Tag Archives: c

Voice Keying with bash, sox, and aplay

There are plenty of times where it is nice to have Linux transmit things out a radio. One obvious example is the digital communication modes, where software acts as a sort of modem. A prominent example of this in Debian is fldigi.

Sometimes, it is nice to transmit voice instead of a digital signal. This is called voice keying. When operating a contest, for instance, a person might call CQ over and over, with just some brief gaps.

Most people that interface a radio with a computer use a sound card interface of some sort. The more modern of these have a simple USB cable that connects to the computer and acts as a USB sound card. So, at a certain level, all that you have to do is play sound out a specific device.

But it’s not quite so easy, because there is one other wrinkle: you have to engage the radio’s transmitter. This is obviously not something that is part of typical sound card APIs. There are all sorts of ways to do it, ranging from dedicated serial or parallel port circuits involving asserting voltage on certain pins, to voice-activated (VOX) circuits.

I have used two of these interfaces: the basic Signalink USB and the more powerful RigExpert TI-5. The Signalink USB integrates a VOX circuit and provides cabling to engage the transmitter when VOX is tripped. The TI-5, on the other hand, emulates three USB serial ports, and if you raise RTS on one of them, it will keep the transmitter engaged as long as RTS is high. This is a more accurate and precise approach.

VOX-based voice keying with the Signalink USB

But let’s first look at the Signalink USB case. The problem here is that its VOX circuit is really tuned for digital transmissions, which tend to be either really loud or completely silent. Human speech rises and falls in volume, and it tends to rapidly assert and drop PTT (Push-To-Talk, the name for the control that engages the radio’s transmitter) when used with VOX.

The solution I hit on was to add a constant, loud tone to the transmitted audio, but one which is outside the range of frequencies that the radio will transmit (which is usually no higher than 3kHz). This can be done using sox and aplay, the ALSA player. Here’s my script to call cq with Signalink USB:

# NOTE: use alsamixer and set playback gain to 99
set -e

playcmd () {
        sox -V0 -m "$1" \
           "| sox -V0 -r 44100 $1 -t wav -c 1 -   synth sine 20000 gain -1" \
            -t wav - | \
           aplay -q  -D default:CARD=CODEC


echo -n "Started at: "

STARTTIME=`date +%s`
while true; do
        printf "\r"
        echo -n $(( (`date +%s`-$STARTTIME) / 60))
        printf "m/${DELAY}s: TRANSMIT"
        playcmd ~/audio/cq/cq.wav
        printf "\r"
        echo -n $(( (`date +%s`-$STARTTIME) / 60))
        printf "m/${DELAY}s: off         "
        sleep $DELAY

Run this, and it will continuously play your message, with a 1.5s gap in between during which the transmitter is not keyed.

The screen will look like this:

Started at: Fri Aug 24 21:17:47 CDT 2012
2m/1.5s: off

The 2m is how long it’s been going this time, and the 1.5s shows the configured gap.

The sox commands are really two nested ones. The -m causes sox to merge the .wav file in $1 with the 20kHz sine wave being generated, and the entire thing is piped to the ALSA player.

Tweaks for RigExpert TI-5

This is actually a much simpler case. We just replace playcmd as follows:

playcmd () {
        ~/bin/raiserts /dev/ttyUSB1 'aplay -q -D default:CARD=CODEC' < "$1"

Where raiserts is a program that simply keeps RTS asserted on the serial port while the given command executes. Here's its source, which I modified a bit from a program I found online:

/* modified from
 * https://www.linuxquestions.org/questions/programming-9/manually-controlling-rts-cts-326590/
 * */

static struct termios oldterminfo;

void closeserial(int fd)
    tcsetattr(fd, TCSANOW, &oldterminfo);
    if (close(fd) < 0)

int openserial(char *devicename)
    int fd;
    struct termios attr;

    if ((fd = open(devicename, O_RDWR)) == -1) {
        perror("openserial(): open()");
        return 0;
    if (tcgetattr(fd, &oldterminfo) == -1) {
        perror("openserial(): tcgetattr()");
        return 0;
    attr = oldterminfo;
    attr.c_cflag |= CRTSCTS | CLOCAL;
    attr.c_oflag = 0;
    if (tcflush(fd, TCIOFLUSH) == -1) {
        perror("openserial(): tcflush()");
        return 0;
    if (tcsetattr(fd, TCSANOW, &attr) == -1) {
        perror("initserial(): tcsetattr()");
        return 0;
    return fd;

int setRTS(int fd, int level)
    int status;

    if (ioctl(fd, TIOCMGET, &status) == -1) {
        perror("setRTS(): TIOCMGET");
        return 0;
    status &= ~TIOCM_DTR;   /* ALWAYS clear DTR */
    if (level)
        status |= TIOCM_RTS;
        status &= ~TIOCM_RTS;
    if (ioctl(fd, TIOCMSET, &status) == -1) {
        perror("setRTS(): TIOCMSET");
        return 0;
    return 1;

int main(int argc, char *argv[])
    int fd, retval;
    char *serialdev;

    if (argc < 3) {
        printf("Syntax: raiserts /dev/ttyname 'command to run while RTS held'\n");
        return 5;
    serialdev = argv[1];
    fd = openserial(serialdev);
    if (!fd) {
        fprintf(stderr, "Error while initializing %s.\n", serialdev);
        return 1;

    setRTS(fd, 1);
    retval = system(argv[2]);
    setRTS(fd, 0);

    return retval;

This compiles to an executable less than 10K in size. I love it when that happens.

