Broadcast Digital TV format overviews – major standards
There are other standards in use throughout the world but not in widespread use. Because most are “country and application specific”, we don’t want to standardize on a non-standard format. Make sense? The major ones are listed below.
- ATSC (8VSB) North America
- DVB-T (CODFM) Europe Terrestrial
- DVB-C (QAM) Europe & USA cable (numerous variations)
- DVB-H (QAM) Europe, (8VSB) USA handheld
- DVB-S (QPSK) Europe and USA Satellite
The main USA ATSC modulation standards for broadcast television are 8-VSB for the transmission of video data, MPEG-2 for video signal compression, and Dolby Digital for audio coding. The 8-VSB (8 level Vestigial Sideband) is an amplitude modulation method which attempts to eliminate the spectral redundancy of pulse amplitude modulated signals. Modulating a real data sequence by a cosine carrier results in a symmetric double-sided passband spectrum. The symmetry implies that one of the sidebands is redundant so removing one sideband with an ideal filter should preserve the ability for perfect demodulation. As brickwall filters with zero transition bands cannot be physically realized, the filtering actually implemented in attempting such a scheme leaves a vestige of the redundant sideband, hence the name “VSB”.
A 6 MHz channel is used which has a fixed symbol rate of 10.76 Mbaud with a gross bit rate of 32 Mbit/s and a net bit rate of 19.39 Mbit/s of usable data. The net bit rate is lower due to the addition of forward error correction codes. The eight signal levels are selected with the use of a trellis encoder.
A significant advantage of 8VSB for broadcasters is that it requires much less power to cover an area comparable to that of the earlier NTSC system, and it is reportedly better at this than the most common alternative system, COFDM (Coded Orthogonal Frequency-Division Multiplexing) which is used for DVB-T in Europe. Part of the advantage is the lower peak to average power ratio needed compared to COFDM. An 8VSB transmitter needs to have a peak power capability of 6 db (four times) its average power. 8VSB is also more resistant to impulse noise. Some stations can cover the same area while transmitting at an effective radiated power of approximately 25% of analog broadcast power. While NTSC and most other analog television systems also use a vestigial sideband technique, the unwanted sideband is filtered much more effectively in ATSC 8VSB transmissions with a Nyquist filter. Reed–Solomon error correction is the primary system used to retain data integrity.
In 2005 the ATSC published standards for Enhanced VSB. Using forward error correction, the E-VSB standard allows DTV reception on low power handheld receivers with smaller antennas in much the same way DVB-H does in Europe, but still using 8VSB transmission. The DVB-H data is encoded within the 8VSB signal so a separate transmitter is not needed to implement the USA handheld standard. However, without the inclusion of the handheld data, motion reception is not practical.
Disadvantages for ATV Use:
- Modulation scheme is very complex
- Fixed 19.4 Mbit data rate and 6MHz bandwidth are not modifiable
- Needs hi linearity amplifiers to create the signal
- Audio channel not receivable on some TV sets
- Ham band not available on standard unmodified TV’s
430-450 no good – cable setting in some TV’s default to QAM
902-915 no good – above TV tuning range & crowded with Wi-Fi
1240-1300 no good – this is above TV tuning range
- Special receive converter required for Ham applications
- Cannot use when receiver is in motion (no mobile operation)
The following material has been compiled and edited with the aid of the descriptions at the Dutch ATV web site along with my verbiage to hopefully make it easier for the beginner. The complete unedited text may be seen at the Dutch Amateur TV web site http://d-atv.com
The DVB-T standard was developed for terrestrial digital television communication and is presently in use by US broadcast television stations for digital and HDTV known best as 8VSB modulation (8 quadrant Vestigial Sideband). The aim is to overcome the destructive effects of multipath reflections from objects in the signal path such as buildings and towers, which produce “ghosts” in the picture of analog transmissions. In digital television transmission, the data rates are very high so multipath reflections will normally be even higher resulting in a partly distorted received signal. Also the multipath reflections cause Inter Symbol Interference because reflections of the received signal interfere with the direct received path. Nevertheless, higher bit rates (symbol rate) produce higher negative effects so, to overcome these disturbances for DVB-T the effective bit rate is spread out over a large amount of digitally modulated carriers. The larger the amount of carriers produces lower effective bit rates for each carrier. The lower the effective bit rate per carrier, the lower the negative effects of multipath reflections which is the basic idea behind DVB-T. This produces a very complex signal so it is the hardest to reproduce and most expensive because it requires very high speed parts. It’s necessary for broadcast TV but not practical for amateur purposes.
