228 IEEE TRANSACTIONS ON BROADCASTING, VOL. 40, NO. 4, DECEMBER 1994

A NOISE RESISTANT SYNCHRONIZATION SCHEME FOR HDTV IMAGES

Panos Nasiopoulos and Rabab K. Ward

Department of E l d e a l Engineering University of British Columbia

Vancouver, B.C. - CANADA V6T 124

ABSTRACT

A method that increases the error resistance of the HDTV sys-

tem and offers graceful picture degradation in the presence of bit

errors, is presented. Due to the nature of the presently proposed

compression schemes for HDTV systems, an error in a data bit does

not only affect the block the bit belongs to, but unfortunately the

effects of this error may perpetuate to the following blocks. This is

because a bit error may cause loss of synchronization between the

data bits and the picture blocks they represent. Our method restricts

the effects of a bit error to a picture block whose size is signifi-

cantly smaller than those used by the HDTV systems. We achieve

synchronization by transmitting a header-word for each such syn-

image has four times the luminance definition of the conventional

NTSC images. Furthermore, the luminance is separated from the

chrominance for excellent color rendition. Transmission of these

digital HDTV images requires a data rate of approximately 1

Gbps; this would occupy a bandwidth too wide to be practical.

Unlike the digital audio signals of the past, the applicability of the

digital HDTV systems depends on the use of data compression.

Powerful compression schemes such as MPEG (Motion Pictures

Experts Group) which are able to compress the digital video HDTV

signals to approximately 15 Mbps were devised [3, 5, 7. 9, 14,

151. Efficient modulation techniques (e.g., QAM - quadrature chronization block. Each header-word contains the number of data

bits representing the compressed block. This header-word is pro-

tected by two levels of FEC code. To decrease the number of ex-

tra bits needed by the header-words, two different synchronization

block sizes are used, a relatively small block size for the reference

frames and a larger size for the inter-frames. The resulting method

improves the quality of the picture in the presence of errors and

defers the SNR at which the HDTV picture suddenly deteriorates

by 2.5 to 3 dB. It also allows operation at higher order modulation

transmission schemes, e.g., 32-QAM instead of 16-QAM, without

the requirement of increased signal power.

amplitude modulation) are used to further compress the data bit

stream into symbols and thus yield symbol rates which fall within

the required 6 MHz bandwidth [3, 6, 7, 9, 11, 181.

At present, cable companies and broadcasters in North America

are considering transmitting two HDTV signals in each 6 MHz

channel [16]. Furthermore, there is consideration of converting the

analog NTSC signals to digital and, by using techniques similar to

those of HDTV, transmitting up to 10 digital NTSC signals within

each 6 MHz channel. With the future additions of video-telephony

and computer information to the system, it is obvious that further

compression of the data will be essential [2, 8, 161. Since video

compression schemes have reached a maximum performance level

for the time being, efforts are being directed toward reducing the

data rates by increasing the order of the modulation schemes (i.e., increasing the number of modulation levels) [ 161. However, as the

number of modulation levels in the transmitted data is increased,

the difficulty in distinguishing between the levels increases [17].

Since we are not free to increase the carrier power as &e order

of modulation is increased, the system becomes more susceptible

to errors. Susceptibility to errors is an important issue in the

case of compressed digital images, since, unlike analog TV images

which deteriorate gradually as the level of interference increases,

the digital HDTV and all other forms of multimedia pictures may

1. INTRODUCTION

High Definition Television (mTv) has as the

a transformation world's most significant new technology*

of markets for information and enttxhhment 12, 4, 8, 13* 191*

ye= of innovations in computer-imaging technoh3', compression,

multimedia, and d i g i d signal Processing have linked the futures

of the computer, television, and telephony industries 2, 8, 1 0 9

13, 24, 251. It began with the development Of the digita1 HDTV

whose main objective is to provide a high quality widescreen image

comparable to that of motion pictures 1 4 - 6 9 129 131. An HDTv

0018-9316/94$04.00 0 1994 IEEE

229

simply vanish [19, 22, 231. This is because the effects of an error in any transmitted bit in the data bit stream may perpetuate

to the following data bits due to the variable length encoding of

the data (each 8x8 pixel block of the picture is represented by a

variable number of bits). Because of the variable length coding

(VLC), an erroneous bit may result in loss of synchronization

of the compressed data with the picture blocks they represent

[20, 22, 231. Also, because a motion compensation process is

used, errors in any frame will propagate to following frames. In

summary, the huge bandwidth demand of the HDTV and full-

motion interactive multimedia prevails as the most critical aspect

of the system. Seeking a solution through greater compression

and higher order modulation techniques leads to an inescapable

increase in the error sensitivity of the system. Error sensitivity

forms a challenging issue for such digital services because they

tend to degrade abruptly in the presence of errors.

