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[Discuss-gnuradio] gr-dtv | DVB-S2 : video bitrate const while symbol ra


From: Kai-Uwe Storek
Subject: [Discuss-gnuradio] gr-dtv | DVB-S2 : video bitrate const while symbol rate increased
Date: Tue, 24 Oct 2017 23:40:46 +0200

Hey folks,

this is just a request for enlightenment. :-)

I was playing around with the dvb-s2 modulation blocks. I started with
the example graph dvs2_tx.grc and the example video (adv16apsk910.ts)
and an ursp. At the receiver side, I used an ericsson rx 8200.

I then increased the symbol rate and noticed three things:

1) The bit rate of the transport stream shown by the rx8200 increased
from 17 mbps to 23 mbps (which is in line with rates calculated via
[1]).

2) A rate probe between the file source block and the BBheader block
shows an increased rate: From 2.1e6 (2.1 megabyte/s) to 2.9e6 (2.9
megabyte/s).

3) The shown video bit rate, analyzed by the rx 8200 (webgui) as well
as by VLC, that plays the transport stream which is distributed by the
rx 8200 via ip multicast, does *not* increase.

4) The playback of the video with VLC works pretty well in both cases.

Especially 3) is very convenient because a user of these gnuradio
blocks must only ensure that the rate of the DVB-S2 stream, determined
by symbol rate, modulation and coding, is higher than the necessary
video bitrate and everything is fine. :-)

Here is my question:

With an increased symbol rate I noticed an increased "reading speed"
of source file / video ( 1) ). With my understanding this is a
contradiction. On one side, the video bit rate seems constant (Rx8200,
VLC) for both symbolsrates tested (which is intuitive right, because
the video does not need "more" bandwidth), but on the other side, the
video data are read out faster by the file source.

So, where is the magic? Who is handling the rate adaption? Where are
the video bytes going that are "overread" by the file source in case
of an increases symbol rate?

A buffer on the receiver side is not the solution, because VLC stops
immediately when I stop the Tx flowgraph.

Hopefully, someone can point me in the right direction.

Best regards,
Kai

[1] https://github.com/drmpeg/dtv-utils/blob/master/dvbs2rate.c



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