IP over ENG: Broadcast Applications and Solutions For Remote IP Connectivity
There is a significant trend in the industry toward the use of IP interfaces in remote editing equipment utilized by broadcasters in the field. This is primarily due to the increased flexibility of the interface to do file transfer, storage and overall management of content with no degradation to video quality. Consequently, there is a secondary need to be able to transfer this IP data from the remote location over the remote path without changing formats.
This paper explores the basics of IP including its advantages for managing content. Additionally, the challenges of implementing portable microwave links from the field including link budgets (carrier to noise vs. transfer rate) and interoperability issues are reviewed. Sample system solutions for transporting IP data back to the studio including ENG solutions and backhaul requirements are presented as well. Recent developments in generic data return link technology will be explored for the purpose of improving overall IP data transfer accuracy and integrity. Also included is a brief review of two key suppliers for field editing equipment and how their equipment could be integrated and configured with the microwave system. These suppliers are Panasonic and AvidŽ.
Legacy Analog Van Systems
For many years, mobile newsgathering vans have utilized 2 GHz terrestrial microwave links for Electronic News Gathering (ENG) applications. Traditionally, these analog news vans have originated and distributed their news content within the van, using an analog one-volt peak-to-peak composite video signal. This signal is typically routed throughout the van via video distribution amplifiers, video switchers and VTR equipment before being passed off to the 2 GHz microwave equipment for transmission back to the studio.
The typical analog method for transmission over the microwave system has been to use FM modulation techniques. These techniques, which have been utilized successfully for many years, are now being supplemented and, in some cases, substituted with an everincreasing amount of digital based product designs. This is primarily due to advances in digital transmission technology coupled with the need for a more efficient use of spectrum. Further to that end, advances in the area of digital microwave transmission technology, compact non-linear editing workstations and the integration of digital video interfaces with Ethernet network capabilities, have spawned an interest in the ability to extend “network” connectivity to the field.
Extending the Network Capabilities for D-ENG
Microwave Radio Communications (MRC) is working toward extending network capabilities to the field utilizing digital radio architectures, which combine robust modulation techniques such as COFDM, with its own network interface technology.
Additionally, Panasonic has introduced its P2 cam which goes beyond the conventional work flow requirements as previously described. The P2 cam has built-in next generation storage drives within its camera that support up to five independent storage cards. The hot-swappable storage cards replace the legacy VTR applications. These storage cards can be directly accessed while in the camera using a USB 2.0 connection from the camera or pulled out of the camera to be inserted into a PCMIA slot of a portable work center or laptop computer editor. This allows each unique storage card to be recognized as an independent drive on the work center. Using the drive flexibility, the user can simultaneously record footage of the news story while still transferring data over the network back to the studio. See Figure 2 for typical flow.
AvidŽ technology has also introduced a wide range of products that are designed to extend the reach of the network newsroom workflow from the studio to the field. Their NewsCutterŽ XP mobile software system is a compact mobile non-linear editing (NLE) system that is designed with the intended capability for the user to stay connected while in the field. It is designed to integrate with media management systems, newsroom computer systems, playback servers and web publishing systems thus allowing the potential for a direct extension of their network capabilities, assuming a wireless high BW connection could be made from the field. The user can move, store and forward material from the field over an Ethernet network connection through their Unity LAN share configuration. Multiple NewsCutterŽ XP clients can be connected within the same LAN configuration, as shown below in Fig. 3, and then interface with the Ethernet connection.
Fig. 4 below depicts one possible connection scenario. AvidŽ also supports many other system application configurations that would allow for connectivity from the remote location to the TV studio, if a connection were available. Currently, many of these applications are supported using transcoder devices or interface converters as depicted below. These devices convert an IEEE 1394 (fire wire) DV25/DV50 type interface to legacy analog base band style interfaces for distribution purposes.
As previously mentioned, both Panasonic and AvidŽ are directing their product development efforts toward enhancing network newsgathering concepts from the field in an effort to maximize workflow efficiency.
The key ingredient to support and fully transition these technologies from AvidŽ and Panasonic is the digital microwave communications link, from the D-ENG van to the studio. The microwave link needs to be robust and able to operate in non-line-of-sight (NLOS) conditions and also support a 12 MHz spectrum allocation while offering the maximum available data rate over the channel.
Digital Microwave Transmission Technology
In 1999, COFDM (Coded Orthogonal Frequency Division Multiplexing) technology was implemented into D-ENG applications, and it subsequently provided the first step in support of the migration to fully digital transmission system architecture.
The implementation and modulator architecture facilitate the interface to most types of digital transport streams, e.g. asynchronous serial interface (ASI), which is used to carry a compressed digital video bit stream; or Internet protocol (IP) network interface, which is used to carry IP data grams over an Ethernet connection. These two interfaces allow the user the flexibility to distribute content as a digital video transport stream or as an IP packet, depending on the configuration within the van. It is also possible and desired to support both types of transmission inputs simultaneously over the same microwave channel on a priority basis. The maximum channel bit rate capacity is shared and prioritized between the 188-byte packets of the digital video transport stream and the IP data grams. As previously mentioned, this allows the simultaneous transfer of FTP data directly from a laptop editor through a network hub or media gateway and/or compressed data from the MPEG encoder.
Digital Transmission Requirements – Capacity vs. Threshold
The advantages of COFDM technology are apparent in its ability to offer error free transmission under severe multi-path conditions and its ability to occupy less overall bandwidth than its previous analog FM counterpart. A typical COFDM waveform occupies 8 MHz of bandwidth at its 1dB bandwidth points. Its digital implementation is based on the ETSI standard EN300-744 for DVB framing, channel coding and modulation architecture. The uniqueness of the standard allows the user to customize his data throughput as a function of three main parameters: forward error correction, guard interval, or delay spread and modulation type.
