Audio Restoration Lab

Microcosm — fixing the muffled audiobook

George Gilder, Microcosm (1989 cassette source, 14h 28m). Same 30-second test passage every time, loudness-matched so you're judging the sound, not the volume.

✅
Winner found — Adobe Podcast (Enhance Speech v2)
After four misses, the Adobe pass sounds genuinely good to you. A/B it against the raw tape below. Next step is batching the full 14.5-hour book through the same engine into a clean .m4b with chapters + cover preserved.

The test passage

"…human imagination and moral revival. The exemplary technology of this era is the microchip. The computer inscribed on a tiny piece of processed material. More than any other invention this device epitomizes the overthrow of matter. Consider a parable of the microchip once told by Gordon Moore, chairman of Intel…"

A/B — the fix vs the raw tape

✓ ADOBE v2 Enhance Speech — the winner "sounds really good"
Adobe's neural restorer rebuilt the full top end naturally — clear consonants, real "air", no underwater smear. His voice, just un-muffled.
BEFORE Original raw tape boxy / muffled
The untouched 1989 source for reference.

Local contenders — offline, no Adobe account

LOCAL · 1 Resemble-Enhance denoise + generative restore
Strongest general local restorer. Rebuilds natural detail; far less "underwater" than VoiceFixer. Keeps his exact voice. Runs fully offline on your M4.
A/B this against the Adobe winner above — does it hold up?
LOCAL · 2 AudioSR — diffusion super-resolution (speech model) strong top-end rebuild
Diffusion model rebuilds the highs iteratively. Bright and detailed; listen for any faint diffusion "shimmer" on sibilants. Local.
LOCAL · 3 AP-BWE — purpose-built bandwidth extension (12k→48k) gentlest / most conservative
GAN trained to rebuild a band-limited signal to 48 kHz. The most restrained of the three — keeps your real content, adds a modest, clean top end. Local, and very fast (~1.4s for 30s).
How they compare on paper (energy rebuilt above 6 kHz, after matching loudness): Resemble −43 dB (most aggressive) · AudioSR −47 · Adobe −52 (natural) · AP-BWE −55 (gentlest) · raw original −58. More lift isn't automatically better — too much can sound hissy/brittle. Trust your ears; this is just the map.

Your idea — resynthesis (know the words + the delivery, rebuild clean)

NEW · RESYNTHESIS IndexTTS-2 — his cloned voice re-reads it, clean fully synthetic
This is the "fill in the blanks with what we know he's saying" idea. We cloned his voice from the Adobe-cleaned clip, fed it the transcript, and steered the tone with his original recording. The output is brand-new, perfectly clean audio — but it's a clone re-performing, not his real waveform. Listen hard: does it sound like him, or like a very good impostor?
The honest tradeoff: this gives perfect fidelity but loses his real voice, and any transcription slip becomes a wrong word. Over 14.5 h that's the risk. Bandwidth-extension (Adobe / the locals above) keeps his actual voice, just clearer.

You can see the fix too

original spectrogram
Before — energy dies above ~5 kHz (dark top band).
adobe spectrogram
After Adobe v2 — top end rebuilt with natural detail.

Next — the full book

Doing all 14.5 h by hand on the website is painful (free tier caps daily minutes; long-file upload is paid). The clean path is Adobe's Enhance API — same engine, batched:
  1. Split the .m4b at its existing chapter marks into ~30–60 min WAV chunks.
  2. Run each chunk through the Enhance API (the exact v2 you just approved).
  3. Loudness-normalize to the −18 LUFS audiobook standard and reassemble.
  4. Re-wrap into a new .m4b with chapters + cover art preserved.
I'll confirm the API access + cost before running anything — you approve the plan first.

Already tried — you rejected these

Expand to re-hear the four that didn't land
A · NODSP remaster (EQ)"same recording, brighter"
B · NOVoiceFixer AI restore"underwater / smeared"
C · NOVoice conversion"AI-processed"
D · NOFull TTS re-narration"lost the human"