by MONISHA GHOSH

Philips J. Res. 50 (1996) 79-90

DIGITAL VIDEO BROADCASTING:CABLE SPECIFICATION

Philips Research, Philips Electronics North America Corp., 345 Scarborough Road,Briarcliff Manor, NY 10510, USA

AbstractIn this paper, we describe the transmission standard selected for transmis-sion of digitally compressed television signals over the cable network inEurope. This standard specifies the error correction coding, modulationand framing structures to be used in the transmitted signal. We reviewthe key features of this standard and describe the architecture of one par-ticular receiver implementation that conforms to this standard.

Keywords: digital television, cable transmission, quadrature amplitudemodulation (QAM), digital video broadcasting (DVB).

1. Introduetion

The three major television standards in the world (pAL, SECAM andNTSC) were designed for an analog transmission environment and are incap-able of transmitting digital data efficiently. With the progress in image com-pression and the formulation of the MPEG-2 worldwide standard, itbecame possible to design a purely digital transmission system capable ofcarrying video, audio and digital data over the same medium. Digital trans-mission has the advantages oflower transmitted power and less required band-width for the same received quality as an equivalent analog signal. Hence it ispossible to send multiple digital programs over the same channel that can beused for only one analog program. The channels that will be used for digitaltransmission are primarily the terrestrial broadcast channel, satellite and cablenetworks. All of these channels are used currently and hence the infrastructureis already in place.

With the move to digital transmission, it became apparent that a whole newset of standards would be required for each of the channels under consideration.

Phllips Journalof Research Vol. 50 No. 1/2 1996 79

M. Ghosh

Under the auspices of the European Digital Television by Cable (DTVC)group, Philips was a major player in the Digital Video Broadcasting (DVB)project, which specified a telecommunication standard describing modulation,channel coding and framing structure for digital multi-program televisiontransmission by cable. This standard is part of a full digital video, audioand data system including baseband image coding, audio coding, data servicecoding, multiplexing, channel modulation and coding for satellite transmis-sion, cable networks and Satellite Master Antenna Television (SMATV),and a common scrambling system. In the United States, no standardizationeffort was carried out for cable transmission; instead two schemes emerged,a Quadrature Amplitude Modulation (QAM) scheme proposed by GeneralInstrument and a Vestigial Sideband (VSB) scheme proposed by Zenith.In this paper, we describe the details of the cable transmission standard as

specified by DVB. Section 2 first describes the environment in which the cablemodem will operate and some of the interferences that are present in cable net-works. Section 3 describes the various elements of the transmission standardsuch as modulation, filtering requirements and forward error correction(FEC). Section 4 describes one possible receiver architecture which complieswith the DVB standard. Finally, conclusions are presented in Section 5.

2. Cable environment

In Europe, the frequency spectrum most commonly used for analog cabletransmission is the band from 45 MHz to 862 MHz with channel spacingsof 7 MHz, 8 MHz and 12 MHz. All channels are not currently used, in orderto minimize intermodulation effects and cochannel interference from terres-trial broadcasts. Since digital signals can be transmitted at a significantly lowerlevel than analog signals for the same picture quality, it is envisioned thatdigital channels can be accommodated in presently unused channels, e.g.one digital channel can be inserted between two analog channels. In theDVB cable standard, compatibility between satellite and cable systems is con-sidered to be an essential component especially with regard to the bit-rateused. Hence, it is proposed that the information contained within one satellitetransponder be carried over a single 8 MHz channel. In order toaccomplishthis, a Quadrature Amplitude Modulation (QAM) scheme with 4, 16,32,64or 256 constellation points is proposed with receivers being able to supportat least 64 QAM.Figure 1 shows the general scenario for digital television transmission. The

MPEG-2 transport multiplexer delivers 188-byte packets of data to the cableheadend. The input to the transport multiplexer is either directly from the

80 PhlIlps Journal of Research Vol.SO No. 1/2 1996

L. _

Satellite Demodulator At Heedend ,

Digital video broadcasting: cable specification

MPEG-2 Transport Multiplex

Fig_ L Digital TV distribution scenario.

