bandwidth (BW) problem with RTL-SDR while receiving DVB-T, DVB-S, and DVB-C signals

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dummy...@gmail.com

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Dec 1, 2013, 1:19:24 PM12/1/13
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RTL-SDR can be used to receive and decode DVB streams, but it couldn't catch the full streams due to the BW limitation of RTL device. However, DVB-S/S2 card has been widely used in PC for receiving and decoding satellite TV streams with no bandwidth issues. Since both RTL and DVB-S/S2 output transport (TS) layer streams, can we use the output from DVB-S/S2 card and further process this TS stream via SDR for the demodulation of this transport layer stream?
 
Work has been done via USRP to get the transport stream and then further process this TS stream via SDR for decoding. However, USRP is much more expensive. Can we simply substitute the USRP with DVB-S2 card (instead of solely using this card to power LNB) and then pass its transportation layer output to SDR for further processing, e.g. demodulation?
 
Is there a difference between the TS layer output stream from USRP and the TS layer output stream from DVB-S2 card so that we couldn't do DVB-S2 and SDR (DVB-SDR) in order to overcome the bandwidth problem related to RTL-SDR and lower the expenses of USRP-SDR?
 
 

Adam Nielsen

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Dec 1, 2013, 2:22:43 PM12/1/13
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> RTL-SDR can be used to receive and decode DVB streams, but it
> couldn't catch the full streams due to the BW limitation of RTL
> device.

This is correct. The RTL chip performs the SDR function and has
limited SDR bandwidth.

> However, DVB-S/S2 card has been widely used in PC for
> receiving and decoding satellite TV streams with no bandwidth issues.

This is because they probably do not use an RTL chip, so they cannot be
used for SDR either. The RTL chip does not have bandwidth problems
when used for DVB reception either (with the hardware demodulator) it
is only the SDR function that has limited bandwidth.

> Since both RTL and DVB-S/S2 output transport (TS) layer streams, can
> we use the output from DVB-S/S2 card and further process this TS
> stream via SDR for the demodulation of this transport layer stream?

No, SDR involves processing the raw RF signal. Once you have a
transport stream this processing has already been done so you cannot do
SDR any more. It doesn't matter whether you use an RTL dongle or a
DVB-S/S2 card, once you have a transport stream it is too late to do
SDR.

> Work has been done via USRP to get the transport stream and then
> further process this TS stream via SDR for decoding.

No, this is not true. The USRP can use SDR to get the transport
stream, but once the transport stream is available the SDR is
finished. The rest of the processing is just normal software codecs
and has nothing to do with SDR.

> However, USRP is much more expensive. Can we simply substitute the
> USRP with DVB-S2 card (instead of solely using this card to power
> LNB) and then pass its transportation layer output to SDR for further
> processing, e.g. demodulation?

No, because the transport stream has *already* been demodulated.
Obtaining a transport stream is the *result* of SDR, once you have it
you cannot do further SDR because you already have your result.

> Is there a difference between the TS layer output stream from USRP
> and the TS layer output stream from DVB-S2 card so that we couldn't
> do DVB-S2 and SDR (DVB-SDR) in order to overcome the bandwidth
> problem related to RTL-SDR and lower the expenses of USRP-SDR?

No, because the transport stream is digital TV data, there is no
difference between USRP, DVB-S2, RTL, etc. They all demodulate the
signal and produce the same transport stream for whatever digital TV
channel you are demodulating. If you are not tuned to a DVB signal
then there is no transport stream present to decode.

There is no such thing as "DVB-SDR". SDR is completely unrelated to
DVB. The cheapest way we have discovered to perform SDR is by using an
undocumented mode hidden away in certain types of USB DVB-T receivers.
We're not using DVB-T or anything to do with TV at all, it's just that
these devices have a hidden SDR mode which we take advantage of.

Cheers,
Adam.

cindy zhang

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Dec 1, 2013, 5:02:22 PM12/1/13
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> However, DVB-S/S2 card has been widely used in PC for
> receiving and decoding satellite TV streams with no bandwidth issues.

