8VSB
8VSB is the
A modulation method specifies how the radio signal fluctuates to convey information. ATSC and DVB-T specify the modulation used for over-the-air digital television; by comparison, QAM is the modulation method used for cable. The specifications for a cable-ready television, then, might state that it supports 8VSB (for broadcast TV) and QAM (for cable TV).
8VSB is an 8-level
Modulation technique
Vestigial sideband modulation (VSB) is a modulation method which attempts to eliminate the spectral redundancy of pulse-amplitude modulation (PAM) signals. Modulating a carrier by a real-valued data sequence results in a sum and a difference frequency, resulting in two symmetrical carrier side-bands. The symmetry means that one of the sidebands is redundant, so removing one sideband still allows for demodulation. As filters with zero transition bandwidth cannot be realized, the filtering implemented leaves a vestige of the redundant sideband, hence the name "VSB".
Throughput
In the 6 MHz (
Power saving advantages]
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.[citation needed] Part of the advantage is the lower peak to average power ratio needed compared to COFDM.[citation needed] An 8VSB transmitter needs to have a peak power capability of 6 dB (four times) its average power.[citation needed] 8VSB is also more resistant to impulse noise.[citation needed] Some stations can cover the same area while transmitting at an effective radiated power of approximately 25% of analog broadcast power.[citation needed] 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. 8VSB uses a Nyquist filter to achieve this. Reed–Solomon error correction is the primary system used to retain data integrity.
In summer of 2005, the ATSC published standards for Enhanced VSB, or
Disputes over ATSC's use
For some period of time, there had been a continuing lobby for changing the
Because of continued adoption of the 8VSB-based ATSC standard in the U.S., and a large growing ATSC receiver population, a switch to COFDM will be challenging. Most analog terrestrial transmissions in the US were turned off in June 2009, and 8VSB tuners are common to all new TVs, further complicating a future transition to COFDM.
However, with the development of ATSC 3.0, an updated version of the American digital television standard designed for mobile reception and better single frequency network performance, the ATSC has decided to make the switch to OFDM with LDPC error correction (essentially COFDM).[4] As a result, ATSC 3.0 will be incompatible with all current ATSC 1.0 receivers, and viewers will need a new TV with a compatible tuner or a converter box.[5] Unlike the previous digital TV transition which was mandated by the FCC, the "transition" to ATSC 3.0 will be completely voluntary. Additionally, the FCC has required that broadcasters who decided to make the switch to ATSC 3.0 continue to make their main channel available through a simulcast agreement with another in-market station (with a similar coverage area) through at least 2022.[6] Sinclair announced an intention to bring ATSC 3.0 to 40 cities by 2020.[7]
8VSB vs COFDM
The previously cited FCC Report also found that COFDM has better performance in dynamic and high level static multipath situations, and offers advantages for single frequency networks and mobile reception. Nonetheless, in 2001, a technical report compiled by the COFDM Technical Group concluded that COFDM did not offer any significant advantages over 8VSB. The report recommended in conclusion that receivers be linked to outdoor antennas raised to roughly 30 feet (9 m) in height. Neither 8VSB nor COFDM performed acceptably in most indoor test installations.[8]
However, there were questions[who?] whether the COFDM receiver selected for these tests − a transmitter monitor [2] Archived 2007-09-27 at the Wayback Machine lacking normal front end filtering − colored these results. Retests that were performed using the same COFDM receivers with the addition of a front end band pass filter gave much improved results for the DVB-T receiver, but further testing was not pursued.[3][permanent dead link]
The debate over 8VSB versus COFDM modulation is still ongoing. Proponents of COFDM argue that it resists multipath far better than 8VSB. This is important property of the modulation for receiving HDTV in e.g. moving vehicles that is not possible with 8VSB. Early 8VSB
The vast majority of U.S. TV stations use COFDM for their studio to transmitter links and news gathering operations[citation needed]. These are point-to-point communication links and not broadcast transmissions.
See also
- ATSC tuner
- ATSC-M/H for Mobile/Handheld receivers
References
- ^ Sparano, David (1997). "WHAT EXACTLY IS 8-VSB ANYWAY?" (PDF). Retrieved 8 Nov 2012.
- ^ DTV REPORT ON COFDM AND 8-VSB PERFORMANCE (PDF), FCC Office of Engineering and Technology, archived (PDF) from the original on 14 April 2007, retrieved 2007-03-04, September 30, 1999.
- ^ Sinclair Claims Wide Support For Dtv Petition, Television Digest with Consumer Electronics, 1999, archived from the original on 2004-09-02, retrieved 2008-06-06, Oct 11, 1999.
- ^ "ATSC 3.0 Technical overview" (PDF). Rohde & Schwarz. 8 March 2021. Archived (PDF) from the original on 8 March 2021. Retrieved 8 March 2021.
- ^ Cohen, Simon. "ATSC 3.0: Everything you need to know about broadcast TV's next big thing". Digital Trends. Designtechnica Corporation. Archived from the original on 29 November 2020. Retrieved 8 March 2021.
- ^ "Report and Order and Further Notice of Proposed Rulemaking". Federal Communications Commission. 20 November 2017. Archived from the original on 18 October 2020. Retrieved 8 March 2021.
- ^ Munson, Ben (8 April 2019). "Sinclair announces 40 U.S. cities getting ATSC 3.0 by 2020". Fierce Video. Archived from the original on 8 March 2021. Retrieved 8 March 2021.
- ^ 8VSB/COFDM Comparison Report Archived 2005-11-22 at the Wayback Machine