MPEG-2 coder or from the satellite receiver after demultiplexing. If the satel-lite and cable payload bit rates are compatible, the output ofthe satellite FEedecoder can be fed directly to the cable transmitter.In general, the cable channel has less distortion as compared to the

terrestrial broadcast channel and hence can support higher data rates. In addi-tion to white noise, some of the main impairments in the cable network aremultipath, phase noise and frequency offset. Multipath is caused by reflectionsof the main signal due to mistermination at taps in the network and activedevices along the cable transmission path. Phase noise and frequency offsetare introduced by tuners at the receiver and can be compensated by carriersynchronization loops in the receiver. These aspects will be discussed furtherin Section 4.

Since the digital signal is compressed before transmission, even a single biterror due to impairments in the transmission channel can cause larger errorsout of the MPEG-2 decoder. The bit error rate (BER) requirements forMPEG-2 to deliver an error free picture is of the order of 10-11• This is anextremely low error rate to accomplish with a high-level modulation aloneand would require a very large signal-to-noise ratio (SNR). Instead, the

TABLE IRequired SNR for various modulations

PhIlIps Journal of Research Vol. SO No. 1/2 1996 81

Modulation SNR for BER = 10-4

16QAM64QAM

256QAM

12.2dB16.5dB21.2dB

M. Ghosh

ISbitshütregister

RS (204,188) 12 b)'1C deep

Fig. 2. DVB cable transmitter.l'~"'·\ ~;~'.""",.\r ,J . ,

approach is to have a relatively high channel BER of 10-4 with a moderateSNR and to achieve the required bit error rate of 10-11with an error correctingcode. Table I gives the theoretical SNR values for BER = 10-4 before FEe for16, 64 and 256 QAM modulation formats.

For each modulation, the corresponding carrier-to-noise ratio (CNR) in an8 MHz bandwidth depends on the transmitted bandwidth and is given byCNR = SNR + 10 10g(DjW) where D is the bit rate including FEe and Wis the bandwidth (8 MHz). For 64 QAM, assuming 15% roll-off, the bit rateincluding FEe is about 41.7 Mbps, giving a required eNR of 23.7 dB.

3. DVB cable specification

The baseline system for digital multi-program television distribution bycable is defined in Ref. [1]. It is based on high-level QAM modulation anduses maximum commonality with the corresponding baseline system designedfor satellite broadcasting. In particular, the same source coding (MPEG-2),channel coding (RS (204,188», interleaving and framing structure are usedin both standards.

The system block diagram as specified by DTVe is shown in Fig. 2. Theinput to the cable headend is I88-byte data packets according to theMPEG-2 system layer specification. The first byte of each packet is a syncbyte. The major blocks in the transmitter are the scrambler, Reed-Solomon(RS) encoder, convolutional interleaver, byte-to-M-tuple converter and differ-ential encoder, square root raised cosine (SQRC) filter and upconverter. Eachof these blocks is described below along with some future modifications thatmay be considered.

3.1. Scrambler

Energy dispersal of the incoming packets is performed by the scrambler,which is a Linear Feedback Shift Register with the following polynomial:1 + xl4 + XiS. Scrambling ensures that the transmitted spectrum is whiteeven if the input spectrum is not and avoids long strings of zeroes in the out-put. The shift register is loaded with the sequence '100101010000000' at thestart of every eight transport packets. In order to provide synchronization

82 Phillps Journal of Research Vol.50 No.1/2 1996

Digital video broadcasting: cable specification

information to the descrambler in the receiver, the sync byte ofthe first packetin every group of eight packets is bitwise inverted. The sync bytes of theremaining seven packets are left unscrambled. Hence, only data bytes (187in each packet) are scrambled.

