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application note aso plus / tea5750 AN551/0593 by jean-yves couet summary page i. introduction .................................... ................... 2 ii. general ............................................................ 2 iii. aso plus principle ................................................. 3 iii.1. active phase shifting . .......................... ................... 3 iii.2. high immunity to tearing effects . ................................. 5 iii.3. monitoring the overall quality of the fm playback signal . . . . .... 7 iv. tea5750 functional description .................................... 8 iv.1. biasing . . . . . . . ............. ......................................... 9 iv.2. phase shifting circuit. . . . . . . . . . . . . . . . . ............................. 9 iv.3. error detector, error memory, softswitch. . . . . . . . . . . . . . . . . . . . . . . 10 v. implementation of the tea5750 in a vhs-vtr application ............ 12 v.1. typical application description. . . . . . . ............................. 12 v.1.a. input filter . . . . . . . . . . . . ............................. ................... 12 v.1.b. aso plus main filter. . . . ............................. ................... 14 v.1.c. permanent correcting filter . . . . . . . . . . . . . . . . . ............................. 16 v.1.d. output amplifier feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................... 16 v.1.e. error memory filter . . . . . . . . . . . . . . ....................... ................ 17 v.1.f. video bandwidth correction . . . . . . . ....................................... 17 v.1.g. error detector adjustment. . . . . . . . . ....................................... 17 v.1.h. input stage gain control. . . . . . . . . . . . . . . . . . . . ............................. 18 v.1.i. other components . . . . . . . . . . . . . . ....................................... 18 v.1.k. typical waveforms and results . . . . ....................... ................ 18 v.2. interfacing with the head amplifier part . . . ....................... 21 v.3. interfacing with the fm demodulator . . . . . . . . . .................... 21 v.4. application circuit options . ....................................... 22 v.5. using the tea5750 without the error detector . . . . . . . . . . . . . ....... 23 v.6. aso plus implementation : problems and solutions . . . . . . . . . ....... 23 v.6.a. ringing effect after luma transients . . . . . . . . . . . . . . . . ....................... 23 v.6.b. unstable picture sharpness . . . . . . . . . . . . . . . . . ............................. 23 v.6.c. pumping effect and long settling times . . . . . . . . . . . . . . . . . ................... 24 v.6.d. high influence of the playback signal level on the video bandwidth . . . . . . . ....... 24 vi. comments and conclusion ......................................... 25 vii. references ..................................... ................... 25 1/25
i - introduction the purpose of this note is to present the tea5750 integrated circuit which implements the aso plus technology (aso : active sideband optimization). this technique does not only intend to replace the fm equalizer circuit (commonly used between the head amplifier and the fm demodulator) but also to bring the following advantagesand functions : - picture sharpness improvement : frequency re- sponse as wide as 3mhz can be realized in vhs standard. - decrease of the sensitivity to the tearing effect (black fishes) - monitoring of the overall quality of the fm play- back signal. in doing so, the tea5750 automat- ically and continuously adapts the frequency spectrum of the fm signal delivered to the de- modulator in such a way that the s/n ratio is improved. this function is particularly active in case of old and aged tapes. ii - general in video recording systems where the luma signal is carried by frequency modulated signal ; a high picture sharpness is performed when the fm play- back signal features fast frequency changes. in practice, the picture sharpness is far lower than expected because the necessary fast frequency transients are slowed down by the limited band- width of the fm playback front end (tape, heads, rotary transformers, h-amps). thus it is necessary to process the fm playback signal before demodu- lation. so far the commonly used solution is the fm equal- izer circuitry. the main purpose of this solution is to improve the balance between the sidebands (upper and lower) and the fm carrier. this tech- nique leads to a picture quality improvement but its efficiency is greatly limited by the following compro- mise : higher picture sharpness against sensitivity to noise and oblack fisheso (tearing effect) : - amplifying the upper sideband improves the pic- ture sharpness but decreases the s/n ratio (s/n ratio of tapes greatly decreases for high frequen- cies). - amplifying the lower side bands improves the picture sharpness but leads to higher sensitivity to the tearing effect (black fishes). from this compromise, it results : - a frequency response of the demodulated signal which starts decreasing for frequencies higher than 1.8 to 2mhz and attenuation at 3mhz is about 8 to 10db. - the sensitivity to black fishes (tearing) remains quite high when playing back aged tapes or in still picture mode. aso plus solution in its principle the aso plus solution is based on an active phase shifting of the fm carrier. this tech- nique intends to compensate the phase errors and delays induced in the fm playback