Fig. 5 shows the relationship between these three parameters, which is listed as Table 17 in the previously referenced ETSI standard. The table shows how a user can increase or decrease his bit rate as a function of his microwave link requirements.
Table 17: Useful bitrate (Mbit/s) for all combinations of guard interval, constellation and code rate for non-hierchical systems for 8 MHz channels
|Modulation||Code rate||Guard interval|
Fig. 5 — COFDM Bit Rate Allocation
Like any other communications application, as the modulation density or bits per hertz efficiency increases, so does the carrier-to-noise (C/N) requirement for its receiver. Meeting the C/N requirement is a major challenge toward achieving a reliable transmission system. The following graph in Figure 6 shows the C/N requirement as a function of bit rate for COFDM technology. The graph compares the three different modulation techniques – QPSK, 16 QAM, 64 QAM – used within a COFDM domain. As expected, QPSK affords the minimum C/N requirement while also offering the lowest bit rate with a C/N of approximately 7 dB and a bit rate of 5.5 Mbits/sec.
Also, as with many other digital communications systems, there are ways to improve the overall system C/N requirement for a given microwave path. The main technique that is employed is antenna diversity.
Digital Transmission Requirements – Antenna Diversity Improvement Factors
Currently, there are a number of diversity receive techniques on the market. They include packet based switching implementations as well as maximal ratio combining techniques. Each of these techniques has their pros and cons for the application. A packet-based implementation, while offering errorless switching, yields minimal improvement in C/N. Whereas maximal ratio combining techniques have the potential to offer a user up to 5dB improvement in his C/N requirements with two antennas and a subsequent 8dB improvement with four antennas. The improvement in C/N offers users the ability to increase their overall data throughput for the application. However if throughputs are held constant, the improvement in C/N will improve overall reliability and robustness of the microwave communications link.
The increase in data throughput allows for a higher quality signal transmission over the link or the potential for a higher speed downstream IP data transfer.
The following graph shows the comparative increase in throughput for a COFDM application with and without two-antenna diversity applied to the microwave communications link.
As previously mentioned, we can now relate our channel capacity requirements from Figure 7 to practical data transfer rates for a digital video compressed bit stream, using an ASI interface and/or an IP network interface.
Remote Digital Electronic Newsgathering (D-ENG) Communications to the Van
Currently, in the 2 GHz licensed spectrum band, there are no requirements or provisions to support an upstream network path from the studio to the D-ENG van. Within the broadcast industry, a separate ATSC (Advanced Television Systems Committee) specialist group has been formed which will focus on Digital Electronic News Gathering. Its charter is to develop a standard or Recommended Practice for private communications services between a DTV station facility and their D-ENG crews, utilizing the ATSC transport stream.
Some of the potential applications suggested for using an ATSC compliant return path include the following:
- LAN or network access for newsroom management systems
- Return acknowledgements for a FTP (file transfer protocol) file transfers from laptop editors as described above in support of the TCP/IP protocol requirements
- Metadata to/from the station to newsroom production systems
- Low resolution proxy video returns to the van from the studio
- Remote monitoring and control of van equipment
- Internet connectivity
- Remote Vehicle Location Data (GPS) to station and return acknowledgement
- IFB (interruptible fold back) communications for program sound and cueing instructions back to talent and crew.
The way this application could potentially work is that an ASI stream would be used to support the digitally encoded video and audio information from an MPEG encoder. At the same time, the modulator would be able to multiplex IP data from a network or Ethernet hub. The ASI and IP data would be simultaneously transmitted over the 2 GHz microwave link from the van. At the central receive site, the ASI and IP data is extracted from the COFDM demodulator and associated circuitry, and then distributed back to the studio over the TSL (transmitter to studio) link in its existing digital format. At the studio, the ASI data is decoded for its video and audio content and passed on to newsroom production. Subsequently, the network IP data is supplied to the station’s LAN or communications hub. The station’s LAN could then supply the network IP data for the return upstream data path back to the van via the ATSC transport stream. At the van, the ATSC off-air signal would be demodulated to complete the return communications path for remote ENG communications.
This technique would allow for a high IP down stream bandwidth from the van to the studio and a significantly lower upstream bandwidth to the van from the studio. See Figure 8 for system drawing.
As previously mentioned, Figure 5 showed the maximum allowable bit rate over the COFDM channel, which is a function of the applicable ETSI standard. Now we can look at the potential IP data requirements for the downstream network traffic content.
The chart above shows some of the potential applications, along with a potential downstream transfer bit rate, for the network connectivity. The FTP application could offer the user the full downstream bandwidth while using the ATSC upstream bandwidth for the return acknowledgements, thus completing the necessary flow control hand shaking that is required.
The future of electronic news gathering is presenting new challenges to the status quo in terms of remote connectivity. Specifically, as field editing capabilities continue to increase overall efficiency and allow further differentiation between markets, the technology utilized to connect news vehicles to the studio will need to adapt in order to keep from becoming the “weakest link.”
As the value of remote IP connectivity continues to grow and transform the existing ENG paradigm, microwave radio equipment manufacturers will subsequently need to adapt to an IP-based front-end hardware environment, thereby enabling a full digital implementation.
However, the key component facing the industry is the accepted standard or recommended practice for private communications services between a DTV station facility and their D-ENG crew for return network connectivity. This standard should supply the necessary upstream data requirements to support some of the network application requirements.