Hello all,
I'm seeing a lot of latency in streaming from a webcam with GNURadio and the
USRP.
The transmit chain looks like:
ffmpeg | ./modulator > tx.fifo &
then I run
sudo ./usrp_stream_tx.py
modulator is an external code I am running that is not part of gnuradio.
uhd_stream_tx is a python script I made with GRC and it reads tx.fifo,
resamples the signal, and then outputs to the USRP.
The receive chain looks like:
sudo ./usrp_stream_rx.py &
then I run
cat rx.fifo | ./demodulator | ffplay -
demodulator is an external code that is not part of gnuradio. uhd_stream_rx is
a python script that reads samples from the usrp and writes them to rx.fifo.
First I run the receive chain, then the transmit chain with about a 1-2 second
delay in between executing the receive and transmit commands. When waiving my
hand in front of the webcam, the above gives me latency in the ffplay video
player of about 5-10 seconds initially, and this keeps growing in time to be
quite high (even up to 30-45 seconds). Someone else named Alex Csete had tried
a similar DVB transmission and said he had seen 10 seconds of lag initially
with his setup. Does anyone know what is causing this building lag? Could it
be the fifo files in the chain?
The external modulator and demodulator code runs in real-time at fixed data
rates that the usrp can handle when sampling a signal at 1MHz. The ffmpeg
encoder bitrate is set to be about the same as the modulator code.
I also tried the same thing without the USRP by connecting a cat5e cable
between the transmitter and receiver laptops. Then I replace
./usrp_stream_rx.py and ./usrp_stream_tx.py in the tx/rx chains with netcat
commands using nc -l 1234 for the receiver, and nc 192.68.0.3 for the
transmitter. This works great and I don't see any of the 5-10 second latency
as with the USRP.
I don't know how or if gnuradio interblock buffers or fusb settings play in to
the latency I'm seeing? The sampling rate for the TX and RX is 1 MHz so it
seems samples are pushed through at a fast enough rate and this latency
shouldn't be buffer related. I don't see any underrun or overrun errors on the
transmitter or receiver so the signal seems to be running real-time.
I appreciate any advice. I'm out of ideas and have searched a lot on latency
related to GNURadio and most of the latency I've read up on seems to be in the
microsecond to millisecond ranges.
Thanks very much, Tom
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