signal delivered by the h-amps. in doing so, the luma transients of the demodulated signal are sharpened. its totally different principle makes the aso plus solution almost independant from the above de- scribed compromise ; here the high picture sharp- ness is not traded off against the sensitivity to noise and tearing effects (black fishes). typically, in vhs recording system, the aso plus solution gives the following results : - the bandwidth of the demodulated signal can exceed 3mhz (at -3db) and very often reaches the limits of the low pass filter used after the demodulator (see figure 1). - signal to noise ratio s/n is not decreased by an additional amplification of the upper sideband. - sensitivity to tearing effects (black fishes) is greatly reduced compared with the fm equalizer case. this improvement is easily noticed when playing back old and aged records and when video heads are getting aged. monitoring the overall quality of the fm playback signal is an additional function of the tea5750 in comparaison with the fm equalizer solution. in the conventional solutions (fm equalizer) the bandwidth of the playback channel remains the same whatever the quality of the playback signal. then the playback of old and aged tapes produces noisy picture because of the constant video band- width. in the case of the tea5750, the video bandwidth is automatically adjusted versus the quality of the fm signal. a wide bandwidth is maintained when playing back good records, and significantly de- creased in case of poor records, in doing so the effect of noise on the picture is attenuated (see figure 2). the signal is decreased but the overall s/n ratio is improved. aso plus / tea5750 2/25
vi deo signal amplitude aso plus fm equalizer f (mhz) (vhs standard cas e) 0123 5750-24.eps figure 1 : extended picture sharpness aso plus (poor records) noisy part of spectrum aso plus (good records) video signal amplitude f (mhz) 0123 fm equalizer 5750-25.eps figure 2 : video bandwidth adjusted versus playback signal quality iii - aso plus principle for clarity this chapter has been divided in three parts corresponding to the three main benefits of the tea5750 : - picture sharpness improvement by active phase shifting, - high immunity to tearing effects (black fishes), - monitoring the overall quality of the playback signal. iii.1 - active phase shifting the amplitude of the signal delivered by the heads decreases when the frequency of the carrier en- creases. in combination with an external lc filter, the tea5750 transforms this natural amplitude modulation into phase shifting which compensates the delays (or phase errors) induced during the fast frequency deviations. iii.1.a - phase shift generation the core of the phase shifting function is given in figure 3. under normal conditions, the v ps voltage at the phase shift input reaches the threshold of the diodes. then for each half cycle, as long as the diodes conduct, a magnetic flux is stored in the inductance l. at the phase shift input pin, the next zero crossing time is delayed respect to the input signal v in for a duration q which is proportional to the energy w l stored in the inductance l. the energy w l increases versus input amplitude and period, consequently q increases when the input frequency decreases. v in v ps threshold control amp from head amplifier to fm demodulator l c c i v in v ps flux in l q t t v in v ps iu 5750-26.eps figure 3 : core of the phase shift function aso plus / tea5750 3/25
iii.1.b - black to white transients (figure 4a) (fast increases of the fm carrier frequency) in this case, the input fm signal changes from large amplitude and long period to small amplitude and short period. consequentlythe delay produced by the phase shifting circuit changes from large values to small values. at the instance of a black to white transient, the phase shifting induces a long period t0 followed by a short period t1. this results in a faster frequency shift and in shorter rise times at the output of the fm demodulator. iii.1.c - white to black transients (figure 4b) (fast decreases of the fm carrier frequency) in this opposite case, the input fm signal changes from small amplitudes and short periods to large amplitudes and long periods. the corresponding delays will increase. so during white to black tran- sients, the period t 1 will be increased respect to the input period t h . this results in a faster frequency change and shorter fall times at the output of the fm demodulator. original luma signal record amplifier input signal actual fm playback signal (output from heads) a) black to white transient b) white to black transient emphasis emphasis 0 t h t 0 t h t phase shift input voltage 1 t 2 t q 2 q 1 q 0 1 h t 1 1 t q 1 q 2 0 t 1 t 2 t q 0 11 2 t 1 h t 1 1 t v ps h' t h' t 5750-27.eps figure 4 : phase shift effect on the fm playback signal aso plus / tea5750 4/25
