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Digital Aaudio Modulation in the PAL AND NTSC Optical Video Disk Coding Formats.

Authors:

Abstract

An extension of the current optical video disk format that includes a digital audio signal is presented. The feasibility of a combined digital audio signal according to the Compact Disc digital audio format and the current analog audio signals in the NTSC video format is described, which permits the realization of a compatible system. For the PAL and SECAM video formats the feasibility of digital audio is shown, but it cannot be combined with the analog audio carriers.
ENGINEERING REPORTS
Digital AudioModulation in the PALand NTSCOptical
Video Disk Coding Formats*
KEES A. SCHOUHAMER IMMINK, AD H. HOOGENDIJK, AND JOOST A.KAHLMAN
Philips Research Laboratories, 5600 JA,Eindhoven, The Netherlands
An extension of the current optical video disk format that includes a digital audio
signal is presented. The feasibility of a combined digital audio signal according to the
Compact Disc digital audio format and the current analog audio signals in the NTSC
video format is described, which permits the realization of a compatible system. For
the PAL and SECAM video formats the feasibility of digital audio is shown, but it
cannot be combined with the analog audio carriers.
0INTRODUCTIONan important role in the overall design. A low modu-
lation index of the digital audio signal results in a poor
In the current optical video disk coding formats signal-to-noise ratio, giving rise to a high bit error
(NTSC,PAL,and SECAM) the analog audio stereo rate. Many experiments have been done to arrive at a
channels are frequency-modulated and added by means compromise on these conflicting parameters.
of pulse-width modulation of the frequency-modulated In Section 1 we briefly describe the optical video
video signal [1]-[3]. formats and derive requirements which a digital audio
The maximum audio signal-to-noise ratio of the Dp- modulation system should meet. We describe the sim-
tical 525-1ine NTSC format at present attainable is ulations used to find quantitatively the constraints for
aproximately 70 dB, which includes a 15-dB improve- the digital audio modulation within the optical coding
ment by the CX noise reduction system [3]. In the 625- format. In Section 2 we describe the experimental results
line PAL and SECAM formats of the optical disk the obtained with actual disks.
audio signal is approximately I0 dB superior to that
in the NTSC optical format. IREQUIREMENTS TOBEMETBY DIGITAL
An improvement in audio quality seems possible only AUDIO MODULATION SYSTEMS IN THE
by going digital. For example, the 16-bit linear quan- OPTICALFORMATS
tization used in the Compact Disc digital audio system
format [4], [5] achieves a signal-to-noise ratio of 96 In this section we describe the optical coding formats
dB. Furthermore the powerful error-correction system and give the particular requirements to be met by a
of this digital format has a beneficial influence on the digital audio modulation system in order that it can be
effect of dropouts. In this paper we report on experi- added to the current optical formats.
ments and simulations to show the feasibility of adding
a digital audio signal according to the Compact Disc 1.1 Descriptionof theOptical Coding Formats
digital audio format in the current optical video disk The signal format of current NTSC and PAL optical
formats, disk is a two-levelsignal (high-frequency),whichis
The bandwidth of the digital audio signal spectrum frequency-modulated, after preemphasis, by the corn-
(approximately 1.5 MHz) and its modulation index on
the main carrier are of great importance. The disturbance posite (luminance and chroma) video signal. Addition
of the stereo sound signal is achieved by means of
of the digital audio signal in the video picture plays pulse-width (duty-cycle) modulation of the high-fre-
* Presented at the 74th Convention of the Audio Engineering quency signal by the two frequency-modulated audio
Society, NewYork, 1983 October 8-12. carriers. Fig. 1 shows a block diagram of the signal
J.AudioEng.Soc.,Vol.32,No.11,1984November 883
IMMINKETAL. ENGINEERINGREPORTS
path of the encoder. The signal Xo(t) is the frequency- J2, found in this low-end frequency range, has to be
modulated composite video signal. Signals xl(t) and removed. An example of an embodiment of such a
x2(t) are the frequency-modulated sound signals. The system is given in [7]. The method (Fig. 4) is basically
sum signal is limited, so that a pulse-width- and fre- a compensation method. The chroma band is filtered
quency-modulated two-level signal y(t) results. Fig. 2 out from the composite video signal. By means of a
depicts the principle of pulse-width modulation; x(t) squarer circuit and bandpass filtering the second har-
is the input signal to the limiter, y(t) is the resulting monic is generated. This frequency-doubled chroma
two-level output signal, signal is now added with the correct phase and amplitude
Fig. 3(b) shows the spectrum of a PAL video signal to the original composite video signal. After frequency
which is frequency-modulated on a carrier of 7.1 MHz modulation the chroma./2 component will be canceled.
