
## Introduction

In multi-speaker audio recordings like meetings, podcasts, or interviews, knowing who spoke when is crucial for many applications. This process, known as Speaker Diarization, partitions an audio stream into segments according to speaker identity.

[PyAnnote Audio](https://github.com/pyannote/pyannote-audio), an open-source toolkit built on PyTorch, offers state-of-the-art models for speaker diarization that are both accurate and accessible. Running these models on Vast.ai provides a cost-effective solution for processing large audio datasets without investing in expensive hardware. This combination gives developers and researchers the tools they need to build sophisticated audio processing pipelines at a fraction of the cost of traditional cloud providers.

### Why Speaker Diarization Matters

Speaker Diarization provides several key benefits for audio processing pipelines:

1. **Speaker Identification**: It identifies different speakers in a conversation, meeting, or any multi-speaker audio recording.

2. **Improved Transcription**: When combined with speech-to-text systems, diarization allows for speaker-attributed transcripts, making it clear who said what.

3. **Processing Efficiency**: By segmenting audio by speaker and removing non-speech portions, diarization can significantly reduce the computational load for downstream tasks like speech recognition, allowing these systems to process only relevant speech segments rather than the entire audio file.

4. **Audio Indexing**: Makes audio content searchable by speaker, allowing users to find all segments where a specific person speaks.

### What This Guide Covers

In this guide, we will:

-   Set up the Pyannote Audio Speaker Diarization pipeline
-   Process audio files to detect different speakers and their speaking turns
-   Calculate speaking time for each identified speaker
-   Identify regions with overlapping speech
-   Extract and save speaker-specific segments from the input audio
-   Play and verify the diarization results

The output will be a collection of audio files separated by speaker, making them ready for further processing in speech-to-text pipelines or speaker-specific analysis.

### Why VAST.ai

VAST.ai offers a marketplace approach to GPU rentals that provides significant advantages for audio processing tasks. Unlike traditional cloud providers, VAST.ai allows you to:

-   Rent precisely the GPU capacity needed for your workload
-   Access GPUs at more affordable rates
-   Access more types of GPU SKUs, particularly lower RAM/more affordable GPUs
-   Avoid long-term commitments for experimental projects

For speaker diarization specifically, VAST.ai is particularly well-suited as these models typically require GPU acceleration but don't demand extensive resources. Users can rent just the right amount of computing power for this specific task without overpaying for unused capacity, making it an economical choice for audio processing workflows that might otherwise be cost-prohibitive on traditional cloud platforms.

## Download This Notebook

To follow along with this tutorial, you can download the complete Jupyter notebook:

[Notebook](https://nbviewer.org/urls/bitbucket.org/%21api/2.0/snippets/jsbcannell/q7MzLr/ee586f7e9bfbcdb2fb48f059b7bf64caf92fa3bd/files/pyannote_diarization_vast.ipynb)

Having the notebook will allow you to execute the code blocks as you read through this guide, making it easier to understand and experiment with the diarization implementation.

## Choosing an Instance

For running the Pyannote Speaker Diarization model on VAST.ai, you'll need a relatively modest GPU setup. The pyannote/speaker-diarization-3.1 model runs in pure PyTorch and is designed to be efficient. Here are the recommended specifications:

-   GPU: A low-end GPU like an RTX 3060 or 4060 would be sufficient.
-   VRAM: 6-8GB of VRAM should be adequate as the Pyannote diarization pipeline is relatively efficient.
-   RAM: 8-16GB system RAM is recommended for processing audio files.
-   Storage: At least 10GB for the model, dependencies, and your audio files.
-   CUDA: Make sure the instance has CUDA installed (version 11.0+ recommended).
-   Python: Python 3.8+ with PyTorch installed.