in other words, the instantaneousdelays produced by the aso core speed up the luma transients compensating the effect of the limited bandwidth of the playback front end. iii.2 - high immunity to tearing effects (black fishes) the aso plus core is a level dependant gain circuit (figure 5). - when the input voltage amplitude is low the aso core behaves as a resonnant lc circuit damped by the resistor r. the input voltage is amplified by resonnance (typically q @ 2 to 6). - when the input voltage amplitude increases the diodes start conducting and clamp the signal. the resulting gain v out v in decreases. thanks to this characteristics, the aso plus core is able to better regenerate the playback signal when drop-outs and missing zero crossing appear. it results from this a high immunity to tearing effects (black fishes) (figure 6). in this field, aso plus core has two benefits com- pared with the conventional fm equalizer solu- tions. - immunity to black fishes is higher. - the signal to noise ratio is not decreased be- cause there is no need to permanently amplify the upper side band. the high gain requested to regenerate the signal, is instantaneously pro- vided when necessary and desappears as soon as the signal recovers a normal amplitude. remark : aso plus core also rejects the owhite fishe effectso : the band pass characteristics of the lc filter reject the high parasitic frequencies that could generate ultra white signals in the demodulated signal. l r c v out v in l r c v out v in v d low amplitude input signal high amplitude input signal v out v in v in t v in amplitude v in t v d g 4q 2 1 1 q g g (q : quality factor of the r, l, c filter ) 5750-28.eps figure 5 : the aso plus core is a level dependant gain circuit. aso plus / tea5750 5/25
missing zero crossing short drop-out aso plus equivalent circuit gain level output signal fundamental low high low high low v in 5750-29.eps figure 6 : aso plus core under disturbed fm signals aso plus / tea5750 6/25
iii.3 - monitoring the overall quality of the playback signal this is an extra function compared withe the fm equalizer. purpose is to adjust the video bandwidth versus the quality of the fm signal. the quality is measured as following : - the signal is poor quality when the frequence of missing zero crossings and drop-outs is high. - quality is judged as good in the opposite case. the tea5750 detectsthe two types of errors (miss- ing zero crossing and drops-out) as following : - a missing zero crossing is detected when no zero crossing is detected for a period longer than tl. tl is the longest normal halfwave. in vhs case tl = 1 2 ? ? ? 1 3mhz ? ? ? @ 160ns an hysteresis level is introduced in the zero crossing detection, in order to consider as an error the waveforms which barely cross the 0v level. those waveforms generates parasitics high frequencies ? ? ? 1 q ? ? ? in the fm demodulator (see figure 7). - a signal drop-out is detected when the signal could not reach the above mentioned hysteresis level for a duration longer than the expected longest halfwave. same as above (see figure 7). with this detection system and in combination with an external r mem c mem filter, the tea5750 pro- duces a dc voltage v mem proportionnal to the average error rate (figure 8). t2 t1 q tea5750 output signal hysteresis t1 : long duration induced by a missing zero crossing t2 : long duration induced by a signal drop-out 5750-30.eps figure 7 : fm playback signal errors detected by the tea5750 application c mem error mem soft switch lc from head amplifier to fm demodulator correcting filter error detector amp v mem r mem 5750-31.eps figure 8 : playback signal quality monitoring aso plus / tea5750 7/25
in case of good records : the dc voltage v mem remains lower than 1v, there is no action ; it results a demodulated video signal with a full bandwidth. in case of poor records : the dc voltage v mem increases, and the tea5750 corrects the playback signal in two steps : - in a first step, when v mem exceeds an internal threshold, by means of a softswitch the aso + circuit change the characteristics of the correct- ing filter. this first correction is intended to reduce the demodulated signal bandwidth in the fre- quency range where noise is supposed to be present and disturbing : 2.7 to 3.2mhz (see figure 9). - in a second step : if v mem continues to increase, the threshold of the diodes of the phase shifting circuit is increased. first consequence, the phase shifting is less effective (less flux in induc- tance l) and this tends to decrease the demodu- lated video bandwidth. second consequence, the equivalent impedance of the node at the junction of l, c and diodes is going to increases, providing so a higher efficiency to the correcting filter. the combined effects of those two conse- quences produce a high attenuation of the video bandwidth. the displayed picture becomes soft (figure 9). 123 demodulated signal full bandwidth encreasing v mem 1st step softswitch action 2nd step diode threshold modulation effect f(mhz) 5750-32.eps figure 9 : automatic video bandwidth adjustment error detector bias 1234 67 8 9 10 11 12 13 14 5 softsw hystlvl out1 err-det phshift gnd in2 ctrlgain err-mem in1 out2 to be connected to gnd i ref v dd input amp out. amp a1 a2 softswitch b 5750-33.eps figure 10 : tea5750 block diagram iv - tea5750 functional description to better understand the herebelow explanations the block diagram of the tea5750 is given in figure 10. aso plus / tea5750 8/25
iv.1 - biasing a precise and stable 1.25v voltage is delivered at pin 14. using an accurate resistor allows to pre- cisely biased the internal functions of the tea5750 i ref = 1.25v r 14 r 14 : 1.1k w /1% is recommanded. iv.2 - phase shifting circuit this function is made of : - the gain controlled input amplifier, - the clamping diodes, - the output amplifier. ? the input amplifier a1 featuresa low input imped- ance (typ. 28 w ) and amplifies the input current into voltage. the gain can be externally control- led at pin 13 (figure 11). control can be done either by a resistor con- nected between pin 13 and ground (typical ap- plication) or by a voltage v 13 v out1 =k.[v 13 ] ? i in v out1 =k[r 13 xi].i in ? clamping diodes according to the principle of the aso plus func- tion the threshold of the diodes is modulated versus the error memory voltage v mem (or v 12 ). characteristics are given in figure 12. ? the output amplifier a2 also features a low input impedance (typ. 13 w ) and then amplifies the input current (issued from the capacitor of the aso core filter) into an output voltage. 4 3 2 1 0.34 12345 v 12 v 5pp clamp level v clamp3 v clamp2 < < 5750-35.eps figure 12 : diode threshold versus v mem on pin 12 5 11 7 3 9 13 from h. amps l c v out1 in i v 13 i 500 m a v d2 v d1 a2 output amp. a1 input amp. 5750-34.eps figure 11 : phase shifting circuitry aso plus / tea5750 9/25
