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| -rw-r--r-- | scenarios.tex | 95 |
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diff --git a/scenarios.tex b/scenarios.tex index 74deb72..bccc8a3 100644 --- a/scenarios.tex +++ b/scenarios.tex @@ -43,8 +43,9 @@ the \texttt{doc/} folder. \subsection{Interfacing Hardware Devices} \subsubsection{Ettus USRP} ODR-DabMod integrates support for the UHD library that can interface with all -USRP devices from Ettus. The following configuration file illustrates how to -send the \texttt{myfirst.eti} over a USRP B200 on channel 13C: +USRP devices from Ettus. The following configuration file \texttt{mod.ini} +illustrates how to send the \texttt{myfirst.eti} over a USRP B200 on channel +13C: \begin{lstlisting} [remotecontrol] @@ -97,10 +98,98 @@ This example also shows more options that the example for the file output: Some of these options are not necessary for the system to work, but they improve the performance. +\paragraph{Remarks concerning the USRP B200} +The USRP B200 is the device we are using most. It's performance is proven in a +production environment, it supports the transmit synchronisation necessary for +SFN and is robust enough for 24/7 operation. + +However, care has to be taken about the host system, especially about the USB +controller. Using USB~2.0 is not a problem for a DAB transmission, both USB~2.0 +and USB~3.0 host controllers can therefore be used. Since USB~2.0 has been +around for longer and is more mature, it is sometimes preferable because it +causes less USB errors. This heavily depends on the exact model of the USB +controller inside the host PC, and has to be tested for each system. + +The txgain on the B200 varies between $0$dB and about $90$dB. Experience shows +that compression effects begin to appear at values around $85$dB. This might be +different from device to device and needs to measured. + +Similarly, the digital gain has to optimised for a given setting. It is +important that there is no digital clipping in the chain, because that leads to +problematic spurious spectrum components, that can disturb or even damage a +power amplifier. + +There are some performance measurements available on the Opendigitalradio +wiki.\footnote{\url{http://wiki.opendigitalradio.org/index.php/USRP\_B200\_Measurements}} + +\paragraph{Remarks concerning other USRP models} +We have used the USRP1, the USRP2 and the USRP B100 with the tools. The WBX is +the most appropriate daughterboard for these models. + +The txgain setting has another range, it is best to start at $0$dB, and increase +it in steps of $3$dB or smaller while measuring the output signal, until the +correct power is reached. + \subsubsection{Other Hardware} +The HackRF can also be used as a transmit platform. It's interfacing is not +integrated with ODR-DabMod, and it only supports 8-bit samples. The +configuration is a bit different. First, we must output signed 8-bit interleaved +I/Q samples instead of 32-bit interleaved floats, and second, we cannot use UHD, +but must go through a pipe to HackRF. The output settings inside the +configuration are as shown: + +\begin{lstlisting} +[output] +output=file +[fileoutput] +format=s8 +filename=/tmp/ofdm.fifo +\end{lstlisting} + +The output fifo has to be created beforehand, and the \texttt{hackrf\_transfer} +utility is then used to transmit the signal to the device. The options needed +for the transmission utility are not complete yet. The rough idea would be: + +\begin{lstlisting} +mkfifo /tmp/ofdm.fifo +odr-dabmod mod.ini & +hackrf_transfer -f <frequency> -t /tmp/odr.fifo +\end{lstlisting} + + +\subsection{Audio Sources} +Preparing a DAB multiplex with different programmes requires that we are able to +read and encode several audio sources. This audio data can reach the multiplexer +in different ways. We will go over different possibilities in this part. -\subsection{Audio sources} \subsubsection{Using Existing Web-Streams} +One common scenario is to transmit radio stations that already are available as +web-radio streams. For simplicity, it makes sense to get these web streams, +which are most often encoded in mp3 and available through HTTP, decode them, and +use them as audio source for the DAB or \dabplus encoder. + +The advantage of this approach is that the radio itself does not need to setup a +new infrastructure if the stream is of good quality. The main disadvantage is +that the audio is encoded twice, and this coding cascading degrades the audio +quality. + +Often, web-streams are encoded in mp3 at $44100\Hz$ sample-rate, whereas DAB +is most often $48000\Hz$ or sometimes $32000\Hz$. A sample-rate conversion is +necessary in the stream decoder. + +There are many different stream decoders, and gstreamer, mpg123 and mplayer have +been tested. We have achieved good results with mplayer, and the dab-scripts +repository\footnote{\url{http://github.com/Opendigitalradio/dab-scripts}} +contain the script \texttt{encode-jack.sh} that uses mplayer, and illustrates +how it is possible to encode a web-stream to \dabplus. JACK\footnote{The JACK + Audio Connection Kit is a virtual audio patch, +\url{http://www.jack-audio.org}} is used to interconnect the stream decoder to +the \dabplus encoder. + +This script is designed for production use, and also contains automatic restart +logic in case of a failure. + \subsubsection{Encoders at Programme Originator Studios} +\subsubsection{Multi-capture Audio Card} \subsection{Single-Frequency Networks} |