What Cascade does · IMD
What it shows. IMD (intermodulation distortion): what two tones played together do to each other. A nonlinear device creates new frequencies at sums and differences of the two tones, which is often more audible than plain harmonics. The standard tests are SMPTE (a low and a high tone), CCIF (two close high tones) and DIM.
The theory. The 2522 generates the two-tone signals in its analog hardware and has a demodulator that measures the products. Chris insisted it be done the way APWIN does it, with no extra audio interface: "i'm going to be stubborn about not using the extra focusrite equipment. APWIN doesn't need it. neitehr should Cascade."
Operating it. Not yet a tab. In the Developer panel (View ▸ Developer Tools): IMD PROBE (T92 SMPTE) replays APWIN's T92 session up to SMPTE 1:1 (60 Hz and 7 kHz, 0 dBu, GenMon), reads the function meter for about six seconds and compares with APWIN. Generator off at the end.
Status: works, one mode (2026-10-18). T92 confirmed the old T90 probe was a D/A sine (its log rejected the IMD settings; T92's did not). Real IMD uses the analog generator: one register selects IMD, one picks the type, one holds the high tone. On the 2522 the probe read a reference level of 17,521 where APWIN read 17,520, and a raw residual within 2 % of APWIN's. Still open: probes for SMPTE 4:1, CCIF, DFD and DIM 30, and the scale APWIN applies to turn the raw reading into the number it reports (about ÷5.5 for SMPTE, estimated from noise-floor readings). The register details are on The USB–APIB link.