Precision Timing and Mobile DTV – An Introduction
Ask any TV engineer if synchronization is important to coverage and the response may surprise you. Synchronization in a broadcast or wireless network is often overlooked because the equipment necessary for timing is taken for granted. The base stations’ timing accuracy in the “broadcast” channel is critical to voice, data or video transmission. Synchronizing a mobile receiver can be achieved by GPS or internet time servers (ITS). A GPS system is more expensive than ITS or modem solutions, however its accuracy and reliability offer substantial benefits. The network timing protocol (NTP) is used in the base station transport streams to synchronize mobile stations to reduce disconnects.
About RF Transmission and Forward Error Correction
Traditional ATSC based 8-VSB modulation is inadequate for mobile or pedestrian-based receivers. The A/153 standard was created for ATSC Mobile/Hand-held (M/H) with additional forward error correction (FEC), advanced video coding, lower video resolution, time slicing (sending video in bursts to minimize receiver battery drain) and IP delivery system. In order for ATSC Mobile DTV to be successful, the signal should have higher immunity to multipath and Doppler shift. To improve reception reliability for moving receivers over that of ATSC A/53, ATSC-M/H uses additional training sequences and FEC. The more FEC added to the signal, the more bits required from the primary DTV service – the efficiency between the payload and the total bits required from the primary service.
Because the total bit rate for an ATSC transmission is fixed at 19.4Mb/s, the M/H data stream must borrow bandwidth from the main channel. This main data rate loss can be set between 0.9Mb/s and 7.3Mb/s. The efficiency of this loss changes since the M/H stream requires overhead for additional error protection. This overhead is set during the transmission and presents a trade-off between the payload data rate (PDR) and error resiliency. Legacy receivers must see the new mobile signal as normal (although unrecognized) data. The Mobile DTV stream must be synchronized. The training sequence has a symbol rate of 800 per second which measures multipath and supports reception at vehicular speeds. The Transmission Parameter Channel (TPC) is described in ATSC A/153 Part 2 Section 5.5 and provides information that lets the receiver align itself on the virtual M/H frame structure and locate the correct slots with desired mobile program. The receiver can use the long, regularly-transmitted training sequences in the data stream for synchronization and the base stations’ timing accuracy is assured with the frame synchronization. The TPC is extra-coded to tell the mobile receiver how much mobile data is present and where it is hidden in the legacy (ATSC) stream. The ATSC M/H channel frame structure is shown below.
For a Multiple Frequency Network (MFN), time slicing enables a receiver to monitor neighboring cells without interrupting service reception. Careful synchronization may be implemented in the headend, so that the same service is transmitted on different slices at the same time in neighboring cells. This would ensure seemingly uninterrupted (zero packet loss) reception when handing over from one cell to another.
For more reliable RF signal coverage, ATSC-M/H can be enhanced with additional transmitters via a Single Frequency Network (SFN). SFNs (A/110 standard) can increase viewer satisfaction by providing enhanced market coverage and in-building penetration. SFN includes precise timing synchronization and trellis initialization so that all transmitters emit the same signal on the same frequency at the same time. All SFN transmitters generate a synchronized signal on one frequency. Transmitters are “adjusted” to compensate for contribution timing differences and to minimize self-interference. In such an SFN, the ATSC-M/H multiplexer and the ATSC-M/H transmitter are synchronized by a GPS reference. The ATSC-M/H multiplexer operates as a network adapter and inserts time stamps in the transport stream. The transmitter receives the transport stream via the Studio Transmitter Link (STL). The transmitter analyzes the time stamp and delays the transport stream before it is modulated and transmitted so different delays in the STL can be compensated.
Significance of the Transport Layer
The ATSC mobile service multiplex and transport subsystem supports various mechanisms for delivering multimedia to portable devices. Whereas the main channel service was designed to carry A/V transport streams, ATSC-M/H was designed to carry IP datagrams. User Datagram Protocol (UDP) is used on top of IP, which is a connectionless protocol that does not set up a dedicated end-to-end connection. UDP requires only a small transport layer and provides packets with definite boundaries, which aids in synchronization. SD/HD MPEG-2 and Mobile DTV share the same transport stream.
The ATSC-M/H standard defines a fixed transport stream structure, based on M/H Frames, which establishes the location of M/H content within the VSB Frames and allows for easier processing by an M/H receiver. This is contrary to the legacy ATSC transport stream, defined in A/53, in which there is no fixed structure to establish the phase of the data relative to VSB Frames.
Equipment Solutions to DTV Timing
Precision timing and synchronization are vital to our internet and wireless infrastructure; efficiently maximizing the use of bandwidth and meshing the flow of network data. Precise timing requirements appear in a variety of other industries, from banking to E911, the healthcare industry and power utilities. Digital television and DTV transmission systems could not function without synchronization to precise time standards. Navigation systems, avionics and military communications are other applications in which precision timing is essential. Primary reference atomic master clock time standards are huge and costly systems. Fortunately, the precision of these master clocks, like the ones maintained by the US Naval Observatory (USNO), is readily available through the GPS satellite system.
The Mobile Digital TV (DTV) standard ATSC A/153 specifies for the mobile/handheld (M/H) time frame to align with the normal video (ATSC A/53 standard) time stamp with a tolerance of less than 1 symbol RMS or 1 x 10-12 ns (15 ns) RMS. Mobile DTV signals must be measured accurately using a reference locked to a GPS rather than a internet server where latency can occur. A GPS signal can also train a crystal or rubidium oscillator so that timing errors are removed.
LBA offers a variety of ultra-precise master clock timing, synchronizing and reference frequency functions. The Schomandl FNX-GPS frequency standard is designed for integration into systems requiring precise time reference or a UTC synchronous time stamp. It incorporates a GPS receiver and 10 MHz OXCO oscillator into a single small, plug-in circuit card.
The R.A.M. Taref is a ultra high precision double Oven-Controlled Crystal Oscillator (OCXO) clock reference. The Taref 3 is configured for a single 19“ chassis that offers high precision frequencies ( 2.048MHz or customized frequencies) with a long term stability of a cesium standard for applications like multiplexer systems and synchronization of cellular and radio transmissions. The double OCXO maintains long term accuracy of less than 10 ns with a GPS antenna. The Taref 30, is designed into a 19“ chassis with 2HE. It is used in systems where a GPS antenna is not applicable like power plants and high safety environments. The clock deviation per year has a maximum of 5 x 10-11 ns with GPS antenna. When used with the Taref 300 GPS transfer standard, the Taref 30 eliminates the need for exterior GPS antenna access. It is ideal for interior building installations, shielded secure spaces and other difficult environments like submarines. The Taref clock reference can also be used as very precise frequency source for standardizing the outputs of the Schomandl ND and SG series of frequency synthesizers. Similarly, the precision timing references may be inputted to the Kathrein MSK-200 digital TV analyzer for precision measurements of DTV systems.
LBA offers a variety of practical, economical systems for provisioning of ultra-precise master clock timing, synchronizing and reference frequency functions. Other test equipment offerings include TV spectrum analyzers, impedance analyzers, signal generators and RF Faraday cages.
*Timing equipment no longer available through LBA.
