DSP Software Development

Follow the development of speech algorithms for digital radios through the complete project life cycle.
Development

Now comes the DSP involvement—for this a DSP starter kit is needed. For simple and easy development, two main contenders are available. Both have patchy support for Linux, cost in the region of £80 and are aimed at the hobbyist, small business or university user.

The original was from Texas Instruments, the TMS320C50 DSK (there was an earlier, less powerful C26 board), with the newer contender being the Analog Devices ADSP2181 EZ-KIT Lite. Both have audio I/O—the latter has 16-bit CD-quality stereo audio, while the former can manage only 14-bit voice quality. On the software side, both provide a nice set of DOS executables—assembler, linker and (for the Analog Devices kit) a simulator. The ADSP has an edge with its assembly language syntax being much more user-friendly than the TI chip. I won't stick my neck out too far and comment on which DSP is more powerful—both are fairly competent.

Linux versions of most DSP development tools are floating around on the Internet, but some are still missing, notably for the ADSP2181. These omissions are the assembler, linker and simulator, which is a pity since I had to use the ADSP.

The freely available cross-assembler as will soon include ADSP21xx compatibility. It already handles TMS320Cxx code along with a staggeringly wide array of other processors, with more added whenever the author, Alfred Arnold, has free time. Analog Devices have been approached about providing Linux versions of assembler and linker, but stated they do not currently have plans to support Linux.

For DSP code development, we need an assembler, linker and a code downloader that sends an executable through the PC serial port to the DSP development board. For the ADSP21xx, few Linux tools are available just now, only the downloader.

The solution is to use DOSEMU, the Linux DOS emulator, which has an impressive feature called the dexe (directly executable DOS application). This is basically a single DOS file or application in a tiny DOS disc image that can be executed within Linux without the user being aware that it is actually a DOS program.

To use this method, the entire ADSP21xx tool set can be incorporated into a single .dexe file. With a little ingenuity, a few simple shell scripts and batch files, the user will never know the assembler and linker he is using are actually DOS programs (see Resources for a HOWTO).

With the newly created dexe, we now have an assembler and a linker for our DSP code. Hidden in the depths of the Analog Devices web site is the source code for a UNIX (Linux/Sun) download monitor to load the DSP executable into the EZ-KIT Lite through the PC serial port. This means the assembler source can be compiled and downloaded all (more or less) under Linux.

The one irritation is the simulator. Analog Devices supply a DOS version of their simulator which will not run under the emulator, but this is no reason to throw Linux out, as we shall see later.

Analog Devices does have a 21xx C compiler based on good old gcc and even released the source. The C code integrates neatly with the assembly language and speeds up development time, but it is quite inefficient both in terms of code size and instruction cycles.

Completion

We now have an algorithm that runs on a DSP system. The complete software package generated by this effort includes:

  • Rlab research and investigation scripts

  • Test vectors and speech files from Rlab

  • Floating-point C implementation

  • Fixed-point C implementation

  • Assembly language version of the code

  • A working DSP executable

Does this list look complete to you? If so, you must be a born programmer like me. Anyone else would realize that documentation is missing.

Documentation

Has this happened to you? When your management says documentation must be in a standard format, you think LaTeX and they think Microsoft Word. ASCII is insufficient because of the lack of text formatting and graphics support.

However, one irrefutable standard that even your boss can agree to is HTML. Once a common standard has been agreed upon, it is time to produce a set of documentation templates. After that, any editor can be used to add content, including Netscape composer, Emacs or even Word. Graphics are more of a problem, but a combination of xfig and GIMP can handle most situations. The resulting web documentation can be read under Linux, Windows, RISC OS, etc. and is even accessible on palmtop computers.

We used RCS to manage our documentation versions too, in order to comply with company quality control standards. This allows a construct such as <li>RCS id: $Id$</li> to be embedded in the HTML. When the HTML document is checked into RCS, the RCS identifier will be inserted between the “$” symbols and will therefore be displayed on the HTML page.

Figure 2. RCS Information Used in Inter/Intranet-Based Documentation

A prettier method is to use JavaScript for display in Netscape to format the page and remove the unwanted $ symbols. The HTML page in Listing 1 forms the front cover to some code documentation, as shown in Figure 2.

Listing 1.

We all know HTML isn't perfect, but at least it is a compromise that can be agreed upon in striving toward a paperless office. Some other features we incorporated were placing the RCS log entries into a scrollable text area on the HTML pages and judicious use of hyperlinks to commented source code, data flow diagrams and flow charts.

To enhance our documentation, the C prototype code was compiled using GCC -pg which inserts extra code to write a profiling information file during program execution. Then gprof was used to interpret this profiling information. xfig was used to manually convert this into a function-call, graph GIF, and a sensitive image map was created for it. A set of HTML templates was created and edited to document each function; these pages can be accessed by clicking on this top-level GIF.

The result was a single HTML page showing the entire code in a pyramidal layer structure starting from main and the calling links between each function, with passed variable names written next to each calling link. The functions were named inside clickable boxes, which pointed to an explanation of that function.

This HTML documentation process is now being automated; see Resources for more information.

As an added bonus, my colleagues used the new documentation standard to justify buying more Linux machines. One was used to serve the documents on the company intranet using the Apache web server. This system can control access to the documents on a need-to-know basis, and keep a log of user accesses versus date and document version. It is even possible to automatically notify affected parties by e-mail when a document they accessed recently has changed.

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