MQN testing/experimentation thread

[jkeny]

We are John, aren't we?

Yep, brain fart - I'm just so used to reading the MQN thread that I thought I was in there & didn't check

[LowOrbit]

We are John, aren't we?

Yep, brain fart - I'm just so used to reading the MQN thread that I thought I was in there & didn't check

I know the feeling, Friday afternoon and all!

Mark

[jkeny]

We also have to be careful what we are analysing.
If we are looking at timing issues then staying in the digital domain to do the measurements is fine
If we are looking at noise issues we need to measure the USB electrical signal itself or examine the analogue out of the RCA from the DAC.

Examining/measuring either is going to be challenging as I imagine we are looking for very low level differences.
How do we differentiate noise from signal, for instance - FFTs?
What dB level will we consider is inaudible & ignore?

[LowOrbit]

Hi John

Those are good questions. I am pretty sure that doing any meaningful analysis in the digital domain would require expensive test equipment.

My suggestion, to keep this in the realm of the achievable (for me at least), is to compare the resultant waveforms created by sending the same file through the same equipment (pc, interface, dac) from two (or more) software players. Capturing the results using another pc (with a pro standard audio card) should allow analysis of the resultant waveform/file down to the sample level.

FFT analysis could be done on those resultant files.

I am open to suggestions. I have the kit to do the above testing (as I am sure to many of us), so it is for me just a question of finding time.

Mark

[jkeny]

I believe a good first step would be to use a USB analyser which allows us to look at the electrical signal as well as the logical signal coming out of the USB port.

I don't have such a device. A very fast oscilloscope may well suffice?

DaveF would you be willing or allowed to do such tests? Do you have access to such equipment?

[jkeny]

Mark,
I have tried this test - using an external Zoom recorder to record the analogue output from my DAC
I played the Oscar Peterson song with MQN & recorded to internal SD card in the Zoom recorded.
Imported the recording off SD card, now on the PC, into Audacity & also imported the original track
Lined them both up to the same sample Not too difficult with this track as it has a nice transient from an isolated triangle sound to line up to) - Inverted one file, rendered the two files together & did an FFT on the resultant combined file.
Nothing in the FFT i.e tracks are exactly the same!

[LowOrbit]

John

That's rather annoying then!

A fast modern digital scope might give an insight into what's happening on the USB cable. I only have access to an old, analogue jobbie - probably has a higher noise floor than than what we're trying to measure.

Mark

[jkeny]

John

That's rather annoying then!

A fast modern digital scope might give an insight into what's happening on the USB cable. I only have access to an old, analogue jobbie - probably has a higher noise floor than than what we're trying to measure.

Mark

Hey but don't let my results stop you doing the same test - try it yourself & see what your results are.
It's good to get verification or not of results across different setups

[jkeny]

One thing though - Audacity FFT spectrum plot only goes down to -90dB which I don't believe is sufficiently low enough for what we want to test here.

[sbgk]

when people say bits are bits they are really talking about values held in bytes, 8 bits to a byte, 16 bit requires 2 bytes and is called a word. The cpu can't load a single byte from memory, it deals in cache lines which are 64 bytes, so get one byte and you get a whole cache line. There are 3 levels of cache which are usually L1 - 32kb, L2 - 512 kb, L3 3 MB, in a multi core cpu L1 and L2 are dedicated to a core and L3 is shared. So the idea is to load a cache line and pass it through the cpu as efficiently as possible. The penalty for accessing data not in cache is several hundred cycles.

There's a lot going on in the CPU and plenty of things to generate noise, the above description is just a brief intro to the complexities of it all.

so it might be some measurement of things like cache misses correlates to sibilance etc