## Selecting an Instance

Follow these steps to set up your environment:

1. Ensure that you have a Vast.ai account
2. Go to the Vast Templates in the Console https://cloud.vast.ai/templates/
3. Select the `PyTorch (CuDNN Runtime)` Template
4. Filter for an instance with:
    - 1 GPU
    - 6-8GB of VRAM
    - 8-16GB system RAM
    - 10GB of storage
5. Select an instance and click rent
6. Install the Vast TLS certificate in your browser to access the notebook server https://docs.vast.ai/instances/jupyter#1SmCz
7. Go to your Instances https://cloud.vast.ai/instances/ and click "Open" to access the jupyter server on your instance.
8. Upload the notebook to the server or create a new notebook

## Installing Dependencies

Let's start by installing the necessary Python packages:

```python
%%bash
pip install pyannote.audio
pip install pydub
pip install librosa
pip install datasets
```

We also need to install FFmpeg for audio processing:

```python
%%bash
apt-get update && apt-get install -y ffmpeg
```

## Setting Up Your Hugging Face Token

Pyannote models are hosted on Hugging Face, so you'll need to set up authentication:

```python
# Make sure you've accepted the user conditions at:
# https://huggingface.co/pyannote/speaker-diarization-3.1
# https://huggingface.co/pyannote/segmentation-3.0

HF_TOKEN = ""  # Add your token here
```

Ensure that you have accepted the terms for the models at the URLs above. The models are free to use, but you must agree to their terms of service.

## Downloading Test Data

For this tutorial, we'll use a sample file from the AMI Meeting Corpus dataset, which is a collection of 100 hours of meeting recordings. This dataset is perfect for testing speaker diarization as it contains natural multi-speaker conversations.

```python
from datasets import load_dataset
import os
import soundfile as sf

# Create a directory to save the files
os.makedirs("ami_samples", exist_ok=True)

# Load the dataset with the correct split
dataset = load_dataset("diarizers-community/ami", "ihm", split="train", streaming=True)

# Load just one sample
n_samples = 1
samples = list(dataset.take(n_samples))

for i, sample in enumerate(samples):
    audio = sample["audio"]
    audio_array = audio["array"]
    sampling_rate = audio["sampling_rate"]

    # Calculate duration in seconds
    duration = len(audio_array) / sampling_rate

    # Use soundfile to save the audio
    output_path = f"ami_samples/sample_{i}.wav"
    sf.write(output_path, audio_array, sampling_rate)

    print(f"Saved {output_path} - Speaker: {sample['speakers']} - Duration: {duration:.2f} seconds")
```

## Speaker Diarization

First, let's set up the Speaker Diarization pipeline:

```python
import torch

device = torch.device("cuda" if torch.cuda.is_available() else "cpu")

from pyannote.audio import Pipeline

pipeline = Pipeline.from_pretrained(
        "pyannote/speaker-diarization-3.1",
        use_auth_token=HF_TOKEN
)

# Move pipeline to appropriate device
pipeline = pipeline.to(device)
```

Next, we'll process our audio file to identify different speakers and their speaking turns:

```python
# Process the audio file
audio_file = "./ami_samples/sample_0.wav"
print(f"Processing {audio_file} on {device}")
output = pipeline(audio_file)

print("Voice activity segments:")

for segment, _, speaker in output.itertracks(yield_label=True):
    result = f"{segment.start:.2f} --> {segment.end:.2f} (duration: {segment.duration:.2f}s) Speaker: {speaker}"
    print(result)
```

The output will look something like this:

```
Processing ./ami_samples/sample_0.wav on cuda
Voice activity segments:
18.36 --> 18.42 (duration: 0.07s) Speaker: SPEAKER_03
23.01 --> 25.63 (duration: 2.62s) Speaker: SPEAKER_03
27.08 --> 27.64 (duration: 0.56s) Speaker: SPEAKER_05
...
```

## Additional Analytics

Now that we have processed our file, let's explore some useful features of the Pyannote SDK:

### Speaker Time

Here we calculate the total speaking time for each speaker:

```python
for speaker in output.labels():
    speaking_time = output.label_duration(speaker)
    print(f"Speaker {speaker} total speaking time: {speaking_time:.2f}s")
```

This will output something like:

```
Speaker SPEAKER_00 total speaking time: 558.98s
Speaker SPEAKER_01 total speaking time: 18.98s
Speaker SPEAKER_02 total speaking time: 22.88s
Speaker SPEAKER_03 total speaking time: 469.68s
Speaker SPEAKER_04 total speaking time: 698.02s
Speaker SPEAKER_05 total speaking time: 190.70s
Speaker SPEAKER_06 total speaking time: 5.74s
...
```

### Speaker Overlap

Pyannote can identify regions where multiple speakers are talking simultaneously:

```python
overlap = output.get_overlap()
print(f"Overlapping speech regions: {overlap}")
```

