PCM Telemetry Standards

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Telemetry | Posted on July 14th, 2010 at 13:52 by 한스 | Modify
PCM Telemetry Standards
Produced by Herley Lancaster -

Pulse Code Modulation (PCM) Telemetry is a way of acquiring data in one location, converting the data samples to digital words, encoding the data in a serial digital format, and transmitting it to another location for decoding and analysis. PCM systems are less susceptible to noise than analog systems, and the digital data is easier to transmit, record, and analyze.

Rapid advances in digital technology, combined with its universal application to many fields, makes it difficult to create standards that don't just as rapidly become obsolete. Serial data transmission, for example, is an essential element of the telecommunications and computer industries. As a result, each industry has independently developed their own standards. Even within the telemetry industry, the many military and commercial aerospace programs, and the usual demand for secrecy, have discouraged the development of international telemetry standards.

Chapter 4 of IRIG Standard 106-96 specifies the requirements for PCM telemetry equipment used on U.S. Government ranges and proving grounds for testing missiles, aircraft, and other vehicles. Because IRIG 106-96 concentrates on defining the essential data structures and serial codes, rather than the content of the data, it has become a recognized standard for PCM telemetry systems used throughout the world.

As a recognized leader in PCM telemetry systems, Herley manufactures equipment for:

  • Airborne flight test systems
  • Mobile and vehicle test systems

  • Telemetry Basics

    Telemetry systems are used to acquire data parameters in one location and encode them for transmission over a serial data link, such as a microwave transmitter of fiber optic cable. At the second location, the serial data is received and decoded to recover the individual data parameters. When making large numbers of measurements, it is desirable to squeeze the data into one signal or data link in order to simplify the transmission and recording.

    Early telemetry systems transmitted analog voltages using a commutator (rotary switch) at one end and a synchronized decommutator at the other end. We still use the words commutator and decommutator though most telemetry systems today use electronic switches and send digital data.

    The figure below shows the basic elements of a modern PCM telemetry system. A PCM encoder converts the input data signals into a serial data format suitable for transmission. At the receiving end, a PCM decoder (or PCM decommutator) converts the serial data back into individual output data signals.

    1279082583.jpg

    PCM systems transmit data as a serial stream of digital words consisting of logic ones and zeros that are easy to transmit and record. The PCM encoder samples the input data and inserts the data words into a PCM frame. Words are assigned specific locations in the PCM frame so the decoder can recover the data samples corresponding to each input signal.

    The simplest PCM frame consists of a frame synchronization word followed by a string of data words. The frame repeats continually to provide new data samples as the input data changes. Frame synchronization enables the PCM decoder to easily locate the start of each frame.


    Definitions

    The following basic definitions cover terms used in PCM telemetry.

    Basic Terms

    IRIG -- Inter-Range Instrumentation Group of the Range Commanders Council (RCC).

    Telemetry -- The science of gathering information at some remote location and transmitting the data to a convenient location to be examined and recorded.

    Time Division Multiplexing (TDM) -- A process by which two or more channels of information are transmitted over the same link by allocating a different time interval for the transmission of each channel.

    Pulse Amplitude Modulation (PAM) -- Pulse modulation in which a voltage is sampled periodically and transmitted as an analog signal whose amplitude is proportional to the sampled voltage. Because PAM is susceptible to transmission noise, it has been replaced by PCM in most telemetry applications.

    Pulse Code Modulation (PCM) -- Pulse modulation in which a signal is sampled periodically, converted to a digital value, and transmitted as a serial binary code.


    Serial Transmission

    PCM Code. Any of several encoding schemes used to convert a parallel digital value into a serially transmitted sequence of binary ones and zeros such that a PCM receiver can decode and recover the original digital value.

    Bit Rate -- The number of PCM output bits transmitted in one second, such as 2 megabits per second (2 Mbps). The bit rate must be very stable in order for the PCM decoder to regenerate the bit clock needed to determine the logic level of each data bit in the PCM code.

