So far, here's what I have done: Used a crystal locked, two-tone generator ( 700 and 1900Hz) to drive a speaker 5 cm from the microphone, adjusting the channel for O dB output. Looking at the generator and channel output on a dual channel scope, I cant' see a difference. Conceptually I know there is, however I can't see it or hear it.
Moving the "B" channel on the scope to the mix+ output (connection to the audio encoder and TX) , I still can't hear a difference, however on the scope, it's very slightly different on the edge (peaks) of the waveform.
Lacking lab grade instruments, what else can I do?
Last Edit: Sept 14, 2019 12:40:07 GMT by sparepart: edit for clarity
In the modern era of digital audio, frequency response and distortion tests aren't done as much, when you think about it, because there are few adjustments in the audio chain. We're more worried about codec quality and that the stream will make it to the transmitter or audience listening on line.
I guess you're doing a two-tone test for intermodulation distortion? Maybe you could drop the signal from the generator to mic level, and send it into the mic channel directly.
My favorite waves are sine, square and sawtooth/triangle. You can get an inexpensive function generator, or a computer with a good sound card and a signal generator program can do it.
On the scope, sine checks for clipping level, or if the peaks are being loaded. You can see harmonics when they get to higher levels. Square can check for frequency response, tilt. The saw can be used for closer checks on linearity in an amplifier or transmitter, making sure the sloping sides are properly straight.
I do those checks, and then comes the wonderful destruction of clipping and pre-emphasis and checking it again after all of that!