This will output the segments where more than one speaker is speaking.

```
Overlapping speech regions:
[[ 00:00:27.672 -->  00:00:27.689]
 [ 00:00:38.337 -->  00:00:38.860]
 [ 00:00:40.395 -->  00:00:40.463]
```

### Filter by Speaker

We can also filter the diarization output to focus on a specific speaker:

```python
speaker = "SPEAKER_06"
speaker_turns = output.label_timeline(speaker)
print(f"Speaker {speaker} speaks at:")
for speaker_turn in speaker_turns:
    print(speaker_turn)
```

This will show the segments where this speaker is speaking.

```
Speaker SPEAKER_06 speaks at:
[ 00:03:45.767 -->  00:03:45.852]
[ 00:03:55.386 -->  00:03:55.521]
[ 00:05:07.257 -->  00:05:07.274]
...
```

## Extracting Speaker Segments

To verify our results and prepare the audio for further processing, let's split the original audio into segments by speaker:

```python
import shutil
from pydub import AudioSegment

def split_audio_by_segments(audio_path, diarization_output, output_dir="output_segments"):
    """
    Split an audio file into multiple files based on diarization output

    Parameters:
    -----------
    audio_path: str
        Path to the input audio file
    diarization_output: Annotation
        Pyannote diarization output
    output_dir: str
        Directory to save the output segments
    """
    # Clear the output directory if it exists
    if os.path.exists(output_dir):
        shutil.rmtree(output_dir)

    # Create output directory
    os.makedirs(output_dir, exist_ok=True)

    # Load the audio file
    audio = AudioSegment.from_file(audio_path)

    # Extract each segment with speaker information
    for i, (segment, _, speaker) in enumerate(diarization_output.itertracks(yield_label=True)):
        # Convert seconds to milliseconds
        start_ms = int(segment.start * 1000)
        end_ms = int(segment.end * 1000)

        # Extract segment
        segment_audio = audio[start_ms:end_ms]

        # Generate output filename with speaker information
        filename = os.path.basename(audio_path)
        name, ext = os.path.splitext(filename)
        output_path = os.path.join(output_dir, f"{name}_segment_{i+1:04d}_{start_ms:08d}ms-{end_ms:08d}ms_speaker_{speaker}{ext}")

        # Export segment
        segment_audio.export(output_path, format=ext.replace('.', ''))
        print(f"Saved segment {i+1} to {output_path} (Speaker: {speaker})")

# Apply the function to our audio file
split_audio_by_segments(audio_file, output)
```

## Inspecting Results

To verify our results, we'll create a function to play audio files in our Jupyter environment:

```python
import librosa
from IPython.display import Audio, display

def play_audio(file_path, sr=None):
    """
    Play an audio file in a Jupyter notebook.
    """
    # Load the audio file
    y, sr = librosa.load(file_path, sr=sr)

    # Display an audio widget to play the sound
    audio_widget = Audio(data=y, rate=sr)
    display(audio_widget)
```

Now, let's listen to a few clips to verify that the speakers were correctly identified and isolated.

```python
import os
audio_dir = "./output_segments/"

audio_files = os.listdir(audio_dir)
audio_files.sort()

n_offset = 21
n_clips = 5

for fname in audio_files[n_offset:n_clips + n_offset]:
    print(f"File: {fname}")

    # Extract speaker info if present in filename
    if "_speaker_" in fname:
        speaker_part = fname.split("_speaker_")[1].split(".")[0]
        print(f"Speaker: {speaker_part}")

    play_audio(audio_dir + fname)
```

Around file 21 or so, we see a 14-second clip of `SPEAKER_00` speaking:

`File: sample_0_segment_0025_00055364ms-00070045ms_speaker_SPEAKER_00.wav Speaker: SPEAKER_00`

A few files later we see a 10-second clip of `SPEAKER_00` speaking again:

`File: sample_0_segment_0029_00071530ms-00081840ms_speaker_SPEAKER_00.wav Speaker: SPEAKER_00`

Note: The filenames may change. Each time we run the model there may be a different number of clips. The diarization model sometimes captures `<1` second of audio and labels it as a speaker.

The 10-second clip is audio of `SPEAKER_00` about the meeting agenda. It is a file of just `SPEAKER_00` talking.

