Les règles d'utilisation et plan de bande du transpondeurs DATV
With PlutoDVB’s built-in analysis tools, we will check that our CBR flow is not too high and also, conversely, if we may increase it a little bit. Click on the Analysis button (or go directly to the dedicated page of PlutoSDR http://192.168.2.1/analysis.php).
On this video, you can compare the two situations.
- On the left a setting of too high a video bit rate. The red dots that appear are information that cannot be processed by PlutoDVB. On reception, the audio signal will show white spots or cuts, the video will be jerky or frozen.
- On the right, a correct data rate, all the points of the buffered data curve are green. Even if, depending on the movement of the curve over time, they approach the horizontal axis at 0, but at no time does the curve pass below the axis. It can also be seen in the data type decomposition graph (transport stream analysis, top) that data packets are null present. This indicates that we might be able to increase the CBR throughput a bit. (Based on the minimum null packet stub, maybe 5 or 10kpbs for this illustration . To be tested by a new iteration of tuning, detailed below).
To refine the CBR video bit rate, I recommend the following technique:
- Calculate your flow rate (or let the rate calculated with online tools as indicated above) using a rather low coefficient (60%).
- Switch to streaming your channel. You are asked not to transmit to the satellite during this tuning phase and to make your settings on a load or another frequency. Personally, my final amplification stage is disabled.
- Consult the dynamic graph of analysis of the PlutoDVB video buffer. On the Transport stream analysis graph, the number of null packets are as many kbit/s that you will be able to add (transfer) to the value of your CBR, to make it useful video data. Turn off your streaming.
- Readjust (increase) your CBR in OBS Studio with the null packet number. Resend your stream to PlutoSDR.
- The null packages must have decreased. You should always have some, to compensate for the vagaries of some videos with highly fluctuating streams, and which is necessary to compensate for the “time constant” of the stream’s arrival at Adalm Pluto.
Depending on the dynamics of your video images, you may notice a larger amplitude of buffer variation. If this is the case and the signal passes under the horizontal axis from time to time but returns to a steady state in the green zone, you can increase the PCR/PTS (buffer) value (1000 – 1500 for example). The disadvantage is the longer transmission delay.
The choice of the FEC
Before seeing how to choose the right FEC, it is necessary to introduce the notion of MER.
MER (Modulation Error Rate)
The modulation error rate or MER is a measure used to quantify the performance of a digital radio (or digital television) transmitter or receiver in a communication system using digital modulation (such as PSK modulation). A signal sent by an ideal transmitter or received by a receiver would have all points of the constellation precisely at the ideal locations, however various imperfections alter the constellation in implementation (such as noise, low image rejection rate, phase noise, carrier suppression, distortion, etc.).
MER can be considered as an indicator of signal strength at reception. If your transmit signal is lowered by 3 dB, you will see a decrease of approximately 3 dB MER at reception. The minimum reception levels (minimum MER) are dependent on the transmitter’s FEC :
You will only be able to decode images at a given MER level if you choose a FEC that does not require more signal. For example, for a signal with an FEC of 3/4, (3 useful data + 1 error correction data), your signal will be decoded only if the MER level is 4.2 dB. Refer to the right column of the table above.
- FEC 1/2 : 1.7 dB
- FEC 2/3 : 3.3 dB
- FEC 3/4 : 4.2 dB
- FEC 5/6 : 5.1 dB
- FEC 6/7 : 5.5 dB
- FEC 7/8 : 5.8 dB
Necessary signal power and width
For a 250 KS signal on a 1.2m dish, it is common to indicate that you need 30W. By doubling the size of your dish, you will divide the power needed by 4. In the table below you can see how the power needed for a 250KS signal evolves according to the diameter of the dish.
|Dish diameter (cm)||Power (W)||Power (dBm)|
But also, according to the transmitted spectrum width, you will multiply the necessary power by the coefficient indicated in the following table to know the necessary power. For example, you can transmit a 125 KS signal and a 1.2m parabola by going to half power, i.e. 15W. You will find the same signal level, just by transmitting less data via the satellite.