MSC3888: Voice Broadcast

proposals/3888-voice-broadcast.md

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+# MSC3888: Voice Broadcast
+
+As a user I want to be able to send a voice broadcast to all users of a room,
+so that I can easily provide information by just talking.
+Compared to already existing voice messages it should be possible to listen to
+voice broadcast at the time they are being recorded.
+Think of this being a live-podcast over Matrix.
+
+Some use-case scenarios for this feature can be to verbally provide updates,
+trainings, or talk to a large number of people.
+
+
+## Proposal
+
+This MSC proposes using chunks of voice messages to implement the Voice 
+Broadcast feature. It will built on top of [MSC3245: Voice Messages][MSC3245]
+and is heavily inspired by [MSC3489: Live Location Sharing][MSC3489].
+
+A new state event `m.voice_broadcast_info` will be introduced. This state event
+identifies a broadcast and provides its state, such as "started" or "paused".
+In addition to that `m.voice` messages will receive a relation to the state
+event to mark them as voice broadcast chunks.
+
+`m.voice_broadcast_info` event example:
+
+```json
+{
+    "type": "m.voice_broadcast_info,
+    "state_key": "@matthew:matrix.org",
+    "content": {
+        "state": "started",
+    }
+}
+```
+
+- `type` is `m.voice_broadcast_info`.
+- `state_key` contains the broadcaster's MXID.
+- `state` describes the broadcast state as listed:
+  - `running` flags a voice broadcast as currently being live.
+  - `paused` stands for a paused broadcast that may be resumed.
+  - `stopped` marks a broadcast as finished.
+
+`m.voice` messages that belong to the broadcast will have a [MSC3267][MSC3267] 
+relation to the identifying state event.
+
+Example of a voice message as part of a broadcast:
+
+```json
+{
+    "type": "m.voice",
+    "content": {
+        "m.relates_to": {
+            "rel_type": "m.reference",
+            "event_id": "$voice_broadcast_info_event_id",
+        }
+    }
+}
+```
+
+
+## Potential Issues
+
+### Not Actually Being Live
+
+Compared to a streaming solution the chunked voice message broadcast is not
+actually live. Instead there will always be an offset of the chosen length of
+the single voice chunks. For example if someone asks for a live response in chat
+during his broadcast, he won't receive an immediate response.
+
+
+### Client Fallback Behaviour
+
+Depending on the chosen length of each broadcast chunk clients not supporting
+this MSC will receive a number of message. Example for an 1 hour broadcast and
+a five minute chunk length: 60 / 5 = 20 messages. This can be quite annoying.
+
+TODO: Can we disable notifications for N > 1 chunk messages?
+
+
+## Alternatives
+
+### Video/Voice rooms with recording
+
+Server-side recording would be required for reliable recording. It would be 
+quite challenging to do this with maintaining end-to-end-encryption.
+
+
+#### Element Call
+
+In order for voice broadcasts to support a large number of listeners, 
+it would rely on SFU (selective forwarding unit), which is not yet ready.
+
+
+### Streaming File Transfer
+
+
+## Unstable Prefix
+
+Until this MSC lands, the following unstable prefixes should be used:
+
+`m.voice_broadcast_info` → `org.matrix.msc3888.voice_broadcast_info`
+`state` → `org.matrix.msc3888.state`
+
+Example of the state event with unstable prefix:
+
+```json
+{
+    "type": "m.voice_broadcast_info,
+    "state_key": "@matthew:matrix.org",
+    "content": {
+        "org.matrix.msc3888.state": "started",
+    }
+}
+```
+
+
+[MSC3245]: https://github.com/matrix-org/matrix-spec-proposals/blob/travis/msc/voice-messages/proposals/3245-voice-messages.md
+[MSC3489]: https://github.com/matrix-org/matrix-spec-proposals/blob/matthew/location-streaming/proposals/3489-location-streaming.md
+[MSC3267]: https://github.com/matrix-org/matrix-spec-proposals/blob/aggregations-references/proposals/3267-reference-relations.md