Ghost

This page is meant to track ideas about low-delay, high-quality audio coding. The work has just started, so don't expect anything in the near future (or at all for that matter).

Signal types

There are many signal types that can be found:

• Sinusoids
  • A few pure (or nearly pure) tones
• Harmonic
  • Periodic waveforms (e.g. voice)
  • Many (sometimes closely spaced) harmonics
• Shapred noise
  • Signals that are (or are indistinguishable) from filtered (coloured) white noise
• Transients
  • Whatever does't fit above I guess

Signal analysis

Sinusoidal

Good when most of the energy is contained in a few sinusoids. May be problematic for very harmonic signals, e.g. a male voice may have close to a hundred harmonics in the full audio band.

Pitch

Good for harmonic signals. Hard to estimate and code when extra sinusoids and noise are present.

MDCT

Very general. Can code anything, but not very good at anything. High delay (2x frame size).

Codec Structure Ideas

Sinusoidal + wavelet

• Preemphasis
• Extract as many sinusoids as possible
• Wavelet transform
• Code wavelet coefs using VQ

Sinusoidal, pitch and noise

• Preemphasis
• Joint pitch + sinusoidal estimation
• LPC analysis
• CELP-like coding of the residual (mainly noise)