Help with video link

GNU Radio
1

Hello all,

I am completely stumped with trying to stream video using gnu radio
(GRC)
and the USRP. I have been at it for an absurdly long time without
success.
I could really use some help. (With Unbuntu and the latest UHD/GNU
radio)

There has got to be someone who has done this, but so far google is not
my
friend.

I have read so many sites of people looking for help and being given
‘help’
that doesn’t work, the internet is now fully spammed when it comes to
this
subject.

After days and days I read things like:
‘To correctly and completely use the RTP payloaders on the sender and
the receiver
you need to write an application. It is not possible to write a full
blown
RTP server with a single gst-launch-1.0 line.’
or that player ‘x’ isn’t really player ‘x’ its a fork due to developers
fighting and is rather broken…

Most of what I read is about streaming over a network with a constant
frame
rate (adaptive bit rate) which really does not relate well to USRP?

I have tried countless permutations of command line strings with various
options using gstreamer, ffmpeg, vlc. mplayer
I have tried UDP and File sink/source.

I once I had it nearly working but could not get the player to stream
with
a constant bitrate (which i think usrp would require?). I have lost
track
of which player that was though.

At this point I am completely desperate for a a solution. I am looking
for
any solution that works (even badly).
All I am to do is put together a demo, and it is becoming clear I have
no
idea what I am doing. Either that or this as far far less trivial than
one
would think.

I would greatly appreciate some help here as this endeavour is now
becoming
quite expensive.

regards
Alexander Buckley

2

Hi Alexander,

The de-facto standard way for sending video over the air (assuming you
can’t use wifi-like links) is to encapsulate all video and audio into
a single, constant bitrate MPEG transport stream (aka. MPEG-TS). It’s
a packetized format designed specifically for transmission over lossy
channels. MPEG-TS is used for DVB-T, DVB-S and probably also ATSC.

If you just want something quick & dirty you can try:
http://www.irrational.net/2014/03/02/digital-atv/

Alex

3

Hello Alexandru

thank you for your reply.

At the moment I am using a loopback cable on the USRP. I have been using
MPEG_TS as well but i noticed after reading your comment I was missing
the
mpegtsdemux on my receiver chain.

I have something extremely crude operating now, though it is mostly a
grey
screen, the image isn’t there yet. This is what I have set up so far:

transmit chain:

gst-launch v4l2src device=/dev/video0 !
‘video/x-raw-yuv,width=160,height=120’ ! x264enc bitrate=1024
quantizer=10
tune=zerolatency ! mpegtsmux ! filesink location=txPIPE.ts

receiver:

gst-launch filesrc location=rxPIPE.ts ! mpegtsdemux ! queue ! h264parse
!
ffdec_h264 ! xvimagesink sync=false

and my GRC flow graph in between is:

TX:
File Source:
repeat: no

OFDM Mod
FFT len: 512
Occ tones: 200
prefix: 20
pad for USRP: yes
Payload length: 0

USRP Sink:
samp rate: 500K

RX:
USRP Source:
samp rate: 500K

OFDM deMod
FFT len: 512
Occ tones: 200
prefix: 20
pad for USRP: yes
Payload length: 0
SNR: 20

File Sink:
Unbuffered: off
Append: Overwrite

I have had trouble with a few things. What i notice when using an FFT
Plot
is that the signal is pulsating. I have been playing with CW values and
gains on the USRP to see if that helps. It does somewhat but not
totally. I
wonder if this is a buffering issue. Though I have no feed back from my
terminal, no underflow or overflow or TIMEOUT, both grstreamers are
reporting to be playing (the rare time I get it through PREROLL into
PLAYING on rx side)

I have been trying all sorts of FFT sizes, occupied tones, grstreamer
bitrate values to no avail. All I have been able to do there is just
make
it worse.

I have the communication channel equations set up in excel so can input
trial values and determine what my data rate through my flow graph
should
be, and then line that number up with gstreamer’s bitrate. No luck
there…

I had also considered using an error coding block (the CCSDS) but I read
in
the notes that comes with it the it wont work with packtised data. Which
I
am assuming then doesn’t work with the MPEG-TS?

I have bypassed the USRP with a throttle block and the signal is better.
It’s more of the really low quality ‘man in the space station’ style. As
it
is not that good, I figure some of my flow graph numbers are off or I’ve
missed something in my gstreamer command string.

Or my OFDM block is not set up right. I’m not sure what payload length
should besides default value or what pad for USRP does.

Alex

4

You should first try and see if you can transmit a static file using
your setup. I doubt you can without using packet encoder/decoder and
some synchronizer on the RX side, though I admint I don’t know if
these are already contained in the OFDM blocks.

Alex