Characterisation of the Octo - noise and harmonic distortion tests

[flatmax]

Hi there,

Here is my analysis of the noise on the Audio Injector Octo. It is pretty good.

I did two experiments, connecting the DAC to the ADC using :

1. Wires connecting pins to pins.
2. RCA cables connecting analogue buffer output to input.

Using a sample rate of 96 kHz, I see the following :

• Dynamic Range of 90 dB (arguably better)
• RCA connector harmonic distortion (1st harmonic) of -70 dB
• Without the RCA connectors (DAC pin to ADC pin connections) 1st harmonic of -Inf dB, the harmonics can't be found because they are in the noise floor.

Here is the graph of the results ...

Audio Injector noise analysis at 96 kHz

au.octo.loopback.noise.test.png (70.24 KiB) Viewed 2132 times

Here are the resulting audio files I recorded :
For the pin test here.
For the RCA test here.

Here is the matlab (octave) script I used for analysis :

Code: Select all

function analysis
% Author : Matt Flax
% Date : 2017 08 26
[xp,fs]=audioread('96kHz.test.v1.pin.wav');
[xr,fs]=audioread('96kHz.test.v1.rca.wav');
Np=size(xp,1);
Nr=size(xr,1);
N=min(Np,Nr)
idxs=round(1*N/8):round(7.2*N/8);
x=[xp(idxs,1) xr(idxs,1)];
N=length(idxs);

% rescale
maxs=max(abs(x));
fact=maxs(1)/maxs(2);
x(:,2)=x(:,2)*fact;

figure(1); clf
plot(idxs/fs,x)

X=20*log10(abs(fft(x/(length(idxs)/fs))));
f=linspace(0,fs,size(X,1));
figure(2); clf
semilogx(f,X)
grid on
set(gca,'xlim',[400 10e3])
title('Audio Injector - octo pin to pin - noise measurements for a 1 kHz tone')
xlabel('f (Hz)')
ylabel('dB')
legend('pin 2 pin','rca 2 rca')
end

[flatmax]

I have done a second test using my PC as the source and sink, with the audio injector octo in loopback mode. I am using the onboard audio from the ASUS X99-A motherboard.

In this test both the PC and the Octo were setup to be running at 96 kHz sample rates. I used stereo RCA cables to connect the two systems together. The Pi was powered from the PC's USB port. The setup is shown in the following image :

Connections between the Octo and the PC

PC.octo.setup.png (62.36 KiB) Viewed 2129 times

The ALSA full duplex loopback code is almost the same as that from the gtkIOStream ALSAFullDuplexTest.C found here. The only difference is that I changed the audio device to "default" and the sample rate to 96 kHz.

The results are similar to the direct loopback on the Octo :

• Dynamic Range of 90 dB (arguably better)
• RCA connector harmonic distortion (1st harmonic) of -70 dB

The results are shown in the following image :

Audio Injector Octo PC loopback test

au.octo.pc.noise.test.png (59.97 KiB) Viewed 2129 times