3.2. Reed-Solomon code

An error correcting code is now applied to the scrambled packets. This codeis a shortened RS (204,188) code, i.e. 16 bytes ofparity are added to each 188-byte packet to form 204 bytes of transmitted data. This code is identical to thatused in the satellite standard in order to simplify receiver design. The 16 bytesof parity allow the code to correct any combination of 8 byte errors out of 204that may occur during transmission. This capability delivers an effective BERof 10-11 at the output of the RS decoder in the receiver. The code generatorpolynomial for the code is: g(x) = (x + aO) (x + al) (x + a2

) ••• (x + alS)and the field generator polynomial is: p(x) = x8 + x4 + x3 + x2 + 1. Theshortened RS code is implemented by adding 51 all-zero bytes to the 188 infor-mation bytes at the input of an RS (255,239) encoder with the above poly-nomials and then dropping the 51 zero bytes after the encoding process iscompleted to obtain 204 transmitted bytes.

3.3. Convolutional interleaving

The RS code can correct only up to 8 error bytes in each 204-byte packet.Hence, in order to protect against longer error bursts caused by impulse noise,it is necessary to distribute these error bytes across several packets so that they

2M=34

From RS Encoder1 byte per line

ToM-tupleconverter

11--lIl I

11M = 18711

Ill-

Fig. 3: Conceptual diagram of 12 byte deep convolutional interleaver.

Philips Journal of Research Vol. SO No. 1/2 1996 83

To RS Decoder

M. Ghosh

11M= 187

2M=34

IO~ [][I~M~=~17~=r]- IIO

I_I 11

Fig. 4. Conceptual diagram of 12 byte deep convolutional deinterleaver.

can be corrected by the RS code. This is accomplished by the convolutionalinterleaver shown in Fig. 3. This scheme is based on Forney's approach [2].Conceptually, the input to the interleaver is broken into 12 parallel bytestreams and the ith stream, i = 0, 1, 2, ... 11, is subject to the delay Di = l7ias shown in Fig. 3. The 12 outputs of the interleaver are then recombined toa serial form before the next stage. Synchronization is performed by routingthe sync byte in each packet to the branch '0'. At the deinterleaver in thereceiver, the first recognized sync byte is routed to the '0' branch. Figure 4 showsthe delays required in the deinterleaver in order to recover the original stream.

3.4. Byte-to-symbol mapping and differential encoding

The byte stream after interleaving is converted into a symbol stream as

Byte toM·tuplcConversion I-----"=---i~

(M·2) bits IbM., br.. ho)

M=4forI6QAMSfor32QAM6for64QAM

Fig. 5. Byte to M-tuple conversion and differential coding.

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Digital video broadcasting: cable specification

Q

101100 101110 100110 100100 001000 001001 001101 001100

101101 101111 100111 100101 001010 001011 00~1I1 001110

I.<lk= 10 I.<lk=oo

101001 101011 100011 100001 000010 000011 000111 000110

101000 101010 100010 100000 00000o 000001 000101 000100

110100 110101 110001 110000 010000 010010 011010 011000

110110 110111 110011 110010 010001 Olooll 011011 011001

It<lk= 11 I.<lk=OI

111110 111111 111011 111010 010101 010111 011111 011101

111100 111101 111001 111000 010100 010110 011110 011100

Fig. 6. Rotationally invariant 64 QAM constellation.

follows. For 2m QAM, k bytes are mapped into n symbols such that8k = n x m for the smallest possible integers k and n. In the case of 64QAM, m = 6, k = 3 and n = 4, i.e. three bytes are mapped into four 64QAM symbols. The two most significant bits (MSBs) of the m-bit symbolare differentially encoded, as shown in Fig. 5. Differential encoding ensuresthat the transmitted constellation is invariant to 90 degree rotations. Figure 6shows the bit-to-symbol mapping for a 64 QAM rotationally invariantconstellation.