ramp generator 9 2 q q1 q 4 q3 reset up/down ramp 1v q2 c2 c1 c3 1 12 memory capacitor charging function d q soft switch error detect out2 hystlvl rpin2 r mem c mem err mem 5750-36.eps figure 13 : error detector (simplified block diagram) iv.3 - error detector, error memory, soft- switch (figure 13) iv.3.a - error detector in order to take into account the correction of the tearing effects naturally performed by the aso plus core (see chapter iii.2.). the signal to be monitored by the error detector must be the output signal (pin 9). as already explained in chapter iii.3., an error is counted when the duration between two zero crossings (taking into account the hysteresis) is longer than a duration fixed by the application. the error detector works as following (figure 14). ? at each rising edge of the output signal (pin 9) an internal ramp starts rising. ? when the next falling edge of the output signal (pin 9) is detected, the comparaison of the ramp signal with two thresholds (comparators c2 and c3) is latched, and then the ramp signal is reset. - the comparator c2 whose threshold is inter- naly fixed at 1v, belongs to a loop which main- tains the slope of the ramp compatible with the frequency of the fm carrier. the comparaison generates an oup/downo sig- nal which increases or decreases the charging current of ramp capacitor. in doing so, the rising time of the ramp (from 0v to 1v) is maintained compatible with the halfwave duration of the fm carrier whatever the recording system (vhs, s-vhs, video 8, hi-8). in the case of vhs standarts, the rising time to 1v is kept betwwen 104ns and 134ns (104 ns @ 1 2 1 4.8mhz ; 134ns @ 1 2 1 3.8mhz ). remark : the speed of this control loop is slow, because the loop is not intended to dynamically follow the carrier frequency. - an error is detected when the ramp signal exceeds the threshold of the comparator c3. the threshold is externally adjusted at pin 4 and typically adjusted at about 3v. aso plus / tea5750 10/25
(pin 9) ramp signal q3 output t t t v out hysteresis level error level 1v 123 5750-37.eps figure 14 : error detection waveforms comments about figure 14 : missing zero crossing a missing zero crossing happening on the negative halfwaves. this type of errors is also detected by the tea5750. a signal drop-out. remark : the hysteresis level of comparator c1 is externally adjustable at pin 2. hsyt level =r 12 x10 m a 0 r 12 10k w ) iv.3.b - error memory (figure 13) each time, an error is detected at the output of q3, the memory capacitorcharging circuit sends a fixed amount of charges d qtoc mem capacitor ( d q remains the same whatever the error shape, length ; d q @ 32nc). the capacitor c mem is discharged by the resistor r mem . in average : v mem = imoy . r mem v mem = d q . ferr . r mem ferr : error frequency iv.3.c - softswitch the voltage level vmem generated by the error detection drives a softswitch : a mos transistor which is used in its variable r ds on area (fig- ure 15). with this softswitch, the tea5750 can gradually attenuate the fm signal lower side band. more details are given in chapter v.1.e. aso plus / tea5750 11/25
20 40 60 80 10 0 12 0 14 0 16 0 18 0 20 0 0 0.5 1 1.5 2 2.5 3 3.5 r( w ) v 1 min - t max max - t min v g t min = 0 c t max = 100 c ds on min - t min v 1 v 1 max - t max v 1 v 12 v 12 = 1 r ds on 5750-38.eps figure 15 : r ds on versus v err-mem on pin 12 v - implementation of the tea5750 in a vhs-vtr application v.1 - typical application description in this chapter, is described only the direct environ- ment of the tea5750. the interface circuits with the head amplifiers and the fm demodulator are decribed in v.2 and v.3 parts. figure 16 illustrates a typical application diagram. v.1.a - input filter (r 4 ,c 2 ) the input filter has 2 purposes : - to provide the input stage with a suitable signal : referring to chapter iv.2, the input stage a1 re- quires a current as input signal. consequently the r 4 c 2 input filter is the impedance which changes the input voltage signal (coming from the h-amps) into the required input current. in order to limit the harmonic distortion to less than -40db, the input current must be less than (0.5map-pk) in the frequency deviation range. (1) for : 3.8mhz f 5mhz v in max ( p - pk ) ```````` r 4 2 + 1 c 2 2 w 2 0.5map - pk additionally c 2 performs a necessary capacitive coupling. - to provide the aso core with a fm signal featur- ing a more appropriate balance between the lower and upper side bands. looking at the frequency response of a tape (figure 17) it can be noticed that the high fre- quencies are greatly attenuated compared with the low frequencies (the attenuation between 3mhz and 6mhz is in the range of -12db/octave for vhs tape). this imbalance leads to important amplitude changes during luma transients (figure 17). those very important changes in the amplitude of the playback signal would induce a too high efficiency of the phase shifting circuit. so the inherant high pass characteristics of the r 4 c 2 input filter are used to decrease this imbalance. r 4 c 2 cut-offfrequency f c is choosen to attenuate the lower side band versus the higher side band. but the cut-off frequency should not be to high in order not to decrease the s/n ratio. in case of vhs standard f c is choosen in the range of 6 to 8mhz. taking into account the previous point r 4 c 2 is sized as following : (figure 17). (1) v in max ( p - pk ) ```````` r 4 2 + 1 c 2 2 w 2 0.5map - pk 3.8mhz < f < 5mhz (2) 6mhz < 1 2 p r 4 c 2 < 8mhz typically : r 4 @ 330 w ,c 2 @ 68pf (v in max. @ 0.25vp-pk) aso plus / tea5750 12/25