This is because they probably do not use an RTL chip, so they cannot be
used for SDR either.  The RTL chip does not have bandwidth problems
when used for DVB reception either (with the hardware demodulator) it
is only the SDR function that has limited bandwidth.
 
Can you make the "SDR function" or "SDR mode" much clear under this context? Are there any references (e.g. documents or papers) that could help to understand all the concepts here?
 
In the situation while DVB-S/S2 card is used, isn't the demodulation done using software (C/C++) after transportation layer streams are received?
 
 



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Adam Nielsen

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Dec 1, 2013, 7:35:47 PM12/1/13
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> Can you make the "*SDR function*" or "*SDR mode*" much clear under
> this context? Are there any references (e.g. documents or papers)
> that could help to understand all the concepts here?

Try Wikipedia: http://en.wikipedia.org/wiki/Software-defined_radio -
there are translations for other languages if that's easier too.

> In the situation while DVB-S/S2 card is used, isn't the demodulation
> done using software (C/C++) after transportation layer streams are
> received?

Demodulation is what you do to obtain the MPEG transport stream. If
you use SDR, then it can take the signal from the antenna and give you
an MPEG transport stream, if you have the correct software. But any DVB
stick can also demodulate the signal and produce a transport stream
without SDR.

Once you have the MPEG transport stream then you (almost) have an .mpg
movie file which has nothing to do with radio - it is just digital
sound and video.

I think perhaps you are misunderstanding what an MPEG transport stream
is. It has nothing to do with radio, it is just a data format that can
hold multiple video and audio tracks (e.g. a single MPEG transport
stream can store 3-4 TV channels.)

When digital TV is broadcast with DVB, the transport stream (with audio
and video for the TV channel) is modulated and transmitted, so when you
receive the signal you demodulate it (with a hardware demodulator or
an SDR device if you want) and you get back the original MPEG transport
stream that contains the TV channel's sound and video.

I hope this explains things a bit better.

Cheers,
Adam.

bulbul...@gmail.com

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Dec 13, 2013, 12:04:35 PM12/13/13
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Hi Adam I'll like to know if i understood well the BW issue there.

When using the stick in "normal" mode (what the stick was sold for) it can receive DVB signals (thanks to the RT820 tunner) and Demodulate it (thanks to the RTL3228U), thus providing an MPEG stream to USB ?
When using he stick in "SDR" mode, it can receive signal from 0 - 1Ghz (Thanks to the RT820 tunner) and bring them to baseband signals (Thanks to RTL3228U) . These signals will be sampled at a given rate (max 2.8Mhz without loss). That is why in SDR mode, because we do not have a BW of 8Mhz, we can not demodulate DVB signals ?

Thanks in advance for your great help !

Adam Nielsen

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Dec 13, 2013, 5:31:36 PM12/13/13
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> When using the stick in "normal" mode (what the stick was sold for)
> it can receive DVB signals (thanks to the RT820 tunner) and
> Demodulate it (thanks to the RTL3228U), thus providing an MPEG stream
> to USB ? When using he stick in "SDR" mode, it can receive signal
> from 0 - 1Ghz (Thanks to the RT820 tunner) and bring them to baseband
> signals (Thanks to RTL3228U) . These signals will be sampled at a
> given rate (max 2.8Mhz without loss). That is why in SDR mode,
> because we do not have a BW of 8Mhz, we can not demodulate DVB
> signals ?

Yes that's pretty much correct. The tuner does not work from 0 - 1GHz
though, the R820T works from 24MHz to ~1.8GHz. So you can't get AM
radio or other signals below 24MHz with this device (although there are
ways around this that involve modifying the device or purchasing
upconverters.)

The RTL2832 then samples the signal as you describe. Technically they
are only baseband with the E4000 tuner, which results in a DC spike in
the middle of the signal. The R820T uses an IF so it's not baseband,
but this means there is no DC spike. Otherwise it's the same.