- Receivers not in use in USA
- Needs high signal to noise ratio receivers
- DVB-T set top boxes not available in US
The DVB-C “standard” was developed for cable digital television transmission using QAM modulation. A cable environment is a relatively protected environment with respect to distortion and signal path attenuation so a higher signal to noise ratio can be achieved. Also, because there is no negative effect from multipath, it is able to implement higher order modulation schemes. DVB-C generally requires higher signal to noise ratios at the receiver side due to the higher order modulation schemes. Because of minimal error correction, it is more susceptible to multipath reflections. This is one reason why DVB-C is not preferred for Digital Amateur Television. It’s too bad because it’s the easiest and cheapest method. Also various cable companies that use 8VSB or QPSK implement DVB-C in a variety of forms so no one uses a common standard. No surprise here because each cable company wants you to use THEIR set top box and not the competition. Therefore a cable box for one cable company will not necessarily work for another. We wouldn’t want to become a part of this fiasco!
- No common standard. Many variants used by each cable company
- Common receiver not available
This standard is intended for hand held video devices both fixed and portable. In the USA it is imbedded into the 8VSB broadcast data stream so a separate transmitter is not needed. That condition makes it unsuitable for use as DATV because it would require sending out an 8VSB signal in order to use it. As discussed elsewhere, 8VSB is generally not a very good match for DATV so that pretty much eliminates DVB-H from any consideration.
The DVB-S standard is developed for satellite digital television transmission using QPSK modulation (Quadrature Phase Shift Keying), which is a type of FM modulation. Why do we recommend DATV use the commercial DVB-S standard? A DVB-S digital system has some distinct advantages and also some disadvantages.
One of the main advantages of a digital ATV system is the fact that picture quality is improved above that of most analog systems. We do not encounter the negative effects of noise. We do not encounter video group delay problems; an item on which much attention has been paid by lots of amateurs and audio quality is improved. With digital ATV we get high quality audio channels and these high quality audio channels don’t disturb picture quality!
Another advantage is it does not extend the occupied bandwidth of our signal, something that is the case with analog where we need some FM modulated audio carriers above our video signal. Other main advantages are the fact that analog ATV systems occupy a lot of bandwidth. A wide occupied bandwidth means several disadvantages among which are less room for others to communicate and a higher noise bandwidth.
The first item is clear. We want to be as efficient as possible. If this can be done without throwing away any quality then this is good. If we can improve quality with less occupied bandwidth then we have even more benefit! The second item is also very interesting. The higher the bandwidth the higher the received noise level will be at the receiver because noise is integrated over bandwidth. Some digital modulation schemes are able to demodulate at lower threshold levels than possible with analog FM ATV systems. One of them is for example QPSK (DVB-S). With QPSK we are able to occupy less bandwidth and also make use of lower thresholds. This means that we can get more out of such a system with less power, better quality and less bandwidth!
A satellite to earth system needs low threshold demodulation and a good signal to noise ratio so only QPSK can be used. QPSK is a very robust modulation scheme because it just has to make a decision in one of four quadrants. The low signal to noise ratio on the other hand will be a source for bit errors, both burst errors as single bit errors. To overcome this weakness, the DVB-S standard uses different layers of Forward Error Correction (FEC) for a very robust protection against any kind of errors. The FEC consists of a Reed Solomon coding that protects against burst errors and also additional convolutional interleaving to spread out the impact of burst errors. The convolutional encoding is better known among users of satellite television and is recognizable in a satellite receiver setup menu under the menu item FEC rate. The fact that satellite communication is basically line of sight communication without obstacles in the transmission path tells us that less attention is paid in this system on multipath effects. Therefore, the DVB-S standard will be moderate when it comes to robustness against multipath reflections.
Mathematically all these carriers are orthogonally spaced from each other with an Inverse Fast Fourier Transform (IFFT). It works like this: The incoming bitstream is encoded with Forward Error Correction blocks like Reed Solomon and convolutional interleaving and finally convolutional encoding. After the FEC the resulting bitstream is mapped on all the constellations for the separate carriers. The resulting constellations are the input for the IFFT processor block which performs the actual transformation from frequency to time domain. After the IFFT a cyclic extension is performed on the resulting OFDM symbol, which is used for the guard interval that gives additional protection against multipath reflections. The resultant complex output of the IFFT block can then be converted to RF with an I/Q modulator. There, did you get all of that? If not, don’t be discouraged because you don’t have to understand it to apply it.