Alright. So, I’ve learned a few things since my last post.

1. The type of work I’m most interested in doing rarely allows for public discussion. For example, for the past 10 months or so, I worked with a research group at the University of Michigan directed by Professor Mike Flynn. It was a fantastic experience, but most of the projects I worked on I could hardly discuss in detail outside of our research group, let alone here. Similar professional standards apply to projects in the commercial sphere as well, so that sort of shuts down the idea of posting as a means of keeping track of (and perhaps inspiring) my work.

2. I hate talking about myself. That’s nothing new to me, but it’s certainly been reaffirmed. I have probably 5 or 6 drafts of posts talking about various school projects and the like that I can’t bring myself to post. It just seems too self-involved.

In response to these problems, I think I may re-purpose this site. Instead of focusing on professional-related nerdy things, I would like to use it as a repository for more random nerdy things I find fascinating. For example, I am particularly enamored with cognitive psychology. The way we process the massive amounts of information we absorb each day – and certainly the relative ease with which we do so – is nothing short of incredible.

Also, I would love for this to become more of a forum for discussion, rather than me just spouting off my own ideas and interpretations of other people’s ideas. So, please please please comment on posts. Add something to them, disagree with them, whatever; just keep it interesting.

 

Weinreich Labs

In the mean time, there are a few happenings that do deserve mentions. The digital violin project has come a long, long way.

–We are now doing business as Weinreich Labs, L.L.C. (named for physicist and friend Gabi Weinreich, without which none of this would have been possible). We should have an official website up soon.
–Over the summer (2010), we presented at 2 conferences for the Violin Society of America (the Oberlin Acoustics Workshop, and the VSA International Convention). My partner, Alex, wrote up nice summaries of each on his blog. You can read those below.

Oberlin Recap
VSA Recap

You can also read a quick summary of what we’re doing (along with sound samples and pictures) here:

Project Summary

 

Nomm… Jazz.

One more thing. For any of you that may be in the Ann Arbor area next week, Nate May will be holding his senior recital at 8:00PM on Monday, April 4th at Canterbury House. He is a fantastic jazz pianist; certainly worth checking out.

I first started working with Nate way back when I was starting my sound engineering degree. I’ve always had a lot of respect for him both as a musician and as a friend. Here are a few examples of recordings we made last year:

Recordame
Speak Low
Stella by Starlight

Enjoy.

 

An Ultra Low Power CMOS Gate-Compressor

At the end of Fall 2009, myself, Chris Conwill, and Daniel Egert created an IC design for an analog audio compressor with an in-line gate. It was designed for IBM’s 0.13um CMOS process, so it uses a 1.2V power supply and draws under 500 uW of power.

Granted, with a supply of 1.2V our compressor isn’t exactly intended for pro-audio. Instead, its extremely low power consumption makes it perfect for battery-operated applications such as hearing aids or cell phones. By putting this compressor just before the ADC, you can get better SNDR on the digital end (since the input has a narrower dynamic range) and save precious DSP time for more complex tasks.

The gain reduction is achieved by a VCA that operates over a log-linear range of 60-dB (ie 60 dB of potential gain reduction). Also, we designed it to have a fixed ratio between the attack time and release times (1:10 for comp, 1:100 for gate). The absolute attack/release times are set by external capacitors. The compression ratio is currently set to be about 2.6:1, but can be adjusted by changing an external resistor value.

Below are some simulations of our circuit using a short recording of a male voice as an input.

Input
—The raw signal, measured just before entering the compressor (to account for any weirdness that could have happened between the original audio and Cadence’s actual simulated signal.

Compressed
—Compression only. I used a fairly low threshold of about -20dBFS (relative to the highest peak of the input signal), so the compressor’s action is more obvious. Attack time = 10ms, Release time = 100ms, Ratio = 2.6:1

Gated
—Gating only. I used a fairly high threshold of about -34dBFS, for the same reason as before. Attack time = 1ms, Release time = 100ms.

Compressed & Gated
—Simultaneous compression and gating. Same settings as before, just showing the two functions working together.

At the beginning of the winter semester, we decided to continue work on this project. Professor Flynn has offered to help get our design fabricated (whom we cannot thank enough), so if all goes well we will have a real chip to test by the end of 2010.

 

Slightly Erratic

Its been quite a while since my last post, but not with out reason. I’ve been plenty busy between school, work, and trying to be at least a little social all the while. I have a couple topics in the works, so here’s the first.

