WO1991015926A1 - Data alignment - Google Patents
Data alignment Download PDFInfo
- Publication number
- WO1991015926A1 WO1991015926A1 PCT/GB1991/000569 GB9100569W WO9115926A1 WO 1991015926 A1 WO1991015926 A1 WO 1991015926A1 GB 9100569 W GB9100569 W GB 9100569W WO 9115926 A1 WO9115926 A1 WO 9115926A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- data
- value
- word
- hybrid
- receiver
- Prior art date
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Classifications
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L7/00—Arrangements for synchronising receiver with transmitter
- H04L7/04—Speed or phase control by synchronisation signals
- H04L7/041—Speed or phase control by synchronisation signals using special codes as synchronising signal
Definitions
- the present invention relates to the alignment of a receiver in a digital data transmission system so that the decoding of words of data in the receiver is synchronised with the transmission of data from the transmitter or other data source.
- the invention is of particular value in the handling of digitally encoded composite video signals distributed over optical networks, but may also be used with other forms of data and different transmission media.
- An input video signal at a head-end station is applied to an encoder which generates and outputs appropriate codewords in accordance with the particular coding scheme adopted. These codewords are converted into a serial bit stream- and modulated onto an optical signal which is transmitted onto the optical network and subsequently detected by an optical receiver. The output from the optical receiver is decoded using a complementary process to that adopted in the head-end station.
- the serial bit stream transmitted over the network has first to be assembled into the appropriate codewords and this requires alignment at the word-level as well as the bit-level between the decoder and the transmitter. Conventionally this has been achieved by transmitting some form of alignment signal in addition to the video data, this however gives rise to an undesirable overhead in the transmission rate. To avoid such an overhead it is possible alternatively to replace some of the least significant bits of the video signal with synchronisation words but this has the disadvantage of introducing small but regular errors into the video signal which may be visible when that signal is decoded.
- a method of aligning a data receiver with a data transmission comprising encoding data for transmission with a coding scheme including at least one invalid codeword, detecting data bits at the receiver in accordance with a local clock, and modifying the phase of the local clock in response to an occurrence in the detected data of the invalid codeword, .
- the data is encoded with a hybrid differential coding scheme, whereby a first code representative of an absolute value and a second code representative of a differential value are assigned to each input signal, the coding scheme for the differential codes including the at least one invalid codeword.
- invalid codeword a word which is never normally assigned to any of the possible input signals. The detection of such an invalid codeword then indicates that data has been corrupted by an error in alignment between the receiver and the transmitter and may be used to prompt a correction in the phase of the decoder clock.
- a hybrid differential coding scheme is of particular advantage when the encoded data is a video signal, in which composite codewords representing both a coarsely quantised absolute level and a difference value are transmitted.
- the invalid codeword(s) is provided only for the differential part of the composite code.
- the differential code is then decoded separately at the receiver and the invalid codeword used to maintain word alignment.
- the differential coding scheme will in general have many quantisation levels so that the loss of one quantisation level in order to provide the invalid codeword does not significantly degrade the performance of the system.
- the present invention is able to maintain word alignment without the disadvantages of a transmission overhead or the introduction of errors in the transmitted data.
- invalid codewords may still be detected occasionally as a result of noise in the transmission medium.
- the receiver compares the rate of occurrence of invalid codewords with a predetermined threshold and modifies the phase of the local clock only when that threshold is exceeded.
- Figure 1 is a block diagram of a data transmission system
- Figure 2 is a block diagram of a codec for use in the system of Figure 1;
- Figure 3 is an encoder suitable for hybrid differential coding
- Figure 4 is a decoder suitable for hybrid differential coding
- Figure 5 is a table showing equivalence of numbers with sign bit omitted
- Figure 6 is a diagram showing the characteristic of a Bostelmann quantiser
- Figure 7 is a diagram showing a full Bostelmaunn characteristic
- Figure 8 is a diagram of quantisation levels for hybrid D-PCM
- Figure 9 is a diagram showing the decoding of hybrid D-PCM signals; and Figure 10 is a diagram showing error propagation for hybrid D-PCM.
- a video signal is transmitted from a head-end station 1 to a number of receivers 2 via a fibre network 3.
- the head-end station 1 includes an A/D converter 4 which receives an input analogue video signal, and a codec (coder/decoder) 5 which encodes the output from the converter 4 for transmission onto the network 3.
- the output from the codec 5 is transmitted onto the network 3 in bit-serial form and is modulated onto an optical carrier in a conventional manner.
- the serial bit stream is assembled into codewords and decoded by the receiver' s codec 6.
- the output from the receiver codec 6 is fed to a D/A converter 7 where the composite video signal is reconstituted.
- the codecs 5, 6 use hybrid differential PCM to encode and decode the video signals.
- a transmitter codec 5 is shown schematically in Figure 2.
- the codec 5 achieves bit rate reduction by having a predictor which makes an estimate of the next sample value based upon the value of one or more previous (and in some cases future) samples. This means that if the prediction is good, the PCM difference words will be small in value.
