What is the ideal test signal to use to get a Static Compression Curve for a
dynamics processor (compressor)?
Can you assume that the processor operates the same across 20Hz-20kHz?
In that case, then a simple 1 kHz sin waveform that is gradually increased in db
amplitude should work as long as its slow enough to not trigger the attack and
release sections of the compressor?
The compressor I am looking to characterize is a peak FET-type, feedback with
relatively medium to fast attack and release. (All way within 1 second).
Test Signal to get Static Compression Curve?
Started by ●April 14, 2009
Reply by ●April 16, 20092009-04-16
There are two main points to bear in mind while making measurements of a
compressor's "Amplitude Transfer Function" curve.
1) It's NOT a transfer function in the same way an EE normally thinks of one. Rather, the axes are normally labelled in dB, not volts, in order to dispense with conceptual issues like "lower part of a sinusoid". Think "level" not "instantaneous value of a voltage wave", and you're on the right track.
It also means the lower left-quadrant is NOT symmetrical with quadrant #1. Minus infinity (in dB) resides in the lower left quadrant, whereas the knee, say, in an above threshold compressor, manifests (e.g., appears) ONLY in one place on the resulting DUT's characteristic curve.
In contrast, the operation of a crude, Plain Jane "clipper" is often described by plotting its transfer function against axes marked with linear volts, in which case the negative half of the sinusoid function gives rise indeed to a knee in BOTH quadrants, not just in quadrant 1.
2) The original poster's question does indeed relate to measuring the steady state response of the DUT to the amplitude level of an input signal, as another reply has already indicated. I'll merely add that a well designed measurement system, like the "Audio Precision" company's products (System ONE; System TWO; etc.) will wait for the DUT output to settle prior to storing the measured output as a data point (making life easy).
If you roll your own measurement setup, just wait for the DUT's output to settle, say, once having bumped the input amplitude by some amount (typically 1 or 2 dB), prior to recording your observation as a data point. Equally, in an automated measurement setup, build this logic into the process, much as Audio Precision does.
As to the answer to the OP's question: be sure to use a steady state sinusoid of a mid-band frequency. Then be prepared to await the DUT settling time after each step in stimulus amplitude (i.e., a stepped-swept amplitude stimulus). Otherwise, if using a continuously swept amplitude stimulus, be sure to use a sufficiently slow sweep rate, namely: one that is slower than the time constant of the DUT (i.e., be aware of the DUT's attack and release times, and their definitions).
Get a copy of Bob Metzler's Handbook of Audio Measurements from Audio Precision's web site library section. He's the man! These guys were the first to suggest and use the unit: "dBFS", and got its definition adopted by the IEEE. Read the story in a back issue of their newsletter: "Audio.tst".
1) It's NOT a transfer function in the same way an EE normally thinks of one. Rather, the axes are normally labelled in dB, not volts, in order to dispense with conceptual issues like "lower part of a sinusoid". Think "level" not "instantaneous value of a voltage wave", and you're on the right track.
It also means the lower left-quadrant is NOT symmetrical with quadrant #1. Minus infinity (in dB) resides in the lower left quadrant, whereas the knee, say, in an above threshold compressor, manifests (e.g., appears) ONLY in one place on the resulting DUT's characteristic curve.
In contrast, the operation of a crude, Plain Jane "clipper" is often described by plotting its transfer function against axes marked with linear volts, in which case the negative half of the sinusoid function gives rise indeed to a knee in BOTH quadrants, not just in quadrant 1.
2) The original poster's question does indeed relate to measuring the steady state response of the DUT to the amplitude level of an input signal, as another reply has already indicated. I'll merely add that a well designed measurement system, like the "Audio Precision" company's products (System ONE; System TWO; etc.) will wait for the DUT output to settle prior to storing the measured output as a data point (making life easy).
If you roll your own measurement setup, just wait for the DUT's output to settle, say, once having bumped the input amplitude by some amount (typically 1 or 2 dB), prior to recording your observation as a data point. Equally, in an automated measurement setup, build this logic into the process, much as Audio Precision does.
As to the answer to the OP's question: be sure to use a steady state sinusoid of a mid-band frequency. Then be prepared to await the DUT settling time after each step in stimulus amplitude (i.e., a stepped-swept amplitude stimulus). Otherwise, if using a continuously swept amplitude stimulus, be sure to use a sufficiently slow sweep rate, namely: one that is slower than the time constant of the DUT (i.e., be aware of the DUT's attack and release times, and their definitions).
Get a copy of Bob Metzler's Handbook of Audio Measurements from Audio Precision's web site library section. He's the man! These guys were the first to suggest and use the unit: "dBFS", and got its definition adopted by the IEEE. Read the story in a back issue of their newsletter: "Audio.tst".
