Communication between Spectrum Lab and other programs


Some sophisticated applications required the control of Spectrum Lab by other programs, and vice versa (Spectrum Lab controlling other programs). This chapter describes the two possibilities: using the tiny HTTP server which is integrated in SpecLab, and a system using windows messages. The latter may be a bit simpler if both programs run on the same machine.

The integrated HTTP server

For remotely controlled operation of Spectrum Lab, you can use the integrated HTTP server. This has the following advantages:

Downside: Sending HTTP requests, and receiving the response (from the client's point of view) is not as fast as exchanging WM_COPYDATA messages locally (as explained in the next chapter). Details about the HTTP server inside SL can be found in the server_pages directory after the installation. There some examples (in HTML with javascript) in the server_pages directory too. If you read this document while SpecLab runs on the same machine, with the server enabled, try this link to check the remote control examples:

( is a dummy IP address of the local machine, and 80 is the default port number for HTTP = hypertext transport protocol )

To configure the built-in HTTP server, select "Options"..."Configure and supervise HTTP Server" in SL's main menu.

Config tab
Network name of this machine :   ( shows the PC's name, as seen in the LAN (local area network) )

Local IP Address (of this PC) :   ( also just a display for convenience, can be used as browser address )

HTTP Server Port (default=80) :  (enter the IP port number under which the server shall be accessed in the LAN)

Root directory for HTTP Server :
( this is the name of the folder on your harddisk,  where the web server's HTML pages, Javascript, and other files are stored.
 The HTTP server also uses this directory to store temporary files, like JPEGs with the current spectrogram, etc)


    [ ] enable HTTP server (must be checked to activate the server)

    [ ] allow _iproc.html and _ifunc.html (check this if you allow others to remotely control contrast, brightness,
                     and remotely call functions in Spectrum Lab's interpreter via Javascript in your browser )

    [ ] allow remote control ("exec") (allows remotely calling the interpreter's "exec" command )
                      Should only be used in a LAN (not if the server is accessable through the internet),
                      because "exec" can be used to launch OS commands !

Max number of simultaneous connections, Update images only when older than X seconds, Bandwidth limit...:
   These parameters should speak for themselves. They can be used to limit the bandwidth
   when this HTTP server is used as a 'real' web server (which can be accessed from the world wide web).


Buttons on the right side of the HTTP server control panel
'Apply' : Apply settings on this tabsheet
'Help' : Show help about the current tabsheet
'Test' : Invokes the HTTP server in the web browser on the local machine (
'Audio' : Opens SL's Audio Stream Server control panel

To let others remotely look at the current spectrogram through their web browsers, it's enough to enable the HTTP server (the first option set, the other two not set). By default, after installing Spectrum Lab, this option (and all other HTTP-server related options) are in their safest state, which means they are 'off' (=not enabled).
To let others remotely adjust the spectrogram's display and brightness, the option 'allow _iproc.html' + '_ifunc.html' must be set, too. Technically, this option allows to call interpreter functions, and interpreter procedures through two special html pages named '_iproc.html' = remote procedure call, and '_ifunc.htm' = remote function calls. An example for remote control via Javascript is in the sample server pages ('server_pages/controls.htm'). A few 'potentially dangerous' interpreter commands can not be called from the HTTP server even if this option is set - see next paragraph.
The option allow remote control ("exec") should only be used in a LAN, because "exec" can be used to launch OS commands ! Do not set this option if the server is accessable through the internet. This (potentially dangerous) option is always off by default, furthermore it is only usable along with the two other options (because from Javascript, "exec" can only be invoked through the '_iproc.html' page mentioned above. Interpreter commands which, from he HTTP server, can only be called with this option are: exec, send, fopen/fread/fwrite .

Again, details about SL's integrated HTTP server, and the 'things you can do with a bit of Javascript' can be found in the server_pages (the 'readme' is intended for developers, please read it) .

Note: If -besides the HTTP server- you also use the Audio-via-TCP/IP-Server, take care not to use the same IP port number for these function. Both servers use the IP address of the machine on which they run, so you must use a different port number for the HTTP server and the audio server. BTW, the default port number for an HTTP server is 80.

