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@seydx/rtsp / BackchannelTranscoder

Class: BackchannelTranscoder ​

Defined in: sinks/rtsp-server/backchannel-transcoder.ts:207

Transcodes inbound viewer talkback audio into the upstream camera's codec.

Bridges the format advertised to RTSP clients and the format an upstream camera expects on its ONVIF backchannel by running an in-process audio pipeline that decodes, resamples, re-encodes, and muxes the received audio. The result is delivered through a caller-supplied callback as either RTP packets or raw container bytes, depending on the chosen target format.

Example ​

typescript
import { BackchannelTranscoder } from '@seydx/rtsp';

const transcoder = new BackchannelTranscoder({
  from: { codec: 'opus', payloadType: 97, clockRate: 48000, channels: 1 },
  to: { codecId: cameraCodecId, sampleRate: 8000, channels: 1 },
  output: (rtp) => camera.sendBackchannel(rtp),
});

await transcoder.start();
transcoder.push(viewerRtpPacket);
await transcoder.close();

See ​

RtspServerSink For the sink that drives talkback delivery

Constructors ​

Constructor ​

new BackchannelTranscoder(options): BackchannelTranscoder

Defined in: sinks/rtsp-server/backchannel-transcoder.ts:227

Create a backchannel transcoder.

The transcoder is inert until BackchannelTranscoder.start is called; the constructor only captures configuration.

Parameters ​

options ​

BackchannelTranscoderOptions

Inbound/target formats, output callback, and tuning

Returns ​

BackchannelTranscoder

Methods ​

close() ​

close(): Promise<void>

Defined in: sinks/rtsp-server/backchannel-transcoder.ts:350

Stop the transcoder and release all pipeline resources.

Marks the transcoder inactive, interrupts the blocked demuxer read so the background loop can unwind, waits for it to finish, and only then closes the input demuxer, decoder, filter, encoder, and output muxer — freeing a native stage while the loop still reads from it would be a use-after-free. Safe to call when the transcoder was never started or is already closed.

Returns ​

Promise<void>

Resolves once all resources are released

Example ​

typescript
await transcoder.close();

push() ​

push(rtp): void

Defined in: sinks/rtsp-server/backchannel-transcoder.ts:330

Feed one inbound RTP packet from a viewer into the pipeline.

Packets are queued into the synthetic input demuxer and consumed by the background transcode loop. Calling push before BackchannelTranscoder.start (or after BackchannelTranscoder.close) is a silent no-op.

Parameters ​

rtp ​

Buffer

A single inbound RTP packet in the advertised viewer codec

Returns ​

void

Example ​

typescript
transcoder.push(viewerRtpPacket);

start() ​

start(): Promise<void>

Defined in: sinks/rtsp-server/backchannel-transcoder.ts:248

Start the transcode pipeline.

Resolves the decoder and encoder, builds a synthetic input SDP for the inbound viewer format, and wires up the decode, resample, encode, and mux stages. Once started, inbound packets fed via BackchannelTranscoder.push flow through the pipeline and produced output is delivered to the configured callback. Calling start when already active is a no-op.

Returns ​

Promise<void>

Resolves once the pipeline is running

Throws ​

If the inbound decoder or target encoder cannot be resolved, or the generated SDP contains no audio stream

Example ​

typescript
await transcoder.start();