Portable four channel mic preamp and A/D converter with matched gains for surround sound recording


This web page documents the development of this unit.  This is a commercial product, development nearly completed.

Board is 4-layer 6.75" x 4.6"
Design timeframe: Early 2006 to present
Last webpage update: May 18, 2007



Top View:
(top view)

Bottom View:
(bottom view)



This product was developed specifically for a new microphone with 4 capsules, designed for portable, affordable, surround sound recording.  In order to make this possible, two important features are needed.
  1. The mic preamp gains per channel need to be tightly matched; +-0.1db or better.  This is not a standard feature on 4 channel preamps, since for all conventional uses gain matching is not required.
  2. A means to losslessly encode 4 audio streams into 2, to enable recording by a common 2 channel digital recorder, thereby avoiding having to purchase an expensive, specialty 4 channel digital recorder.
4ch has a number of other features, listed briefly below.


Design Overview

The design was dictated by two overriding factors: the gain matching and running for 4+ hours on two standard 9V alkaline batteries.  Various power-saving design features were implemented, including multiple dc/dc converters, shutdown of unused chips when possible, selection of the newer versions of "classic" audio chips and Toslink transmitters,  LED display on/off switch, headphone shutdown, and the like.  Gain matching is done by using popular pro-audio instrumentation amps, with gain set by a switch that selects between a series of fixed gain steps (selected by another slide switch), variable gain via 2 dual-shaft pots, or line-level.

How are 4 channels converted losslessly into a single stereo pair?  Multiplexing.  Consider this: 4 channels of 48KHz/24bit audio can be sent down a 2 channel 96KHz/24bit line using some bit-swizzling magic.  Channel 1 of the 48KHz stream becomes the "Left" in the 96KHz stream, channel 2 becomes the "Right", channel 3 becomes the next "Left", channel 4 the next "Right".  Since 96KHz is running 2x the speed of 48KHz, everything is synchronized and no samples nor bits are dropped.  The 96KHz "stereo" recording is not playable or monitor-able as is, since the left and right channels are each actually two channels switching back and forth sample by sample, but a simple utility application splits the recording back out into the 4 separate tracks.  Since the generic 2-track, 96KHz recorder can't monitor this multiplexed stream this unit has flexible meters and monitoring.


Major design blocks

Handling 4 streams of 24bit, up to 192KHz serially, with a variety of clocks to create, flexible signal routing, and sample-by-sample analysis for the LED meters is a job microcontrollers are not well suited for.  Instead of using an expensive, power-hungry 32bit (something) with a few unavoidable external logic gates a better approach was taken: a garden-variety lower-power microcontroller  (PIC18F87J10) and a low-end, low-power FPGA (XC2S30).  FPGAs are "programmable hardware", where the digital logic one needs is described with a high-level language (VHDL or Verilog) and then synthesized into gates and flip-flops, which are then "placed and routed" into the mesh of generic elements in the chip.  A $12 FPGA runs most of this unit and does so at 1/3 the speed it is capable of.  The microcontroller does the initialization and configuration of the various chips (including the FPGA) and responds to switches being flipped, the headphone volume control and displays the battery level, sleeping whenever possible to save power.  The FPGA handles everything else.


Board layout / testing

This design has evolved across 3 prototypes, has over 500 components and over 1400 nets, everything placed and routed by hand in Eagle.  There are 7 separate grounds, a "daughterboard" for the gain selection switches, and some components on the backside to facilitate a symmetric mic preamp layout for each channel.  A 4-layer board is an absolute necessity for a design of this complexity and analog performance requirements.  Making models of parts in Eagle, managing the BOM, ordering parts, soldering up the prototypes (around 400 tssop pins per board), debugging, and extensive analog performance testing with an Audio Precision System Two Cascade Dual Domain SYS-2522 were all done by yours truly.


Software/Firmware

There are three major sections to this.
  1. Microcontroller firmware: written in CCS C, which (until recently anyway, I'm unsure about the latest improvements) is not ANSI and therefore has some problematic quirks, this manages the overall functionality and initialization / configuration of all chips.  The configuration data for the FPGA is stored in the program space of the micro, avoiding the need for an external flash chip for it.  The mico communicates with the various chips and the FPGA over a shared SPI bus.
  2. FPGA "firmware": written in VHDL, using Xilinx's ISE version 9.1.  I personally have found their software to be reliable in terms of no synthesis or place & route bugs, but pretty poor in general UI responsiveness.  I've found UI bugs and have had it crash or hang a number of times from parsing incorrect VHDL or other slightly out of the ordinary (but not at all unexpected in development) situations.  It is pretty clear that big designs (100x of what this unit needs) don't use the IDE much, and instead call the individual tools from a custom batch build/test process.  The IDE is obviously a UI over these tools and is definitely clunky to occasionally unusable.  There is a steep learning curve, documentation is ok but not great, a lot to learn and having the tools be definitely shaky at times is a big hindrance.
  3. Windows (only, for now) utility application: Written in C++ in the circa 2000 Microsoft VC++ 6.0, this handles the de-multiplexing of the 2 channel wave file into 4 channels (details TBD) and walks the user through updating the firmware from a downloaded file.  The firmware update is fast, has smooth progress indicators, CRC checking, and encryption.

Conclusion

This is a major project, by far the largest I've done.  The testing and tweaks required to get something of this scale to be low power, high performance, relatively small, and relatively inexpensive are innumerable.  The limitations of Eagle for larger designs were clearly reached; there are a number of  simple features that are missing that would have made the design significantly faster to do.


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