fig1 shows the spectrum of a pal video signal which is frequency - modulated on a carrier wave of 7 . 1 mhz ( frequency in the case of modulation by the black level of the video signal ), where j 0 is the principal component representing the frequency as a function of the amplitude of the video signal , j 1 is the first - order lower side - band , also referred to as chroma band , which is situated at 4 . 43 mhz from the principal component j 0 , 4 . 43 mhz being the frequency of the chrominance carrier in the video signal , and j 2 is the second - order sideband which is mirror - inverted relative to the frequency zero - point . in the known apparatus , two fm - modulated audio carrier waves of 0 . 683 and 1 . 066 mhz are added to this signal . fig2 shows the spectrum of an ntsc - video signal which is frequency - modulated on an 8 - mhz carrier wave ( frequency in the case of modulation by the black level of the video signal ). since in an ntsc - signal the chrominance carrier wave has a frequency of 3 . 58 mhz , the lower sidebands j 1 and j 2 are now spaced at distances of 3 . 58 and 7 . 16 mhz , respectively . in the known apparatus and audio signals are added as fm - carrier waves at 2 . 3 and 2 . 8 mhz . fig3 shows the results of a measurement of the effect of a pulse - width modulation of the carrier wave which is frequency - modulated by the video signal ( in the present case pal ), on the picture quality . the relative amplitude is ploted of the signals with which said carrier wave is pulse - width modulated , the influence on the picture being shown as a function of the frequency . the graph shows that the influence remains fairly constant up to approximately 1 . 5 mhz and increases above this frequency . the graph illustrates that with a maximum signal level , pulse - width modulation may be applied when the signal is passed through a low - pass filter with a cut - off frequency in the range from 1 . 5 to 2 mhz . in optical recording , the signal - to - noise ratio at low frequencies deteriorates as a result of the interference produced by the he - ne laser . since for the coding in according with the invention a comparatively weak signal strength of the digital audio signal is desirable , it is advantageous to emphasize the signal strength at low frequencies relative to the signal strength at higher frequencies . a suitable cut - off frequency is situated in the range from 100 khz - 1 mhz , in particular at 500 khz , because at 500 khz the efm - spectrum exhibits a maximum and rolls off below this frequency . fig4 shows the frequency diagram of a suitable low - frequency pre - emphasis filter . the cut - off frequency is situated at 500 khz . below this frequency the signal strength is boosted by 5 db / octave , which is easy to achieve . the cross - over frequency at which the characteristic becomes flat again ( in the present example at approximately 32 khz ), is determined by the pre - emphasis level for low frequencies , which level is dictated by the visibility limit ( fig3 ) and its possible influence on control systems , such as the radial tracking . in the present example , the pre - emphasis is approximately 23 db . fig5 shows the resulting efm - spectrum after low - pass filtering with a cut - off frequency of 1 . 75 mhz and after the passage through a low - frequency pre - emphasis filter having a characteristic as shown in fig4 . this spectrum increases gradually with approximately 23 db to 500 khz , after which the low - frequency pre - emphasis filter ensures that the spectrum remains substantially constant down to very low frequencies . the spectrum thus obtained can be accommodated in the frequency band below the j 1 component ( fig1 and 2 ) without any significant problems , while in the case of an ntsc - signal , the two audio fm - carriers can be maintained , so that it is possible to manufacture a record carrier with both digital audio and fm - audio , which can read on both types of players . a broken line in fig4 denotes a roll - off for frequencies lower than , for example 5 khz , which may be useful in order to preclude a disturbance of the various low - frequency control systems . fig6 shows an apparatus for encoding a recording signal in accordance with the invention . it comprises an input 1 for receiving a composite video signal . by means of a sync - separator 2 , a signal of a frequency ( f h ) equal to the frequency for the horizontal synchronizing signals in the composite video signal , is extracted , which is applied to a frequency generator 3 which comprises , for example , a phase - locked loop . this frequency generator 3 supplies a signal of a frequency f 1 for the processing of the digital audio signal ; in the present example , a signal of a frequency of 4 . 2336 mhz (= 96 × 44 . 1 khz ; 44 . 1 khz being the sampling frequency of the analog audio signal ). further , the composite video signal is applied to an fm - demodulator 4 , for example an oscillator which is voltage - controlled by the video signal . subsequently , the fm - signal is applied to a device 9 which in a manner known per se records this fm - signal on a record carrier , in particular a video disc , by means of a laser . the analog audio signal is applied to the signal processing unit 6 , which inter alia comprises an analog - to - digital converter and an efm - modulator , which may be of a type as described in the article in &# 34 ; philips technical review &# 34 ; mentioned in the introduction . this processing unit receives the signal of the frequency f 1 = 4 . 