Digital TV Definitions

There are five major digital broadcast standards in widespread use worldwide. Other standards exist but are country- or application-specific. The major formats are compared below, with notes on their suitability for Digital ATV (DATV).

The main USA ATSC modulation standard uses 8-VSB (8-level Vestigial Sideband) for video transmission, MPEG-2 for compression, and Dolby Digital (AC3) for audio. 8-VSB is an amplitude modulation method that suppresses one sideband to reduce occupied bandwidth while retaining demodulability. A vestige of the suppressed sideband remains — hence the name.

A 6 MHz channel carries a fixed symbol rate of 10.76 Mbaud, yielding a gross bit rate of 32 Mbit/s and a net usable rate of 19.39 Mbit/s after forward error correction. Compared to NTSC, 8-VSB stations can cover the same area at approximately 25% of the analog effective radiated power. It is more resistant to impulse noise than COFDM and requires a lower peak-to-average power ratio (∼6 dB above average).

Enhanced VSB (E-VSB, published 2005) encodes handheld DTV data within the 8-VSB signal, enabling low-power portable reception without a separate transmitter.

DVB-T was developed for terrestrial digital television and uses COFDM (Coded Orthogonal Frequency-Division Multiplexing). It is designed specifically to overcome multipath reflections from buildings and terrain, which cause “ghosts” in analog pictures and Inter-Symbol Interference in digital. The solution is to spread the high data rate across a large number of individually modulated carriers at low bit rate each — typically 1,705 in 6 MHz mode — so that multipath delays represent only a small fraction of each symbol’s duration.

DVB-T produces a very complex signal and is the hardest to reproduce, requiring very high-speed components. It is necessary for broadcast TV but is not practical for amateur purposes. The US commercial HDTV standard (ATSC 8-VSB) is sometimes described as DVB-T’s counterpart, though they use different modulation methods.

DVB-C was developed for cable digital television using QAM (Quadrature Amplitude Modulation). A cable environment is well-protected from path loss and distortion, so higher-order modulation schemes (up to 256QAM) are practical without needing large amounts of error correction. This makes DVB-C the simplest and cheapest digital TV method — but unsuitable for amateur use for several reasons.

The minimal error correction makes it susceptible to multipath reflections that do occur in cable plants. More problematically, there is no common DVB-C standard: each cable company implements its own variant so that only their proprietary set-top boxes work on their network. This proprietary fragmentation means no universal receiver is available for ham use.

DVB-H is intended for hand-held video devices, both fixed and portable. In the USA, DVB-H data is embedded within the ATSC 8-VSB broadcast data stream, so a separate transmitter is not required to implement handheld reception.

This embedding is the key reason DVB-H is unsuitable for DATV: it requires an 8-VSB signal as its carrier, and as discussed above, 8-VSB is generally a poor match for ham radio ATV. DVB-H is therefore not a practical option for amateur digital television.

DVB-S was developed for satellite digital television using QPSK (Quadrature Phase Shift Keying) modulation. It is the standard recommended for Digital ATV by most ham groups in Europe and the United States, including ATCO.

A satellite-to-earth link involves high path attenuation, line-of-sight geometry, and relatively low multipath. QPSK is a very robust modulation scheme: a demodulator only needs to decide which of four phase quadrants a received symbol falls in. This simplicity enables demodulation at lower signal-to-noise ratios than most other schemes. The standard allows the symbol rate — and therefore the RF bandwidth — to be freely adjusted to suit the available spectrum and signal path, unlike ATSC’s fixed 6 MHz/19.4 Mbit/s.

Compiled and edited from the Dutch Amateur TV web site d-atv.com. See also the DVB-S Details page for in-depth articles on symbol rates, FEC, and bandwidth.