    Synchronization Pattern -- A pattern of ones and zeros that are chosen because they are easy to distinguish from randomly occurring data words. The pattern is used to synchronize the PCM decoder so that it can accurately locate the positions of the data words in the PCM stream. Appendix C of IRIG 106-96 lists recommended PCM synchronization patterns.

    Bandwidth -- The frequency range occupied or required by a signal; the frequency range containing useful information. PCM signals contain harmonics that are usually removed by premodulation filtering to reduce the bandwidth needed for telemetry transmission or tape recording. However, insufficient bandwidth may result in the loss of essential information and prevent the recovery of PCM encoded data.

    Premodulation Filtering -- The use of filters to limit the bandwidth or frequency spectrum of the PCM signal before sending it to a telemetry transmitter or data link. Appendix A of IRIG 106-96 describes frequency considerations for telemetry systems.


    Data Structures

    Channel -- A signal that carries data information, such as temperature. This term normally refers to a specific data input or output of the PCM encoder/decoder system as well as the associated data word(s) in the frame format.

    Word -- A digital or binary value appearing in the PCM output and occupying a specific time interval. Numeric values are usually expressed in binary or twos complement format.

    Word Length -- The number of bits in a PCM output word. (Note:  Synchronization patterns are counted as one word and typically twice as long as the average data word.) All words for a particular data channel are the same length. Many PCM systems have fixed word lengths for all channels, such as all words being 12 bits long. Other PCM systems support variable word lengths, in which each data channel can have an independently defined word length.

    Frame Structure-- The PCM output is arranged into a data structure consisting of one or more frames. Each frame starts with a synchronization pattern followed by the data channel words. The synchronization pattern enables the PCM decoder to locate the beginning of each frame. When there are many data channels, the PCM output is structured into a Major Frame comprising two or more Minor Frames. (See PCM frame illustration.)  Complex frame formats (Class II) may use several different formats indicated by a frame format identification word.

    Minor Frame -- The number of bit intervals starting with a frame synchronization pattern and continuing to the occurrence of the next synchronization pattern. A frame count word is usually included to identify each frame. The most simple PCM format contains one Minor Frame.

    Major Frame -- One complete cycle of data sampling in which all parameters are sampled at least once. A Major Frame contains enough Minor Frames to sample every parameter.

    Commutator -- Originally a rotating mechanical switch with many contacts used for sequentially switching voltages. Early telemetry systems used two synchronized units (called a commutator and decommutator) to pass analog voltage samples through a link between the units. Commutators can be connected in series and driven at different speeds to allow channels to be sampled at different rates. Modern telemetry systems employ electronic switches and multiplexers.

    Commutation -- The process of sampling data channels. Channels can be sampled at different rates to accommodate different data bandwidths. A normal channel is sampled once each Minor Frame. Channels can also be sampled at multiples (supercommutation) and submultiples (subcommutation) of the Minor Frame rate.

    Normal Commutation -- A channel is sampled exactly once in each and every Minor Frame.

    Subcommutation -- A channel is sampled one or more times in each Major Frame, but not in every Minor Frame.

    Supercommutation -- A channel is sampled two or more times in each Minor Frame.

    Subframe -- One cycle of the parameters from a commutator whose rate is a submultiple of the Minor Frame rate.


    Class II Data Structures

    Fragmented Word -- A word divided into segments that occupy various locations in the same minor frame. For example, a 64-bit floating-point data word could be divided into 4 segments of 16 bits.

    Format Change -- The frame format may change with regard to structure, word length or location, commutation sequence, sample interval, or minor frame boundaries, and may not have a definable major frame length. The current format structure is indicated by a frame format identification (FFI) word placed in a fixed position in every minor frame.

    Asynchronous Embedded Format -- A secondary data stream inserted into specified word positions in the major frame. The internal data structure of the embedded format is independent of the major frame.

    Asynchronous Data Merge -- Data, such as serial RS-232 and parallel MIL-STD-1553 messages, are inserted into specified word positions in the major frame. To aid in recovering the data, flag bits indicating stale and overflow conditions are appended to each data word.