2336 mhz from the generator 3 for synchronizing the signal processing . the efm - modulated signal is subsequently passed through a low - pass filter 7 with a cut - off frequency of 1 . 75 mhz and a low - frequency pre - emphasis filter 8 with a characteristic as shown in fig4 and is then applied to a pulse - width control input of the fm - modulator 4 . in its simplest form , this pulse - width control is obtained by addition prior to limiting , which will be explained with reference to fig8 . in fig8 a shows a signal , in the present example a sinewave signal , representing the fm - modulated composite video signal ; b shows a digital signal of a frequency lower than the frequency of the video signal a and also having a lower amplitude ; and c shows the sum of these two signals . if after suitable amplification this signal is limited , a signal as shown in d is obtained . this signal d is a square - wave with a repetition rate corresponding to the instantaneous frequency of the fm - modulated video signal and a relative pulse - width corresponding to the digital signal . if this signal d controls a laser which inscribes an optically readable disc 10 , a pattern as shown in e is obtained with a track 13 of optically detectable areas 11 of a length corresponding to the pulses in the signal d and with intermediate areas 12 . in order to compensate for the second - order lower side - band j 2 , a compensation circuit 10 may be added to the pulse - width modulator , for example as described in u . s . pat . no . 4 , 223 , 282 . fig7 shows an embodiment of an apparatus for decoding a signal encoded by means of the apparatus shown in fig6 . this apparatus comprises a player 17 which reads the record carrier . the output signal is applied to a low - pass filter 15 which functions as pulse - width demodulator . indeed , the signal a shown in fig8 can be filtered out of the signal d by means of a low - pass filter having a cut - off frequency situated between the frequency bands occupied by these signals . the digital audio signal thus obtained is passed through a high - frequency pre - emphasis filter 16 which is complementary to the low - frequency pre - emphasis filter 8 ( fig6 ) and subsequently it is converted into an analog signal by means of a circuit 18 comprising an efm - demodulator and a digital - to - analog converter , the signal processing being synchronized by a reference frequency f 1 . the circuit 18 may be as described in the aforementioned issue of &# 34 ; philips technical review &# 34 ; and may be equppied with ic &# 39 ; s which are commercially available under type numbers saaa 7000 , saa 7010 , saa 7020 , saa 7030 , mab 8410 , tda 1540 and random access memory 6116 . the analog audio signal ( in stereo version ) is then available on output 19 . the output signal of the player 17 is also applied to an fm - demodulator 19 whose output signal , via a sync separator circuit 20 , becomes available on an output 21 . the extracted synchronizing signal , the horizontal synchronization of the frequency f h , is applied to a motor control circuit 22 , which controls the speed of the player 17 in such a way that the frequency f h corresponds to a reference frequency f 2 within narrow limits . the reference frequencies f 1 and f 2 may in principle be supplied by two separate generators . however , this is problematic because stringent requirements are imposed on the stability and mutual relationship of these two reference frequencies . one solution is to derive the two frequencies from one reference oscillator 23 by frequency division , if this is possible , so that phase - locked loops , which are not stable enough , are no longer necessary . in the embodiment shown in fig7 the oscillator 23 is tuned directly to the frequency f 1 and f 2 is obtained by dividing the frequency f 1 , by means of a divider stage 24 . for pal the frequency f 2 must be 15 . 625 khz , while f 1 , when the said ic &# 39 ; s are used , must be equal to 4 . 2336 mhz ( 96 times the sampling frequency of 44 . 1 khz ) within specific tolerances . it has been found that this is possible by tuning the oscillator to 4 . 2342375 mhz and dividing it by 271 , so that precisely the correct value is found for f 2 and f 1 exhibits a deviation of 2 × 10 - 4 , which is acceptable . acceptable values for ntsc are obtained by means of an oscillator 23 which is tuned to 4 . 232518 mhz and by the use of a divider 24 which divides by 269 . the deviation of the frequency f 1 is then 10 - 4 . however , the slight deviations of these frequencies , may lead to a time error in the decoding of the audio signal . indeed , if the digital signal is decoded with a different frequency than the frequency with which it has been encoded , a time error arises which accumulates in reproduction . this may be compensated for by introducing a similar time error during encoding , for example by means of a sampling - frequency conversion .