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Blackmagic Embedded Tally Control Protocol
Version 1.0 (30/04/14)
This section is for third party developers or users who may wish to add support for the Blackmagic
Embedded Tally Control Protocol to their products or system. It describes the protocol for sending tally
information embedded in the non-active picture region of a digital video stream.
Data Flow
A master device such as a broadcast switcher embeds tally information into its program feed which is
broadcast to a number of slave devices such as cameras or camera controllers. The output from the slave
devices is typically fed back to the master device, but may also be sent to a video monitor.
The primary flow of tally information is from the master device to the slaves. Each slave device may use its
device id to extract and display the relevant tally information.
Slave devices pass through the tally packet on their output and update the monitor tally status, so that
monitor devices connected to that individual output may display tally status without knowledge of the device
id they are monitoring.
Assumptions
Any data alignment / padding is explicit in the protocol. Bit fields are packed from LSB first.
Blanking Encoding
One tally control packet may be sent per video frame. Packets are encoded as a SMPTE 291M packet with
DID/SDID x51/x52 in the active region of VANC line 15. A tally control packet may contain up to 256 bytes of
tally information.
Packet Format
Each tally status consists of 4 bits of information:
uint4
bit 0:
bit 1:
bit 2-3:
The first byte of the tally packet contains the monitor device tally status and a version number.
Subsequent bytes of the tally packet contain tally status for pairs of slave devices. The master device sends
tally status for the number of devices configured/supported, up to a maximum of 510.
struct tally
uint8
bit 0:
bit 1:
bit 2-3:
bit 4-7:
uint8[0]
bit 0:
bit 1:
bit 2-3:
bit 4:
bit 5:
bit 6-7:
program tally status (0=off, 1=on)
preview tally status (0=off, 1=on)
reserved (0x0)
monitor device program tally status (0=off, 1=on)
monitor device preview tally status (0=off, 1=on)
reserved (0b00)
protocol version (0b0000)
slave device 1 program tally status (0=off, 1=on)
slave device 1 device preview tally status (0=off, 1=on)
reserved (0b00)
slave device 2 program tally status (0=off, 1=on)
slave device 2 preview tally status (0=off, 1=on)
reserved (0b00)
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