Digital Television
Digital Television
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Digital Video Broadcast (DVB) |
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There are two formats that are used for Ham radio. The first format is DVB-S. This uses Q.P.S.K. (Quadrature Phase Shift Keying) modulation which works well on the 13 / 23cm bands. DVB-S is primarily used for satellite communications. The symbol rate can be adjusted from 3k to 45k depending on the data rate required and bandwidth available. The second is DVB-T. This uses COFDM (Coded Orthogonal Frequency Division Multiplexing) modulation designed for UHF television channels with 8 MHz bandwidth. 1705 or 6817 carriers are fitted into an 8 MHz channel with guard intervals in the time domain. These guard intervals are there to improve the robustness of the wanted signal against any multi-path problems. Each carrier can be QPSK or QAM (Quadrature Amplitude Modulation) modulated up 64QAM. COFDM will fit into the 70 / 50cm bands. |
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The DVB system is common to all types modulation used (C, S, and T formats). So it possible to transcode between formats. A DATV repeater can simply put together with a receiver module connected to a modulator module. The receiver corrects for any errors that may be in the received data. Then the modulator re-adds in the forward error correction for the transmit side. Multiplexing of transport stems into a DVB format. Most multiplexes have a number of ASI inputs and one or two ASI outputs. The data rate is set for each of the inputs within the multiplex unit. Types of digital input could be Data, MPEG2 video & sound, or Teletext. MPEG2 encoder formats are 352 x 288 and 720 x 576. Analogue input for standard definition DATV 720 x 576 is used, in high definition, the format 1920 x 1080 is used. High definition is fed into the encoder as serial data DVI or HDSDI formats. Video files can also be streamed from your PC hard drive by plugging in a PCI to DVI or a PCI to ASI card. The DVI card could be connected to a MPEG2 encoder. An ASI card could send out as DVB formatted serial data to the multiplex unit or direct to a modulator. |
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Available bitrates (Mbit/s) for DVB-T system in a 8 MHz bandwidth
Modulation |
Coding rate |
Guard interval |
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1/4 |
1/8 |
1/16 |
1/32 |
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QPSK |
1/2 |
4.976 |
5.529 |
5.855 |
6.032 |
2/3 |
6.635 |
7.373 |
7.806 |
8.043 |
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3/4 |
7.465 |
8.294 |
8.782 |
9.048 |
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5/6 |
8.294 |
9.216 |
9.758 |
10.053 |
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7/8 |
8.709 |
9.676 |
10.246 |
10.556 |
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16QAM |
1/2 |
9.953 |
11.059 |
11.709 |
12.064 |
2/3 |
13.271 |
14.745 |
15.612 |
16.086 |
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3/4 |
14.929 |
16.588 |
17.564 |
18.096 |
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5/6 |
16.588 |
18.431 |
19.516 |
20.107 |
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7/8 |
17.418 |
19.353 |
20.491 |
21.112 |
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64QAM |
1/2 |
14.929 |
16.588 |
17.564 |
18.096 |
2/3 |
19.906 |
22.118 |
23.419 |
24.128 |
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3/4 |
22.394 |
24.882 |
26.346 |
27.144 |
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5/6 |
24.882 |
27.647 |
29.273 |
30.160 |
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7/8 |
26.126 |
29.029 |
30.737 |
31.668 |
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For more information on digital television have a look at this web site |
D-ATV.com |
Digital Radio Mondiale (DRM)
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DRM Data DRM is designed to work on frequencies below 30 MHz. The concept of DRM is a digital replacement for AM Broadcast radio on Long Wave, Medium Wave and Short Wave bands. The digital signal on HF uses 10 kHz of bandwidth MF and LF bands are 9 kHz wide. On LF and MF broadcast bands DRM is transmitted in mode A on HF bands modes B, C, or D are used for sky wave propagation. DRM modes are design for different propagation effects. Mode A is for ground wave propagation, modes B and C are for sky wave and mode D is for near vertical intendance sky wave. The robustness of signal comes from the addition of pilot carries within the digital signal. When a DRM receiver locks on to the signal it looks at relative levels of the pilot carries. In this way you can counter the fading effects though out the pass band. Digital encoding let you encode a number of transport stems into one. Sound encoding uses mpg 4 with AAC for mono or AAC+ (SBR) for stereo. AAC+ also gives greater high-end audio frequency response. Data stems can multiplex into signal at different bit rates. Image files can also sent as compressed jpg files at different bit rates within the signal. Receiving DRM can done by converting it down to an IF of 12 kHz. The simples way to do this is modifying a broadcast receiver at the IF frequency. Most IF down converters use a NE612 mixer to do this. The 12kHz IF frequency is feed into a PC sound card in my case into the microphone input. There is two type of DRM receiver software available that can be used DRM Radio or DRM dream. DRM dream is what I am using it gives you the ability to receive analog signals as well. DRM dream |
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Mode |
Modulation (nQAM) |
Robustness |
Nominal Signal Bandwidth (kHz) |
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4.5 |
5.0 |
9.0 |
10.0 |
18.0 |
20.0 |
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Approx. available bit rate kb/s |
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A |
64 |
Max |
9.4 |
10.6 |
19.7 |
22.1 |
40.9 |
45.8 |
64 |
Min |
14.7 |
16.7 |
30.9 |
34.8 |
64.3 |
72.0 |
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16 |
Max |
6.3 |
7.1 |
13.1 |
14.8 |
27.3 |
30.6 |
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16 |
Min |
7.8 |
8.9 |
16.4 |
18.5 |
34.1 |
38.2 |
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B |
64 |
Max |
7.2 |
8.3 |
15.3 |
17.5 |
31.8 |
35.8 |
64 |
Min |
11.3 |
13.0 |
24.1 |
27.5 |
50.0 |
56.1 |
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16 |
Max |
4.8 |
5.5 |
10.2 |
11.7 |
21.2 |
23.8 |
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16 |
Min |
6.0 |
6.9 |
12.8 |
14.6 |
26.5 |
29.8 |
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C |
64 |
Max |
13.8 |
29.0 |
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64 |
Min |
21.6 |
45.5 |
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16 |
Max |
9.2 |
19.3 |
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16 |
Min |
11.5 |
24.1 |
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D |
64 |
Max |
9.2 |
19.5 |
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64 |
Min |
14.4 |
30.6 |
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16 |
Max |
6.1 |
13.0 |
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16 |
Min |
7.6 |
16.3 |