Generate subtitle for a video

[TOC]

Requirement

• Adding a subtitle for a video automatically
  1. Extract sound from video
     1. Input: video
        • Format: any video format allowed by FFmpge
        • Size: no longer than 1min, if it is longer than 1min, we may upload output audio into Google Cloud Storage, and use gcs_uri in convert speech step with Google Cloud Speech-To-Text API
     2. Output: audio
        • Format:
          • Ecoding: FLAC
          • sample_rate_hertz=16000 or more
          • Language=Mandarin
  2. Convert speech into text
     1. Input: audio
        • Format: above output audio format
     2. Output: text
        • Format: Plain text and str file
  3. Generate subtitle using above source
     • .str file is the target subtitle file

Component

1. Extract sound from video using FFmpeg

• Install:

• $ brew install ffmpeg

• Usage:

  ffmpeg -i video.mp4 -f mp3 -ab 192000 -vn audio.mp3

  • -i input file
  • -f convert to format, flac is recommanded by Google Cloud Speech-To-Tex API
  • -ar conver to sampleRateHertz, 16000 is recommanded by Google Cloud Speech-To-Tex API
  • -vn output file is not video
  • -ac 1 only 1 channel audio would be allowed by Cloud Speech

• Allow format

   ffmpeg -formats

  D  3dostr          3DO STR
    E 3g2             3GP2 (3GPP2 file format)
    E 3gp             3GP (3GPP file format)
   D  4xm             4X Technologies
    E a64             a64 - video for Commodore 64
   D  aa              Audible AA format files
   D  aac             raw ADTS AAC (Advanced Audio Coding)
   DE ac3             raw AC-3
   D  acm             Interplay ACM
   
  ...

2. Convert audio into text using Google Cloud Speech-To-Text API

• Install: follow official website Set up a GCP Console projec, Set the environment variable GOOGLE_APPLICATION_CREDENTIALS and install and initialize Google Cloud SDK

  Install the client library (for python):

  pip3 install --upgrade google-cloud-speech

• In this project, our test video is longer than 1 min, we need the asynchronously transcribes and use gcs_uri for speech file

• we refer the following code from official website

• # [START speech_transcribe_async_gcs]
  def transcribe_gcs(gcs_uri):
      """Asynchronously transcribes the audio file specified by the gcs_uri."""
      from google.cloud import speech
      from google.cloud.speech import enums
      from google.cloud.speech import types
      client = speech.SpeechClient()
  
      audio = types.RecognitionAudio(uri=gcs_uri)
      config = types.RecognitionConfig(
          encoding=enums.RecognitionConfig.AudioEncoding.FLAC,
          sample_rate_hertz=16000,
          language_code='en-US')
  
      operation = client.long_running_recognize(config, audio)
  
      print('Waiting for operation to complete...')
      response = operation.result(timeout=90)
  
      # Each result is for a consecutive portion of the audio. Iterate through
      # them to get the transcripts for the entire audio file.
      for result in response.results:
          # The first alternative is the most likely one for this portion.
          print(u'Transcript: {}'.format(result.alternatives[0].transcript))
          print('Confidence: {}'.format(result.alternatives[0].confidence))
  # [END speech_transcribe_async_gcs]

• In config,

  • We use as following:

    • sample_rate_hertz=16000
    • language_code='zh'
    • encoding='FLAC'
    • this config would set several phrases for specific vedio "Savvy _June Cut_final.mp4"
    • enable_word_time_offsets=True, which include timestamp used for generate subtitle
    • enable_automatic_punctuation=True, which include punctuation in transcript field

  • config = types.RecognitionConfig(
            encoding=enums.RecognitionConfig.AudioEncoding.FLAC,
            sample_rate_hertz=16000,
            language_code='zh',
            speech_contexts=[
                speech.types.SpeechContext(phrases=[
                    '思睿', '在思睿', '海外教育', '双师', '辅导', '授课', '云台录播', '讲义', '赢取'
                ])
            ],
            enable_word_time_offsets=True,
            enable_automatic_punctuation=True)

Usage

extract-audio.py

python3 extract-audio.py Savvy\ _June\ Cut_final.mp4

• This script would recogonize whether the file exists, whether the format allowed by ffmpeg

• Furthermore, this script can handle the input name with whitespace and output coutain original file name

• The output audio is obtain each config which needed by Google Cloud Speech-To-Text API(see: Component2)

• The output file name would be audio-inputFileName.flac , which would also be upload into gs://test-convert-audio/audio-inputFileName.flac(used in convert step)

audio-to-text.py

For shorter audio (no longer 1 min) using synchronous speech recognition.

python3 audio-to-text.py localFile.flac

For longer audio (longer than 1 min) using asynchronous speech recognition.

python3 audio-to-text.py "gs://test-convert-audio/audio-Savvy _June Cut_final.flac"

Note: if filename with whitespace plase use ""

Step1: convert to text

This script would send a recognize request to Cloud Speech-to-Text and obtain the response, and we can also write into a plain text in output folder ( transcript-text.txt)

operation = client.long_running_recognize(config, audio)
response = operation.result(timeout=90)

Step2: format text as subtitle file .srt

This script add sequence number for each line and timpesamp for each line (e.g 0:00:1.012 —> 0: 00: 3.211) and the words, which format is need by .str file

helper modual timestr.py would help us convert the start_timeor end_time as allowed string, start_timeor end_time both are from response information

Because the response.result.alternatives[0].word only contain word information, so reading output file transcript-text.txt, which including punctuation, and add punctuation into subtitle file or leave white space at punctuation position.

Note

• There provide two version audio-to-text.py available

  • Subtitle with punctuation audio-to-text-with-punctuation.py:

    • python3 audio-to-text-with-punctuation.py "gs://test-convert-audio/audio-Savvy _June Cut_final.flac"

  • Subtitle no punctuation audio-to-text-no-punctuation.py

    • python3 audio-to-text-no-punctuation.py "gs://test-convert-audio/audio-Savvy _June Cut_final.flac"