What Cascade does

What Cascade does, and how

Cascade is a native macOS app that runs an Audio Precision SYS-2522 audio analyzer directly over USB. No Windows PC is involved, and neither is APWIN, AP's own Windows software. It makes the measurements an audio engineer uses the instrument for: spectrum, distortion, frequency response, level and power. Chris, a hardware engineer who owns the 2522, built it with Claude Code between April and October 2026. Every command it sends was worked out from USB captures of APWIN talking to the instrument.

This page is for readers who know audio or electronics but not necessarily software. It covers what the app measures, how it gets the numbers, and how far each number can be trusted today.

Why build it

The hardware is excellent; the software path to it is narrow. The SYS-2522 talks over APIB, AP's own bus, not GPIB. A third-party adapter turns APIB into USB, but the only software that drives the instrument is APWIN, which runs on Windows. No Mac software existed. Using the analyzer meant using a Windows machine.

Native, with nothing bridged. In August 2026, while the FFT looked out of reach, Claude suggested a fallback: let APWIN compute the spectrum and pass the result across. Chris turned it down:

"Oh got it - the bridge is a non-starter. I want this Mac app independent from APWIN and a PC. It needs to run on its own and be just as accurate and precise as APWIN is"

That constraint shapes everything in this section. Each capability had to be reverse-engineered from the USB traffic and rebuilt on the Mac. APWIN is still used as the reference the app is checked against, but the app never depends on it.

Something another person could trust. The longer-term goal is portability. In Chris's words (September 2026):

"ideally, we can get this software to a place where i can give it to someone else to run with their hardware, and they can trust that it is accurate and precise and functions to spec."

The app is not there yet. Some calibration constants belong to this one unit, and there is no built-in self-check. See Where it's going.

Learning to work with an AI coding tool. Chris also started the project "partly to learn how to work better with claude code." That half of the story, including the corrections and the retracted conclusions, is told in The Correction Log.

The instrument, in one minute

An audio analyzer has two halves:

A generator makes a very clean test signal, usually a sine wave at a frequency and level you choose.
An analyzer measures what comes back: its level, its frequency, and how much it differs from the pure tone that went in.

You connect the generator to a DUT (device under test: a preamp, an amplifier, a cable), connect the DUT's output to the analyzer, and the difference between the two tells you what the device does to a signal.

The SYS-2522 is a two-channel, dual-domain System Two Cascade. Things that matter on this page:

Two channels, A and B. Each has a generator output and an analyzer input, so stereo devices can be tested in one pass.
GenMon or XLR input. GenMon (generator monitor) means the analyzer reads the AP's own generator internally, with no cable. It is the cleanest possible signal and the app uses it to calibrate itself at connect. XLR is the front-panel input where a DUT's output plugs in. The app calls this the DUT input.
dBV is level in decibels relative to 1 volt RMS: 0 dBV is 1 V, −20 dBV is 0.1 V. Cascade's generator covers −50 to +22 dBV, and both ends were measured.
Inside the box, the analyzer computes its own spectrum. The app reads back numbers the instrument has already calculated. It does not do the transform on the Mac.

In this section

How Cascade talks to the instrument: capture, replay, commands, decoding, and the safety rule.
The measurements: the terms, then one page per measurement: FFT spectrum, Multitone, Frequency response, THD vs frequency and level, Power & THD, IMD.
Running it, and trusting the numbers: unattended scripts, how the numbers are checked, and where it is going.