code: def _ensure_single_channel(signal, filename, rate):
      """
      Ensure that the audio signal has only one channel.
  
      Arguments
      ---------
      signal: Tensor
          The audio signal.
      filename: str
          The path to the audio file.
      rate: int
          The sampling frequency of the signal.
  
      Returns:
      ---------
          Torch.Tensor
          The audio signal with a single channel.
      """
      if signal.shape[0] > 1:
          signal = signal[0].unsqueeze(0)
          torchaudio.save(filename, signal, rate)
      return signal
  
file_path: speechbrain/speechbrain/augment/preparation.py
imports: import os
  import logging
  import torchaudio
  from speechbrain.utils.data_utils import download_file
  from speechbrain.utils.data_utils import get_all_files
label_desc: Use image node_id 5 for calling a global function with example usage: _ensure_single_channel(signal, filename, rate) and returns: signal