The 14-second clip has two other speakers speaking. This seems like an error at first, but we will notice two other files that capture the additional speakers:

`sample_0_segment_0026_00059228ms-00060325ms_speaker_SPEAKER_02.wav`
`sample_0_segment_0027_00061793ms-00062924ms_speaker_SPEAKER_02.wav`

With further processing, we could remove these from the `SPEAKER_00` file if necessary for our application.

We can also use the Pyannote overlap function to verify that we did have overlapping speech at this time in the audio file.

## Verifying Speaker Overlap

Finally, let's check for regions where multiple speakers are talking simultaneously:

```python
overlap = output.get_overlap()
for overlap_ts in overlap:
    print(f"Overlapping speech regions: {overlap_ts}")
```

Here we see two overlapping segments that match up with the overlapping files we found above.

`sample_0_segment_0026_00059228ms-00060325ms_speaker_SPEAKER_02.wav`
`sample_0_segment_0027_00061793ms-00062924ms_speaker_SPEAKER_02.wav`

```
Overlapping speech regions: [ 00:00:59.228 -->  00:01:00.325]
Overlapping speech regions: [ 00:01:01.793 -->  00:01:02.924]
```

## Conclusion

This tutorial demonstrates how Pyannote's speaker diarization on VAST.ai provides a powerful solution for identifying "who spoke when" in multi-speaker recordings. The implementation offers several advantages:

-   **Accuracy and Efficiency**: Pyannote accurately identifies different speakers and their speaking times, even detecting overlapping speech
-   **Practical Applications**: Enables speaker-attributed transcription, conversation analytics, and content indexing for meetings, podcasts, and interviews
-   **Cost-Effective Processing**: VAST.ai's affordable GPU rentals make processing large audio datasets accessible without expensive hardware investments

The Pyannote diarization model provides impressive results out of the box, and running it on VAST.ai makes it accessible and affordable for a wide range of applications. Whether you're building a meeting transcription service, analyzing call center interactions, or researching conversation dynamics, this approach gives you a solid starting point.

Look out for more content from Vast about other audio processing tasks!


Run speaker diarization with Pyannote on VAST.ai to detect who spoke when in audio files—cost-effective, accurate, and easy to integrate with your pipeline.

Speaker Diarization with Pyannote on VAST


## Introduction

In the world of audio processing and speech recognition, identifying when someone is speaking versus when there's silence or background noise is a critical first step. This process, known as Voice Activity Detection (VAD), serves as the foundation for many speech-related applications, from transcription services to voice assistants. While conceptually simple, implementing an efficient and accurate VAD system can significantly improve downstream tasks and reduce computational costs.

[PyAnnote Audio](https://github.com/pyannote/pyannote-audio), an open-source toolkit built on PyTorch, offers state-of-the-art models for VAD that are both accurate and accessible. Running these models on Vast.ai provides a cost-effective solution for processing large audio datasets without investing in expensive hardware. This combination gives developers and researchers the tools they need to build sophisticated audio processing pipelines at a fraction of the cost of traditional cloud providers.

### Why Voice Activity Detection Matters

VAD provides several key benefits for speech processing pipelines:

1. **Reduced Computation Load**: By filtering out non-speech segments before running speech-to-text (STT) models, we significantly reduce the computational resources needed for transcription.

2. **Improved Accuracy**: Many STT models perform better when processing only speech segments rather than trying to interpret silence or background noise.

3. **Efficient Storage**: Extracting only the speech segments can reduce storage requirements for large audio datasets.

4. **Better User Experience**: For applications like voice assistants or transcription services, VAD helps eliminate unnecessary processing of silence.

### What This Guide Covers

In this guide, we will:

-   Set up the Pyannote Audio VAD pipeline
-   Process audio files to detect speech segments
-   Extract and save only the speech portions of the input audio
-   Visualize the results

The output will be a collection of audio files containing only the detected speech segments from the original recording, making them ready for further processing in speech-to-text pipelines.

### Why VAST.ai

VAST.ai offers a marketplace approach to GPU rentals that provides significant advantages for audio processing tasks. Unlike traditional cloud providers, VAST.ai allows you to:

-   Rent precisely the GPU capacity needed for your workload
-   Access GPU's at more affordable rates
-   Access more types of SKU's of GPU's, particularly lower RAM/more affordable GPUs
-   Avoid long-term commitments for experimental projects

For VAD specifically, VAST.ai offers an ideal balance of performance and cost-effectiveness, as these models benefit from GPU acceleration without requiring the most expensive hardware tiers.

