Session story · 2026-09-26-b
Session: 2026-09-26, 07:11 → 09:27 and 16:36 → 17:41 UTC (one conversation, two sittings with a night between) · Bench items: V11, V12
The arb hunt had just ended for the night on finding 8: the generator ignored EC mode writes, so it could not be put into arb mode. Multitone had been described for weeks as "blocked on arb". The analysis code, Multitone.analyse, was implemented and tested, and nothing in the app called it.
Chris asked: "so in the quest to get multitone testing, the FFT test needed a lot fo work. i think we broke it and then fixed it again. What is the current status of the FFT test and the multitone test?"
The working belief was: no multitone result until the AP's generator can play one.
Can Cascade measure a multitone today, from a source other than the AP?
A regression check first. Claude found that every completed acquisition that night had also entered arb mode. A plain sine FFT had not been run since the changes. Chris ran one (V11): −20.00 dBV at 1024.0 Hz, complete in 3 polls.
The reframe. "I'm not sure what is the most long term thorough path" Claude checked the FFT path and found no generator-derived constant in it. On DUT Input the analyzer measures whatever is on the XLR. Multitone was two halves: measuring was not blocked, and generating was. Claude proposed four steps, offline first.
Step 1 found a bug nobody was looking for. A synthetic, on-bin, distortion-free multitone reported −37.2 dB of residual. guardBins: 2 was narrower than the Blackman-Harris mainlobe, so the clipped skirts counted as distortion. No test asserted on residualDB. The new test failed on the old default at −36.3 dB, which Claude checked before trusting it.
Step 2 found a bug Claude had just introduced. "so next is step 2 and that does not need the machine. go ahead" Widening the guard to 4 made the bottom tone windows overlap at 4 Hz bins. A clamp to half the neighbour distance fixed it. Tone-finding was added so a source on its own clock could be followed.
Step 3, and Chris's design question. The readout was wired in, with a test that fails if the wiring is removed. "is the APWIN implementation like yours? A variation on FFT?" Claude read T84: APWIN's FASTTEST acquires once and reprocesses. Chris: "I think it will make sense to have it as its own configurable test. But it makes sense to make sure it works first, since it seems you're using the same mechanics for both" Checking AP's tone sets showed the generated set was unresolvable at the default N=2048, so the generator was made resolution-aware.
A Scarlett 2i2 as the source. The plan used the interface only as a DAC. Claude's setup instructions kept describing a panel it hadn't seen. "the gain knobs do not have a "full counter clockwise" because they are relative." Then: ""set the monitor control anywhere reasonable," I'm not sure how to do that. what is the "monitor control"" Claude stopped guessing and moved level control into the WAV file.
Four "check X" instructions with no X. The first baseline ran in Generator Mon. The DUT baseline timed out, which exposed a missing post-reload DSP wait on the sine path at N=16384, caused by Claude's own reload change; that was fixed. Then: "i'm not sure what status line you mean, but i am running the test 1 as specified" The status line was never displayed in any view. ""drag the WAV file from Finder onto the Terminal window. " this is the problem. Where is that file?" It had never been created. Claude generated it, and later caught that Python's banker's rounding made its first tone 16 Hz where the app's was 24 Hz. Finally: "what i mean is = you said to check it, but i don't know where that is or how to do that." "FFT input check" was a console line. Claude recorded all four in feedback_actionable_instructions.md as one pattern.
Real bugs from real signal. The first run with signal read a positive residual (+4.6 dB): binHz was taken from point spacing (2 Hz) where the true resolution at N=16384 is 4 Hz. Then the DC bins at −32.56 dBV dominated the residual, a skirt the FFT noise path already guarded and multitone didn't. With both fixed: 13 of 13 tones, flatness 0.79 dB, residual −64.2 dB. The tone frequencies read about −168 ppm across eight consecutive tones, against the +160 ppm sample-rate offset found earlier by frequency counter.
The morning sitting: setting the level. "what if i put a voltmeter on the scarlett to do an initial setting? That is live feedback." Claude's −5 to 0 dBV target turned out wrong: with a 12.6 dB crest factor the peaks would clip, so it revised to 400 mVrms. "the outputs on this scarlett are 1/4". i am playing the test tone and 208mv is as high as it goes" Chris's meter chain went through RCA, which is single-ended, so the 208 mV was the hot leg only.
Are you sure? Claude said the channels were nearly 12 dB apart, from two mantissas. "are you sure the 2 channels are 12db apart? doesn't look like it on the interface" They weren't. The two mantissas were on different range codes (3 and 5) and not comparable, a trap the project had already recorded. That capture was also flagged invalid by the app itself.
A level sweep. At ~200 mV the spectrum was a forest of spikes (−17.6 dB). At 165 mV, −67.4 dB; at 100 mV, −71.4 dB, with relay clicking that Chris attributed to a range boundary: "i think that might be right on an auto-ranging boundary. just a guess." The log showed autoranged to code 4 (requested 3). "where do you see the -71.4db ? I see it says that as "residual" but the graph itself looks like down below -120 for non-tones. am i reading that wrong?" He wasn't. Per-bin depth (−131.9 dBV) and power integrated over 4,884 bins differ by 36.9 dB.
The orange halo. Chris flagged that the Scarlett's level halo went orange. Claude designed a one-variable test: a file 12 dB quieter digitally, knob turned up to match. The voltmeter read 52 mV against 208 mV, a ratio of 12.04 dB. The residuals were −69.3 and −69.1 dB, 0.2 dB apart. The residual was independent of analog and digital level, which Claude read as source jitter, and the halo as meter scaling.
| Believed | Then | What overturned it |
|---|---|---|
| multitone is blocked on arb | only generation is | reading fftWordToDBV |
| the analysis is done | residual off by ~100 dB on a perfect signal | synthetic on-bin test |
| point spacing is resolution | the grid was oversampled 2× | positive residual on hardware |
| target −5 to 0 dBV | the multitone would clip | crest factor 12.6 dB |
| the channels are 12 dB apart | they overlap | Chris's screenshot; range codes 3 vs 5 |
| the orange halo means overload | residual unchanged with 12 dB less digital level | quiet-file A/B |
session_20260926-001338 (V11), -012036, -013634, -015508, -020648, -021630, -095208, -102235docs/mutitone.png, mt2, mt3, mt4 in docs/~/Desktop/multitone-13-48k.wav, multitone-13-48k-QUIET.wav, setlevel-1khz.wavdocs/DESIGN_DIFFERENCES.md (multitone divergences), docs/BACKLOG.md; commits 35ee274 (V11 recorded) and fbfabab (the −64.2 dB result recorded, before the morning sitting)Multitone measured on hardware from an external source: 13 tones, 24 Hz to 12.3 kHz, flatness 0.8 dB, residual −69 to −71 dB, with 82 tests. Open: making multitone its own configurable test, the EC→module 0x40 question, and how AP handles tone spacing below about 128 Hz.