Magicboxes Descrambler 3-chip Theory of Operation

I lived in an area where Zenith ZTACs (2601s) were used to encode video signals on premium channels. In 1990 I started designing my own ZTAC descrambler and built one using about 60 chips! (pictures to follow). I spent a little over a year designing and adjusting my SSAVI monster. After reducing the part count to 10-20 chips it was still to much of a burden to build such a device in volume. One day I got the idea to look in my business directory so I started calling Cable Television warranty shops. After a few tries, I *socially engineered* 50 ZTACs for $35 ea out of a shop as they felt I was not a threat to any of their clients. When I received the 50 ZTACs I designed the 2-wire test chip to provide a cheap, easy way to *test* these units (for educational purposes only). In the load of the 50 ZTACs I got a letterhead from the cable company which sold the 50 ZTACs to the warranty shop. Well, I decided its now or never, so I created a company which remanufactured Zenith converters in NY (no, I am not going to give out it's name) and called the manager of the Cable Company to inquire about more ZTACs. After 30 minutes of social engineering the manager decided to sell me 72 more ZTACs at $15 ea. Heaven. I told the cable company I was unlike other warranty shops as I pay in advance for all orders, will take any volume at a moments notice(gulp), and will purchase complete systems. Well, after receiving the 72 ZTACs I got a call from the manager to see if I would take another 862 ZTACs (wow!) ; at $12ea. Well, the next day I sent off a large (well, at the time) certified check and a week later 3000 pounds of ZTACs arrived. I made an agreement with the MSO that I purchase ALL ZTACs they had (as they were upgrading). Over the following months I purchased a few thousand ZTACs until the "well" went dry. I now had many large customers and no ZTACs. Hence, an easy, cheap, ZTAC solution has to be found in order to accomidate my happy customers. A few months after all *reasonably* priced ZTACs were purchased I had to do something so I went back to my mother of a home-built ZTAC and decided to redesign and re-examine my approach. As I do most of my work on paper, I will describe in this document the steps I took to create, tune, and finalize the 3-chip descrambler. To ensure we are all thinking on the same wavelength I am first going to discuss the fundamentals of NTSC video. (I got some good pictures on other web sites to demonstrate.)

The relationships between vertical, horizontal, and subcarrier components within a video signal are established by RS-170A (NTSC). For proper lockup (shift-free viewing), these relationships must be maintained throughout any descrambler.

NTSC video displays 29.97 frames per second. Each frame is made up of two fields yielding a total of 59.94 fields per second. Each field consists of 262.5 horizontal lines scanned across the field from left to right, top to bottom, at a rate of 15,734.2637 per second. The half line at the end of the first field and at the beginning of the second field causes the lines of the two fields to "interlace." The two interlaced fields combine to form a frame made up of 525 distinct scan lines (figure 1).

This clever system updates the image viewed on a TV screen ~60 times per second to avoid a flickering or jumpy appearance, provides 525 scan lines for good resolution, and yet requires one half of the bandwidth of a non-interlaced system with the same image update rate and resolution characteristics.

It is important to note that our discussion of NTSC pertains to an "unmodulated" video signal. An unmodulated video signal compares to a modulated video signal in the same way as an unmodulated audio signal compares to an AM radio signal. Modulated signals must be received by a tuner in either your audio system or TV. Unmodulated signals must be provided directly to your audio or video system via what engineers call "base band" inputs.

The base band video signal is a voltage measured in IRE units. There are 140 IRE per volt. As the voltage approaches 100 IRE the picture becomes brighter. As the voltage falls to 7.5 IRE the picture approaches black. During retrace (the time during which the picture tube's electron beam returns to the left after scanning a line or to the top after scanning a field) the signal drops to 0 IRE in order to "blank" the beam and make the retrace invisible. Sync pulses contained within the blanking interval fall to -40 IRE. The -40 to 100 IRE range corresponds to a one volt peak-to-peak signal (between -0.286 to 0.714 VDC if the signal is properly clamped).
RS-170A alters and augments RS-170 to enable color video transmissions. The key component is the addition of a color subcarrier signal to the monochrome signal to specify the color hue and saturation of objects in an image.

The color subcarrier is a 3,579,545±10 Hz sine wave impressed upon the monochrome signal. The net effect is that the video signal is a sine wave of varying amplitude, DC offset, and phase (see figure 2). The amplitude (peak-to-peak size) of the sine wave describes how deeply an object in an image is saturated with color (is it deep red or just a hint?). As the peak-to-peak voltage of the sine wave increases the displayed color becomes deeper. The phase (precise timing) of the sine wave describes the hue of the object (is it red, blue or some other color?). The DC offset voltage of the sine wave determines how bright an object is. As the offset voltage increases the displayed object becomes brighter.