To protect the transmitted data from errors, Forward Error Cor-

rection (FEC) codes are used [3, 7, 9, 111. These codes mini-

mize the effects of transmission errors and allow operation at lower

signal-to-noise ratio (SNR) levels. FEC codes do not, however,

correct or detect all errors which may arise during transmission.

Thus all HDTV proposals provide measures to synchronize the

transmitted data with the picture blocks. Each frame (image) is

divided into sub-images called macroblocks or slices. An error is

only allowed to perpetuate to within the boundaries of the slice the

errors belongs to. The parameters of each slice are encoded inde-

pendently of those of other slices. Thus, for any parameter which is encoded differentially, i.e., as the difference between its actual

values in the present block and a previous block, such as DC coef-

ficient and motion compensation vector, the original true values of

these parameters are re-initiated at the beginning of every slice. A

video buffer of fixed bit-length (data-line) is used to form a whole

codeword consisting of the video data bits and the corresponding

FEC bits. Each buffer codeword also contains a pointer which

identifies the picture slice the present buffer data belong to. The

re-initialization of the true values of differential parameters and the

use of the pointer restrict the effects of undetected errors to the

slice in which the errors occurred. In other words, synchronization

at the slice (or macroblock) level is achieved.

The DigiCipher and Channel Compatible DigiCipher (CCDC)

systems (by General Instruments and by the Massachusetts Institute

of Technology in conjunction with General Instruments, respec-

tively, use macroblocks or slices, each of size equal to 5632 pixels

and comprised of 16 lines high and 352 pixels wide [7, 91. The

compressed data are transmitted as consecutive data-lines of 106

bytes (for 16-QAM). Each data-line includes a 16-bit macroblock

pointer which points to the next macroblock in the bit stream [7,

91. The large size of the macroblock ensures that the number of

bits representing it is always larger than the bit length of the data-

line and thus each data-line cannot contain more than one complete

macroblock. This mechanism guarantees that the maximum image

area lost because of an error is one macroblock. We evaluated the

picture performance of this system using computer simulation. In

terms of transmission, our model employs 16-QAM and a Reed-

Solomon (1 16,106) FEC code.' Figure 1 shows the reference image

obtained by the DigiCipher and CCDC HDTV method at 15 dE3

SNR in the channel. Figure 2 shows the fifth frame from the refer-

ence frame (Figure l), obtained by using inter-frame compression

utilizing motion compensation.

Figure 1 shows that if an undetected error occurs, its effects

perpetuate to the consecutive blocks within the macroblock, result- ing in block streaking effects in the picture. The horizontal width of

the resultant erroneous block may be less or equal to 352 pixels and

its vertical length is 16 lines. Figure 2 contains errors which have

perpetuated from the reference frame Figure 1 as well as newly

introduced errors in the inter-frame data.

The Advanced Digital Television (ADTV) synchronization

scheme (by David Sarnoff and Philips laboratories) is much more

complex than that of the DigiCipher and CCDC [3]. It includes

prioritization of the Discrete Cosine Transform (DCT) coefficients

into two streams, the high and the standard priority data bit streams,

and a spectrally shaped QAM channel for the transmission of the

high priority and the standard priority DCT bits. For each of the

two data streams a 960-bit data-line (cell) is used. The image is

divided into 64 pixels wide x 16 pixels high slices and the system

aims at providing synchronization at the slice level. Each data-line

contains the compressed video and audio data and the FEC bits. It

also contains a IC-bit pointer which points to where slice 2 starts,

i.e., number of bits in slice 1 in this cell, and a IC-bit slice-number

that identifies the position of slice 2 in the frame (Figure 3). The

small size of the slices allows up to 5 compressed slices to fit in

one data-line. As a result, if an error occurs in slice 2, and since

the pointer points only to the end of slice 1, slice 2 and the follow-

ing slices in the data-line will be lost (see Figure 3). In the case

of high compression, i.e., inter-frames, this could result in a total

loss of 4096 pixels.