So these examples support voice keying both with VOX circuits and with serial-controlled PTT. raiserts.c could be trivially modified to control other serial pins as well, should you have an interface which uses different ones.

If Programming Languages Were Christmas Carols

Last spring, I posted If Version Contol Systems Were Airlines, which I really enjoyed. Now, because I seem to have a desire to take a good joke way too far, it’s time for:


I apologize in advance. (Feel free to add your own verses/carols in the comments.)

Away in a Pointer (C)

(to Away in a Manger)

Away in a pointer, the bits in a row.
A little dereference to see where they go.
I look down upon thee, and what do I see?
A segfault and core dump, right there just for me.

I saw thy init there, a reaping away
My process, from its address space, so sorry to say.
I thought I had saved thee, from void pointers all,
But maybe I missed one, and doomed you to fall.

Be near me, debugger, I ask thee to stay
Close by my terminal, and help me, I pray;
To find all the bugs and the void pointers too,
And if my kernel oopses, help me reboot for you.

Joy to the Wall (Perl)

(to Joy to the World)

Joy to the Wall, the Perl is come!
Let awk receive her King;
Let every grep prepare him room,
And bash and sed shall sing,
And bash and sed shall sing,
And bash, and bash, and sed shall sing.

Joy to the keyboard, we’ll use it all!
Let men, shift keys, employ;
Implicit variables, and globals never fall.
Repeat the line noise now,
Repeat the line noise now,
Repeat, repeat, the line noise now.

Perl rules the world with truth and ASCII,
And makes the doctors prove
The glories of carpal tunnel hands,
And we do it more than one way,
And we do it more than one way,
And we do it, and we do it, more than one way.

Hark! The Herald Coders Sing (Haskell)

(to Hark! The Herald Angels Sing)

Hark! The herald coders sing,
“Map and fold, recursive King;
Recursion and patterns wild,
Pure and IO — they’re reconciled!”
Joyful, all ye functions rise,
Join the typeclasses of the types,
With recursion, do proclaim,
“Laziness is born in this domain.”

Hark! The herald coders sing,
“Map and fold, recursive king!”

Monads, by highest Heav’n adored;
Monads, their depths still unexplored;
Late in time, behold they’re good,
Never once were understood.
Veiled in functions, the Monads stay,
Used for IO, and more, each day,
With excitement, Monads say,
“Arrows are stranger, so with us stay.”


Hail the glorious compiler of Glasgow!
Hail the threaded run-time system!
Join the beautiful Cabal of Hackage,
Upload there thy perfect package.
We know best, what we will Handle,
You’re safe with us: no pointers, no vandals.
Born to make your exceptions throw,
Unless you unsafePerformIO.


Lispy the Paren

(to Frosty the Snowman)

Lispy the paren was a jolly happy soul,
With a lot of cars and a little cons
And two ends made out of curves.
Lispy the paren is a fairy tale, they say,
He was just common, but the children know
how he came to life one day.
There must have been some magic in that
Old Symbolics they found.
For when they placed him on its disk,
It recursed around and ’round.

O, Lispy the paren,
Was recursive as can be.
And the coders say it would take a day
To put his parens away.
Clunkety clunk clunk,
Clunkety clunk clunk,
Look at Lispy go.
Clunkety clunk clunk,
Clunkety clunk clunk,
Consing on the car.

Lispy the snowman knew
The keyboard was hot the day,
So he said, “Let’s cons and we’ll have some fun
now before they Scheme away.”
Down to the function,
With a list there in his RAM,
Running here and there,
all around the LAN, saying
“cdr me if you can.”
He led them down the streets of disk
Right to the traffic bus.
And only paused a moment when
He heard them holler (quit).

Oh BASIC Night

(to O Holy Night)

Oh BASIC night, the LEDs are brightly glinting;
It is the night of the dear GOSUB’s birth!
Long lay the world in sin and error printing,
Till you appeared and the RAM felt its worth.
Shiver of fear, line numbers do inspire,
For yonder breaks a mostly harmless GOTO.
Fall on your bits, O hear the Visual voices!
O BASIC divine, O BASIC where GOTO was born!
O BASIC, O Holy BASIC, O BASIC, you’re mine!

Some want to say, “GOTO is harmful always,”
But what of them, in their post-modern world.
We PRINT the truth, in the line-numbered goodness,
But Dijkstra appeared, and the faith, it was lost.
A thrill of hope, when .NET BASIC announces,
But Visual BASIC, what kind of thing are you?
Fall on your GUI, O see the old line numbers!
Behold BASICA, O BASIC when DOS was born!
O numbers, O lines, spaghetti divine!

Guido We Have Heard on High (Python)

(to Angels We Have Heard on High)

Guido we have hard on high
Sweetly indenting o’re the code,
And the functions in reply
Their exceptions sweetly flowed.


Indent….. in your whitespace careful!
Indent…… in your whitespace careful!

Spaces, why this jubilee?
Why semicolons have you so wronged?
What backslashes must we use
If we want our lines so long?


Come to Guido here to see
“One Right Way” is good, of course.
There’s no need for Perl, you know,
We have to be more verbose.


Now the PEP will show the way
To the future, we shall see.
Banish lambda and the rest
Of the things we liked the best.