To determine color hue and saturation, video monitors and other color decoding equipment compare the color subcarrier's phase and amplitude with a reference signal. The reference signal is called the "burst" and is transmitted in the video signal during the blanked retrace period between horizontal lines known as the horizontal interval (figure 3).

Specific Relationships: The burst defines a sine wave of 40 IRE peak-to-peak amplitude and 180° phase. The timing of the burst relative to horizontal sync (called SC/H Phase) is clearly defined by RS-170A Note 7 (see figure 4). The horizontal and color subcarrier frequencies were chosen so that there are 227.5 cycles of the subcarrier sine wave during the entire horizontal line (including blanking). The half cycle causes the phase of the burst to alternate 180° with each successive scan line within a field. Alternating the relationship in this manner minimizes the visibility of the color subcarrier within the brightness components of the picture.

Subcarrier's timing relationship with the rest of the video signal is defined by NTSC during the vertical interval which spans a period equivalent to 20 horizontal lines at the beginning of each field (see figure 5). There are 59.94 color fields and 29.97 color frames per second. The starts of color field one is defined by a whole line between the first equalizing pulse and the preceding horizontal sync pulse. The start of color field two is defined by a half line between the first equalizing pulse and the preceding horizontal sync pulse. These half lines give rise to interlacing.

The relationships between components within the NTSC video signal are complex, fragile, and important. A descrambler must be carefully designed, built, and used in order for these relationships to remain intact. If not, problems will creep in to the finished descrambled video product.

At the time of design I decided to build a "descrambler" using minimal parts so volume production could be attained at a low cost. In order to build a descrambler with few parts there must be an intelligent part (like a microcontroller) and a few other parts to feed it information it would need to reconstruct video. In looking at the NTSC standard, let's examine what kind of information we would need to collect in order for a microcontroller to *understand* it's surroundings.

1. How to detect the horizontal blanking interval with normal and sync-supressed video.
2. Where are we in the frame? Best way to find out.
3. Simple video clamp.

1. Should I just shift the sync or re-insert it?
2. How to best capture data (cheaply)
3. Invert the video for under $.50.
4. Run all the above while trying to create a video frame.
5. Create a list of priorities in the 3-chip descrambler world.

For the NTSC issues we shall look at parts of the schematic of the 3-chip to see how all three issues were resolved.

1. How to detect the horizontal blanking interval with normal and sync-supressed video.

1. Most importantly, how do I tell where the horizontal blanking interval is in both suppressed and normal sync? The EASY answer might be the color burst. After repeated attempts by people to use the color burst and the incredible cost to lock on such a *small* signal I felt it would be pointless to do so. I therefore determined that another not-so-apparant pattern in NTSC video was the fact that the SYNC portion of the video had to have a VERY fast fall and rise time (like 150nS). Since this is the ONLY pattern of fast rise and fall in a video line over time I chose this to be the method for finding a horizontal blanking interval.

A closer look at the negative edge detector (with only black video present):

As you can see from the above waveforms that we can sync up our circuit using the positive and negative edge detects as it ALWAYS occurs in the same place for both normal and sync-supressed syncs.

This one took some thought. The NTSC standard recommends using the last line of the field to determine which field in the frame you are looking at. Since we have a neg/pos detector it would only be reasonable to look for some other event that happens which could be EASILY detected while we create a line. Line 6 of the second field offers an interesting (short time) detectable signal. Look below:

Notice this whole event lasts ONLY 7uS (perfect for a few reads from a port on a PIC)

Looking through the commented code you will find

So the video does not float everywhere I kind of needed to design a type of clamp which would keep the video in one place even though the dc reference was lost. This was rather simple as I chose to "reinsert" the sync rather than lower it (see descrambler challenges). The clamp control is low ONLY when sync is present, and the entire VBI. The circuit below shows the clamp circuit with the invert amplifier (for circuit reference):

This is how the clamp works from the last sync of field 1 to the vertical blanking interval of field 2.

The above demonstrates a simple clamp worth no more than $.12. Mission accomplished.

