This is an M4 recently bought off Amazon Warehousedeals. It has firmware 2.0 and the additional screws on the combi connectors, so I suspect it is v3 but haven't opened it up.
Performance in loopback is better than what Amir measured (I suspect on v1), however, I get strange spurious tones such as twin peaks at 2.9 and 3.0 kHz instead of the pure third harmonic I'm getting at 48 kHz sampling rate. Also note the 4.9 kHz tone. This happens at both 44.1 and 88.2 kHz but not at 48, 96 and 192 kHz. Does the Motu have only a multiple of 48 kHz as a clock and use a sample rate converter or analog PPL to generate 44.1 and multiples?
Interestingly, the THD and THD+N numbers do not change significantly. I'm even getting slightly higher THD with 48 kHz, e.g. 0.0020 instead of 0.0018%. Some of the energy of the harmonics ends up away from the harmonic frequency, but ARTA should account for everything that is not at the fundamental.
This was measured with a 6.3 mm symmetric cable from Line Out 3 to Line In 3 on the left channel. Setting 0 dB as a generator level resulted in a wide floor of spuriae at -100 dB. Setting -1 dB results in -3.9 dB being displayed in ARTA in the dB fs setting. Not sure why this is, so far I thought dB fs ingnores the calibration.
Setting generator level at -3 dB in ARTA results in the peak sitting at -5.9 dB. THD+N increases from 0.0005% to 0.0006 but THD improves from 0.00018 to 0.00013, with the 2nd harmonic at roughly -124 dB from the fundamental and 3rd at -120 dB.
Performance in loopback is better than what Amir measured (I suspect on v1), however, I get strange spurious tones such as twin peaks at 2.9 and 3.0 kHz instead of the pure third harmonic I'm getting at 48 kHz sampling rate. Also note the 4.9 kHz tone. This happens at both 44.1 and 88.2 kHz but not at 48, 96 and 192 kHz. Does the Motu have only a multiple of 48 kHz as a clock and use a sample rate converter or analog PPL to generate 44.1 and multiples?
Interestingly, the THD and THD+N numbers do not change significantly. I'm even getting slightly higher THD with 48 kHz, e.g. 0.0020 instead of 0.0018%. Some of the energy of the harmonics ends up away from the harmonic frequency, but ARTA should account for everything that is not at the fundamental.
This was measured with a 6.3 mm symmetric cable from Line Out 3 to Line In 3 on the left channel. Setting 0 dB as a generator level resulted in a wide floor of spuriae at -100 dB. Setting -1 dB results in -3.9 dB being displayed in ARTA in the dB fs setting. Not sure why this is, so far I thought dB fs ingnores the calibration.
Setting generator level at -3 dB in ARTA results in the peak sitting at -5.9 dB. THD+N increases from 0.0005% to 0.0006 but THD improves from 0.00018 to 0.00013, with the 2nd harmonic at roughly -124 dB from the fundamental and 3rd at -120 dB.