[Discuss-gnuradio] variable transmission time of a single frequency

[Affan Syed]

Basically, I am trying to send a single frequency tone for a specified duration (currently 5ms) whenever a user wants to send one. My current solution uses messages by basically sending a single byte of ones no matter what a person enters on the cmd line. I convert that byte to symbols (representing one due to my above hardcode) and add to it the frequency of transmission. I then use a fixed number of repeaters to extend this symbol and use that to ourput from a vco that particular frequency. I have the code at the end of my email. 

What I am observing is that there seems to be some fundamental limit to how short that tranmission can be. In my code below, I am trying to send an 18Khz tone for 5ms. However, no matter what I do:

 a) the transmitted signal lasts for ~10-11ms (from scope and wav file) 

b) not every key press generates the tone. I generally miss one or two keypress. This happens even if the keypress are well spaced in time. 

So two questions to the community. What could be causing such a behavior? and is there an alternative way to get a similar result than what I am currently doing?

Thank you. 

Affan

Code 

====================

TONE_FREQ=18000 #hz

AUDIO_RATE=48000 #samples/sec

TONE_DUR=5/1000#in sec

BITS_IN_BYTES=8

REPEAT_TIME= TONE_DUR*AUDIO_RATE/BITS_IN_BYTES

# Main Graph

class tone_graph(gr.top_block):

    def __init__(self):

        gr.top_block.__init__(self)

        audio_rate = AUDIO_RATE

    

        self.src = "" msgq_limit)

    

        src = "">

        

        b_to_syms = gr.bytes_to_syms()

        

        

#we know that there should always be just one byte of code 

        add = gr.add_const_ff(TONE_FREQ)

                    

        repeater = gr.repeat(gr.sizeof_float,REPEAT_TIME)

    

        fsk_f = gr.vco_f(audio_rate, 2*pi,1.0)

    

        speaker = audio.sink(audio_rate);

        dst = gr.wavfile_sink("tone_tx.wav", 1, audio_rate, 16)

        

        self.connect(src,b_to_syms,add,repeater,fsk_f,speaker)

        self.connect(fsk_f,dst)

        

def main():

    

    fg = tone_graph()

    

    fg.start()                       # start flow graph

    

    try:

        while True:

            message = raw_input(" transmit a 5ms tone (Ctrl-D to exit): ") 

#make a one byte packet to indicate the tone transmission.

            pkt = struct.pack('B', 0xff)

    

            msg = gr.message_from_string(pkt)

    

            fg.src.msgq().insert_tail(msg)

 

    except EOFError:

        print "\nExiting."

        fg.src.msgq().insert_tail(gr.message(1))

        fg.wait()

    

# Main python entry

if __name__ == '__main__':

    try:

        main()

    except KeyboardInterrupt:

        pass