(that is, the frequency corresponding to the black level
of the video signal), where Jo is the principal component 1.2 Picture Quality
representing the frequency as a function of the amplitude To find the effect of a pulse-width modulation of the
of the video signal. J1 is the first-order lower sideband, carrier on the picture quality we designed the experi-
also referred to as chroma band, which is situated at mental setup shown in Fig. 5. The video modulator
4.43 MHz from the principal component J0, 4.43 MHz supplies a composite PAL video signal to the frequency
being the frequency of the chrominance carrier in the modulator. The sine wave from the signal generator is
PAL video signal, and J2is the second-order sideband fed to the pulse-width input of the modulator. The fre-
which is mirror-inverted [6[ relative to the frequency
quency- and pulse-width-modulated carrier passes ,a
zero point. In the optical PAL format two frequency- circuit which simulates the frequency rolloff of the
modulated audio carriers of 0.683 and 1.066 MHz are
optical readout system. The video signal can be studied
added to this signal. The amplitude of the sound carriers with a video demodulator and a video monitor. An
is chosen as -26 dB relative to the main carrier.
Fig. 3(a) shows the spectrum of an NTSC video signal informal panel examined the effect of the pulse-width
which is frequency-modulated on an 8.1-MHz carrier, modulation on the picture quality as a function of the
Since the chrominance carrier in an NTSC signal has amplitude and frequency of the signal. We determined
the pulse-width level at which it became just visible
a frequency of 3.58 MHz, the lower sidebands J1 and
J2 are now spaced at distances of 3.58 and 7.16 MHz, in the video picture. To achieve the maximum visibility
respectively, in the NTSC video format the audio signals of the disturbance of the pulse-width modulation, the
are added as frequency-modulated carriers at 2.3 and frequencies of the generator were chosen at even mul-
tiples of the line frequency (15 625 Hz for PAL). The
2.8 MHz. The basic idea of the addition of digital
audio, which will be explained in detail in the following, disturbance in the video signal is caused by the second-
is to use the low-end frequency range up to 1.75 MHz. order sideband of the pulse-width modulation.
We may therefore conclude that, unlike in the case of
PAL, the analog carriers can even remain in the case sy_Jo
of NTSC, so that compatibility with analog sound is de 0
possible. -20 A,A2
The interference caused by the second-order sideband ?.L___,[
-/.c
-60
_--' ', ', ,_--,L_j.!3dL-t ,
Ii B i i i I I I 1 i i i
, B _ i i i ti i i i _ i I i
'_.i-Trnj-L.FLlnj yin'o , 2 ,. s :, ,,9 ;o
------.- freq. MHz
Fig. 1. Generationof modulatorsignal. The sumsignal is (a)
limited so that a pulse-width- and frequency-modulated two-
s_c Jo
level signal results, de 0
video FM _ -z,0 J[_-'_
-60
-_ _ _ -80
limiter e,
sound 2 3 /. S 6 7 8 9 10
freq.MHz
x2(t)(b)
Fig. 2. Principle of pulse-width modulation, x(t)--input Fig. 3. Frequency spectra of optical video formats. (a) NTSC.
signal to limiter; y(t)--resultiug two-level output signal. (b) PAL.