This one took a while to figure out. If I reinsert the sync will it happily co-exist with now foreign video? Will the color burst get hacked on video which is not up to spec? Will the sync to video relationships get screwed up even with a clamp? Well, after some experimenting the answer is no. I found that if this unit was to CREATE TWO frames of video and then so to speak SHIFT that frame UNTIL it matches with the original video AND can be dynamic enough to allow for small timing differences there should be no problem. Easier said than done. This brought about my worst nightmare. Build a circuit which can essentially rebuild the sync including the front porch, back porch, sync, and re-insert the color burst. (That's easy part). Timing all those decisions in a PIC was rough. Below is the circuit which re-creates the sync from scratch:

R19 controls either SYNC or BLACKLEVEL for the sync. (front porch, back porch, sync)
The color burst switch controls a switch allowing the chroma signal to pass

The actual NEW sync at pin 1 of the switch looks like this:
(notice the color burst at pin 1, WHY? Well look at the commented code
The best way to ensure the color burst was at the SAME BLACK LEVEL
was to actually open the color burst switch and the SYNC SWITCH (BLACKLEVEL SELECTED)
at the same time. This essentially placed the color burst at the same level as the black level thereby
making a happy :-) final sync.

Fortunately the data has fast slew rates so we can use our neg/pos edge detector to tell us if there is data.
In the commented source code you will find that this data on Lines 10 through 13 is used to determine if
we are on a SSAVI channel and if we should sample line 20.

Well, the negative or positive edge detector work WELL for fast slew rates. Look below:

Below is Line 20 (the active invert line) and with it on trace two is the negative edge detector.
Notice how we can use the neg/pos edge detector not only for lockup but also inversion/
data.

Using a simple polling procedure of the negative edge detector on line 20 we can determine whether this field
is inverted or not (this field is normal video).

Well, that's a cheap method for detecting inversion/data $0.00 (cost absorbed in sync correction)

Now for the contest. Design an amplifier using ONLY transistors for input, inversion, and output (follower). The circuit which best passes normal video/inverted with the least distortion and part count (cost) wins. Your design MUST work in the exsisting 3-chip descrambler and will replace ALL current transistor amplifiers.

The prize is $100 and a free kit (or a free kit for the new 3-chip member). I am taking all costs for this contest out of my pocket, so it will not cost this project a dime.

All entries will get posted on this website and the winners design will be incorporated in the new 2-chip descrambler (ooppss)?

Now that you know what the circuit does lets look at commented code, line by line.

The code is VERY difficult to read so I have made MANY pictures so ALL of you can make changes you need. There are many routines to accomplish the rebuilding of video and make decisions. The program consists of a MAIN program which is an endless loop creating 2 fields of a video signal. MAKEHLINE actually makes a horizontal sync, line, inverts the video, checks for inverted video, changes the VCO, etc. MAKEHALFLINE creates the halfline ..... SETMODE makes decisions based on information gathered in HLINE like ..... MAKEVBI makes the vertical blanking interval. We will look at the most complex routine first MAKEHLINE (make horizontal line), this will give you an idea what is going on inside the 3-chip most of the time.

; Port A
; 0 - PLL frequency control
; 1 - VidLevel - an experimental port used to try and clamp video better (not used in this code)
; 2 - Troubleshooting
; 3 - Manual Invert control (MANUALINVERT)

; Port B
; 0 - VNX-    Normal Video Switch (this passes video through to output as it comes in)
; 1 - CLV-    Clamp Voltage (this selects 0=0V and 1=2.5V)
; 2 - NED-    Negative Edge Detect (rapid falling signal)
; 3 - PED-    Positive Edge Detect (rapid rising signal)
; 4 - BSV-    Black/Sync Voltage (Black =1 , Sync = 0)
; 5 - VIX-      Inverted Video Switch (this passes the inverted video from Q2)
; 6 - BUX-    Burst Switch (this passes the burst through, which is AC coupled so it can be clamped!)
; 7 - BSX-    Black Level/Sync Switch

; SOME USEFUL EQUATES
;
; MAKESYNC = BSX (just make sync level)
; MAKEBURST = BSV + BUX + BSX (this puts the BURST right on the black level)
; PASSVIDEO = VNX + CLV + BSV (normal video passes, its clamped, Black Voltage)
; PASSINVERT = CLV + BSV + VIX (inverted video passes, its clamped, Black Voltage)
; PASSBLACK = BSV + BSX (video is just black level)
;
;
;
;
; NED = 00000100b - Negative Edge Detector
; PED = 00001000b - Positive Edge Detector
;
; BIT EQUATES
; VIDEOKAY          EQU    MODE.0    - if 1 then 3-chip is genlocked onto video signal
; VIDEOINVERT   EQU    MODE.1    - invert the video during active lines
; VIDEOBLACK    EQU    MODE.2    - make the video black during active lines
; WANTSINVERT    EQU    MODE.7    - user wants to invert video

