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 = gr.message_source(gr.sizeof_char, msgq_limit)
src = self.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
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