Packet and Page Teletext Data Reception Using the SAA5250
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Philips Semiconductors Video Products Interface information Packet and Page Teletext data reception using the SAA5250 Authors: J.R. Kinghorn Philips Product Concept & Applications Lab, Southampton England A. Guenot Philips Semiconductors, Paris France Revised: M. T. Schneider Philips Product Concept & Applications Lab, California SUMMARY can be used for many purposes. For through the conventional television receiving Two methods of transmitting serial data in instance, a nationwide one way data circuitry of tuner, I.F., and demodulator stages World System Teletext (WST) format are distribution service could provide customer to produce a baseband composite video available. These are the independent data support for a computer software signal (CVBS). This is applied to the Teletext line or ‘Packet 31’ method, and the page manufacture, sending both new product data slicer and acquisition decoder, which format technique. For universal application in announcements, software updates and bug provides a data output to the host computer a subscription Teletext environment the fixes automatically. rather than the usual text display output. receiving equipment must be able to accept At the receiving end, a variety of terminal The Teletext decoder can use a both forms of transmission. The equipment types can be envisaged microprocessor to format the data output and Multistandard acquisition circuit CIDAC depending on the application. A common provide control of the system. Tuning in the (SAA5250) or CMOS Interface for Data requirement, however, is for a ‘black box’ local broadcast station and selection of the Acquisition and Control, is available to which can be connected to an aerial input required service can be done through the simplify the receiver design. signal and deliver a parallel data output to a controls on the adapter unit, or alternatively desktop computer. This unit can be largely by sending commands from the host transparent to the application, being the computer through a suitable interface logic. INTRODUCTION equivalent of a data link in a conventional Facilities for descrambling the data and Recently there has been a great upsurge in wired computer installation. A block diagram access control can be provided in the interest in using Teletext to transmit serial of such an adapter unit is shown in Figure 1. software of the adapter unit or in the host data. This can be achieved without interfering computer according to the particular with the normal Teletext service, and the data The UHF or VHF aerial signal is passed requirements of a service. RF INPUT DATA TUNER / IF SLICER TELETEXT HOST INTERFACE ACQUISITION (OR µC) DECODER Figure 1. Data Adapter Unit 2-213 Revised: June 1994 Philips Semiconductors Video Products Interface information Packet and Page Teletext data reception using the SAA5250 The Teletext Decoder performance (i.e. speed) and adaptability packets of data is under software control. As mentioned earlier, the requirements for a may be prerequisites in a competitive design. Data is retrieved from the RAM via CIDAC’s parallel interface to a host interface or a Teletext decoder to receive serial data are The multistandard acquisition circuit CIDAC microcontroller. quite different from a conventional decoder in (SAA5250) provides a solution to these a television set, or set-top decoder. To begin problems. Originally designed for the with, an RGB text display output is not reception of the French ANTIOPE and the If a microcontroller is used, the microcode is usually required as perfectly adequate World System Teletext formats, it is equally responsible for formatting the data into the character generating capacity is available on capable of acquiring data using the form required to interface with the host the host computer’s display. The data output independent data line transmission computer (i.e. an RS-232 serial interface at is the main requirement, demanding technique. The device can be programmed to 9600 baud). The microcontroller itself can be reasonably direct access to the acquisition operate in various modes and two of these one of several standard types, 8051, 8049, memory from the host. Facilities for access are suitable for the independent data line and 6801, 6805, etc. Any controls (i.e. to select control and descrambling the data may be the page format transmissions respectively. A the service) can be implemented locally in the needed, together with special error-checking block diagram of a multistandard Teletext decoder using port pins of the algorithms. decoder for serial data is shown in Figure 2. microcontroller; alternatively if the output interface is made bi-directional, selections As a further complication, two entirely Composite video is supplied to a standard can be made using the host computer different transmission methods are used for Philips data slicer (SAA5231 or SAA5191) externally. Access controls and descrambling serial data; the page format