AssemblyAISTTService ignores PipelineParams.audio_in_sample_rate by default

[timofey-TK]

Service issue

Pipecat version: 1.5.0; the same constructor default is present on current main

Service: AssemblyAI Streaming STT (AssemblyAISTTService)

Model: universal-3-5-pro

Issue description

AssemblyAISTTService does not inherit PipelineParams.audio_in_sample_rate when the service sample rate is omitted.

The base STTService supports pipeline sample-rate inference:

self._sample_rate = self._init_sample_rate or frame.audio_in_sample_rate

However, AssemblyAISTTService.__init__ declares sample_rate: int = 16000 and passes it to the base class. This always sets _init_sample_rate to 16000, preventing the StartFrame value from being used. In comparison, DeepgramSTTService declares sample_rate: int | None = None, so it inherits the pipeline rate as expected.

This is especially problematic for telephony. With raw 8 kHz PCM input and PipelineParams(audio_in_sample_rate=8000), AssemblyAI connects with sample_rate=16000 and computes its chunk size for 16 kHz, while the bytes are still 8 kHz PCM. There is no resampling in this path.

Reproduction steps

1. Construct AssemblyAISTTService without an explicit sample_rate.
2. Run it in a pipeline configured with PipelineParams(audio_in_sample_rate=8000).
3. Inspect service.sample_rate or the AssemblyAI WebSocket query after start().
4. Observe that the declared sample rate is 16000 rather than 8000.

In a real telephony session this caused provider-reported audio duration to be almost exactly half the actual recording duration, along with severely degraded transcription and turn behavior. Passing sample_rate=8000 explicitly fixed the sample-rate mismatch.

Expected behavior

When sample_rate is omitted, AssemblyAISTTService should inherit StartFrame.audio_in_sample_rate, consistent with the base STTService, PipelineParams, and DeepgramSTTService.

An explicitly supplied service sample rate should continue to override the pipeline rate.

Suggested fix

Change the constructor default to:

sample_rate: int | None = None

and add coverage for:

• omitted service rate with an 8000 Hz StartFrame -> 8000 Hz
• explicit service rate -> explicit value overrides StartFrame