' simulation of the digital transmission process [ZI].

The Signal Processing WorkSyslemPD (SPWTM) software package was used for the computer

230

p o I nt e r to next lice I .lice

10 2 20 960 b i t e I ;It". k data 4 bits I b b / bits I

slice 2 slice3 slice4 lice 5 I1

Finally, the Digital Spectrum Compatible HDTV (DSC-HDTV)

proposal (by Zenith and AT&T) uses a complex transmission sys-

tem that multiplexes between 1 bit per symbol and 2 bits per symbol transmission, resulting in more robust transmission for the binary

portion (1 bit per symbol) of the video data [Il l . The image is

divided into slices which correspond to 3072 pixel regions, 64 pix-

els horizontally x 48 pixels vertically. The encoded data stream is divided into fixed length (648 bytes) data-lines. Synchronization

for each data-line is provided by four repeated sync-interval sym-

bols. These repeated sync symbols are the only ones which recur

with the same pattern. Their periodic identical recurrence is used

to provide synchronization. These symbols are not provided with

any FEC protection. We note that the complexity of the transmis-

sion scheme of the system is higher than the DigiCipher and CCDC

systems, while an error may still affect a large area of the image

(3072 pixels).

We observe that all four HDTV systems (which presently form

the Grand Alliance) allow a bit error to affect large areas of

the image. As the SNR drops below a threshold value, sudden

picture degradation results. Higher order modulation schemes will

further increase the error rate and this increases the rate of the

deterioration of the picture quality. System robustness is essential

so that noise interference will not produce catastrophic effects, but

rather a gradual degradation of the picture.

In this paper, we present a novel synchronization method that

increases the e m r resistance of the HDTV system and offers grace

ful picture degradation in the presence of bit emrs. Our method is

similar to the HDTV systems in that it provides synchronization of

the data at the block levels. Our blocks are however significantly

smaller than those used by the HDTV systems. Furthermore, we

use two block sizes. The smaller synchronization block is assigned

to the reference frames since the quality of a reference frame affects

all consecutive frames. The larger synchronization block is assigned

to the inter-frames. In Section II.A, we describe the new synchro-

nization method. In Section ILB, we show how the efficiency of

this method increases by using two different synchronization block

sizes: a relatively small size for the reference frames and a larger

size for the inter-frames. Trade-off combinations between the er- ror protection and the overhead used for synchronization are also

studied. In Section II.C, we study the performance of our method

when higher order modulation schemes are used.

I I . A NOISE-RESISTANT SYNCHRONIZATION METHOD FOR REFERENCE

FRAMES AND INTER-FRAMES

We propose a new method that provides synchronization of data

with picture blocks which are much smaller than the macroblock or

the slice. To distinguish this kind of a block from others we call it

a sync block. Our method achieves synchronization by transmitting

a header-word for each sync block. This header-word contains the

exact number of bits assigned to the sync block. By knowing the

exact number of bits belonging to each sync block, the decoder is

able to find the beginning of the following sync block, thus limiting

the effects of an undetected error to within the sync block the errors

belong to. It is clear that sync blocks of smaller sizes offer better

error protection than larger ones. However, the smaller the sync

block size is, the greater the number of the codewords that should

be added to the data stream. Thus, the size of the sync block affects

the total video rate which should not exceed the required bandwidth

limit of 6 MHz [3, 4, 7, 9, 111. Therefore, the size of the sync

block should be chosen carefully.