3.5. SQRC filter and upconverter

The I and Q signals after differential encoding are input to an SQRC filter.The roll-off factor for this filter is 15%. The theoretical transfer function isdefined by the following expressions:

H(f) = 11

If - fol < 2T (1 - a)1

If - fol > 2T (1 +a)H(f) = 0

H(f) =1 1 [_1 -If - fol]- + -sin 1rT ~2",-T _2 2 a

PhIIIps Journal of Research Vol. 50 No. 1/2 1996 85

M. Ghosh

OdB

I

• r, r, -,In-band ripple ro < 0.4 dB

B "", t r,-, Out of band rejection> 43

0.8SrN rN I.ISrN

dB

IH(I)

·3.01 d

rN : Nyquist Frequency

Fig. 7. Template for transmitter SQRC filter.

wherefo is the centre frequency ofthe digital spectrum, liTis the symbol fre-quency and Cl! = 0.15 is the roll-off factor. A practical implementation can onlyapproximate the above transfer function. Hence, the template in Fig. 7 is usedfor a hardware implementation on the transmitter side. In addition to meetingthe amplitude spectrum described in Fig. 7, the filter should be phase-linearwith the group delay ripple less than IOns. Finally, the signal is convertedto analog and upconverted to the channel over which it will be transmittedover the cable network.

3.6. Cable bandwidth organization

The baseline system described above is considered for 8 MHz channel band-width with 15% roll-off and can accommodate almost the whole range of bitrates of the baseline satellite system. Table II gives the symbol rates and band-width requirements for each of the satellite bit rates using 16, 32, 64 or 256QAM constellations. The shaded values are of less practical significaneeover 8 MHz channels since a lower-order QAM would be more robust. Thevalues in the bold frames indicate the appropriate constellation for a givensatellite bit rate.

Variable symbol rates imply the need for receiver SQRC filters that can varyover a range of symbol rates. These are typically more difficult to implement atreasonable cost than fixed symbol rate filters. Hence one scenario under con-sideration is to have a fixed number of symbol rates and different filters corres-ponding to these rates.

86 Philip. Journal of Research Vol.50 No. 1/2 1996

Philips Journalof Research Vol. 50 No.IJ2 1996 87

Digital video broadcasting: cable specification

TABLE IIPossible bit rates over satellite and corresponding symbol rates and bandwidth

requirements over cable for different modulation schemes

SatelUteBit

3.7. Future modifications

An extension ofthe baseline system described above to accommodate higherbit rates from wide-band satellite transponders may be required. This can be

provided by expanding the signal constellation to include 256 QAM whichrequires a higher CNR as well as better tuners in the receivers, since higherlevel modulations are more susceptible to phase noise. In addition, it is pos-sible that a training signal may be required for equalizing 256 QAM. Thisneed can be met in the future by introducing a training sequence as anMPEG-2 transport packet with a unique ID. Though a training sequenceadds a slight overhead ('" 0.5%) in terms of reducing the useful data rate, itcan be very beneficial in reducing acquisition times as well as acquiringchannels with a lot of distortion. If a lower CNR is required in order to accom-modate 256 QAM, it may be necessary to insert a trellis code as the inner codeafter the RS outer code. This would add complexity to the receiver but pro-vide lower CNR thresholds.

M. Ghosh

4. Cable receiver architecture

The DVB standard described above does not specify receiver architecturesand hence various designs are possible. In this section we will briefly describeone architecture which has been implemented by Philips [3].

The basic block diagram of the receiver is shown in Fig. 8. Most of theblocks are inverses of corresponding blocks in the transmitter, for examplethe differential decoder, symbol-to-byte mapper, convolutional deinterleaver,Reed-Solomon decoder and descrambler. The additional blocks that arerequired in the receiver are the carrier recovery and clock recovery circuitsand the equalizer. Different receiver designs vary in the algorithms used toimplement these synchronization and equalization functions.

+1.0.·1.0",.

Fig, 8. QAM receiver architecture.

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Digital video broadcasting: cable specification

The input signal is first downconverted to a low IF which is at the symbolfrequency. A single analog-to-digital converter (AjD) converts this signal toa digital signal at 4 times the symbol rate. Since the IF is the symbol frequency,the final conversion to baseband is accomplished in the digital domain bysimple multiplications by + 1, -lor 0 as shown in Fig. 8. The I and Q portionsof the baseband signal are then input to the even and odd portions of theSQRC filter respectively. The outputs of the filters are at twice the symbolrate and are used as such by the clock recovery circuit. However, the signalsare downsampled by 2 to the symbol rate before being used by the equalizer,which is a T-spaced complex equalizer.