    Tagged Data Format -- A format with fixed length containing a stream of data words, or blocks of words, with associated identifiers (tags). The stream may contain alternating tag and data words, or blocks of MIL-STD-1553 bus data.


    IRIG 106-96 Telemetry Standards

    This is the primary telemetry standard used throughout the world by both government and industry. IRIG Standard 106-96 covers all aspects of frequency modulation (FM) and pulse code modulation (PCM) telemetry, including transmitters, receivers, and tape recorders. This is one of many comprehensive standards prepared by the Telemetry Group of the Range Commanders Council (RCC) to foster the compatibility of telemetry transmitting, receiving, and signal processing equipment at member ranges. Owing to its success as a proven standard and its wide support by telemetry equipment manufacturers, most commercial data acquisition systems also use the same IRIG standard PCM formats and definitions.

    U.S. Government agencies can order IRIG publications from:

    Secretariat
    Range Commanders Council
    Attn: STEWS-RCC 0944
    U.S. Army White Sands Missile Range,
    New Mexico 88002-5110
    Telephone: (505) 678-1107
    DSN: 258-1107
    http://tecnet0.jcte.jcs.mil:9000/RCC/index.htm

    Non-government agencies can order IRIG publications from:

    Defense Technical Information Center
    Attn: DTIC-OCP
    John J Kingman Road Suite 0944
    Fort Belvoir, Virginia 22060-6218
    Telephone: (703) 767-8019/8021
    DSN: 427-8019
    FAX: (703) 767-8032
    http://tecnet0.jcte.jcs.mil:9000/RCC/index.htm

    IRIG PCM Specification Summary

    The following table provides a brief summary of relevant PCM specifications. Refer to Chapter 4 of IRIG 106-96 for detailed specifications and descriptions.

    Specifications Class I Class II
    Class Format Support

    Class I (simple formats)
    supported on all ranges

    Class II (complex formats) requires
    concurrence of range involved
    Binary Bit Representation 
    (PCM codes)
    NRZ-L, NRZ-M, NRZ-S,
    RNRZ-L (per Appendix D), 
    Biᅢリ-M, Biᅢリ-M, Biᅢリ-M
    Same as Class I
    Bit Rate 10 bps to 5 Mbps 10 pbs to >5 Mbps
    Bit Rate Accuracy and Stability 0.1% Same as Class I
    Bit Jitter
    ᅡᄆ0.1 bit
    Sames as Class I
    Bit Numbering MSB is bit number 1 Same as Class I
    Word Length (Data) 4 to 16 bits 4 to 64 bits
    Fragmented Words Not allowed Up to 8 segments each. All segments of a word must be located in the same minor frame.
    Word Numbering
    First word after synchronization
    is number 1. Following words
    are numbered sequentially within
    each minor frame.
    Same as Class I
    Frame Structure
    PCM data is formatted into
    fixed length frames containing
    a fixed number of equal
    duration bit intervals.
    Same as Class I
    Minor Frame Length
    Up to 8192 bits or 1024 words
    including sync word
    Up to 16384 bits
    including sync word
    Minor Frame Composition
    Minor frame synchronization
    pattern, data words, and subframe
    synchronization if used.
    Same as Class I plus other words
    such as frame format identifiers
    Minor Frame 
    Synchronization
    Minor frame sync pattern is
    16 to 33 bits long
    Same as Class I

    Transmitted 
    Frame Counter 
    (Optional)

    Binary count located in fixed
    word position increments to
    indicate minor frame number.
    Can use Subframe ID Counter.
    Same as Class I