- the current PCM word and the predicted value are compared in a comparator
- the predictor 9 is arranged such that it codes small values accurately and larger values less accurately, provided that the predictor makes a good estimate most input signals will be accurately coded at a reduced bit-rate. Additionally, the eye is less sensitive to errors in parts of the picture which are rapidly changing, hence any error introduced by the coarse grained coding of larger changes is generally acceptable. It is mainly the complexity of the predictor which determines how many output codewords are required, and hence the overall transmission rate.
- the major spectral components in a composite video signal are in the vicinity of the colour sub-carriers (4.433 MHz for PAL, 3.58 MHz for NTSC, and 4.25 MHz and 4.41 MHz for SECAM).
- the predictor must be well suited to predicting the colour sub-carrier.
- a number of different structures are possible for the predictor, and it is found to be particularly advantageous to sample the input signal at an integral multiple (n) of the sub-carrier frequency and to use the nth previous sample as the prediction value.
- the differential signal is encoded using a Bostelmann quantiser 10 having a reflected characteristic.
- a Bostelmann quantiser 10 having a reflected characteristic.
- the output from the codec 5 is a composite codeword e' +i, where e' is the differential codeword as quantised by the Bostelmann quantiser 10 and i is the coarsely quantised predicted absolute value output by the predictor 11.
- the composite codeword is then converted to a serial bit stream by a parallel-to-serial converter 12 before being output onto the network 3.
- a complementary structure is used in the receiver codec 6 shown in Figure 4 .
- the serial data received from the network 3 is converted into parallel codewords by a serial-to-parallel converter 13.
- the output of a predictor 14 matched to that in the head-end station is then subtracted from the composite codeword e' +i to leave the differential codeword e' which is applied to a de-quantiser 15 having characteristics matched to the Bostelmann quantiser 10 in the head-end station 1. Further subtraction of the predictor value i then leaves the true absolute value i which is fed to the D/A converter 7 and output.
- the receiver described above must be aligned at the word level with the transmitter.
- the Bostelmann quantiser 10 used to encode the differential part of the composite codeword is modified so that only 31 of the 32 levels possible with a five bit system are assigned, leaving one codeword which is "invalid" in the sense that it would never be generated in response to any input signal in the head-end station 1.
- the alignment of the serial-to-parallel converter 13 is governed by a local clock 17.
- a monitor circuit 16 is provided between the output of the serial-to-parallel converter and the de-quantiser 15. This monitor detects any occurrences of the invalid codeword. When the rate at which any such invalid codewords are detected exceeds a predetermined threshold then the monitor outputs a signal to the local clock for the serial-to-parallel converter 13, advancing the phase of that clock step-wise by one bit. If after that correction to the phase of the local clock the serial-to-parallel converter 13 is correctly aligned then invalid codewords will cease to appear at the input to the monitor and the current phase of the local clock is maintained until any further error in alignment occurs.
- the phase of the local clock can be modified by advancing it stepwise by an integral number of bits.
- the number of bits in each step must not be equal to, or an integral multiple of, the number of bits in each codeword.
- the difference (e) between the prediction and incoming signal can be either positive or negative, then a sign bit is needed in the DPCM difference word.
- the Bostelmann quantiser makes it possible to omit this sign bit, and still be able to decode the signal correctly.
- ' 24' in decimal represented in 8-bits, and prefixed by a sign bit is written as '000011000'. Remove the sign bit and the binary word ' 00011000' is obtained, which still represents decimal ' 24' . However, if we take the decimal value ' -24' , then this is written in binary as ' 111101000'. If the sign bit is omitted, then this becomes ' 11101000' , which is equivalent to ' 232' in decimal. (This is the result obtained from 256-24. )
- each binary word which will be in the range 0 to 2 -1, will have two possible values, one positive and one negative (e. g. the pair ' -24' and ' 232' ).
- the word can easily be decoded by omitting the carry bit ffrroomm tthhee ssuumm,, wwhhiicchh iiss eeqquuivalent to subtracting 2 , giving the result required.
- the error word is quantised without a sign bit, and is reconstructed without using the carry bit.
- the Bostelmann quantiser has the characteristic shown in Figure 6. It is symmetrical about the mid-point because of the sign ambiguity. As positive and negative errors (e. g. ' -1' and ' +1' ) should be encoded with the same accuracy, then these must be represented by the same step sizes. As e is equivalent to 2 n -&, then these must also be represented by the same step size. Therefore +e, -e, 2 +e and 2 -e must all be encoded with the same accuracy. This leads to the full effective characteristic shown in Figure 7, with the negative portion being implied due to this sign ambiguity.
- hybrid D-PCM as suggested by Van Buul is to combine PCM and DCPM to achieve the bit rate reduction of DPCM while retaining the low error propagation of PCM.
- this sum can take any value between -(2 n -l) and 2 n+ -1.
- this hybrid value is restricted to lie in the range 0 to 2 -1, requring two less bits to represent the information than if the full range was used, though this means that not all the difference values are available.
- the first incoming level measures ' 2' on the fixed scale.
- the second sample is thus represented by the hybrid word ' 7' , which is generated by adding the previous fixed scale value and the difference value between the previous and current samples.