1I.A. Synchronization at 32x16 Pixel Level for All Frames

Let us first consider the case where the size of the sync block

is the same for both the reference frames and the inter-frames

and is 32x16 pixels, i.e., the same size as the superblock. As it

will be shown later, this sync block size significantly improves

the error resistance of the system, while maintaining the overall

transmission rate to within the required bandwidth limits. For

each sync block, a header-codeword indicating the number of bits

belonging to the sync block, is added. For coding efficiency the

R, G and B components are converted to one luminance (Y) and

two chrominance (U and V) components by a color conversion

matrix [6]. The chrominance components are Iowpass filtered and subsampled by a factor of 4:1 horizontally and by a factor

of 2:l vertically. A 12-bit fixed-size word is large enough to

accommodate the number of bits representing a luminance sync

block of this size and the corresponding chrominance information

for the block. To ensure the fidelity of this 12-bit word, we protect

it by using a (6,4) Reed-Solomon code (12 information-bits, 18

bits total). Thus, each of these special header-codewords has a

fixed length of 18 bits. This process provides an extra layer of

error protection for the header-codewords, since the whole data

stream (the original data and the header-codewords) is protected

later by the channel error protection code. The channel noise

has two effects on the picture: 1) an erroneous data bit changes

the information of the block it belongs to, and 2) the effects of

the error may perpetuate to the following blocks causing loss of

synchronization of the information bits with the blocks. If the

second effect (loss of synchronization) is not present, it has been

found experimentally that the SNR at which the picture deteriorates

to the level of being unrecognizable is advanced by approximately

3 dB. Thus, the synchronization scheme should improve the system

by 3 dB only, since beyond that level the picture becomes of

unrecognizable quality and there is no point in protecting the

synchronization of such a totally degraded picture. To obtain this 3

dB improvement, performance evaluation results have shown that a

(6,4) Reed-Solomon code (in addition to the channel Reed-Solomon

code) ensures the errorless recovery of the header-codewords, i.e.,

ensures synchronization up to noise levels at which the original

signal does not yield images of recognizable quality. The DC

coefficients in this case are only differentially encoded within each

sync block. This eliminates the DC error propagation between

consecutive sync blocks and makes each sync block independent

of the others. The use of the 18-bit header-codeword for each sync

block adds 47.5 Kbits to every 1408x960 pixel image for a total

of 1.42 Mbps.

Figure 4 shows the reference frame obtained by the DigiCipher

scheme in conjunction with our 32x16 synchronization method at

15 dB channel noise, i.e., under the same noise conditions as that of Figure 1 , Similarly, the fifth frame obtained by DigiCipher utilizing

inter-frame motion compensation and our block synchronization

method, is shown in Figure 5. Comparing Figures 4 and 5 with

Figures 1 and 2, we find that our method improves the quality of the

decompressed picture by restricting the effects of errors to within

much smaller block boundaries. In the following section we present

a way to improve the trade-off between the error protection and the

total number of overhead bits used by this synchronization method.

1I.B. Two Sync Block Sizes Method

So far we presented a method that provides synchronization at

the 32x16 pixel level (superblock) for both the reference frames

and the inter-frames, (i.e., the same synchronization block size was

used for all the frames). However, it is more efficient to provide

better protection to those frames whose quality affects that of the

other frames the most. Generally, for every 10 frames a reference

frame (intra-frame) is transmitted, the other 9 frames (inter-frames)

are encoded using inter-frame motion compensation techniques.

Motion compensation involves the estimation of the motion vectors

between two consecutive frames of each block of predefined size.

Once a motion vector is estimated, the values of the corresponding

block in the subsequent frame are predicted. The predicted values of

the block are subtracted from the values of the corresponding block

in the new frame. Then the resulting difference (error data) are

transmitted after being (DCT) transformed, quantized, and encoded

in exactly the same way as in the reference frames [3, 5 , 7, 9,

11, 151. The fidelity of the reference frames is crucial since the

effects of any error in a reference frame will perpetuate and affect

all subsequent 9 inter-frames because of the motion compensation

process. Errors occurring in an inter-frame will also propagate to

subsequent frames (until the next reference frame is encountered).

Thus, while an error in a reference frame affects all 10 frames,

an error in an inter-frame, on average, affects 5 frames. Another

important factor is that the probability of an error occurring in a

reference frame is around 4 times greater than an error occurring in

an inter-frame. This is because most of the video compression is

due to the motion compensation process. We have experimentally

found that an average of around 30% of the total number of

compressed bits belong to the reference frames and around 70%

belong to the inter-frames. Thus the probability of an error affecting

a reference frame is equal to 0.30Np where N is the number of bits

in a 10 frames sequence and p is the probability of a bit error.