884 J.AudioEng.Soc.,Vol.32,No.11,1984November
ENGINEERING REPORTS DIGITAL AUDIO MODULATION IN OPTICAL DISK FORMATS
Fig. 6 gives the maximum pulse-width level that can this has the advantage of enabling current equipment
be allowed as a function of the generator frequency, to be used for encoding and decoding the digital audio
The graph shows that the influence remains fairly con- signal. Accordingly the digital audio signal is CIRC
stunt up to approximately 1.5 MHz and increases rapidly encoded and EFM modulated, and the subcode generator
above this frequency, due to direct interference in the supplied the additional information. The two-level
video frequency-modulated spectrum. The vertical ampli- output of the EFM modulator is low-pass-filtered with
tude axis is given relative to the amplitude of the main a cut-off frequency of approximately 1.75 MHz. After
carrier, thelow-frequencypreemphasisthe signalis appliedto
Fig. 6 illustrates that with a maximum signal level the pulse-width input of the video modulator. Fig. 8
pulse-width modulation may be applied when the signal holds functionally for both the NTSC and the PAL cases,
is passed through a low-pass filter with a cut-off fre- with, of course, some altered parameters. Note, how-
quency in the range of 1.5-2 MHz. ever, that in the PAL video format the analog audio
carriers have to be removed. The level of the digital
1.3 Interference and Noise audio signal with respect to the main carrier is ap-
In optical recording the signal-to-noise ratio at low proximately -22 dB. Fig. 9 depicts the resulting spectra
in both the NTSC and the PAL formats.
frequencies (<500 kHz) deteriorates as a result of the Fig. 10 shows the block diagram of the decoder. The
interference produced by the He-Ne laser which is used
to read out the video disk. Since a comparatively weak digital audio signal can be reconstructed easily by
deemphasis and low-pass filtering. The low-pass filter
signal strength is desirable for the coding of the digital
audio signal (Fig. 6), it is advantageous to boost the cut-off frequency is 1.75 MHz. After this filtering the
signal is passed to a normal Compact Disc decoder
signal strength at low frequencies relative to the signal which eventually supplies the audio signal. The video
strength at higher frequencies. A suitable cut-off fre- decoder needs no functional changes with respect to
quency is situated in the range of 100 kHz to 1 MHz, the current one.
in particular at 500 kHz, because at approximately 500
kHz the eight-to-fourteen modulation (EFM) spectrum 2 EXPERIMENTS
(EFM is the modulation system used in the Compact
Disc) exhibits a maximum and rolls off below this fre- The digital audio signal can be disturbed by different
quency [8]-[10]. Fig. 7 shows the frequency diagram sources. These disturbances may originate from:
of a suitable low-frequency preemphasis filter. The roll- 1) Irregularities on the video disk surface
off frequency is situated at 500 kHz. Below this fre- 2) Birefringence of the disk substrate
quency the signal is boosted by 6 dB per octave, which 3) Crosstalk.
is easy to achieve. The crossover frequency at which We recorded many experimental disks (NTSC and
the characteristic becomes flat again (in the present PAL) to measure the effect of these sources of inter-
example 30 kHz) is determined by the visibility limit ference.
(Fig. 6) and its possible influence on control systems
such as the radial tracking. During our experiments we 2.1 Irregularities on the Video Disk Surface
found a preemphasis of 23 dB an optimal choice.
