Just following on from what people have been saying re the M7 USB module, I think there may be some flaws with it and bypassing it/disconnecting it seems worthwhile.
Had some spare time this morning so I hooked up my scope to the M7 to measure waveforms to quantitatively check different sampling rates, filter settings, oversampling settings, etc (I wasn't particularly satisfied with my past square wave measurements so just wanted to explore that further for my own sake).
I didn't really get anywhere re improving my square wave measurements, but I did notice that when I used the USB input (my default input), a 100 Hz sine waveform was visible on the rising and falling edges of my square waves (seemed like amplitude modulation of the square wave edges). The 100 Hz signal was continuously visible when I tested with 96 kHz and 192 kHz .wav files but was only intermittent at 44.1 kHz.
It may be a mains harmonic (50 Hz mains in Aus) - and I might test this further. However, the 100 Hz waveform was near negligible when I switched to optical input suggesting that it's related to the USB module only.
I also noticed visible jitter when viewing waveforms (USB and optical) - which makes me want to test with I2S input and an external clock source.
Edit:
I didn't capture any waveforms at the time, but will do and post them up - I understand most of the above is irrelevant without some pictures.
Waveforms for 1 kHz square wave at various sampling rates. M7 jumper settings were default (e.g. 130 dB filter, 8 x OS, PLL on, dither on).
Waveform captures were undertaken at 5 ms time resolution so that waveform envelope distortion could be easily seen.
44.1 kHz Optical - No issues. The rising and falling edge 'spikes' you see are acceptable and due to the waveform being band limited.
44.1 kHz USB - Generally fine, however notice intermittent distortion to waveform envelope.
44.1 kHz USB - Zoomed in on one channel to better see the distortion/dip.
192 kHz Optical - Notice distortion at peaks/envelope of waveform. ~100 Hz sine wave amplitude modulation.
192 kHz Optical - Zoomed in one channel. Distortion, ~80 mVolts peak-peak.
192 kHz USB - Distortion is more evident/more pronounced.
192 kHz USB - Zoomed in on one channel. Distortion ~110 mVolts peak-peak.
Based on the above, I think the optical input is better than the USB. There seems to be intermittent glitching/distortion issue with USB at 44.1 kHz which doesn't occur with optical. And both suffer from the AM distortion issue, but it is measurably less with optical.
I am not sure on the reason for the AM distortion which seems to appear at frequencies > 44.1 Khz and get worst with higher sample rates. If it were a mains/ground loop issue, I'd think it would appear at all times, irrespective of sample rate?
I thought maybe the USB module was causing this problem based on reports from some to disable the USB module - but when I unplugged the USB module, the distortion remained when using optical.
I was also unable to EQ out the 100 Hz signal (thought I'd try it).
Maybe the distortion is being added by the DSP. Guess the next things to try are:
1. Bypass mode (bypass the DSP)
2. Different filter settings
Note, I'm also fully aware that the distortion could be due to my setup too. I don't have any power conditioning, and when on USB, I'm connecting directly from laptop USB (noisy)... just odd that I'm getting various degrees of distortion with different variables.
