60 lines
3.1 KiB
Markdown
60 lines
3.1 KiB
Markdown
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# Cable live media
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Cable live media travels beside Cable room chat and Dat archives. A live
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session is not an archive, does not append to a user's permanent chat feed, and does not
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advance a Dat version. It is sent as volatile Cable records over authenticated
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Cesspool/uTP peer sessions. The current audio path fans packets out from the
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broadcaster to authenticated room peers; multi-hop relay is not yet wired.
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## Audio, first slice
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`cable-thang`'s `BROADCAST` button opens the default SDL microphone as 48 kHz,
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mono, signed 16-bit PCM. FFmpeg's AAC-LC encoder converts 1,024-sample frames
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to 96 kbps AAC and wraps each access unit in an ADTS header. The `cable-live`
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packet header carries a random session id, sequence number, GUI timestamp,
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media kind, and bounded payload. The C viewer validates packets, decodes AAC,
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places PCM in a half-second maximum SDL playback queue, and plots those decoded
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samples in the waveform panel. `Ctrl+Shift+A` toggles the local broadcast.
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Received room audio begins playing as soon as its start packet is seen.
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Cable audio and video are experimental and disabled by default. A normal SDL
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build has no FFmpeg dependency. Enable both together with
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`make ENABLE_SDL2=1 ENABLE_CABLE_MEDIA=1`; the makefile requires compatible
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FFmpeg 5.0+ development files (`libavcodec`, `libavformat`, `libavutil`,
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`libswresample`, and `libswscale`). To build the SDL clients without compiling
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either media feature, use `make ENABLE_SDL2=1 ENABLE_CABLE_MEDIA=0`.
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On Debian or Ubuntu, install the development headers and enable the feature:
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```sh
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sudo apt install libavcodec-dev libavformat-dev libavutil-dev \
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libswresample-dev libswscale-dev
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make ENABLE_SDL2=1 ENABLE_CABLE_MEDIA=1
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```
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With media enabled, the Cable video pane can listen to a local OBS MPEG-TS
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output. Click `OBS INPUT` or press `Ctrl+Shift+V`; Cable listens on
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`udp://127.0.0.1:5000`, decodes the video on a
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worker thread, scales the local preview to at most 640x360, and displays the
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latest frame. New frames are reduced to 160x90 JPEG and sent in 4,000-byte
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fragments over authenticated Cesspool/uTP sessions; receivers reassemble and
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decode them in the video pane. Transmission is capped at five frames per
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second. In OBS, add a Custom FFmpeg Output with container `mpegts`, video
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encoder `libx264`, and output URL `udp://127.0.0.1:5000?pkt_size=1316`. OBS
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input provides local preview and direct broadcast to authenticated peers.
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Camera capture without OBS and multi-hop forwarding are not implemented.
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## Storage boundary
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No media packets are written to `.dat`, test-only Cabal JSON chat feeds, or the
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versioned archive metadata/content feeds. Closing the room discards the
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decoder and playback queue. This avoids turning a live broadcast into
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permanent archive history. Hypervision's
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historic approach chunked WebM media and appended the chunks to a dedicated
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Hypercore feed. That is a useful precedent for chunking and peer redistribution,
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but an append-only archive is not the right retention policy for this live
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session path.
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The packet kinds cover audio and video start/frame/stop. Direct camera capture,
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multi-hop relay, remote stream selection, and adaptive bitrate are follow-on
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work.
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