EFM test sequences (pseudo music) written directly
on the disk as a pulse-width modulation of the pits,
1.4 Complete Diagram of Encoder and Decoder
with levels varying from - 30 to - 20 dB with respect
After the preliminaries of the preceding sections it to the main carrier, are hardly detectable with sufficient
is now quite easy to draw the block diagram of the bit error rate without preemphasis. The low-frequency
complete system. Fig. 8 shows the diagram of the eom-disturbances are due to scratches and rapid fluctuations
bined video and digital audio encoder. We decided to of the reflection coefficient of the video disk. After
use the Compact Disc encoder as the line encoder. In preemphasis as in Fig. 7, we found bit error rates in
other words the bit stream supplied to the pulse-width the range of 10-5-10 -6 without the EFM signal being
modulation input is bit to bit compatible with the normal visible in the video picture.
Compact Disc modulation bit stream. It is clear that
2.2 Birefringence of the Disk Substrate
5MHz w$ 2w$ AA A source of interference with the EFM signal is the
v.o
comp. to opticolbirefringence of the video disk when a He-Ne laser is
video moduIotor
gen_ t
cD
encoder
Fig. 4. Example of compensationof second-ordersideband Fig.5.Block diagramofexperimentalsetuptostudyinfluence
J2. of pulse-width modulation on picture quality.
J. Audio Eng. Soc., Vol. 32,No. 11,1984 November885
IMMINK ET AL. ENGINEERINGREPORTS
dB]'0i
_20 ¸
o o
o o
oooo
o o
o
-40
o
o o
-60
_ I I I
0.5 .1 1.5 2' )
NHz
Fig. 6. Relative amplitude of sinusoidal pulse-width modulation when it becomes just visible in picture. Video decoder is
unmodified model LV 720, Mk-1 PAL decoder.
3o
i/ / ,t/ /I'll
10 "// _ t
/0 sync J0
dB digital J1
0''I 20L audioA1 A2 I_
l
lk 10k 32k10ak 500k 1M ._' 40-
freq
Fig. 7. Amplitude Bode diagram of preemphasis filter. 60-
80
t
1 2 3 4 5 6 7 8 9 10
freq MHz
Composite
igital J_
IEnc°deq I Filter I I empl' 2 3 s 6 78' lb
Fig. 8. Block diagram of combined video anddigital audio = freq. MHz
encoder. The,block diagram holds for PAL and NTSC video
formats with some altered parameters. Note, however, that (b)
the analog audio carriers have to be removed in the PAL case.
The video frequency modulator is extended using J2 corn- Fig. 9. Spectra of combined digital audio and video formats.
pensation. (a)NTSC.(b)PAL.
886 J. AudioEng. Soc., Vol. 32, No. 11, 1984 November
ENGINEERINGREPORTS DIGITALAUDIOMODULATIONINOPTICALDISKFORMATS
_M directions,of upto 1° we wereunableto measureany
TF
_FM_mposite increase of the bit error rates of the EFM test sequences
Video (written at levels varying from -26 to -20 dB).
co.c.us,o.s
[ M_rF Thecurrentopticalvideodisksystemcanbecombined
with digital audio according to the Compact Disc audio
t = Audio_ system, fulfilling the requirements that (1) the digital
audio signal be not visible in the video picture, and
_...LP_e 7 Digital AudioI (2) the digital audio signal can be read out well within
' I _,,ter I I emph.I O[....e_e_Audio]X the limits of the CIRC error-correction system, even
with disturbing influences originating from imperfec-
Fig. 10. Block diagram of decoder. The EFM signal is simply tions of the disk and/or the readout system.
reconstructed by deemphasis and low-pass filtering. Contrary to the case with PAL, the analog carriers
can even remain in the case of NTSC, so that compat-
ibility with analog sound is possible.
The digital audio signal can be reconstructed easily
used for readout. The birefringence of the measured by filtering and decoding by the EFM and CIRC decoder.
disks reached up to 25°, resulting in a certain fraction 4 REFERENCES
of light intensity being reflected by the disk, but not
reflected to the signal detector by the polarizing beam [1] P. B6gels, "System Coding Parameters, Me-
splitter and hence fed back toward the laser. The fre- chanics and Electromechanics of the Reflective Video
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Optical Videodisk System," Proc.4th Int.Conf.on
2sn Video and Data Recording (Southampton, 1982), pp.
f_ 351-365; see also IEC Draft UAR-1605-223/8304.