bias 14 13 1 12 11 10 9 8 2 345 67 input amp a1 output amp a2 error detector soft switch c3 100nf v cc c10 10nf output to fm demodulator c4 10nf l1 120 m h output amp feedback r2 2k w 1% r1 1.1k w 1% input stage gain contr ol r3 1m w c1 1 m f error memory filter r4 330 w c2 82pf input from h-amps input filter r5 22k w r6 27k w r7 1k w error detector adjustment c5 220pf video bandwidth correc tion c9 100pf l3 47 m f r9 820 w r10 2.7k w l2 47 m f c7 39pf c8 10nf c6 aso plus main fil ter permanent correc ting filter v cc 5750-39.eps figure 16 : tea5750 typical application aso plus / tea5750 13/25
video signal input signal (from h. amps) v (after input filter) 3 black white disturbed transients a) waveforms b) typical tape frequency response c) equalization by r c 42 d 12db f (mhz) g (db) 246 f (mhz) fm signal bandwidth corrected bandwidth original bandwidth high noise part of the spectrum (normal quality) 246 5750-40.eps figure 17 : input filter equalizing effect v.1.b - aso main filter (l 2 ,c 7 ,r 10 ) the function of the filter l 2 c 7 r 10 has been already explained in details in chapter iii. basically, under normal playback conditions, the l 2 inductance performs a flux storage (phase shift- ing), the l 2 c 7 resonnant circuit performs an ampli- fication of low amplitude waveforms (tearing effect suppression). r10 provides damping. in the typical application, the frequency charac- teristics of the aso main filter are influenced by the correcting filter (chapter v.1.c) (see figure 18). for an appropriate operation in the application, the total filter (main filter + correcting filter) is normally dimensionned as following. - resonnance frequency is typically choosen at the center of the frequency deviation of the car- rier. in vhs case : f resonnance @ 4.3mhz. remark : in some applications, f resonnance can be slightly under this typical value in order to better protect the synchro signal against tearing effects. looking at figure 18 it can be seen that the resonnance frequency of the total filter (main filter + correcting filter) is about 4.3mhz (l 2 c 7 resonnance frequency is about 3.5mhz). aso plus / tea5750 14/25
- q factor of the filter is choosen in order to cover prop erly the carrier frequency swing range (3.8mhz to 4.8mhz). f resonnance f deviation 4.3mhz 1mhz 4.3 q 4.3 in practice in order to provide margin, q factor is in the following range : 2 q 3.5 q r 10 l 2 w res - free transient response of the complete filter : in case of signal drop outs, there is no signal at the input of the filter which consequentlygener- ates its own free transient response (figure 19). effect of this transient response on the picture are minimized when : - the transient response is strongly damped : short duration. damping is normally enough when 2 q 3.5. - the free frequency of the filter is in the range of 4.3mhz producing then a medium grey spot less disturbing than an ultra black or ultra white spot. r9 c9 l3 r r10 l2 c7 c6 c8 i 5 correcting filter main filter r : input signal t : current i 2 1 2 1 main filter alone main filter + correcting filter 5750-41.eps figure 18 : main filter + correcting filter typical frequency response 7 9 l2 c8 l3 c9 r9 c7 r10 input voltage 0.2v signal at pin 9 (output) after a voltage step 225ns 5750-42.eps figure 19 : main filter + correcting filter free transient response aso plus / tea5750 15/25
remark : it is possible to locate the correcting filter outside the tea5750 environment. in this case l 2 c 7 r 10 values are simply calculated as following -f resonnance = 1 2 p ````` ` l 2 c 7 -q= r 10 l 2 w res v.1.c - correcting filter (l 3 ,r 9 ,c 9 ) (figure 18) l 3 c 9 r 9 filter simply performs an spectrum equali- zation of the playback signal ; it attenuates the spectrum in the range of 1.5mhz to 3mhz (center frequency @ 2.2mhz) where vhs tape have a high efficiency. results is a better balance between the upper and lower side band. this filter is often located in the environment of the tea5750 (like in figure 16) because aso plus replaces the conventional fm equalizer. however it can be located in an other part of the fm signal path. the filter is dimensioned to compensate the soft peaking which appears in the demodulated signal frequency response (figure 20). consequently the requested q factor has a low value (about 1.5). v.1.d - output filter feedback (l 1 c 4 ) (figure 21) the output amplifier converts the current i c of the main aso filter into the output voltage. the typical waveforms of the current i c show a slow slope of video signal without correction with correction f (mhz) 123 0 5750-43.eps figure 20 : correcting filter effect l1 c4 c7 i c v out c4p l1p 1 output voltage 5750-44.eps figure 21 : inductive feedback of the output amplifier the signal in the area of the zero crossing. this induces a high sensitivity to phase noise. in order to avoid this problem, the feedback net- work of the output amplifier is an inductance. con- sequently the output contains a component of input signal derivate. v out crosses zero with a higher slope, therefore the sensitivity to jitter is reduced. in figure 22, is given the output voltage waveforms when the feedback is made by a resistor, this waveforms would lead to high sensitivity to phase noise (to be avoided). c4 capacitor only provides capacitive coupling, the resonnance frequency of l 1 c 4 is choosen in low frequencies ( @ 150khz) so that the filter is almost equivalent to l1 in the fm playback spectrum (1mhz to 10mhz). typical values are c4 : 10nf, l1 : 120 m h. aso plus / tea5750 16/25