method and the circuit which performs adaptive data slicing are dealt with using the appropriate software independent data line or ‘packet 31’ method. and supplies serial data and clock to the in the decoder’s microcontroller acting on the Each of these methods has its advantages CIDAC (SAA5250). The other section of the corresponding received data. and disadvantages, but it appears likely that data slicer is concerned with display timing both will be used commercially. synchronization is not used. All of the CIDAC Alternatively, these functions may be timing is derived from the 5.7273Mhz If only page format data reception is performed by the host computer, with the (6.9375Mhz in 625 line) data clock. required, a standard Teletext decoder chip decoder simply acting as a transparent data Acquisition of the data is performed by the can be used with appropriate control link and no microcontroller is used in this CIDAC circuit (SAA5250). software, see Reference 1. However, the configuration. The same hardware adapter designer may require a universal The received data is buffered in a standard configuration can be used as a receiver for decoder capable of operating on either form low cost 2K x 8 static RAM connected to the downloadable software, or as a standard of transmission. For reasons of economy, CIDAC. The chip performs appropriate prefix acquisition unit for normal World System duplication of circuitry should be avoided if processing according to the operating mode Teletext or pages with the host computer possible. On the other hand, good selected, and the storage of particular used as the display unit. COMPOSITE DATA VIDEO DATA SLICER HOST INTERFACE DATA OUTPUT CVBS SAA5191 CLOCK CIDAC OR or SAA5250 SYNC MICROCONTROLLER SAA5231 2K x 8 STATIC RAM Figure 2. Serial Data Teletext Decoder 2-214 Philips Semiconductors Video Products Interface information Packet and Page Teletext data reception using the SAA5250 The CIDAC Circuit Some data bytes are Hamming protected, stored in memory regardless of any further The CIDAC itself performs the acquisition and these are passed through Hamming conditions. It is then up to the host software functions and interfaces with the memory and correction logic. Most of the operating modes to search for the appropriate data by looking host. A block diagram of the device is shown have hardware recognition of a channel or for a start-of-page flags, packet number in Figure 3. magazine, so the appropriate input data is recognition, etc. This method is suitable for compared with the requested magazine modest operating speeds such as the packet The received serial data from the data slicer number in the channel comparator. This 31 system, in which the host has no difficulty is checked for framing code (which is ensures that only data from the selected in keeping up with the overall data programmed from the host) during a line magazines is loaded into memory, and that throughput. timing window derived from the VCS sync the acquisition process is not burdened with signal. This timing window can be moved irrelevant data. Alternatively if fast mode is chosen, data is within limits under software control to only stored after recognition by the CIDAC The format counter is used to count the compensate for the different framing code hardware of an appropriate ‘start-of-page’ number of bytes loaded into memory on each delays. After detection of the framing code, flag. This flag depends on the system; codes data line; this value can be loaded by the the information is converted into 8 bit parallel SOH, RS for Didon, a bit in the PS byte for software. In long and short Didon (ANTIOPE) form. In addition, the VAL OUT output (pin 2) NABTS, or row 0 (page header) for World modes this information is taken from the will reflect the position of the programmed System Teletext. Using fast mode broadcast format byte via the format framing window. considerably simplifies the host’s task in page transcoder. recognition, so the position of the data to be The functions performed by the data depend Storage of the data in memory also depends checked becomes defined in memory. Fast on the operating mode selected, and are on the selection of slow or fast mode. In slow mode is implemented by page flag detection controlled by the sequence controller circuit. mode, all data from the selected magazine is circuitry in the sequence controller. PAGE DETECTION FRAME CODE PROGRAM DETECTOR REGISTER DB<7..0> SERIAL SEQUENCE SD REGISTER REGISTER CONTROLLER CHANNEL PARALLEL COMPARATOR REGISTER HAMMING ALE CORRECTOR SERIAL/PARALLEL HOST CONVERTER INTERFACE CS FORMAT PROCESSOR CLOCK RD DCK GENERATION 2 K BYTE FIFO MEMORY FORMAT CONTROLLER TRANSCODER WR FORMAT COUNTER VAL IN / SYNC VALIDATION SIGNAL PROCESSING VAL OUT CBB MEMORY INTERFACE SAA5250 MS WE AD<10..0> DB<7..0> VDD VSS Figure 3. The SAA5250 (CIDAC) 2-215 Philips Semiconductors Video Products Interface information Packet and Page Teletext data reception using the SAA5250 Fast mode is suitable for the page format the memory interface and the timing components.