Abstract
The top band of both channels is given zero bits. A zero-bit band codes nothing, decodes as silence, and its band coder never moves; after a stereo-domain switch it is put back to its power-on state. The filter tree then acts as the codec's low-pass (−3 dB at about 17.2 kHz). The four freed bits go to the middle bands: A [8,6,3,0] / B [6,5,3,0] as the default, and A [8,5,4,0] / B [7,4,3,0] as an operator-selected mode for good signal conditions. Below 18 kHz every track measured the same (within 0.1 dB) or better. The step-size field now cycles over the coded bands only.
The encoder may additionally feed the lowest band's coding error back into that band's own quantiser input: the coder is given u = x + Σ c_k·e[n−k], with e = recon − x the reconstruction minus the original band sample, and 1 − Σ c_k z^−k = A(z/γ), where A is a 4th-order LPC of the band's own input and γ is fixed. The coding error becomes the quantiser's white noise coloured by the band's own spectral envelope. Nothing is sent and no delay is added. On a masked noise-to-mask measure it improved every reference track by 1.4 to 5.5 dB.
Creative Commons License
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Recommended Citation
Anonymous, "A frameless sub-band ADPCM codec shaped for a block radio link: a zero-bit top band, a step-size field over the coded bands only, encoder-only in-band noise feedback through a bandwidth-expanded LPC of the band's own input (floating point, batched, and a bit-exact integer form with an FPGA unit), fixed loading thresholds per link profile, and 10-bit lowest-band allocations (second version)", Technical Disclosure Commons, (October 20, 2026)
https://www.tdcommons.org/dpubs_series/12316