BECKHOFF DK9222-0213-0063 User Manual

XFC

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Application Note DK9222-0213-0063

Oversampling

SMPTE
timecode
EL3702
entertainment
stage
show
light
technology
light control
decoding
time information
synchronising
LED

Reading SMPTE timecode information

This application example provides basics on the SMPTE timecode and contains a TwinCAT sample code for

Keywords

extracting the time information sent by a SMPTE master. Using an EL3702 EtherCAT oversampling terminal

it is possible to “listen” to the analog signal the SMPTE master broadcasts to the slaves. For the sample

code see DK9222-0213-0063_TwinCAT_sample.zip.

Preamble

Please keep in mind that in this document the term frame is used in a double meaning:

– Part or unit of a protocol.

– Snippet or single picture as unit of a movie.

What is SMPTE-Timecode?

Defined by the Society of Motion Picture and Television Engineers, the SMPTE timecode is a standard to provide an absolute

time reference for media metadata. Mainly it is used to synchronise different layers in media productions. A SMPTE frame

consists of 80 bits and is always broadcast from a SMPTE master to the slaves. Via an analog signal (±1…10 V), the SMPTE

timecode provides time information in a binary coded decimal identification on an absolute 24-hour time base (hh:mm:ss:ff).

Depending on the signal coding, there are different rates of frames per second (fps); the most common values are 24, 25, and

30 fps.

SMPTE master

hh:mm:ss:ff

Broadcast

Audio

h:mm:ss:

Video

h:mm:ss:

Automation

hh:mm:ss:ff

Other

h:mm:ss:

Fig. 1 SMPTE timecode: Broadcasting an absolute time reference to the slaves

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Application Note DK9222-0213-0063

Oversampling

Protocol details | Duration of a single bit

The first step is to calculate the duration of a single bit in relation to the frame rate. Because of the different picture frame

rates (24, 25, 30), the duration of a single bit differs and is calculated as follows:

XFC

Frames per second

30

25

24

Table 1 Duration of one bit

Duration of one bit Duration of a frame

4162⁄3 µs

500 µs

520 µs

80 x 4162⁄3 µs = 331⁄3 ms

80 x 500 µs = 40 ms

80 x 520 µs = 412⁄3 ms

Protocol details | High or low bit

The second step is to count the transitions within this duration. The bits are encoded as biphase: A „0“ bit has a single

transition at the start of the bit period. A „1“ bit has two transitions, at the beginning and middle of the period. So by counting

the transitions within a defined period of time, it can be determined whether the bit is high or low. When the signal is stable

during 417 µs at a rate of 30 fps, it is 0. When there are two constant signals of ~208 µs detected, it is 1.

417 µs 417 µs

“Zero“ “One“

Fig. 2 Sample at 30 fps

208 µs

transition

Protocol details | Frame setup

Aa a timecode frame always consists of 80 bits, it is 10 byte long. To mark the ending of a frame, the last two bytes follow a

defined sequence called SyncWord: 0011.1111.1111.1101. In table 2 you can see the setup of a complete timecode frame. In

the first row an overview of the time segmentation is given. To enable a fast overview of time-relevant information, the bits

containing other information are marked „x“.

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Application Note DK9222-0213-0063

Oversampling

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Frame
units

Bit 0

Bit 1

Bit 2

Bit 3

Bit 4

Bit 5

Bit 6

Bit 7

Table 2 Binary coded time information in the SMPTE frame. Bits containing other information than timecode are marked x.

Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 Byte 6 Byte 7 Byte 8 Byte 9 Byte 10

Frame
tens

1

2

4

8

x

x

x

x

Second
units

10

20

x

x

x

x

x

x

Second
tens

1

2

4

8

x

x

x

x

10

20

40

Minute
units

x

x

x

x

x

Minute
tens

1

2

4

8

x

x

x

x

10

20

40

Hour
units

Hour
tens

1

2

4

x

x

x

x

x

8

x

x

x

x

Sync word
(fixed sequence of bits)

10

20

x

x

x

x

x

x

binary 0

binary 0

binary 1

binary 1

binary 1

binary 1

binary 1

binary 1

binary 1

binary 1

binary 1

binary 1

binary 1

binary 1

binary 0

binary 1

Analysing a frame

In the following figures two sections of a frame are decoded as an example. Figure 3 shows the complete SMPTE frame, figure

4 focusses on a frame detail (the frame units and the frame tens) to analyse the binary coded time information and transfer it

into the decimal based count of frames.

Frame

User

Frame

User

Second

User

Second

User

Minute

User

Minute

User

Hour

User

Hour

SyncWord

64 68 72 76 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76

n+1

Units

Bits

Tens

Bits

Units

Bits

Tens

Bits

Units

Bits

Tens

Bits

Units

Bits

one SMPTE frame

n

User

Tens

Bits

SyncWord

Frame
Units

0

User

Frame

Bits

Tens

4 8 12

n-1

Fig. 3 A single SMPTE sample frame

Bit 0

Bit 1

Bit 2

Bit 3

Frame
units

1

0

0

1

Frame
tens

01

10

1

2

4

8

20

x

x

frame units = 9 frame tens = 20

1 0 0 1

0 1 x x

Fig. 4 Bit sequence 1001 | frame unit = 9; bit sequence 01xx | frame tens = 20, as both last bits do not give information about time

To obtain the complete frame value, both values are added: 20 + 9 = 29. It is the 29th frame in this second. Extracting further

time data out of the complete sample frame shown in fig. 3 results in: 05:38:14:29. Having understood the coding of the frame,

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