- the third word measures - 6' on the fixed scale, and the difference between this, and the second, is ' +2' . Summing this with the previous fixed scale value of ' 5' gives a transmitted hybrid word of ' 7' . And so this process is repeated to give the hybrid words for transmission shown.
- the hybrid words arriving at the receiver are decoded using the preceding fixed scale value p. It is assumed that the previous sample was decoded correctly, so that is amplitude can be measured on the fixed scale, to give the same value p as was obtained in the encoder. This is subtracted from the incoming hybrid word h, to regenerate the difference number d, as measured on the non-linear sliding scale. This value is added to the fixed scale value, to give the amplitude of the sample being decoded. This gives the reverse of the encoding process, and is demonstrated by Figure 9. The error introduced by this system is indicated by arrows.
- the coarse scale with which the PCM word is coded has little effect on the accuracy with which the signal is decoded, as the output level obtained is constructed from the sum of the PCM values and the more finely quantised difference values, which effectively gives more levels than are available on just the fixed scale. This means that, for example, near eight-bit accuracy can be obtained from a hybrid system transmitting just five bits.
- Hybrid DPCM has improved error recovery compared with DPCM. This is because each transmitted word contains both absolute and relative information, and so the effect of transmission errors are reduced.
- Figure 10 demonstrates the effect of an error in the first transmitted word of Figure 8 and shows that it is corrected within three samples.
- Bostelmann By using the folded quantisation characteristic described by Bostelmann, the overload problem is eliminated. This is because it is no longer necessary to limit the range of the differential words allowable so that the transmitted word lies in a certain range. If a negative difference occurs, then the Bostelmann quantiser will represent this without the sign bit, so it will appear as a positive number. Thus the resulting hybrid word will still be in the required range, without the difference values having to lie in a limited range, due to positive and negative values being indistinguishable.
Abstract
Description
Claims
Priority Applications (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP91506999A JPH05506133A (en) | 1990-04-10 | 1991-04-10 | data alignment |
FI924540A FI924540A0 (en) | 1990-04-10 | 1992-10-08 | DATASYNKRONISERING |
NO92923938A NO923938L (en) | 1990-04-10 | 1992-10-09 | DATA ADJUSTMENT |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GB909008161A GB9008161D0 (en) | 1990-04-10 | 1990-04-10 | Data alignment |
GB9008161.3 | 1990-04-10 |
Publications (1)
Publication Number | Publication Date |
---|---|
WO1991015926A1 true WO1991015926A1 (en) | 1991-10-17 |
Family
ID=10674243
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/GB1991/000569 WO1991015926A1 (en) | 1990-04-10 | 1991-04-10 | Data alignment |
Country Status (6)
Country | Link |
---|---|
EP (1) | EP0524237A1 (en) |
JP (1) | JPH05506133A (en) |
CA (1) | CA2079475A1 (en) |
FI (1) | FI924540A0 (en) |
GB (1) | GB9008161D0 (en) |
WO (1) | WO1991015926A1 (en) |
Cited By (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO2006126121A2 (en) * | 2005-05-24 | 2006-11-30 | Koninklijke Philips Electronics N.V. | Compression and decompression using corrections of predicted values |
Citations (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0126384A2 (en) * | 1983-05-13 | 1984-11-28 | Siemens Aktiengesellschaft | Method and arrangement for decoding an nB/(n+1)B-coded data stream |
-
1990
- 1990-04-10 GB GB909008161A patent/GB9008161D0/en active Pending
-
1991
- 1991-04-10 JP JP91506999A patent/JPH05506133A/en active Pending
- 1991-04-10 WO PCT/GB1991/000569 patent/WO1991015926A1/en not_active Application Discontinuation
- 1991-04-10 EP EP19910907680 patent/EP0524237A1/en not_active Withdrawn
- 1991-04-10 CA CA002079475A patent/CA2079475A1/en not_active Abandoned
-
1992
- 1992-10-08 FI FI924540A patent/FI924540A0/en not_active Application Discontinuation
Patent Citations (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0126384A2 (en) * | 1983-05-13 | 1984-11-28 | Siemens Aktiengesellschaft | Method and arrangement for decoding an nB/(n+1)B-coded data stream |
Non-Patent Citations (1)
Title |
---|
IEEE Transactions on Communications, volume COM-26, no. 3, March 1978, IEEE, (New York, US), M.C. Van Buul: "Hybrid D-PCM, a combination of PCM and DPCM", pages 362-368 * |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO2006126121A2 (en) * | 2005-05-24 | 2006-11-30 | Koninklijke Philips Electronics N.V. | Compression and decompression using corrections of predicted values |
WO2006126121A3 (en) * | 2005-05-24 | 2007-03-08 | Koninkl Philips Electronics Nv | Compression and decompression using corrections of predicted values |
Also Published As
Publication number | Publication date |
---|---|
JPH05506133A (en) | 1993-09-02 |
FI924540A (en) | 1992-10-08 |
FI924540A0 (en) | 1992-10-08 |
GB9008161D0 (en) | 1990-06-06 |
EP0524237A1 (en) | 1993-01-27 |
CA2079475A1 (en) | 1991-10-11 |
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