Setting sequence number of LE Audio broadcast stream

[gab987]

Hello, 
when sending audio packets using a broadcast stream one can provide the seq_num of the packet (using the function `int bt_bap_stream_send(struct bt_bap_stream *stream, struct net_buf *buf, uint16_t seq_num)`.
I am trying to relay a broadcast (with only 1 BIS), where also the sequence number should be the same. However, when the relayed stream is started, the sequence lways begins at 0. When following the code execution the sn is set in the ```bt_iso_chan_send``` function, so I assume the controller actually sets the sn to 0 for a new stream.

I am using a nrf5340-dk with the nordic softdevice-controller on the network core. 
Is there a way to actually set the seq_nr of the stream?

[Edvin]

Hello,

The initial reply that I got from our LE Audio team was that you should not alter the sequence numbers. May I ask why you need to change this?

Best regards,

Edvin

[gab987]

I am looking into "auracast roaming", where i would need to synchronize multiple broadcast audio streams. My idea was to use the sequence number of the streams to match the audio packets.

[Edvin]

What do you mean by synchronize? Like copy one audio stream broadcasting on another device? If the idea is that you want any device that looses connection to the original stream to jump over to this one, I don't think that is how it works. What is the purpose of changing the sequence number?

Best regards,

Edvin

[gab987]

Basically yes.
If the second device, that copies the audio stream, could also set the sequence number to the same value, the transition between the two devices would be as simple as: receive packet with SN x from device A and then receive the next packet with SN x + 1 from device B. 
Currently I try to match the packets from device A and B and determine which payloads are identical. (packet from dev A with SN x = packet from dev B with SN y, then i can assume that packet from dev A with SN x + n = packet from dev B with SN y + n)

[Edvin]

I understand. The clear recommendation from our SoftDevice Team (the Bluetooth stack) is:

Sequence numbers (in ISO, and thus also LE audio when using ISO over HCI) shall be initialized to 0 when the CIS or BIS is created. There is no way for 2 broadcast sources (device A and device B) to reliably coordinate sequence numbers or use that to identity or determine payloads

So this is not possible, unfortunately.

Best regards,

Edvin

[gab987]

Okay. Thank you