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of the Laser Video Disk," presented at the 72nd Con-
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wavelength of the He-Ne laser. At s= 2 mm, h=Eng.Soc.(Abstracts), vol. 30, p. 952 (1982 Dec.),
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harmonics. In practical situations nonflat disks can give Hoeve, J. Timmermans, L. M. Driessen, T. T. Doi,
K. Odaka, S. Furukawa, I. K. Iwamoto, Y. Sako, H.
amplitude modulations of the light output of the He-
Agawa, and T. Itoh, "The Digital Compact Disk:
Ne laser at frequencies up to 500 kHz. Modulation and Error-correction Schemes," presented
With deliberately warped test disks we found am- at the 67th Convention of the Audio Engineering So-
plitude variations at frequencies up to 350 kHz. We ciety, J.Audio Eng.Soc.(Abstracts), vol. 28, p. 931
measured the maximum tolerable birefringence of the (1980 Dec.), preprint 1674.
disk substrate, where the digital audio signal is just [5] M. G. Carasso, J. B. H. Peek, and J. P. Sinjou,
disturbed (the appearance of interpolations and mutes "The Compact Disc Digital Audio System," Philips
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rotation of the quarter-wave plate in the video disk Philips Res.Reps., vol. 32, pp. 436-459 (1977).
[7] C. H. Coleman, "Moir6 Interference Reducing
player around the optical axis. Our measurements
Circuit for FM Video Recorders," U.S. patent
showedthat the bit error rates of the EFM pseudo music 4,052,740 (1977).
signal, at a level of - 26 dB, are not seriously perturbed [8] J. P. J. Heemskerk and K. A. Schouhamer Im-
by the birefringence of the disk up to 30°. mink, "Compact Disc: System Aspects and Modula-
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Another possible source of interference with the EFM
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pp. 63-66 (1983).
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optical axis of the readout objective lens in the video "EFM--The Modulation for the Compact Disc Digital
disk player. At the nominal track pitch of 1.67 gm and Audio System," DigitalAudio, Collected Papers Pre-
at skew angles, in the radial as well in the tangential mier AES Conf. (Rye, NY, 1982), pp. 117-124.
J. Audio Eng. Soc., Vol. 32, No. 11, 1984 November 887
IMMINKETAL. ENGINEERINGREPORTS
THE AUTHORS
J. Kahlman
Joost Kahlman was born in Tilburg, The Netherlands, Mr.Immink's biography appeared in the July/Aug.
in 1956. In 1979 he received a B.Sc. degree in elec- issue.
tronics in Eindhoven. He joined the Philips Research Mr. Hoogendijk's biography was not available at
Laboratories in 1980 after serving in the Dutch Army. press time.
888 J. AudioEng.Soc.,Vol.32,No. 11,1984November
Article
Full-text available
A description is given of the eight to fourteen modulation system (EFM) designed for the Compact Disc Digital Audio System with optical read-out. EFM combines high information density and immunity to tolerances in the light path with low power at the low-frequency end of the modulation bit stream spectrum. In this modulation scheme, blocks of eight data input bits are transformed into fourteen channel bits, which follow certain minimum and maximum run-length constraints by using a code book. To prevent violation of the minimum and maximum run-length constraints a certain number of merging bits are needed to concatenate the blocks. There are cases where the merging bits are not uniquely determined by the concatenation rules. This freedom of choice thus created is used for minimizing the power of the modulated bit sequence at low frequencies. The paper presents the results of algorithms that were used to minimize this low-frequency content.