Similarly, the probability of an error affecting an inter-frame is

0.70Np/9 = 0.077Np. Thus, the probability of an error in the

reference frame is greater than that in an inter-frame by a factor

of 3.89. We conclude that, preserving the quality of the reference

frames is more important than that of the average inter-frame since

an error in the reference frames, on average, affects double the

number of frames and since errors in the reference frames are 4 times more likely to occur than in an inter-frame. This means

that on average we should provide the reference frames with error

protection greater than the inter-frame by a factor of 8.

Motivated by the above, we propose a method that provides

synchronization using two different sync block sizes: a relatively

small size for the sync blocks of the reference frames and a larger

size for the sync blocks of the inter-frames. This scheme offers

higher protection to the reference frames than the rest. Choosing the

sizes of the two sync blocks requires a trade-off decision between

the achieved degree of error protection and the number of the extra

bits of the header-codewords to be added to the data stream.

Table 1 shows the bit length of the header codewords for

different sync block sizes, the number of the corresponding Reed-

Solomon protection bits, and the total number of extra bits required

for one frame. Table 2 shows the number of extra bits required

232

for different combinations of sync block sizes for the reference and

inter-frames. From this table we observe that as the sizes of the

sync blocks increase the number of extra bits needed decreases.

Sync block pixel size

Blt length of I Reed-Solomon I Number of extra header-codewords code bltdreference frame

n b l e 1. This table shows different combinations of block sizes, the length of the header-codewords for each size, the Reed-Solomon protection code

needed, and the total number of extra bits required for one reference frame.

Reference frame Inter-frame Number of , extrabltdsec

8x8 32x16 ' 2.47 Yb%s

8x1 6 32x1 6 1.99 Mbits

8x1 6 , 64x16 1.46 Mbits

16x1 6 32x1 6 1.56 Mblts

16x1 6 64x1 6 1.03 Mblts

Table 2. This table shows the number of extra bits required for different combinations of sync block sizes for the reference and inter-frames.

Let us first consider the case which uses the smallest synchro-

nization block size possible (8x8 pixels) for the reference frames

and a larger 32x16 pixels synchronization block for the inter-frames.

This implementation offers the required (eight folds) more protec-

tion to the reference frames than the inter-frames. A 9-bit fixed-size

header-word is large enough to accommodate the number of bits

representing an 8x8 luminance or chrominance block. An error pro-

tection for this header-word is provided by a (53) Reed-Solomon

code (9 information bits, 15 bits total). Thus, after the addition of

the parity bits, each of the resultant header-codewords has a fixed

length of 15 bits. The resultant data stream is now composed of blocks of bits, each block consists of the 15 (header codeword)

bits followed by the bits representing the 8x8 pixel block. For

each inter-frame, synchronization at the 32x16 superblock level is

provided by an 18-bit header-codeword, as previously outlined in

Section 1I.A.

Under the same noise conditions as those of Figure 1 and Figure

2 and using our 8x8 header-codeword synchronization scheme for

the reference frame, Figure 6 is obtained. Figure 7 depicts the fifth

frame (from the reference) obtained by using the 32x16 header-

codeword scheme for all the inter-frames.

The root mean-sauare errors N-1 K-1

?;R loriginal (x,y)-reconstructed (x,y) lz) of the reconstructed

images are 3.81 and 28.53 for Figures 6 and 7 respectively, while

for Figures 1 and 2 the RMSE are 34.76 and 49.57 respectively.

In the case of the DigiCipher and CCDC systems (Figure 1 and

Figure 2), each of the errors affects an area of up to 88 (8x8 pixels)

blocks (one macroblock), With the addition of our method the

effects of errors are limited to within 1 or 8 (8x8 pixel) block

boundaries. We observe that using our synchronization scheme

much improves the picture quality. As mentioned earlier, the

performance of HDTV picture is expected to be very good up to

a certain SNR. Below that ratio the quality of the HDTV picture

suddenly deteriorates. Performance evaluations have shown that

by using our synchronization method the SNR at which the HDTV

picture suddenly deteriorates is deferrer by approximately 3 dB.

