ATSC transmission
With the exception of quantum physics, it’s an analog world. Digital modulation techniques are mappings of digital data onto analog voltage levels. These voltage levels then modulate an RF carrier signal, the fundamental technology that has facilitated broadcasting since its inception.
Vestigial sideband modulation (VSB), developed by Zenith, has been the target of much debate about performance since the adoption of the ATSC standard in 1996. Subsequent generations of VSB demodulator chips with adaptive capabilities, as well as significant industry and political maneuvering has helped ease many early doubts.
A two-step process
Transport packets must be converted to transmission packets. MPEG transport streams first undergo a series of digital coding transformations to permit correction of lost data during transmission. Next, analog filtering and modulation techniques are used to fit the data into a standard 6MHz channel.
For an in depth description of ATSC transmission techniques an SBE DTV booklet [1] is an excellent place to start. A tutorial [2] by Gary Sgrignoli, a member of the Zenith Grand Alliance design team, offers an insider’s expertise. The ATSC standard, A/53 [3] and A/54 Guide [4] are the definite sources for technical specifications.
Data packet preparation
The 188 byte MPEG-2 transport stream, as specified in SMPTE 310M [5], feeds the transmission encoding process. The first step includes randomizing, forward error correction, interleaving and trellis encoding steps and concludes with multiplexing data packets and synchronization fields.
- Data randomizing
Randomization uses a 16-bit pseudo random number generator, feedback loop and exclusive or gates on incoming data to scramble the stream. Its purpose is to avoid patterns and to spread energy equally across the spectrum. The data appears noise-like when observed on a spectrum analyzer. The TS sync byte (0x47) resets the pseudo random generator. - Forward Error Correction
Being a one-way digital data stream, Forward Error Correction (FEC) is required to help insure the validity of data in a noisy, over-the-air transmission environment. Reed Solomon encoding is used and adds 20 bytes of parity data. It is specified as a t= 10 (207, 187). The data packet is now 207 bytes (with sync, 208). The algorithm is capable of correcting up to 10 bytes with burst errors. - Data interleaving
A complex convolutional byte interleaver disperses data over 52 data segments Whole bytes are time-dispersed and occur over a four-minute period. With this technique, even single bit errors can be corrected in a DTV receiver. - Trellis encoding
Trellis encoding divides 8-bit byte codes into four 2-bit sections. Each 2-bit data word generates 3-bit symbols. The first input bit is encoded into two bits, while the other bit has been pre-coded. - Segments, fields and sync
Data segments consist of: a Data Segment Sync (DSS) byte; 207 bytes of payload data; and FEC for a total of 208 bytes — the equivalent of 832 2-bit symbols. 313 data segments are assembled into a Data Field. The Data Field Sync (DFS) is the first Data Field segment (832 bytes) and carries the additional VSB mode data. The DSS and DFS are not Reed Solomon encoded, trellis encoded or interleaved.
Ultimately analog
The multiplexer output is converted from 3-bit digital symbols to eight distinct analog voltage levels.
- Nyquist filtering
Raised cosine (Nyquist) filtering of the signal is necessary because of the sharp data transitions. The signal is shaped into precisely timed impulses, weighed by the value of the symbol. The ultimate goal is to achieve zero inter-symbol interference. At this point the bandwidth is 5.38MHz and therefore will fit easily in a 6MHz terrestrial TV channel. - Pilot insertion
Adding a +1.25V offset produces a small pilot signal 11.3dB down from the average DTV power level. This helps the receiver lock on the carrier and the receive antenna. The offset also minimizes lower channel and co-channel interference between NTSC and DTV stations. - VSB modulator and RF upconversion
The eight distinct voltage levels modulate I (In-phase) and Q (Quadrature) IF carriers. A VSB IF is created through side band cancellation. The resultant upper side band signal is upconverted to the assigned channel frequency and results in an 8-VSB signal ready for transmission over a 6MHz channel.
By the numbers
As described in ATSC A/54 [4], the symbol rate is related to NTSC scanning and color frequencies. The symbol clock can be used to generate an NTSC color subcarrier to simplify receiver design to include both DTV and NTSC decoding capabilities.
The 10.76MHz symbol rate is arrived at using the following formula:
4.5MHz/286x684 = 10.7MSymbols/sec:
where: 4.5MHz is the center frequency of the audio carrier offset in NTSC
4.5MHz/286 is the horizontal NTSC scan rate, 15,734KHz
(15734KHz * 455/2 = 3.579MHZ, the NTSC color subcarrier)
684 produces a symbol rate that maximizes use of the 6MHz channel
The resulting 10.76MSymbols/sec can be transmitted as a VSB signal at half the bandwidth, 5.38MHz (per Nyquist), which fits nicely inside a 6MHz channel while providing 620KHz for guard zones.
Working backwards from the symbol rate, the transport stream data rate of 19.39Mb/s can be derived.
First, 2-bits per symbol at 10.76MSymbols/sec = 21.52Mb/s.
Each term in the equation is described as:
Data Field – Sync/ Data Field = 312/313 (one sync byte)
Data Segment - Sync/ Data Segment = 828/832 (four sync symbols)
Transport Payload / Payload + FEC = 187/207 (both terms without sync byte)
The full equation is:
21.52Mb/s x 312/313 x 828/832 x 187/207 = 19.28Mb/s of payload data
When the data rate is adjusted for the 188th sync byte, voila!
19.28Mb/s x 188/187 = 19.39Mb/s (the transport stream bit rate)
The second channel
To enable the simulcast of NTSC and DTV until the transition to digital is complete, now predicted to be Dec. 31, 2008, a second 6MHz channel has been granted to each broadcaster. This allocation ensures that existing service contours will be maintained when a station completes its transition to full power DTV transmission.
One important characteristic of the digital transmission is the cliff effect. As an NTSC’s signal strength weakens, the picture gets snowier and the audio sounds noisier. However, digital transmission will be perfect until there are too many uncorrectable bit errors. At that point, the signal has fallen off the cliff and a receiver goes black and silent!
Into the digital age
This tutorial concludes our 50,000ft overview of the ATSC standards that have ushered in the transition to digital TV. In many ways, they are the culmination of a century of communications engineering and computer technology. This modular system will enable integration of features and capabilities we can’t even dream of today.
In the next newsletter, we will begin a discussion about the impact of the transition to digital on BOCs.
References
[1] Introduction to DTV RF, Douglas W. Garlinger, CPBE, Society of Broadcast Engineers, 1998
[2] ATSC Transmission System: VSB Tutorial, Gary Sgrignoli www.zenith.com/sub_hdtv/hdtv_papers_atsc_system.html
[3] ATSC Standard A/53C with Amendment No. 1 and Corrigendum No. 1: ATSC Digital Television Standard, Rev. C, 21 May 2004, (Amendment No. 1 dated 13 July 2004, Corrigendum No. 1 dated 23 March 2005) www.atsc.org/standards/a_53c_amend-1_corr-1.pdf
[4] ATSC Recommended Practice A/54A: Guide to the Use of the ATSC Digital Television Standard, 4 December 2003 www.atsc.org/standards/a_54a.pdf
[5] SMPTE 310M-1998, for Television, Synchronous Serial Interface for MPEG-2 Digital Transport Stream
http://smpte.org
