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React native – How to send ICY format message in audio stream from server in Python?

I implemented a StreamingResponse in FastAPI with audio bytes from async generator sources. But besides need to insert some messages for client side audio player (currently, React Native) just in the stream.
Read about ICY format and it looks like appropriate stuff for this. So what headers are required for stream endpoint and what format a message for AudioPlayer should be to trigger an event (like Event.MetadataCommonReceived)?

@router.get(
    "/stream/{session_id}",
    response_class=StreamingResponse,
    responses={200: {"content": {"audio/mpeg": {}}, "description": "An audio file in MP3 format"}},
)
async def stream_audio(session_id: str):
...
    return StreamingResponse(
        stream_from_queue(<some asyncio.Queue>, session_id),
        headers={
            "content-type": "audio/mpeg",
            "icy-metaint": "16000",
            "icy-name": "MyAudioStream",
            "icy-genre": "Podcast",
            "icy-url": "http://localhost:8000"
        },
        media_type="audio/mpeg",
    )


async def stream_from_queue(queue: Queue, session_id: str):

     ... # get an audio chunk from queue
     ... # send some metadata
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2

Answers


  1. Chosen as BEST ANSWER
    0

    Base on the protocol description link (thank to Rauuun) I implemented this algorithm.

    ICY_METADATA_INTERVAL = 16000 # bytes
ICY_BYTES_BLOCK_SIZE = 16  # bytes
ICY_METADATA_SIGNAL = "META_EVENT".encode()

async def stream_from_queue(queue: Queue, session_id: str):
    buffer = b""
    ...
        for chunk in <stream_queue>:
            ... # get an audio chunk from queue
                if chunk == ICY_METADATA_SIGNAL:  # if get a special signal we can send some metadata
                    # flush buffer padded with zeros to ICY_METADATA_INTERVAL length
                    yield buffer + (ICY_METADATA_INTERVAL - len(buffer)) * (0).to_bytes()
                    buffer = b""
                    # send a meta message
                    yield preprocess_metadata()
                else:  # send raw audio data
                    buffer += chunk
                    if len(buffer) < ICY_METADATA_INTERVAL:
                        continue
                    yield buffer[:ICY_METADATA_INTERVAL]
                    yield (0).to_bytes() # we have to send at least zero byte as metadata after every ICY_METADATA_INTERVAL
                    buffer = buffer[ICY_METADATA_INTERVAL:]
...


def preprocess_metadata(metadata: str = "META_EVENT") -> bytes:
    icy_metadata_formatted = f"StreamTitle='{metadata}';".encode()
    icy_metadata_block_length = len(icy_metadata_formatted) + 1
    return (
        # number of ICY_BYTES_BLOCK_SIZE blocks needed for this meta message (including this byte)
        (1 + icy_metadata_block_length // ICY_BYTES_BLOCK_SIZE).to_bytes(1, "big")
        # meta message encoded
        + icy_metadata_formatted
        # zero-padded tail to fill the last ICY_BYTES_BLOCK_SIZE
        + (ICY_BYTES_BLOCK_SIZE - icy_metadata_block_length % ICY_BYTES_BLOCK_SIZE)
        * (0).to_bytes(1, "big")
    )

    screenshot of a stream with META_EVENT message


  2. 0

    After reading https://gist.github.com/niko/2a1d7b2d109ebe7f7ca2f860c3505ef0 it seems that your method stream_from_queue needs to pad the initial message that you will yield with the metadata information you desire to send.

    1. Calculate how many bytes are used in the metadata.
    2. Pad them to the next message
    3. Once that is sent, you can keep streaming data

    I understand the encoding is ascii

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