On 12/09/2011 03:42 PM, Yan Nie wrote:
Dear all,
I am trying to transmit a sequence at various frequencies on USRP N200 with
LFTX.
When I use the uhd.single_usrp_source.set_center_freq() to change the frequency
then insert the the sequence as a message in the msgq buffer in another thread
other than top_block, underrun occurs. The amplitude of the transmitting signal
on oscilloscope varies some, some of the signal have the full amplitude that I
set, some of them only reach half amplitude. The following is the thread that I
open for the sequence transmitting.
The mgq are created by:
data=gr.message_source(gr.sizeof_char, msg_limit) #where msg_limit is defined
as 2;
msgq = data.msgq()
transmit_loop(tb)
repeat = 40
payload_13barker = repeat * '\x01'
payload_13barker += repeat * '\x01'
payload_13barker += repeat * '\x01'
payload_13barker += repeat * '\x00'
payload_13barker += repeat * '\x00'
stop_sign = 5*repeat*'\x02'
i = 0
while(i<5):
payload_13barker += stop_sign
i += 1
msg_13bit = gr.message_from_string(payload_13barker)
tb._ls.msgq.insert_tail(msg_13bit)
time.sleep(1.0)
print "info inserted"
while 1:
freq_cnt = 4
while (freq_cnt< 7):
i += 1
if i==40: #this counter is used to slow down the rate
the transmission frequency changes
i = 0
tx_freq = freq_cnt * 10e6
tb.set_freq_tx(tx_freq,0.0)
tb._ls.msgq.insert_tail(msg_13bit)
time.sleep(1.0) # suspend to wait for the
payload insert into buffer
freq_cnt += 1
I am curious
1. what causes the underrun and how to avoid the underrun?
2. How to defined the msgq_limit? Is the underrun cause by the wrongly set of
the msgq_limit, as I already set to let the program to wait for buffering?
Really appreciate any of your suggestion!
Underrun is caused by your flow-graph not being able to provide samples
to the hardware at the specified rate. It looks to me like in
your flow-graph above, your message-queue can easily get starved for
new bits, and so the entire flow-graph stalls. But that's just
after a superficial inspection. Since you actually *go to sleep* for
an entire second, it seems like it would be pretty easy for the flow-graph
to stall.
--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org
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