Article
Digital processing of the audio signal and optical scanning in the Compact Disc system yield significant advantages: insensitivity to surface damage of the disk, compactness of disk and player, exellent signal-to-noise ratio and channel separation (both 90 db) and a flat response over a wide range of frequencies (up to 20,000 Hz). The Compact Disc, with a diameter of only 120 mm, gives a continuous playing time of an hour or more. The analog audio signal is converted into a digital signal suitable for transcription on the disk. After the digital signal has been read from the disk by an optical 'pick-up' the original audio signal is recreated in the player.
presented at the 72nd Conwhere s is the total stroke of the nonflat disk and h the vention of the Audio Engineering Society, J. Audio wavelength of the He-Ne laser. At s = 2 mm
  • G Badger
  • R G Allen
G. Badger and R. G. Allen, "The Audio Side of the Laser Video Disk," presented at the 72nd Conwhere s is the total stroke of the nonflat disk and h the vention of the Audio Engineering Society, J. Audio wavelength of the He-Ne laser. At s = 2 mm, h = Eng. Soc. (Abstracts), vol. 30, p. 952 (1982 Dec.),
The Digital Compact Disk: Ne laser at frequencies up to 500 kHz. Modulation and Error-correction Schemes," presented With deliberately warped test disks we found amat the 67th Convention of the Audio Engineering Soplitude variations at frequencies up to 350 kHz. We ciety
  • L B Vries
  • K A Immink
  • J G Nijboer
  • H Timmermans
  • L M Driessen
  • T T Doi
  • K Odaka
  • S Furukawa
  • I K Iwamoto
  • Y Sako
L. B. Vries, K. A. Immink, J. G. Nijboer, H. harmonics. In practical situations nonflat disks can give Hoeve, J. Timmermans, L. M. Driessen, T. T. Doi, K. Odaka, S. Furukawa, I. K. Iwamoto, Y. Sako, H. amplitude modulations of the light output of the He-Agawa, and T. Itoh, "The Digital Compact Disk: Ne laser at frequencies up to 500 kHz. Modulation and Error-correction Schemes," presented With deliberately warped test disks we found amat the 67th Convention of the Audio Engineering Soplitude variations at frequencies up to 350 kHz. We ciety, J. Audio Eng. Soc. (Abstracts), vol. 28, p. 931 measured the maximum tolerable birefringence of the (1980 Dec.), preprint 1674. disk substrate, where the digital audio signal is just
light fed back to the laser. This can be obtained by a rotation of the quarter-wave plate in the video disk
  • M R De Haan
  • C H F Velzel
M. R. de Haan and C. H. F. Velzel, "Intermodulation and Moir6 Effects on Optical Video Recording," light fed back to the laser. This can be obtained by a rotation of the quarter-wave plate in the video disk Philips Res. Reps., vol. 32, pp. 436-459 (1977).
patent showedthat the bit error rates of the EFM pseudo music 4
  • C H Coleman
C. H. Coleman, "Moir6 Interference Reducing player around the optical axis. Our measurements Circuit for FM Video Recorders," U.S. patent showedthat the bit error rates of the EFM pseudo music 4,052,740 (1977).
Schouhamer Imby the birefringence of the disk up to 30°. mink
  • J P J Heemskerk
J. P. J. Heemskerk and K. A. Schouhamer Imby the birefringence of the disk up to 30°. mink, "Compact Disc: System Aspects and Modulation,'' Philips Tech. Rev., vol. 40, pp. 157-164 (1982).
Schouhamer Immink, optical axis of the readout objective lens in the video "EFM--The Modulation for the Compact Disc Digital disk player
  • H Ogawa
substrate relative to the perpendicular position of the [10] H. Ogawa and K. A. Schouhamer Immink, optical axis of the readout objective lens in the video "EFM--The Modulation for the Compact Disc Digital disk player. At the nominal track pitch of 1.67 gm and Audio System," DigitalAudio, Collected Papers Preat skew angles, in the radial as well in the tangential mier AES Conf. (Rye, NY, 1982), pp. 117-124.