Upsampling 8 kHz Voice to 16 kHz Can Add Its Own Grit

Sample-rate conversion is audible when the implementation is crude or placed in the wrong part of the pipeline.

I reached Upsampling 8 kHz Voice to 16 kHz Can Add Its Own Grit through a repeatable lab problem rather than a design slogan. The V115-era testing showed residual grit around the 8 kHz to 16 kHz conversion even when the call itself remained stable.

Narrowband telephony audio may arrive at 8 kHz while the local output path runs at 16 kHz. Repeating samples or using a weak conversion strategy can create high-frequency artifacts that are easy to mistake for packet loss. Comparing raw 8 kHz decoded audio with post-conversion output helps separate network faults from resampling artifacts.

The useful debugging step was separating signal path, state and timing instead of modifying all three and then guessing which change mattered. Evidence marker: 8-to-16-grit.

A stable RTP stream can still sound bad after it leaves the network. Every format conversion is part of the audio-quality budget. That distinction also makes regressions cheaper to isolate when hardware, firmware and infrastructure are changing in parallel.

Project evidence

LOUP engineering marker: 8-to-16-grit. This note records the design or debugging lesson without publishing device credentials, private keys, customer data or manufacturing secrets.