STARTOFDIGITALACTIVELINE
NEXTLINE
EAVCODE
BLANKING
SAVCODE
CO-SITED
CO-SITED
F
F
0
0
0
0
X
Y
8
0
1
0
8
0
1
0
8
0
1
0
F
F
0
0
0
0
X
Y
C
B
Y C
R
Y C
B
Y C
R
Y
C
R
Y
F
F
4
272
4
1560
1280
DIGITAL
VIDEO
STREAM
FIGURE7. 8-BITDATAFORMATFORSQUAREPIXEL525/60VIDEOSYSTEMS
STARTOFDIGITALLINE
STARTOFDIGITALACTIVELINE
NEXTLINE
HCONTROLSIGNAL
EAVCODE
BLANKING
SAVCODE
CO-SITED
CO-SITED
F
F
0
0
0
0
X
Y
8
0
1
0
8
0
1
0
8
0
1
0
F
F
0
0
0
0
X
Y
C
B
Y C
R
Y C
B
Y C
R
Y
C
R
Y
F
F
4
344
4
1888
1536
DIGITAL
VIDEO
STREAM
FIGURE8. 8-BITDATAFORMATFORSQUAREPIXEL625/50VIDEOSYSTEMS
T=1/12.27MHz
23T
50%SYNC
LEVEL
DIGITAL
BLANKING
140T
(640-779)
DIGITALACTIVELINE
640T
(0-639)
TOTALLINE
780T
(0-779)
T=1/14.75MHz
23T
50%SYNC
LEVEL
DIGITAL
BLANKING
176T
(768-943)
DIGITALACTIVELINE
768T
(0-767)
TOTALLINE
944T
(0-943)
FIGURE9. HORIZONTALTIMINGRELATIONSHIPFORSQUARE
FIGURE10. HORIZONTALTIMINGRELATIONSHIPFORSQUARE
PIXEL525/60VIDEOSYSTEMS
PIXEL625/50VIDEOSYSTEMS
4
ApplicationNote9728
BLANKING
FIELD1
ACTIVEVIDEO
BLANKING
FIELD2
ACTIVEVIDEO
LINE1(V=1)
LINE23(V=0)
LINE263(V=1)
LINE286(V=0)
LINE525(V=0)
LINE
NUMBER
1-3
4-22
23-262
263-265
266-285
286-525
F
1
0
0
0
1
1
V
1
1
0
1
1
0
H
H
(EAV)
(SAV)
1
1
1
1
1
1
0
0
0
0
0
0
H=1
EAV
H=0
SAV
FIGURE11. TYPICALVERTICALBLANKINGINTERVALSFORSQUAREPIXEL525/60VIDEOSYSTEMS
LINE1(V=1)
LINE23(V=0)
LINE311(V=1)
LINE336(V=0)
LINE624(V=1)
LINE625(V=1)
LINE
NUMBER
1-22
23-310
311-312
313-335
336-623
624-625
F
0
0
0
1
1
1
V
1
0
1
1
0
1
H
H
(EAV)
(SAV)
1
1
1
1
1
1
0
0
0
0
0
0
LINE4
FIELD1
(F=0)
ODD
LINE266
FIELD2
(F=1)
EVEN
LINE3
LINE1
FIELD1
(F=0)
ODD
BLANKING
FIELD1
ACTIVEVIDEO
LINE313
BLANKING
FIELD2
ACTIVEVIDEO
BLANKING
FIELD2
(F=1)
EVEN
LINE625
H=1
EAV
H=0
SAV
FIGURE12. TYPICALVERTICALBLANKINGINTERVALSFORSQUAREPIXEL625/50VIDEOSYSTEMS
AncillaryData
Ancillarydatapacketsareusedtotransmitinformationsuch
as digital audio, captioning information, and teletext data.
BT.656discussesancillarydataonlyinverygeneralterms;
theSMPTE291Mstandardgoesintomuchmoredetail.
On scan lines containing active video data, ancillary data
maybetransmittedinthetimebetweentheEAVsequence
andSAVsequence(horizontalblanking).Onblankedscan
linesduringtheverticalblankingintervals,ancillarydatamay
betransmittedatanytime.
Therearetwotypesofancillarydataformats,asshownin
Tables2and3.Ingeneral,Type1(anolderformat)usesa
5
single data ID word to indicate the type of ancillary data,
Type2(anewerformat)usestwowordsforthedataID.
DataID(DID)
TheDataIDwordindicatesthetypeofdatabeingsent.The
assignmentofmostoftheDataIDvaluesiscontrolledbythe
ITUandSMPTEtoensureequipmentcompatibility.AfewID
valuesareavailableforuserapplicationsthatdon’trequire
registration. Video ICs for the PC multimedia market use
theseuser-applicationIDcodestoavoidcontentionwithpro-
fessionalvideoequipment.
ApplicationNote9728
TABLE2. TYPE1ANCILLARYDATAPACKETFORMAT
D9(MSB)
0
1
1
PAR
PAR
PAR
D8
0
1
1
PAR
PAR
PAR
Preamble
DataID
DataBlockNumber
DataWordCount
UserDataWord0
:
UserDataWordN
D7
0
1
1
8-BITDATA
D6
0
D5
0
1
1
1
1
D4
0
1
1
D3
0
1
1
D2
0
1
1
10-BITDATA
D0
D1
0
0
1
1
1
1
Valueof00000000to11111111
Valueof00000000to11111111
Valueof00000000to11111111
Valueof0000000100to1111111011
:
Valueof0000000100to1111111011
CheckSum
PAR
SumofD0–D8ofdataIDthroughlastuserdataword.Presettoallzeros;carryisignored.