    Major Frame Length

    Up to 256 minor frames
    Same as Class I

    Minor Frame 
    Numbering

    First minor frame in each
    major frame is number 1
    Same as Class I
    Subcommutation 
    (Subframes)
    Parameters may be sampled
    at submultiple rates (1/D)
    where D is an integer between
    2 and Z, the number of minor
    frames in each major frame
    Same as Class I
    Subframe 
    Synchronization 
    (Subframe ID Counter)
    Standard method is to use a
    Subframe ID Counter, a binary
    count located in a fixed position
    in every minor frame and which
    increments or decrements at the
    minor frame rate and is reset to
    max or min count at the start of
    each major frame.
    Same as Class I
    Supercommutation
    Parameters may be sampled
    at a multiple of the minor
    frame rate (supercom) or at a
    multiple of the subframe rate
    (supercom on a subframe).
    Samples must be evenly spaced.
    Samples should be as evenly
    spaced as practical
    Format Change
    Not allowed
    Frame structure is
    specified by frame format
    identification (FFI) word
    in every minor frame.
    Asynchronous 
    Embedded Format
    Not allowed
    Up to 2 embedded formats
    per major frame. 
    Embedded formats must 
    occupy same word locations 
    in every minor frame.
    Tagged Data Formats
    Not allowed
    Alternating tag and data, or
    MIL-STD-1553 data blocks
    Time Words
    Standardized time format uses
    three 16-bit words designated
    high order time, low order time
    and microsecond time. It is
    recommended that the time
    words be inserted before the first
    data word in the minor frame.
    For PCM word sizes other than
    16 bits, the data must be inserted
    into the PCM stream as 48 con-
    tiguous bits with zeros added
    at end to fill any unused bits.
    Same as Class I

    Asynchronous 
    Data Merge

    Regarded as Class II feature
    External sequential data (such
    as RS-232) can be inserted
    into the PCM frame format
    in fixed word positions.


    IRIG PCM Frames

    1279082734.jpg


    IRIG PCM Codes

    The following PCM codes are used world wide in industry and government systems.

    The only codes allowed by IRIG 106-96 for PCM bit streams are: NRZ-L, NRZ-M, NRZ-S, RNRZ-L, Biᅢリ-M, Biᅢリ-M, and Biᅢリ-M. For tape recording PCM data, the only permissible codes are: RNRZ-L, Biᅢリ-M, Biᅢリ-M, and Biᅢリ-M.

    Randomized NRZ-L code (RNRZ-L) is not illustrated since it is derived from NRZ-L code by using the randomizer circuit defined in Chapter 6 and Appendix D of IRIG 106-96. Despite its name, RNRZ-L is not truly random because it is completely predictable. The main advantage of RNRZ-L code is that prevents the occurrence of long strings of consecutive ones or zeros that would make it difficult to decode or record the PCM signal. It is also a low bandwidth code for economical PCM tape recording.

    1279082762.jpg

    Note: Bi-phase codes can be derived from NRZ codes
    by inverting the level for the last half of each interval

    PCM Synchronization Patterns

    Appendix C of IRIG 106-96 lists the following optimum frame synchronization patterns for PCM telemetry. In systems with fixed length data words, the pattern length is usually two times the data word length. For example, a system with 12-bit data words would use a 24-bit synchronization pattern. Pattern lengths shorter than 16 bits are primarily used for frame format identification when necessary.

     1279082857.jpg


     PCM Tape Recording

    Telemetry data is usually recorded onto magnetic tape for later analysis. When recording PCM data, it is important to ensure that the tape recorder provides sufficient frequency response to capture and reproduce the PCM signal. The following general rules may be used to calculate the maximum bit rate for various PCM codes.

    • NRZ, RNRZ, and DM codes: Maximum bit rate is 1.4 times the tape recorder frequency response.
    • All Bi-phase codes: Maximum bit rate is 0.7 times the tape recorder frequency response.

    • PCM Telemetry Transmission

      PCM is a digital signal with sharp transitions that produce many harmonics. To limit the transmission bandwidth, the PCM signal is usually passed through a pre-modulation filter before being applied to the input of the transmitter. The following general rules may be used to calculate the filter cut-off frequency for various PCM codes.

      • NRZ, RNRZ, and DM codes: Filter cut-off frequency is 0.7 times the PCM bit rate.
      • All Bi-phase codes: Filter cut-off frequency is 1.4 times the PCM bit rate.

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