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DIY Audio Analyser using CS4398 / CS5381

Posted 21st August 2015 at 05:16 AM by googlyone

Over the years I have played with measuring - well - just about everything in audio.

distortion measurement has been one of these preoccupations, not the least because it is hard to do, and using commercial gear, expensive.

I have built analogue hear to do this, used mixes of analogue and digital and of late played with using modified commercial sound cards. My efforts on the Sound Blaster x-Fi were interesting but ultimately not rewarding enough for me to leave the mods in there.

I have also been playing with using the A/D and D/A converters from my DSP crossover for this purpose. These are modular and use a common I/O plug with power MLCK, SCLK, LRCLK and Data (alone with I2S for the CS4398 and sometimes a digital volume control).

--- notwithstanding the fact that in previous tests I have shown the digital volume control is the MAJOR source of distortion - I will plug ahead with this, and likely leave that out of the test system.

A recent thread / discussion with JENS here:

kicked me into a higher gear on this. The interesting thing was the ability to get data in and out of my PC without using a Sample Rate Converter. Tests I have done showed that the spectral impact of this was both obvious and unacceptable for a decent measurement system.

An additional issue I found was that many clock sources used in USB interfaces are absolutely awful, with jitter all over the place. When I built my oscillator, I intentionally used an odd clock rate for the DACS, as this allowed me to use no jittering / odd timing on the clocks. The effect on the spectral output of the sinewave was unbelievable. (Refer earlier blog).

A last thing that was essential was to galvanically isolate the thing from my computer. Connecting grounds in the test setup almost always ended in a "grasslike smear" of spurs.

These three things rolled into the design, which is an expension of the simple PIC microcontroller interface I built for my ADC / DACs.

Where am I at?
- I have built up an interface card that allows me to use a local clock;
- Provides power
- Provides galvanic isolation from the PC
- Interfaces to the minidsp streamer. I did talk to Jens about his work in this area, which confirmed a bunch of my design decisions. He does some clever stuff in extracting the sample rate from the Minidsp device - I decided not to include this, as this is a very dedicated piece of kit for me - and will be set to one sample rate. (Talk to him if you are interested in this.)

I was going to send this out to a fab - but I had a spare day and wanted to get my hands dirty. So I made the double sided PCB in my shed.

Click image for larger version

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It is not perfect, and I had to put wire links in for all the vias took an hour or so, but I find this sort of thing therapeutic.

The interface is all set to work now, so tonight it is time to give the minidsp streamer a whirl!

I will be really interested to see what performance I can extract from the CS devices. (note that the analogue interfaces to the CS4398 / CS5381 are pretty much "off the shelf" CS circuits.)
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