Worklog for task "Research qualitative programmatic human voice synthesis in the browser"

The fundamental physical limit for high-quality speech reproduction and benchmarks for the Russian language have been investigated.

Key takeaways:

• A single audio channel (mono) is sufficient for reproducing the human voice: at any single point in space, air pressure is described by a single scalar time function. Stereo is needed for spatial features, not for the timbre or phonemic intelligibility of the voice itself.
• ITU wideband speech: approximately 50–7000 Hz. According to the Nyquist theorem, preserving this band requires a sample rate >14 kHz; the practical standard is 16 kHz. This yields raw PCM 16-bit mono = 16,000 * 16 = 256 kbps.
• ITU super-wideband: 50–14,000 Hz; practical sample rate >=32 kHz, raw PCM 16-bit mono = 512 kbps. 48 kHz/16-bit mono = 768 kbps, which covers the fullband of human hearing much wider than typically necessary for speech.
• For compressed speech, Opus RFC 6716 specifies sweet spots: 8–12 kbps for narrowband speech, 16–20 kbps for wideband speech, 28–40 kbps for fullband speech.
• G.722 defines high-quality wideband speech at 50–7000 Hz at 64 kbps (an older SB-ADPCM codec); modern Opus achieves comparable or better subjective efficiency at a noticeably lower bitrate.
• The Russian OpenSTT (~20 thousand hours) uses mono, 16 kHz, int16 as its main practical format; their former MP3 profile was 16 kHz mono 32 kbps. This is a useful Russian empirical baseline, although not a fundamental minimum.

It is important to distinguish between storage/transmission bitrate and synthesizer complexity: a single output audio channel can be derived from many internal sources/filters (glottal source, aspiration, frication, resonances, nasal branch, etc.). The number of internal DSP components does not equal the number of output audio channels.