r c4 c7 i c v out c4p l1p 1 output voltage 5750-45.eps figure 22 : resistive feedback of the output amplifier v.1.e - error memory filter (c 1 r 3 ) each time an error is detected in the playback signal, the error detector generates a fixed amount of charge d q (32nc : 40 m ax800 m s). so by means of the parallel (c 1 r 3 ) filter, a dc voltage propor- tional to the error rate will appear at pin 12. v 12 =r 3 (i 12 ) average =r 3 ? ? ? ? 1 t 0 t d q ? ? ? during the playback of normal quality, v 12 must be kept below the triggering threshold of the softswitch (typ. 0.8v). under those conditions, errors are nor- mally in the range of 25 per second. r 3 1m w c 1 value influences the charge and discharge times of the filter. a compromize has to be found between low time constants which lead to fast fluctuations of the picture resolution (unstable sharpness) and long time constants which induce long delays be- fore picture sharpness in definitively stabilized. typically a 1 seconde time constant is used : r 3 c 1 @ 1s v.1.f - video bandwidth correction (c 5 ) (fig- ure 23) in case of medium error rate (0.8 < v 12 < 1.2v) softswitch is turned on and then c5 is added to the l3 r9 c9 correcting filter. the resulting filter is choosen to produce an attenuation of the lower sideband of the fm playback signal in the range of 1 to 1.6mhz. center frequency f c @ 1.3mhz 2 p f c = 1 ``````````` l 3 ( c 9 + c 5 ) this correction corresponds to an attenuation of the upper spectrum (above 2.5mhz) of the de- modulated video signal. in doing so, the expected noisiest part of the spectrum is reduced. the signal spectral density is reduced but s/n is improved. when the error rates increases to high values v 12 > 1.2v, the correction performed by the l 3 ,r 9 , c 9 +c 5 filter is not enough. the video bandwidth is again adjusted by decreasing the phase shifting, this is performed by increasing the threshold of the diodes. v.1.g - error detector adjustment (r 5 ,r 6 ,r 7 ) (figure 16) referring to the detailed description of the error detector (chapter iv.3.) : -v 4 is the threshold voltage of the error detector and selected as following : v 4 1v expected longest hardware duration average halfwave duration v 4 1v x 166ns 116ns in practice at least a double margin is taken v 4 @ v cc r 6 r 6 + r 5 @ 3v -v 2 gives the hysteresis of the comparator hyst = r 7 x10 m a 0 hyst typ 300mv aso plus / tea5750 17/25
v.1.h - input stage gain control (r 2 ) r2 is intended to control the gain of the input stage. gain is usually adjusted in order to get about 1.2vp-pk at pin 3 during the synchro (3.8mhz). v.1.i - other components c 6 ,c 8 ,c 10 are coupling capacitors ; their value are not critical (10nf to 100nf are typical values). c 3 filters the v cc line. v.1.k - typical waveforms and results the following results and waveforms have been checked in a vhs set and with the typical applica- tion circuit of he tea5750 (see figure 16). 1 c9 c5 l3 r9 softswitch fm demodulator input spectrum fm demodulator output spectrum 24 13 123 fc f (mhz) f (mhz) softswitch-off softswitch-on softswitch-off 5750-46.eps figure 23 : video bandwidth adjustment demodulated signal input signal from h-amps tea5750 v 5 tea5750 v 9 5750-47.eps figure 24 : waveforms in case of medium and good quality tape. case of a black to white transient. aso plus / tea5750 18/25
input signal from h-amps 50mv/div output signal v 100mv/div (500ns/div) 9 mm m m missing zero crossing 5750-48.eps figure 25 : waveforms in case of badly disturbed playback signal (ex : still picture mode of an aged tape) most of the missing zero crossing are recovered. the picture features a far low rate of black fishes (tearing effect). out in out in 5750-49.eps figure 26 : upper side band recovery. input and output signals spectrum of the aso plus function. (video pattern is a vertical black to white transient in the middle of the line) aso plus / tea5750 19/25
recorded signal is : video sweep with 1, 2, 3mhz markers. 5750-50 to 53.eps figure 27 : demodulated video signal bandwidth versus voltage at pin 12 (envelope signal) 5750-54 to 57.eps figure 28 : demodulated video signal bandwidth versus the input signal amplitude reference level (0db) corresponds to 28a) : 1.2vp-pk at pin 3 during synchro. results : the video bandwidth is not very sensitive to the playback signal amplitude. 1mhz 2mhz 3mhz v 12 =0v a) v 12 =1v 1mhz 2mhz 3mhz b) 1mhz 2mhz 3mhz v 12 = 1.4v c) v 12 =2v 3mhz 2mhz 1mhz d) 3mhz 2mhz 1mhz reference input level (0db) v = 1.2vp-pk 3 a) 1mhz 3mhz 2mhz v in ref - 6db c) 1mhz 3mhz 2mhz v in ref + 5.3db b) 1mhz 3mhz 2mhz v in ref - 15db d) aso plus / tea5750 20/25