sir0 #LO

For the proposed 1408 x 960 pixel HDTV image, and using

our method with the present sync block sizes, an extra 2.47 Mbps

is needed for the synchronization control bits. These constitute

26400 (8x8) luminance and chrominance blocks for each reference

frame, each block having 15 bits header-codeword + 2640 (32x16)

luminance blocks which include the corresponding chrominance

information, each block having 18 bits header-codewords for each

of the 27 inter-frames. If this scheme is simply added to the

compression scheme of the DigiCipher system, the total data rate

becomes 13.09+0.25+ 2.47 = 15.81 Mbps. The combination which provides synchronization at the 16x16

pixel level for the reference frames and 64x16 pixel level for the inter-frames requires 1.03 Mbps only. Let us now examine the

error protection performance of this scheme. Under the same

noise conditions as before, i.e., 15 dB SNR, Figure 8 shows the

reference frame obtained by using the 16x16 header-codewords

scheme. Figure 9 depicts the fifth frame from the reference (Figure

8) when 64x16 pixel sync blocks are used for the inter-frames.

Comparing the present synchronization method (which uses

16x16 and 64x16 pixels for the reference and inter frames respec-

tively) with that presented in Section II.A which uses the same sizes

(32x16 pixel sync blocks) for both the reference and inter frames,

we observe that the latter method does not offers better picture performance. More importantly, the present method reduces the

overhead synchronization bits added to the data stream by approx-

imately 28 % (1.03 Mbps compared to 1.42 Mbps). Table 3 illus-

trates the video data rates for the DigiCipher and the DigiCipher in

233

conjunction with different versions of our synchronization method,

as well as the corresponding total video rates obtained after the ad-

dition of the synchronization control bits. Clearly, providing better

protection to the reference frames than the inter-frames improves

the efficiency of the synchronization method. Performance evalua-

tions have shown that by using 16x16 and 64x16 pixel sizes of sync

blocks the SNR at which the HDTV picture suddenly deteriorates

is deferred by 2.5 dB.

SNR. Figures 10 and 11 show the reference and fifth frames, ob- tained using Our 8x8 and 32x16 blocks for the reference frames and

the inter-frames, respectively. In this case, the resulting transmis- sion symbol rate is 4.40 MBaud (21.98 Mbps 1 5 bits per symbol).

Figures 12 and 13 show the reference and fifth frames, respectively,

obtained by Our 16x16 and 64x16 b b ~ k s . Using these Sizes Of sync

blocks we obtain a total Symbol rate of 4-10 ".Id (20.54 M P S

/ 5) . We observe that at 15 dB SNR, the picture quality obtained

Table 3. Video data rates (Mbps) for the DigiCipher and the DigiCipber in conjunction with different version of our synchronization method

1I.C. Using Higher Order Modulation Schemes

Higher order modulation schemes have the advantage of reduc-

ing the transmission rate, i.e., the bandwidth. An increase in the

number of modulation levels from 2" to 2"+' improves the transmis-

sion symbol rate by x 100%. For example, the DigiCipher sys-

tem would require 3.90 MBaud at 32-QAM instead of 4.88 MBaud

at 16-QAM. However, by going to 32-QAM from 16-QAM the

performance of the system will deteriorate by 2.5 to 3 dB. This is

because as the order of modulation levels is increased the system

becomes more susceptible to channel errors. To maintain the same

performance at the higher modulation level as in the lower level,

the signal power must be increased. For an increase in the number

of modulation levels from 2" to 2"+l the signal power has to be in-

creased by 2.5 to 3 dB [ 171 or the same amount of dB deterioration

in the system performance is expected.

Our synchronization scheme produces a more graceful deteri-

oration and defers the SNR at which the HDTV picture suddenly

deteriorates by 2.5 to 3 dB. Thus, our scheme would allow op-

eration at a higher modulation level (2"") while maintaining the

picture performance at the 2" modulation level scheme. The cost is

the number of extra bits needed for the synchronization. This cost

, however, is still favorable as seen below.

Let us examine the picture performance of DigiCipher in con-

junction with our synchronization method, using 32-QAM at 15 dB

by our ( 16x 16164~ 16) synchronization method at 32-QAM is com-

parable to that of DigiCipher at 16-QAM. The advantage of using

our method at 32-QAM, however, is that it reduces the transmis- sion rate to 4.40 and 4.10 MBaud, down from 4.88 obtained by

DigiCipher at 16-QAM (Table 3). Thus, when our synchronization

method is used at 32-QAM, it improves the overall system com-

pression while it still provides more graceful picture deterioration

than DigiCipher at 16-QAM. For comparison reasons, Figures 13

and 14 show the reference and fifth frame obtained by DigiCipher

at 32-QAM and 15 dB SNR. As expected, the DigiCipher system

is unable to handle the increased number of errors, resulting in a

complete loss of the picture.

