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This guide covers supported encoding formats, sample rates, and recommendations for optimal real-time transcription quality.

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Supported Encoding Formats

The Lightning STT WebSocket API supports the following audio encoding formats for real-time streaming:
EncodingDescriptionUse Case
linear1616-bit linear PCMRecommended for best quality
linear3232-bit linear PCMHigh-fidelity audio
alawA-law encodingTelephony systems
mulawμ-law encodingTelephony systems (North America)
opusOpus compressed audioLow bandwidth, high quality
ogg_opusOgg Opus containerOgg container with Opus codec

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Supported Sample Rates

Sample rate is the number of times the audio signal is measured per second. A higher sample rate naturally implies audio of better detail and higher quality. However it increases the size of the audio file. The WebSocket API supports the following sample rates:
  • 8000 Hz
  • 16000 Hz
  • 22050 Hz
  • 24000 Hz
  • 44100 Hz
  • 48000 Hz

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Audio Requirements

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Chunk Size

The recommended size is 4096 bytes per chunk. Sending audio in consistent 4096-byte chunks helps maintain optimal latency and processing efficiency. It minimizes the tradeoff between processing latency and network latency, finding the right fit between number of requests and the size of each request.

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Channels

Currently, we support only single-channel (mono) transcription. Multi-channel support is coming soon.

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Streaming Rate

For optimal real-time performance:
  • Stream chunks at regular intervals (e.g., every 50-100ms)
  • Maintain consistent chunk sizes when possible
  • Avoid sending chunks too rapidly or too slowly

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Format Recommendations

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Best Quality (Default)

Use 16 kHz mono Linear PCM (linear16) for the optimal mix of accuracy and processing speed: 
Encoding: linear16
Sample Rate: 16000 Hz
Channels: Mono
Chunk Size: 4096 bytes

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Telephony Quality

Use 8 kHz μ-law or A-law encoding for low bandwidth usage: 
Encoding: mulaw or alaw
Sample Rate: 8000 Hz
Channels: Mono
Chunk Size: 4096 bytes

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High Fidelity

For broadcast or high-quality scenarios, use higher sample rates: 
Encoding: linear16 or linear32
Sample Rate: 44100 or 48000 Hz
Channels: Mono
Chunk Size: 4096 bytes

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Audio Preprocessing

Before streaming audio to the WebSocket API, ensure your audio is:
  1. Converted to the correct format: Use the specified encoding (linear16, linear32, alaw, mulaw, opus, or ogg_opus)
  2. Set to the correct sample rate: Match the sample_rate parameter in your WebSocket URL
  3. Mono channel: Downmix stereo or multi-channel audio to mono
  4. Properly chunked: Split audio into 4096-byte chunks for streaming

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Example: Converting Audio for Streaming

import numpy as np
import soundfile as sf

# Read audio file
audio, sample_rate = sf.read('input.wav')

# Convert to mono if stereo
if len(audio.shape) > 1:
    audio = np.mean(audio, axis=1)

# Resample to 16 kHz if needed
if sample_rate != 16000:
    from scipy import signal
    audio = signal.resample(audio, int(len(audio) * 16000 / sample_rate))

# Convert to 16-bit PCM
audio_int16 = (audio * 32767).astype(np.int16)

# Split into 4096-byte chunks
chunk_size = 4096
chunks = [audio_int16[i:i+chunk_size//2] for i in range(0, len(audio_int16), chunk_size//2)]

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Query Parameters

Specify encoding and sample rate in the WebSocket connection URL: 
const url = new URL("wss://waves-api.smallest.ai/api/v1/lightning/get_text");
url.searchParams.append("encoding", "linear16");
url.searchParams.append("sample_rate", "16000");