## Download This Notebook

To follow along with this tutorial, you can download the complete Jupyter notebook:

[Notebook](https://nbviewer.org/urls/bitbucket.org/%21api/2.0/snippets/jsbcannell/6q8akx/6b8dc846a00d27decad5a60a6159466813f6d445/files/pyannote_vad_vast.ipynb)

Having the notebook will allow you to execute the code blocks as you read through this guide, making it easier to understand and experiment with the VAD implementation.

## Choosing an Instance

For running the Pyannote Voice Activity Detection model on VAST.ai, you'll need a relatively modest GPU setup since VAD models are computationally efficient compared to larger AI tasks. Here are the recommended specifications:

-   GPU: A mid-range GPU like an RTX 3060 or 4060 would be sufficient.
-   VRAM: 6-8GB of VRAM should be adequate as the Pyannote VAD model is relatively small.
-   RAM: 8-16GB system RAM is recommended for processing audio files.
-   Storage: At least 10GB for the model, dependencies, and your audio files.
-   CUDA: Make sure the instance has CUDA installed (version 11.0+ recommended).
-   Python: Python 3.8+ with PyTorch installed.

## Selecting an Instance

Follow these steps to set up your environment:

1. Ensure that you have a Vast.ai account
2. Go to the Vast Templates in the Console https://cloud.vast.ai/templates/
3. Select the `PyTorch (CuDNN Runtime)` Template
4. Filter for an instance with:
    - 1 GPU
    - 6-8GB of VRAM
    - 8-16GB system RAM
    - 10GB of storage
5. Select an instance and click rent
6. Install the Vast TLS certificate in your browser to access the notebook server https://docs.vast.ai/instances/jupyter#1SmCz
7. Go to your Instances https://cloud.vast.ai/instances/ and click "Open" to access the jupyter server on your instance.
8. Upload the notebook to the server or create a new notebook

## Installing Dependencies

Let's start by installing the necessary Python packages:

```python
%%bash
pip install pyannote.audio
pip install pydub
pip install librosa
pip install yt-dlp
```

We also need to install FFmpeg for audio processing:

```python
%%bash
apt-get update && apt-get install -y ffmpeg
```

## Setting Up Your Hugging Face Token

Pyannote models are hosted on Hugging Face, so you'll need to set up authentication:

```python
# Make sure you've accepted the user conditions at:
# https://hf.co/pyannote/voice-activity-detection
# https://hf.co/pyannote/segmentation

HF_TOKEN = ""  # Add your token here
```

Ensure that you have accepted the terms for the models at the URLs above. The models are free to use, but you must agree to their terms of service.

## Generating Test Data

For this tutorial, we'll download a sample audio file from Vast.ai's YouTube channel. You can also use your own audio file if you prefer:

```bash
yt-dlp -f "bestaudio" --extract-audio --audio-format wav -o "test.wav" https://www.youtube.com/watch?v=542xENIxKFU
```

## Voice Activity Detection

First, let's set up the VAD pipeline:

```python
import torch

device = torch.device("cuda" if torch.cuda.is_available() else "cpu")

from pyannote.audio import Pipeline

pipeline = Pipeline.from_pretrained(
        "pyannote/voice-activity-detection",
        use_auth_token=HF_TOKEN
    )

# Move pipeline to appropriate device
pipeline = pipeline.to(device)
```

Next, we'll process our audio file to identify speech segments:

```python
# Process the audio file
audio_file = "test.wav"
output = pipeline(audio_file)

print(f"Processing {audio_file} on {device}")
print("Voice activity segments:")

# Get all speech segments
speech_segments = list(output.get_timeline().support())

for i, speech in enumerate(speech_segments):
    # active speech between speech.start and speech.end
    print(f"Segment {i+1}: Speech from {speech.start:.2f}s to {speech.end:.2f}s (duration: {speech.duration:.2f}s)")
```

The output will look like this when using the default audio file:

```
Processing test.wav on cuda
Voice activity segments:
Segment 1: Speech from 6.78s to 51.62s (duration: 44.84s)
Segment 2: Speech from 53.56s to 54.27s (duration: 0.71s)
Segment 3: Speech from 55.55s to 84.76s (duration: 29.21s)
Segment 4: Speech from 86.53s to 89.03s (duration: 2.50s)
...
```

