The USB protocol

The ATEM Mini series of switchers can be controlled over USB instead of ethernet. This variant of the protocol is a simplified version of the UDP protocol used for the other ATEM hardware.

The USB protocol drops all the retransmission and framing from the UDP protocol since USB is a reliable transport and provides framing already.

The ATEM hardware uses a standard pair of bulk endpoints for all the communication except the initial handshake.

The interfaces exposed by an ATEM Mini:

Interface	Class	Notes
0	Vendor specific	Unknown
1	Vendor specific	Unknown
2	Vendor specific	ATEM control protocol
3	DFU	Firmware updates
4	Video control	Virtual webcam settings
5	Video stream	Virtual webcam stream
6	Audio control	Virtual soundcard settings
7	Audio stream	Virtual soundcard stream

Starting a connection

The communication is established by sending a vendor USB control transfer to the device and then atem commands are sent using regular bulk packets. The ATEM protocol is on interface 2.

Type	Control Transfer
bmRequestType	0x21	host-to-device, class, interface
bRequest	0
wValue	0x0000
wIndex	0x0002
wLength	0

After this initial packet it should be possible to do an URB Bulk read to get the full initial sync in one packet.

The pyatem implementation does reads with a size of 32768 bytes.

Packet protocol

The USB protocol is simplified a lot compared to the older UDP protocol. Since USB already provides a robust, ordered and framed protocol a lot of the error recovery features could be dropped.

Offset	Length	Note
0	4	Chunk length as int32 (n after this)
4	n	Atem messages concatinated together
4+n	4	(optional) Chunk length of the second chunk. These chunks and data blocks repeat

The contents of the ATEM messages are what’s defined as The higher level protocol on the UDP page. All further parsing is exactly the same as the UDP protocl.

For sending commands to the ATEM the inverse can be done. Prepend a 32 bit length header to the normal ATEM command structure and write it as an USB Bulk message