And yes, the SDR sample rate is 2.8MHz max, which is well below the
8MHz needed to demodulate DVB in software. But there are other more
expensive SDR devices that have much higher bandwidths, and these can
be used to demodulate DVB entirely in software.

Cheers,
Adam.

cindy zhang

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Dec 13, 2013, 8:29:35 PM12/13/13
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So, if RTL3228U is not in SDR mode, its max output sampling rate could be up to 8MHz (demodulated RF signal / TS signal in IF band). To continue the demodulation in order to recover the original information bits, we will need to convert the signal into baseband, do physical demodulation, and finally send the demodulated information bits to the display. 
 
Are all the concepts here correct? I am trying to further clarify the difference between SDR mode and non-SDR mode.
 
Thanks



Cheers,
Adam.

jdow

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Dec 13, 2013, 10:15:47 PM12/13/13
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Indications are that the sample rate is related to 28.8 MHz. It gets
decimated down to narrower bandwidths before sending the data out. It
may get decimated before it performs the decoding. But I can't even
guess on that issue.

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cindy zhang

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Dec 13, 2013, 10:48:35 PM12/13/13
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"The RTL2832U complies with NorDig Unified 1.0.3, D-Book 5.0, and EN300 744 (ETSI Specification). It supports 2K or 8K mode with 6, 7, and 8MHz bandwidth"



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Adam Nielsen

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Dec 14, 2013, 3:25:21 AM12/14/13
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> So, if RTL3228U is not in SDR mode, its max output sampling rate
> could be up to 8MHz (demodulated RF signal / TS signal in IF band).

If the RTL2832 is not in SDR mode, there is no output sampling rate.
It outputs demodulated and decoded MPEG-TS data.

> To continue the demodulation in order to recover the original
> information bits, we will need to convert the signal into baseband,
> do physical demodulation, and finally send the demodulated
> information bits to the display.

The tuner (R820T or E4000) converts the signal into baseband (or IF) in
all modes (SDR and non-SDR.) The RTL2832 then samples this signal at
28.8MHz, and then does one of two things:

- in SDR mode, decimates it to ~2MHz sampling rate and outputs a
digitized version of the RF signal over USB, so that demodulation
can be done by a software application. Any signals at the tuned
frequency can be decoded depending on the SDR software used.

- in non-SDR mode, performs demodulation and decoding (only if a
DVB-T signal is present at the tuned frequency), outputting an
MPEG-TS stream over USB. If there is no DVB-T signal, no data is
output.

> Are all the concepts here correct? I am trying to further clarify the
> difference between SDR mode and non-SDR mode.

I hope this helps. It is probably wrong to call it SDR and non-SDR
mode, because these are completely different concepts. Think of the
RTL2832 as one IC with two completely different unrelated functions.
One fully receives a DVB signal, and the other is a high speed ADC. The
two have some similarities but are implemented in the chip as completely
independent functions.

Cheers,
Adam.

jdow

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Dec 15, 2013, 5:53:28 PM12/15/13
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How, then, do you explain dongle performance in direct sampling mode?

Do note that the specified bandwidths are not happy with the dongles as
designed. It's far easier to sample at a higher rate and digitally filter
it to the desired bandwidth. There will be some analog filtering. The
digital filtering would finish the adjacent channel job very nicely.
The specification you cite is for the whole unit's performance not
the RTL2832 D/A.

As I say, indications are that it samples a 28.8 MHz, the crystal
frequency. This is born out by performance in the direct sampling SDR
modes for HF reception.

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cindy zhang

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Dec 28, 2013, 10:46:15 PM12/28/13
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Does Ettus LLC have a substitute for this USB 2.0 bandwidth problem? For example, how about a Ethernet connection instead of a USB connection?




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Siegfried Jackstien

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Dec 29, 2013, 11:09:04 AM12/29/13
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www.ettus.com

look and see that "some" ot the receiver or txrx combinations have ethernet
(or usb 3.0) ...

dg9bfc

sigi


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