## Saving Speech Segments

Now we'll create a function to extract the identified speech segments from our audio file:

```python
import os
import shutil
from pydub import AudioSegment


def split_audio_by_segments(audio_path, segments, output_dir="output_segments"):
    """
    Split an audio file into multiple files based on speech segments

    Parameters:
    -----------
    audio_path: str
        Path to the input audio file
    segments: list
        List of speech segments (with start and end attributes)
    output_dir: str
        Directory to save the output segments
    """
    # Clear the output directory if it exists
    if os.path.exists(output_dir):
        shutil.rmtree(output_dir)

    # Create output directory
    os.makedirs(output_dir, exist_ok=True)

    # Load the audio file
    audio = AudioSegment.from_file(audio_path)

    # Extract each segment
    for i, segment in enumerate(segments):
        # Convert seconds to milliseconds
        start_ms = int(segment.start * 1000)
        end_ms = int(segment.end * 1000)

        # Extract segment
        segment_audio = audio[start_ms:end_ms]

        # Generate output filename
        filename = os.path.basename(audio_path)
        name, ext = os.path.splitext(filename)
        output_path = os.path.join(output_dir, f"{name}_segment_{i+1:04d}_{start_ms:08d}ms-{end_ms:08d}ms{ext}")

        # Export segment
        segment_audio.export(output_path, format=ext.replace('.', ''))
        print(f"Saved segment {i+1} to {output_path}")
```

Let's apply this function to extract our speech segments:

```python
split_audio_by_segments(audio_file, speech_segments)
```

This will be the output when using the default audio file:

```
Saved segment 1 to output_segments/test_segment_0001_00006780ms-00051617ms.wav
Saved segment 2 to output_segments/test_segment_0002_00053558ms-00054267ms.wav
Saved segment 3 to output_segments/test_segment_0003_00055549ms-00084760ms.wav
Saved segment 4 to output_segments/test_segment_0004_00086532ms-00089029ms.wav
...
```

## Inspecting Results

To verify our results, we'll create a function to play audio files in our Jupyter environment:

```python
import librosa
from IPython.display import Audio, display

def play_audio(file_path, sr=None):
    """
    Play an audio file in a Jupyter notebook.

    Parameters:
    -----------
    file_path : str
        Path to the audio file to play
    sr : int, optional
        Sample rate to load the audio with. If None, uses the file's native sample rate.

    Returns:
    --------
    Audio widget that can be played in the notebook

    Example:
    --------
    >>> play_audio('path/to/audio.wav')
    """
    # Load the audio file
    y, sr = librosa.load(file_path, sr=sr)


    # Return an audio widget to play the sound
    audio_widget = Audio(data=y, rate=sr)
    display(audio_widget)
```

First, we'll play the original audio file. Listen to the first minute or so to get an idea of what it sounds like before VAD.

```python
play_audio(audio_file)
```

Next, we'll listen to the first three clips to verify that we have isolated the speech in our test file:

```python
import os
audio_dir = "./output_segments/"

audio_files = os.listdir(audio_dir)
audio_files.sort()

n_clips = 3

for fname in audio_files[0:n_clips]:
    play_audio(audio_dir + fname)
```

You'll see that the audio lengths match up with the `speech_segments` output:

```
Segment 1: Speech from 6.78s to 51.62s (duration: 44.84s)
Segment 2: Speech from 53.56s to 54.27s (duration: 0.71s)
Segment 3: Speech from 55.55s to 84.76s (duration: 29.21s)
...
```

Listening to the output files, we can see that we have effectively isolated the speech.

## Conclusion

With this implementation, you now have a working Voice Activity Detection system that can identify and extract speech segments from audio files. This forms an excellent foundation for more advanced audio processing tasks like speech recognition, speaker diarization, or audio content analysis.

Look out for more content from Vast about all these other types of tasks!


Run state-of-the-art Voice Activity Detection (VAD) with PyAnnote Audio on Vast.ai for cost-effective speech processing. Extract speech segments easily!

Posts about: Audio Processing

Speaker Diarization with Pyannote on VAST

Voice Activity Detection (VAD) with Pyannote on VAST