Philbo King
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I've been thinking about how to test outboard effects boxes. I haven't worked out comprehensive methods for everything. But I'll start with this.
Dynamics processors (gates, compressors, expanders)
There are a few relevant items for a functional test (not getting into Zin, Zout, noise floor or similar 'figure of merit' tests, which would be nice but consume a large amount of time).
- Time response and curve of attack and release controls
- Steady-state amplitude response to signals below and above the threshold level, including frequency response.
- Frequency response of side-chain input (if present)
- Threshold control accuracy
- Ratio control accuracy
In doing testing in the past I constructed custom test signals using a DAW program. These consisted of bursts of swept frequency and bursts of 400 Hz sine tone mixed with a constant 1 KHz tone 20 dB below the primary tone, at various levels, in 1 dB increments, with a couple seconds of silence between each burst. This would be fed in to the unit under test while recording the output of the unit back into the DAW.
Amplitude droops/peaks during a sweep gave an indication of frequency response issues, indicating a more detailed FR test should be done at that particular level.
The attack and release time measurement show how the unit reacts when the input signal exceeds the threshold level (for compressors, at least).
For attack time, I'd measure the amount of time it takes to start reducing gain after that event (for a compressor).
The attack and relaease 'waveshape' is mostly a point of interest: is it linear, exponential, log, or whatever. There is no right or wrong on this, it mostly is a characterization of 'type' (optical vs. FET vs. VCA compressors)
For release time, I'd measure the time between when a 400 Hz burst ends till the 1KHz steady tone returns to the pre-burst gain level.
Because of the number of controls and how they can interact, a number of tests must be run, one for each knob combination. So you end up with multidimensional results. I keep track of all that in multiple Excel spreadsheets. Tedious stuff...
I need to think on this further, and may add to this post if any lights come on for me. Any comments, suggestions and thoughts are welcome.
Dynamics processors (gates, compressors, expanders)
There are a few relevant items for a functional test (not getting into Zin, Zout, noise floor or similar 'figure of merit' tests, which would be nice but consume a large amount of time).
- Time response and curve of attack and release controls
- Steady-state amplitude response to signals below and above the threshold level, including frequency response.
- Frequency response of side-chain input (if present)
- Threshold control accuracy
- Ratio control accuracy
In doing testing in the past I constructed custom test signals using a DAW program. These consisted of bursts of swept frequency and bursts of 400 Hz sine tone mixed with a constant 1 KHz tone 20 dB below the primary tone, at various levels, in 1 dB increments, with a couple seconds of silence between each burst. This would be fed in to the unit under test while recording the output of the unit back into the DAW.
Amplitude droops/peaks during a sweep gave an indication of frequency response issues, indicating a more detailed FR test should be done at that particular level.
The attack and release time measurement show how the unit reacts when the input signal exceeds the threshold level (for compressors, at least).
For attack time, I'd measure the amount of time it takes to start reducing gain after that event (for a compressor).
The attack and relaease 'waveshape' is mostly a point of interest: is it linear, exponential, log, or whatever. There is no right or wrong on this, it mostly is a characterization of 'type' (optical vs. FET vs. VCA compressors)
For release time, I'd measure the time between when a 400 Hz burst ends till the 1KHz steady tone returns to the pre-burst gain level.
Because of the number of controls and how they can interact, a number of tests must be run, one for each knob combination. So you end up with multidimensional results. I keep track of all that in multiple Excel spreadsheets. Tedious stuff...
I need to think on this further, and may add to this post if any lights come on for me. Any comments, suggestions and thoughts are welcome.
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