NOTES:
1. PAR=evenparityforD0–D7.
2. D8=invertedvalueofD8bit.
3. 8-bitdataistransferredusingtheD2–D9bits
TABLE3. TYPE2ANCILLARYDATAPACKETFORMAT
D8
0
1
1
PAR
PAR
PAR
Preamble
DataID
SecondaryID
D9(MSB)
0
1
1
PAR
PAR
DataWordCount
PAR
UserDataWord0
:
UserDataWordN
D7
0
1
1
8-BITDATA
D6
0
D5
0
1
1
1
1
D4
0
1
1
D3
0
1
1
D2
0
1
1
10-BITDATA
D0
D1
0
0
1
1
1
1
Valueof00000000to11111111
Valueof00000000to11111111
Valueof00000000to11111111
Valueof0000000100to1111111011
:
Valueof0000000100to1111111011
CheckSum
PAR
SumofD0–D8ofdataIDthroughlastuserdataword.Presettoallzeros;carryisignored.
NOTES:
4. PAR=evenparityforD0–D7.
5. 8-bitdataistransferredusingtheD2–D9bits.
6
ApplicationNote9728
SecondaryID(SDID):Type2FormatOnly
TheSecondaryDataIDwordisalsopartofthedataIDfor
Type2ancillaryformats.TheassignmentofmostoftheSec-
ondary Data ID values is also controlled by the ITU and
SMPTEtoensureequipmentcompatibility.AfewSDIDval-
uesareavailableforuserapplicationsthatdon’trequirereg-
istration.VideoICsforthePCmultimediamarketusethese
user-applicationSDIDcodestoavoidcontentionwithprofes-
sionalvideoequipment.
DataBlockNumber(DBN):Type1FormatOnly
TheDataBlockNumberwordisusedtoallowmultipleancil-
lary packets (sharing the same data ID) to be put back
together at the receiver. This is the case when there are
morethan255userdatawordsrequiredtobetransmitted,
thus requiring morethan one ancillarypacket to be used.
TheDataBlockNumbervalueincrementsbyoneforeach
consecutiveancillarypacket.
DataWordCount(DC)
The Data Word Count word specifies the number of user
datawordsinthepacket.
UserDataWords(UDW)
Upto255userdatawordsmaybepresentinthepacket.8-
bitvaluesof00 HandFF Harenotallowed;10-bitvaluesof
000H-003Hand3FC H-3FFHarenotallowed.
DataLimits
YCbCr and ancillary datamaynot usethe 8-bitvaluesof
00HandFF Hsincethosevaluesareusedfortiminginforma-
tion.For10-bitsystems,the10-bitvalues000
3FCH-3FFHmaynotbeusedtoavoidcontentionwith8-bit
systems.
H-003Hand
Duringblankingintervals,unlessancillarydataispresent,Y
values must be set to 10
CbCrvaluessetto80
H (040 H if a 10-bit system) and
H(200 Hifa10-bitsystem).
ImplementationConsiderations
VideoICReceivers
OnlytheEAVandSAVsequencesshouldbeusedtorecover
thevideotiming.Assumptionsshouldnotbemadeaboutthe
numberofclockcyclesperlineorhorizontalblankinginter-
val.Otherwise,theimplementationmaynotworkwithreal-
worldvideosignalsandproposedvariationssuchasscaled
video.
Someoldervideosourcesalsoindicatesynctimingbyhav-
ingYdatabeavalueoflessthan16.However,newvideo
ICshavetheoptionofoutputtingdigitaldata(suchasclosed
captioning and teletext) during the blanking intervals. In
addition, to allow real-world video and test signals to be
passed through with minimum disruption, many also now
allowtheYdatatohaveavaluelessthan16duringactive
video.Thus,receiverdesignsassumingsynctimingwillbe
present on the Y channel will no longer work with these
newerICs.
Formaximumcompatibilitywhenprocessingancillarydata,
oncetheDataWordCountvalue(N)isread,itshoulddis-
ablethePreambledetectioncircuitryforN+1words.Some
proposed variations allow the User Datato have the8-bit
valuesof00 HandFF Hand10-bitvaluesof000
3FCH-3FFH.
H-003Hand
VideoICSources
Toensuremaximumcompatibility,activevideodatashould
betransmittedasacontiguousstreamofdata,withavalid
datawordeachclockcycle.Ifthereareanyvariationsinthe
linelength,thehorizontaldigitalblankinginterval(theinter-
valbetweentheEAVandSAVsequences)shouldbeshort-
enedorlengthenedtoreflectthetruelinetiming.
Summary
ThisApplicationNotepresentedsomeofthecapabilitiesof
theBT.656interfaceasappliedforICs.
UsingNTSCandPALdecoders(suchastheHMP8116)and
encoders(suchastheHMP8171andHMP8173)thatsup-
port the BT.656 interface can simplify system design and
lowercost,whileallowingnewcapabilities.
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7
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