v.2 - interfacing with the head amplifier part in its principle, the aso plus function is sensitive to the amplitude changes of the playback signal . for this reason, a chroma trap filter is necessary be- tween the head amplifier part and the tea5750 application, in order to avoid parasitic phase shifts induced by the chrominance signal. in pal application, the trap center frequency is 637khz and an attenuation higher than 30db is recommanded (figure 29). figure 30 shows an example of an interface circuit used between the head amplifier part and the tea5750 application. remark : the group delay compensation circuit (q102, c104, l102, r107) is very often inserted in the interface circuit but is not specially requested by the aso plus circuit. very often the head amplifier section provides two types of playback signal : with and without agc. when available it is generally recommended to use the output without gain control because of the two following reasons : - this signal offers a higher s/n ratio, - a gain control is recommended between the aso plus function and the fm demodulator (see chap- ter v.3.). in this case, the total gain would become too high if the gain controlled output of the head amplifier is used. a high gain of the channel is not suitable for stability, sensitivity to parasitics. remark : generally, between 2 and 7mhz the frequency response of the interface circuit is flat, however in some application, a small peaking effect helps to better shape the playback signal (especially for applications without peaking with the head impedance). v.3 - interfacing with the fm demodulator in case of disturbed playback signal, the clamping diode threshold level changes (see chapter iv.2). consequentlythe output signal amplitude is modu- lated by the error rate, to compensate this very low frequency modulation an agc function can be used between the aso plus circuit and the fm demodulator (see figure 31). 5750-58.tif figure 29 : chroma trap filter typical characteristics in case of pal standard aso plus / tea5750 21/25
c101 390pf r101 1.8k w q101 bc557b r102 330 w c102 330pf r109 27k w r110 22k w c106 10nf from h-amps r103 22k w r104 22k w l101 100 m h q102 bc550b c101 22pf r106 390 w r105 390 w q103 bc557b r107 1.8k w r108 1.8k w c105 100nf v dd l102 68 m h to aso plus function 5750-59.eps figure 30 : example of interface circuit between the head amplifier section and the aso + fm demodulator agc function aso plus chroma trap h-amps output without agc to chroma processing 5750-60.eps figure 31 : fm playback channel structure maximum theorical output level increase is : d db = 20 log q ````` ` 1 - 1 4q 2 q = quality factor of aso main filter 2 < q < 3.5 ? d dbmax. = 10db in pratice this modulation does not exceed 6db. v.4 - application circuit options the tea5750 application provides a high flexibility, and the following part is just intended to give some suggestions. remark : those suggestions are just small im- provements of the typical application (see fig- ure 16), and are not really requested. v.4.a - input stage gain adjustment versus the playback signal envelop signal as described in chapter iv.2. the input stage gain can be controlled by the voltage at pin 13. interface circuit lpf env. detect h-amps to pin 13 of tea5750 5750-61.eps figure 32 : adapting the input stage gain versus playback signal amplitude v out @ =k[v 13 ]xi in v out =v 3 and i in =i 11 then it is possible to adjust the gain of the input stage with the envelope detectionsignal (very often provided by the h-amps) (see figure 32). a very low pass filter is normally requested in the interface circuit in order to avoid frame frequency modulation,time constant of the filter is in the range of 1s. aso plus / tea5750 22/25
1 11 r4 c2 r l c5 tea5750 5750-62.eps figure 33 : video bandwidth correction filter (option) v.4.b - video bandwidth correcting filter in the typical application, c5 is directly connected the permanent correcting filter (l3, r9, c9) (see chapter v.1.f). it may happen that the correction requested cannotbe easily realized with this typical solution. in this case it is possible to use a rlc 5 filter connected to the filter (figure 33). with this configuration the correction becomes independant from the characteristics of the permanent correct- ing filter. v.5 - using the tea5750 without the error de- tector this type of application is no longer typical, but can useful in the following case : - the vtr system already features a function which monitors the quality of playback signals. - there is a need to change the picture sharpness versus the quality of the playback signal. - to keep a high immunity to tearing effects (black fishes). for this type of application, the tea5750 offers a simple and smart solution. by driving the voltage at pin 12, it is possible to easily control the video bandwidthwhile keeping a high immunity to tearing effect, without degrading s/n and inducing para- sitic group delay (see figure 34). in this case, the error detector ofthe tea5750 must have the lowest sensitivity as possible. to do so : - pin 4 must be connected to v cc - pin 2 must be grounded. signal processing aso plus h-amps playback signal quality monitoring to pin 12 of tea5750 5750-63.eps figure 34 : architecture with external playback signal quality monitoring v.6 - aso plus implementation : problems and solutions this chapter does not claim to be comprehensive. this part only claims to describe four common problems with their solutions. v.6.a - ringing effect after luma transients because of the wide video bandwidth provided by the aso plus, same parasitic effects of the luma channel may be displayed. for instance the tran- sient response of the low pass filter used in the fm demodulation; in some applications the transient response of this filter generatessignal components in the range of 3 to 3.2mhz (inside the maximum bandwidth achievable with the aso plus) (see fig- ure 