WORLD is a C-based open-source speech synthesis system that belongs to the category of parameter synthesis. Unlike waveform concatenation, which relies on pre-recorded speech segments, parameter synthesis uses mathematical models to generate speech from features such as fundamental frequency (F0), spectral envelope, and aperiodic components. WORLD stands out for its efficiency and real-time capabilities, making it suitable for applications where speed and resource optimization are important. While STRAIGHT is another well-known parameter synthesis system, it is not open source. In comparison, WORLD offers similar or better performance in both audio quality and processing speed, so we will focus solely on explaining the principles and usage of WORLD here.
The WORLD system consists of three core modules: DIO, CheapTrick, and PLATINUM. Each module plays a specific role in the speech synthesis process. DIO is responsible for estimating the fundamental frequency (F0) of an input signal. CheapTrick calculates the spectral envelope, which represents the overall shape of the frequency spectrum. Finally, PLATINUM estimates the aperiodic parameters, which capture non-harmonic components of the sound.
Let’s start with the F0 estimation using the DIO algorithm. F0 is the lowest frequency component in a sound, representing the perceived pitch. In WORLD, DIO works by filtering the input signal with multiple low-pass filters at different cutoff frequencies. For each filtered signal, it identifies potential F0 values and evaluates their reliability based on the consistency of the signal’s zero-crossing intervals. The most reliable candidate is selected as the final F0 value.
Next, the CheapTrick algorithm is used to estimate the spectral envelope. This involves applying a Hanning window to the signal, computing the power spectrum, smoothing it with a rectangular window, and then calculating the cepstrum. This process helps extract the smooth curve that represents the formants of the speech signal.
Finally, the PLATINUM algorithm calculates the aperiodic parameters. It uses the waveform, F0, and spectral envelope to separate the periodic and aperiodic components of the signal. This allows for more natural-sounding synthesis by incorporating both harmonic and noise-like elements.
To use WORLD in practice, developers often rely on PyWorld, a Python wrapper for the C-based library. Installing PyWorld is straightforward—just run `pip install pyworld` and `pip install soundfile`. With PyWorld, you can easily extract F0, spectral envelope, and aperiodic parameters from an audio file and then synthesize new speech. Here’s a simple example:
```python
import soundfile as sf
import pyworld as pw
# Load audio
x, fs = sf.read('utterance/vaiueo2d.wav')
# Estimate F0 using DIO
f0, t = pw.dio(x, fs, f0_floor=50.0, f0_ceil=600.0)
# Estimate spectral envelope using CheapTrick
sp = pw.cheaptrick(x, f0, t, fs)
# Estimate aperiodic parameters using D4C
ap = pw.d4c(x, f0, t, fs)
# Synthesize speech
y = pw.synthesize(f0, sp, ap, fs)
# Save synthesized audio
sf.write('Test/y_without_f0_refinement.wav', y, fs)
```
After running this code, you can compare the original and synthesized waveforms. The results show that WORLD is capable of producing high-quality speech with minimal distortion. This makes it a powerful tool for research and development in text-to-speech systems. By leveraging deep learning techniques, researchers can train models to predict F0, spectral envelope, and aperiodic parameters from text, and then use WORLD to convert those features into natural-sounding speech.
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