111. CONCLUSIONS

We presented a synchronization method which increases the

error resistance of a full-motion digital system by restricting the

effects of bit errors to block levels significantly smaller than those

used by the HDTV systems. A trade-off between the error protec-

tion and the overhead used is obtained by using a relatively small

synchronization block size for the reference frames and a large

size block for the inter-frames. For each block, synchronization is

achieved by transmitting an error protected header-codeword which

contains the number of data in the block. This method improves

the quality of the picture in the presence of errors and defers the

234

SNR at which the HDTV picture suddenly deteriorates by 2.5 to 3

dB. Thus, it has special advantage when higher order modulation

schemes are used.

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Figure 1. Reference image obtained using DigiCiphedCCDC macroblock-pointer method at channel SNR = IS dB (RMSE = 39.21)

235

Figure 2. Fifth frame from the reference frame (Figure 1)

using DigiCipherKCDC at 15 dB channel SNR (RMSE = 69.22).

Figure 6. Reference image obtained using our synchronization scheme with 8x8 pixel sync blocks at channel SNR = 15 dB (RMSE = 3.81).

Figure 4. Image obtained using our 32x16 superblock header-ccdewords at 15 dB SNR channel noise (RMSE = 12.01).

Figure 7. Fifth frame from the reference frame (Figure 6) obtained using the

32x16 inter-frames sync blocks at 15 dB channel SNR (RMSE = 28.53).

Figure 8. Reference image obtained using Digicipher and our 16x16 header-ccdewords scheme at channel SNR = I5 dE3 (RMSE = 9.11).

Figure 5 . Fifth frame from the reference frame (Figure 4) obtained using the 32x16 superblock header-ccdewords at 15 dB SNR channel noise (RMSE = 30.72).

Figure 9. Fifth frame from the reference frame (Figure 8) obtained using 64x16 inter-frames sync blocks at 15 dEl channel SNR (RMSE = 30.01).

Figure 12. Same as Figure 8 but at 32-QAM instead of I W A M (RMSE = 24.68).

) _ _ -

Figure 10. Same as Figure 6 but at 32-QAM instead of l6-QAM (RMSE = 10.09). Figure 13. Same as Figure 9 but at 32-QAM instead of laQAM (RMSE = 49.74).

Figure 1 1 . Same as Figure 7 but at 32-QAM instead of 16-QAM (RMSE = 40.01). Figure 14. Reference image obtained by DigiCipher at 32-QAM and I5 dB SNR.

237

Figure 15. Fifth frame from the reference frame (Figure 14) obtained by DigiCipher at 32-QAM and 15 dB SNR

Panos Nasiopoulos was bom in Greece, in 1956. He received his B.Sc. degree in physics from the University of Thessalon- iki. Greece. in 1980, and the B.A.Sc. and M.A.Sc. degrees in electrical engineering from the University of British Columbia, Canada, in 1985 and 1988, respectively.

From 1988 to summer 1990 he was with the Computer Science department of Lan- gara College, Vancouver, Canada. In Sep- tember 1990, he joined the department of Electrical Engineering at the University of British Columbia, Canada, as a Sessional

Lecturer. He received his Ph.D. degree from the same department, in May 1994. Currently, he is an Assistant Professor in the Electrical Engineering department at the University of British Columbia.

His current research activities focus on the development of digital video coding, synchronization, and transmission schemes for full-motion multi- media applications, including HDTV, digital TV. video telephony, and tele- conferencing.

Rabab Kriedieh Ward was bom in Beirut, Lebanon. She received the B. Eng. degree from the University of Cairo, Egypt and her Masters and Ph.D. degrees from the University of California, Berkley. She is a professor in the Electrical Engineering department at the University of British Columbia and a member of the Centre for Integrated Computer Systems Research there. Her research is mainly in the area of digital image processing including detec- tion, recognition, encoding, restoration and enhancement and their applications to cable TV, HDTV, medical images and

astronomical images. She holds 3 patents related to the cable television pic- ture monitoring, measurement and noise reduction.