Version 13 of Fox Board Tcl: a audio mixer application

by Theo Verelst

This page is about two essential tcl scripts for a big project of mine, which is a computer and otherwise controlled audio mixer.

The prototype hardware in buildup

Both the control side in the mixer machine and the control program on a PC are pure Tcl, which is fun and well maintainable.

The prototype with its tcl-scripts currently controls (in very high quality) one stereo volume via a slider on the screen of a Linux (I used Fedora Core 6/64) or Windows XP (professional):

The first tryout script is this one:

 proc slidsend { {n} {v} } {
  set w [format "%.2x" [expr $v+191]]
  send "00$w$w$w"

 }

 proc slidsendupdate { } {

 #   send update

 } 



 set sock {}
 proc send { {line {\n}} } {
   global sock
   if {$sock == -1} {
     # log "(failed attempt to send:\n$line)\n"
      return
   }
   puts $sock $line
   flush $sock
   # log $line\n
 } 

 proc log a {puts -nonewline $a}



 set sock [socket 192.168.1.38 4448]

 #sliders {{vol 255}}



 wm geometry . 88x478
 scale .sl
 .sl conf -from 64 -to -191 -tickinterval 32 -command {slidsend 0 }
 pack .sl -expand y -fill y

The slider above is shown on the screen, moving it immedeately changes the volume of the single text channel of the mixer.

The ticks are in 0.5 dB steps ( a small volume change), which is not to lose resolution, when the slider values would be in Decibels, (-96 .. 31.5) the accuracy would become a decibel per slider step.

The send command computes the hardware setting and assumes exactly 4 bytes must be send over the socket per volume change. The local IP address is of the mixer machine.

The following picture shows the machinery setup:

The computer runs a script which on the screen shows sliders, controls etc, which is currently only one long Tk slider. It connects over ethernet to a Fox board (see [L1 ] and Fox Board Tcl), which runs another Tcl (not Tk) script which can access storage and has a direct serial connection (115kbps) with a FPGA (programmable logic) where messages can be put into serial driver commands for the high grade volume control chip(s).

Using Tcl in the fox board makes it possible to store and save configurations easily, do computations which can be changed on the fly, have web page access to the mixer where a tcl cgi can directly acces the mixer and display (in buildup) settings, and with the front pannel controls the machine can be used stand-alone, and is then extremely easy to customize and to build almost any user interface in which could be required.

On a PC (or several PCs in a network) the whole mixer, which will have dozens of sliders) can be shown on a large screen, all under Tcl control, and all freely programmable, also by users.

For the current prototype, this is the script in the machine's Fox Board, which is automatically started at boot time

 set sock [socket -server connect 4448]
 set s1 {}

 proc connect {s host port} {
    global s1
    set s1 $s
    fconfigure $s1 -blocking 0 -buffering line
    fileevent $s1 readable [list handleSocket $s1]
 }

 proc handleSocket {s} {
    gets $s line
    if {[eof $s]} {
        close $s
        set s -1
        return
    }
    if {$line eq ""} return
    w $line
 }


 set fh [open /dev/ttyS2 RDWR]

 fconfigure $fh -blocking 0 -mode 115200,n,8,1 -translation binary \
    -buffering  full

 fileevent $fh readable {
    set w [read $fh] ;
    foreach c [split $w {}] {
       binary scan $c H* a; set v $a
       if {$s1 != {}} {puts $s1 "$v"}
       # flush stdout
    }
 }
 proc w h {
    global fh
    puts -nonewline $fh [binary format H* "$h"] ; flush $fh
 }
 w 00 ; w C0 ; w C0 ; w C0
 # after 500 exit
 vwait forever

I'm not sure this machine will be all open source, thus far they're simple but very well working (and fast) programs are free to use.