Communication using WM_COPYDATA messages

For this task, a simple communication protocol which uses WM_COPYDATA messages was implemented. Detailed information about the protocol (for fellow programmers) can be found on the author's website, search for the file yhf_comm_info.htm, titled

Communication between windows programs using WM_COPYDATA messages

(If the link fails due to server problems, copy the title into your favourite search engine - you may find the file on my "backup" site too)

I didn't include it in the Spectrum Lab documentation because it is used in other projects too (and there shall not be outdated copies of that document all over the place.. ;-) . So follow the above link for the latest up-to-date description of DL4YHF's "YHF_COMM" protocol if you are considering to...

No idea what such an application could be ? You can send any interpreter command which is mentioned in the SL manual in a WM_COPYDATA message, so you could..

The other way around, you can also

All this is possible ... but, due to the unavoidable time lag in the windows message processing system, this message system is not suitable for "tough" real-time operation. Expect a delay of some milliseconds until the command you sent to SL has been executed, and your program receives the result message.

All you have to know -in addition to the general message protocol- are the MODULE-ID's and COMMAND-ID's which are used to invoke the interpreter commands and -functions through WM_COPYDATA messages. The next chapter gives an overview of these values.

To send a windows message to Spectrum Lab, you must know its windows handle. This can be found with a WinAPI function called "FindWindow". For convenience, don't search for the window's title (which may change depending on the version), use the window's class name (lpClassName, which is the first parameter for "FindWindow"). The class name of Spectrum Lab's main window is TSpectrumLab. But this is not the only possibility:
To support communication between two instances of SpecLab running on the same PC, the program now uses the following additional names (for programmers: SL creates an extra invisible window for this purpose, which has a unique class name) :

The old class name "TSpectrumLab" (for the main window) remains for backward compatibility.

For testing purposes, open the "Command Window" in Spectrum Lab, and let it show all received and sent traffic in the message list (in the command window: "Options".."show Inter-Application Comm's").

Module- and Command Identifiers in Spectrum Lab

Module Identifiers for DL4YHF's inter-application message handler
Module Identifier Meaning
CL Command Line Interpreter
AU Audio Data

The third and fourth character in the message's dwData parameter contain the "command". They are specific to the "module ID". Here are the most important "command IDs" for the Command Line Interpreter built inside DL4YHF's Spectrum Lab:

Command Identifiers for the command line interpreter in DL4YHF's Spectrum Lab
Command Identifier Meaning
CF Calculate Function (with result returned in response message)
EC Execute Command (no result, will not send a response message)

A practical example: To stop the spectrum analyser, send the following message..

 // Set module ID "CL"  + command ID "EC" :
 cds.dwData = ('C')+((DWORD)'L'>>8)+((DWORD)'E'>>16)+((DWORD)'C'>>24);
 cds.lpData = "spectrum.pause=1";        // command sent to SpecLab's interpreter
 cds.cbData = strlen((char*)cds.lpData); // count of bytes in data block

How to send audio streams per WM_COPYDATA is explained in the next chapter.

See also: Spectrum Lab's main index, Spectrum Lab's interpreter commands, interpreter functions (in separate documents).

Sending uncompressed Audio via WM_COPYDATA

Note: If possible, do not use this feature, because it is not available under non-windows operating systems; and both sender and receiver must be running on the same PC. Better use one of the UDP or TCP/IP-based methods mentioned here : They allow sending audio over a local network, and even work between PCs running different operating systems (like Windows and Linux).

The first use for this principle was the "winamp-to-SpecLab" plugin; the WM_COPYDATA method was chosen for simplicity.

As explained in the previous chapter, the four bytes (here: four characters) in the dwData parameter of the WM_COPYDATA message must contain the "module identifier" (here: "AU" for audio data), followed by the "command identifier" (here: "SD" = stream data).

Below is the sourcecode of a C function which sends a block of audio samples through a WM_COPYDATA message (actually taken from audiomsg.c). The T_AudioMsgBuffer structure contains the audio block, and some information about the audio stream (like the number of channels, the sampling rate, etc). It is defined in the file audiomsg.h which is available in the archive "".
The same data structures are used when sending audio streams via UDP or TCP/IP. An introduction to the sound utilities is here.
Details, header files, and "C" sourcecodes of a simple test application which uses WM_COPYDATA to send a test signal to Spectrum Lab are included in the zipped archive.

See also: Spectrum Lab's main index, Spectrum Lab's interpreter commands, interpreter functions (in separate documents).

Last modified: 2011-08-12 (YYYY-MM-DD)