I’m currently taking an analog circuit design class. More specifically, monolithic circuit design – that is, IC circuits. For our first of two projects, we were to design a video repeater with a switchable output impedance for interfacing with different types of cables. The design specs were:

-Voltage Gain (loaded) = 2.5
-Bandwidth = 300MHz
-Input Impedance = 50 ohms
-Output Impedance = 50/75 ohms (switchable)

All the above specs were to stay within +/- 20% of the nominal value over process variation (inaccuracies in transistor manufacturing), supply voltage variation (1.08-1.32V), and temperature variation (25-85 deg. Celsius). My design maintains +/-10% for most specs, the only exception being the input impedance, maintaining +20/-5%.

Being that we can’t exactly afford to fabricate the ICs, we didn’t get quite that far. However, we did everything up to that point from conceptual design to initial SPICE simulation to physical layout. We finished up by extracting the parasitics from our layout and doing final simulations of a fabricated IC.

Here’s the paper I wrote summarizing my design strategy and results. It was limited to 4 pages, so its a bit hand-wavy in places, but the basic ideas are there.

 

Digital Pains…

Lately I’ve been writing a codec to communicate over a digital audio bus called a I2S. I2S actually stands for Inter-IC Sound and is meant to pass digital audio between ICs (duh) in large signal processing systems. Its actually fairly simple, but debugging the particular chip I’m sending data to is a pain. So I spent much of today tracking down random clicks and pops that have been showing up in my output.

As I was debuging, I would listen to the output post- Digital to Analog Converter (which happens to be what I’m sending data to), and had noticed it sounded a bit off. I shrugged it off at the time as the output was significantly quieter than the input I was comparing it to. After a certain point, however, I hit a wall and needed something to take my mind off the problem for a bit so I decided to measure the frequency response of the converter.

The device in question here is TI’s TLV320AIC23. Its an all-in-one Analog/Digital interface chip; that is, it contains A/D (Analog to Digital) converters, D/A (Digital to Analog) converters, headphone amps, and a microphone input. For this test, I was passing audio into one of the A/D converters, then immediately back out one of the D/A converters. Thus, the resulting frequecy response is that of an A/D and a D/A converter in series. Here is the measurement taken in Smaart 6.1.

The upper graph is phase response; the lower one is magnitude.

Now, in all fairness to TI, A/D converters are stupid hard to get flat. That ripple shown above is caused by the need for an extremely sharp roll-off in the upper end of the spectrum. That’s actually why, often times, higher sampling rates sound better (this is sampled at 48KHz, btw). Contrary to popular belief, it doesn’t have anything to do with sampling “more” of the waveform. Rather, its because with higher sampling rates, the resitrictions relax significantly on how sharp the rolloff needs to be at the top, allowing designers to end up with acceptable stop-band rejection (roll off) while minimizing pass-band ripple (see above).

That having been said, I’ve certainly heard much better converters and have no intention of using this particular converter in any final designs. However, for the time being I’m stuck being that the TLV320 is the only audio I/O built into the development kit I’m using.

After completing that, I thought it’d be interesting to find out what the response of my USB pre (M-Audio FastTrack Pro). It was a bit more difficult, but long story short, I recorded the output looped back to the input, dropped the wave files into Matlab and BAM!

Upper is Magnitude; Lower is phase.

Zoomed in on 1KHz to 22KHz.

The ripple response on this guy is much much better (almost +/- 0.05 dB). The only elephant in the room here is a big ole scoop from about 50Hz to about 200Hz. Uppon further examination (a larger FFT), the scoop only drops about -0.8 dB at its lowest; contrary to what this graph says. Still odd… Fortunately, I really only use this pre for measurement (similar to the Smaart plot above), which means as long as both channels are the same, the response of the device itself doesn’t actually matter.

Anyway, what’s the lesson to be learned today? If ever you should hear an audio engineer talking about how a certain preamp or converter sounds better than another, don’t just write it off as a placebo. Different pieces of gear do actually sound significantly different. It can be especially heated when you’re talking about the difference between analog and digital gear. I think the above plots say all that really needs to be said on that topic.

 

cout << "Hello World!";

Hey all.

Ya know, I was never really into the whole blogging thing and to be honest, I never thought I’d wind up with one. That being said, here I am… writing in semi-stream of consciousness wondering how much of an ass I’ll make myself look like.

Anyway, I’ve been somewhat inspired as of late and I’m hoping this will serve to feed said inspiration in the future.  Hopefully having a place to rant about my various projects and musings will allow me to move forward more consistently in my work.

The current plan is to talk about all the nerdy goings on in my life, including active design projects and other random (likely audio or electronics-related) musings. I’ll probably throw out bits from gigs once in a while if there’s anything particularly interesting happening.

I think that’s all for now. I’ll post some actual content within the next few days. For the time being, I’m going to wander about this site and see what kinda horsepower I have to play with.