The clock recovery algorithm is based on the principle of energy maximiza-tion with respect to the sampling time [4]. However, other clock recovery algo-rithms are also possible [5]. The output ofthis block is converted to an analogvalue, loop filtered and then used to control the AjD VCXO.

The equalizer is a 'blind' equalizer since the DVB standard currently doesnot specify a training signal. The number of taps in the equalizer is a receiverdesign issue and is usually of the order of 20 taps for the cable channel. Thereare many algorithms for adapting the equalizer taps in the absence of a train-ing signal, like Godard's algorithm [6], Stop-and-Go [7], etc. These are all adap-tive algorithms that compensate for slowly varying changes in the channel byminimizing an error signal which is a function of the equalizer output signal.

Carrier recovery is performed at the output of the equalizer using a phase-frequency detector. The output of this detector is converted to analog, loopfiltered and is then used to control the oscillator that down-converts the signalto low IF. In this particular design, equalization is performed at baseband andhence the carrier recovery loop controls the oscillator that converts the signalto low IF. However, other designs are possible where the concept ofpassbandequalization is used and the carrier recovery is done completely in the digitaldomain [8].

The rest of the data path is simply the reverse of the transmitted data path.After equalization, the signal is sliced to the nearest constellation point, differ-entially decoded, converted to bytes, deinterleaved and then decoded to givethe received bit stream that is input to the MPEG-2 decoder.

PbIJlps Journal of Research Vol. SO No. 1/2 1996 89

5. Conclusion

The DVB cable standard has been designed to transmit quasi-error freedigital video over existing cable networks. It has much in common with thesatellite standard in order to reduce receiver complexity. Provisions havebeen made in the standard to accommodate different bit rates by using 4,

REFERENCES[1] Digital broadcasting systems for television, sound and data services - framing structure,

channel coding and modulation for cable systems, EBU/ETSI JTC, DE/JTC-DVB-7, April(1994).

[2] G.D. Forney, Interleavers, U.S. Patent no. 3,652,998 (1972).[3) S. Brand and W. Weltersbach, A QAM demodulation concept for digital video cable transmis-

sion, ICCE Digest of technical papers, June 1995.[4] F.M. Gardner, A BPSK/QPSK timing error detector for sampled receivers, IEEE Trans.

Commun., COM-34, 423-429, May (1986).[5] D.N. Godard, Passband timing recovery in an all-digital modem receiver, IEEE Trans.

Commun., COM-26, 517-523, May (1978).[6] D.N. Godard, Self-recovering equalization and carrier tracking in two-dimensional data com-

munication systems, IEEE Trans. Commun., COM-28, 1867-1875, Nov. (1980).[7] G. Picchi and G. Prati, Blind equalization and carrier recovery using a 'stop-and-go' decision-

directed algorithm, IEEE Trans. Commun., COM-35, 877-887, Sept. (1987).[8] J.G. Proakis, Digital Communications, McGraw-Hill (1983).

M. Ghosh

16, 32, 64 and, in the future, 256 QAM constellations. There are alreadymany ICs on the market conforming to this standard, thus ensuring its wideacceptance in the cable industry worldwide. Most recently, the Digital AudioVisual Council (DAVIC) has adopted DVB as the cable standard for its sys-tem specification.

Acknowledgements

The author would like to thank the management and staff of PhilipsLaboratories, Briarcliff for their support and the staff of Laboratoires d'Elec-tronique Philips (LEP) for providing material regarding the DVB standard.

AuthorMonisha Ghosh received her B.Tech. in Electrical and Electrical Communication Engineering fromthe Indian Institute of Technology, Kharagpur, India in 1986, and MS.and PhD. in ElectricalEngineering in 1988 and 1991, respectively, from the University of Southern California, LosAngeles. Since 1991, she has been a Senior Member ofthe Research Staffin the Video Communi-cations Department at Philips Laboratories, NY, where she has been involved with digital trans-mission of high data rate signals over terrestrial, satellite and cable channels. Her research interestsinclude estimation and information theory, error-correction and digital signal processing.

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