35). solutions : - either the user wants to keep a very wide band- width, in such a case, the low pass filter transients characteristics have to be improved. - or to cut the video bandwidth with external filter so that the attenuation is high enough above 3mhz. fm demo black white demod. signal ringing induced by the transient response of the lpf aso plus lpf 5750-64.eps figure 35 : origine of the ringing effect v.6.b - unstable picture sharpness this effect may be described as following : the picture sharpness easly changes versus the time and tape quality. this effect likely happens when the error detector is adjusted in a too sensitive way ; so that small changes in the quality of the fm playback signal induces variations of the error voltage (pin 12). to correct this effect, the error detector sensitivity has to be adjusted in the way it has been primarly intended : -v 12 < 0.6v for medium and good quality tape (normal) to do so : -v 4 is adjusted in the range of 3 to 3.5v, -v 2 is adjusted in the range of 0 to 250mv (typ.). aso plus / tea5750 23/25
v.6.c - pumping effect and long settling time this effect can be described as following : pumping effect : the picture sharpness widely changesfrom soft to sharp (especially in still picture mode). long settling time : it takes a few seconds (1 to 5s) before the picture sharpnessreaches its final state. this specially occurs : at the beginning of a play- back sequence occuring after stop conditions or after a fast search sequence. root cause of the effect is : in the typical application, v 12 voltage can have a large swing : 0 to 5v, and the time constant c 1 r 3 is large ( @ 1s) ; the combined effect of those pa- rameters induce slow and large changes of the phase shift circuits. solution : when such an effect is not acceptable for the user. it is possible to greatly limit it by clamping the voltage at pin 12 (see figures 36 and 37). it is usually recommanded to clamp v 12 in the range of 1.6 to 2v. this value is high enough to control the bandwidth(enough control of the internal clamping diodes) and to limit the time constant so that it is no longer noticible. v.6.d. high influence of the playback level on the video bandwidth description of the effect : when the playback level widely changes (case of signal delivered by the non-gain controlled output of the head amplifier part) the bandwidth of the demodulated signal may be influenced : low play- back level induces low bandwidth, high playback level induces wide bandwidth and some peaking in the range 2mhz to 2.5mhz. origin of the effect : in the typical application the permanent correcting filter l 3 r 9 c 9 is directly connected to the aso plus main filter l 2 c 7 , so the efficiency of the correction in a large extent depends on the amount of the phase shift (so depends on the playback signal amplitude). solution : since the location of the permanent correcting filter is not critical, the above described effect can be greatly attenuated by moving l 3 r 9 c 9 filter. fig- ure 38 gives an example of solution. tea5750 12 14 t1 t2 without clamp v clamp 1.8v 1.2v t t1 t2 3to4 roughtly 5750-65.eps figure 36 : time constant induced in the error signal. effect of clamping. corrected bandwidth with v 12 1.8v full bandwidth v 12 =5v demod. signal f(mhz) 123 5750-66.eps figure 37 : excessive sharpness variation with v 12 =5v aso plus / tea5750 24/25
11 tea5750 input r4 c2 l3 r9 c9 5750-67.eps figure 38 : permanent correcting filter in the in- put section for a better rejection of the playback influence vi - comments and conclusion the aso plus is a new function, and then a lot of detailed explanations have been given in this note, but in practice, the aso plus function is quite easy to implement. all the examples given in this note are relevant to vhs pal applications, but obviously can be ex- tended to ntsc ones and to s-vhs applications where wide bandwidth with high immunity to tear- ing effects is even more attractive. vii - references ovhs improved reproduction by using active phase shifto by gerd reime nokia consumer electronics. ieee chicago) oaso plus improvement of the vcr picture quality by fm phase controlo. nokia vcr r&d labora- tory pfozheim - germany. june 24th, 1992. otea5750o technical report by s. gornati and l. lietar sgs-thomson microelectronics. romainville - france . january 10th, 1991. oaso system operation analysiso by m.j. gay miee. april 24th, 1992. information furnished is believed to be accurate and reliable. however, sgs-thomson microelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no licence is granted by implication or otherwise under any patent or patent rights of sgs-thomson microelectronics. specifications mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. sgs-thomson microelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of sgs-thomson microelectronics. ? 1994 sgs-thomson microelectronics - all rights reserved purchase of i 2 c components of sgs-thomson microelectronics, conveys a license under the philips i 2 c patent. rights to use these components in a i 2 c system, is granted provided that the system conforms to the i 2 c standard specifications as defined by philips. sgs-thomson microelectronics group of companies australia - brazil - china - france - germany - hong kong - italy - japan - korea - malaysia - malta - morocco the netherlands - singapore - spain - sweden - switzerland - taiwan - thailand - united kingdom - u.s.a. ? asoplus is a trademark of nokia consumer electronics aso plus / tea5750 25/25

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