Re: [Discuss-gnuradio] bit stuffing DVB-S TX example ?

[Ron Economos]

Alban,

    It's absolutely critical that you use the -muxrate option in ffmpeg. If you don't, you get a variable rate Transport Stream, which is guaranteed to cause problems.

Also, I would specify the muxrate to 1 bps of accuracy. ffmpeg supports it. So for 300 kSyms/s, it would be 276471.

You don't really need the -nal-hrd cbr parameter. The video bitrate can be variable, but the peak video  bitrate (combined with the audio bitrate) can't exceed the TS rate.

BTW, the RF power output is not changing. It's just the player not being able to decode the bitstream properly.

Ron

On 2/8/19 09:59, Alban Meffre wrote:

hi thanks for your answers

my symbol rate is 300 ksps

using this calculator 

http://www.satbroadcasts.com/DVB-S_Bitrate_and_Bandwidth_Calculator.html

and these settings : QPSK rate 1/2 FEC frame 64800

i get 276.5 kbps of TS bitrate

here are my settings for ffmpeg :

ffmpeg -re -f lavfi -i testsrc=s=640x480:r=25:n=1 -vf "drawtext=text=%{localtime}:x=(w-tw)/2:y=h-th-37:fontsize=20:fontcolor=yellow:borderw=2:bordercolor=black" -pix_fmt yuv420p -an -g 25  -b:v 228k -c:v libx264 -tune zerolatency -maxrate 228k -bufsize 230k -nal-hrd cbr -f mpegts udp://127.0.0.1:1234?pkt_size=188

this generates a test pattern very close to 275 kbps

as i'm very close to the DVB-S net bitrate i get a near constant output power (with some small gaps) and SDRAngel is quite happy

i will play with the -muxrate option to see if this keeps tightly the bitrate at maximum limit

best regards

Bob

Le ven. 8 févr. 2019 à 13:22, Raydel Abreu (CM2ESP) <address@hidden> a écrit :

Hi Ron

That formula it's very useful! Thanks. I have a silly question (sorry for it). If I understand correctly 8000000 is the symbol rate and 12901961 is the TS rate? Correct?

The multiply by 2 is because Tx signal is QPSK? If that's so then it will be 1 for BPSK, 3 for 8PSK and so on.... Am I right?

Thanks!

Raydel

El jue., 7 de feb. de 2019 7:26 PM, Ron Economos <address@hidden> escribió:

First, you have to match the TS rate to the symbol rate. The equation is:

Symbol rate * 2 * 188/204 * code rate

For the example flow graph:

8000000 * 2 * 188/204 * 7/8 = 12.901961 Mbps

Second, you need to constrain the video bitrate to fit into the selected TS bitrate. With ffmpeg, something like this:

ffmpeg -i test.mp4 -c:v libx264 -b:v 10M -minrate 11M -maxrate 11M -bufsize 8M -c:a copy -muxrate  12901961 test.ts

For SD video, a smaller bufsize like 2M should be used.

Ron

On 2/7/19 15:15, Alban Meffre wrote:

Hi All

did some transmission test in DVB-S tonight

TX : ffmpeg + gnuradio + pluto SDR

RX : RTLSDR + sdrangel

it works but there are some gaps in the received signal because the TS stream bitrate is slightly less to the maximum usable bitrate

is it possible to add some bit stuffing at DVB-S modulator side to make to output power constant ?

Bob

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Alban MEFFRE F4GSW

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