Meta-Instrument
Sun, 2006-09-03 12:17 — Peufeu
We have oscilloscopes, multimeters, function generators and distortion analyzers.
All these share some common concepts. It would be nice to have a computer-controlled measurement rig which would be able to emulate all these functions, automate measurements, and perform interesting calculations on the measured values.
So, this will be a spinoff project of the computer-controlled WTF DAC. It will use the same comcept : FPGA controller board connected to a computer, driving a number of peripherals, mostly DACs, ADCs, and switches ; coupled to various flavors of analog input and output stages.
Its purpose is to measure things that are usually not measured for lack of the required equipment. As Hans Reiser would say, for the last century of audio development, people have been measuring what is measurable rather than what is relevant. Sometimes, what is measurable gives perfectly relevant information. Sometimes, it has absolutely no worrelation with the sonics.
It should be able, via simple software configuration and wire plugging, to do the following measurements :
Standard functions :
- Distortion analysis
- Transfer function analysis
- Arbitrary waveform generation
- Oscilloscope
- Multimeter
- Injecting a signal in any part of a circuit, under any form :
- known-impedance voltage disturbance
- modulated current source
- stray magnetic field
- vibrations
- etc
- Measuring various things in a circuit, being the result of the injected perturbation or not :
- current
- voltage
- differential voltage
- stray magnetic fields
- sound pressure
- vibrations
- etc
- Synchronizing with any event related to one of the measured values :
- rectifier diode switching
- zero-crossing of a waveform
- etc
- Being controlled by a computer
- Being able to measure itself for simple calibration
- A board to connect to the computer, with :
- Ethernet or USB2 port
- FPGA for fast, real-time processing, and easy reconfiguration
- Many output pins
- RAM to store the experimental signals and results
- Many output pins
- An analog board with :
- Low-jitter clock
- Multiple precision A/D and D/A chips for data acquisition and signal generation:
- high-precision audio converters operating at audio frequencies (24/192k)
- high-speed low-bit converters for digital oscilloscope
- low-speed converters for offset compensation
- Digital Inputs and Outputs (not digital audio, but just signal pins)
- I/O modules with the relevant capabilities :
- DAC buffering with voltage output, software controlled offset and maybe even gain
- ADC buffering and protection with voltage input (single ended and differential) and software-controlled offset and maybe even gain
- Power amplifiers (to inject stuff into power supplies)
- Various level adapters
- Comparators for triggering
- The usual THD measurements
- Non-stationary, uncommon signal measurements
- Measuring, and listening to amplifiers or amplifier stages open-loop to magnify defects (use a software to compensate the offset and drive with an input signal)
- What is the impedance and transfer function of a power supply, depending on wether the rectifier diodes conduct or not ? How much crap goes through the transformer ?
- What is the output impedance and transfer function of a power amplifier, depending on wether one or both of the output transistors conduct ?
- Measuring thermal inertia effects in active and passive components : bias point drifts, distortion spectrum drifts, etc.
- Measuring transistors (characterizing each parameter according to voltage, current, frequency and temperature)
- Spying inside live gear : Inject arbitrary signal (voltage, current) at the input, output, power supply rail, ground, or any other point in a live system. Record various currents and voltages (output, feedback error, driver current, etc) and perform FFT, correlation, or statistical analysis.
- Measuring microphony (stick a piezoelectric transducer on a component, drive it with various signals, record the effect)
- Measuring signal distortions which are correlated with other variables
- Gremlin microscope : measure the errors and distortions introduced by a circuit, by measuring its input and output signals, using software control to match offset and gain, using numerical analysis to compensate for linear errors (transfer function and phase), then, in software, multiply the error signal and add it to the original music, then listen.
- Anything, really.