;Description of Variables
; MODE defines the mode the unit is currently in (look above for specifics)
; OscDecision holds the value of the NED+PED for running the oscillator during a sample time after negedge.
; FastCnt is incremented every time the oscillator is sped up, if it is sped up 128 times then it concludes that
; we are no where near locking up and invokes a code timing adjustment of .2uS (faster lockup)
; SlowCnt (same as fastcnt) but used to quickly slow down oscillator.
; temp is a crap var for counting, etc...
; Data is a place to store the result if data was actually found on the line (HL10 to HL13)
; Line6 holds temp data telling if we found field 2 , line 6 (line6=ped+ned if found)
; isInverted tells the main program if the line was inverted/not (0 = inverted) (HL 20)
; FindLine6 (PED bit) is set if we dont know where we are and MakeHLine has to find Field2, line 6
; makelines is passed to this routine telling it how many Hlines to build...

        movlw       MAKESYNC ; normally make sync (horizontal sync pulse)
        btfss         VIDEOKAY    ; IF 3-chip is not locked on THEN
        movlw       PASSVIDEO ; let video pass through (errors)

        movwf       PORTB           ; Start hsync pulse (cool)

        movlw      00001100b    ; read comparitors (rise/fall) , everything else out.
        tris            PORTB ;

                                               ; turn on my troubleshooting pulses
        movlw       11111011b
        tris             porta             ; turn on my troubleshooting pulses
        bsf             troubleshoot
        bcf             troubleshoot    ; pulse 1 on OSCILLOSCOPE

        ; Look below at the FIRST pulse, this is where we are in a line

The following description of the code and how it maintains lock is incorrect. A corrected description will follow this one.

        ; the code below works as follows: (O = OscDecision)
        ; if the neg. edge of a horizontal sync is detected in section A but not B then O.2 is 0 (slow down oscillator)
        ; if the neg. edge of a horizontal sync is detected in section A and B then O.2 is 1 (speed up oscillator)
        ; if the neg. edge of a horizontal sync is detected in section B but not A then O.2 is 1 (speed up oscillator)
        ; if the neg. edge of a horizontal sync not detected in either section then O.2 is 1 (speed up oscillator)
        ; if the pos. edge of a horizontal sync is detected then O.3 is 1 (slow down oscillator)
        ;
        ;
        ; in other words, if the neg pulse is found in A but not B then we are doing good, slow down and let it
        ; cross into section b so we can speed it up again!   (*** Wrong ***)
        ; This provides a very accurate PLL.

        ; section a
        movf    PORTB, 0        ; samples for negative edge of hsync
        iorwf   PORTB, 0
        iorwf   PORTB, 0        ;
        xorlw   NED                    ;

        ; section b
        iorwf   PORTB, 0        ; There should be no neg here if we are on time...
        iorwf   PORTB, 0        ; There is some delay until edge detect
        iorwf   PORTB, 0        ;   will return back to normal.
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        movwf   OscDecision    ; this is where the final sample is stored.

        movlw   MAKEBURST    ; time to make the burst
        btfss      VIDEOKAY        ; IF video is not locked up THEN
        movlw   PASSVIDEO    ; just pass the video
        movwf   PORTB           ; Sync up, let burst pass by

        btfsc   OscDecision, NED        ; IF negative edge found in a & b (where it should not be) THEN
        bsf     PORTA, 0                        ; speed up oscillator 50Khz
        btfss   OscDecision, NED        ; ELSE (negative edge found in section a but not b, where it should be)
        bcf     PORTA, 0                        ; slow down oscillator 50Khz
        btfsc   OscDecision, PED        ; IF we heard a positive edge THEN
        bcf     PORTA, 0                        ;   slow down oscillator 50Khz (were okay)

        incf    FastCnt,1                         ; how often are we speeding up?
        btfss   OscDecision, NED        ; IF we are slowing down THEN
        clrf    FastCnt                             ; reset fast count

        incf    SlowCnt, 1                          ; how often are we slowing down?
        btfsc   OscDecision, NED        ; IF we are speeding up THEN
        clrf    SlowCnt                                ; reset SlowCnt

        ; After 127 fast oscillators or 127 slow oscillators we have to do something to speed this lockup!
        ; Notice this routine provides .2uS of course code time tuning!

        btfsc   FastCnt, 7       ; IF we have done 128 fast oscillators (1/2 a field) THEN
        goto    ActiveLine                  ; use up .6uS total!
        btfsc   SlowCnt, 7      ; IF we have done 128 slow oscillators (1/2 a field) THEN
        goto    ActiveLine                ; use up 1.0uS total!

To understand how lock is maintained, you need to understand the relationship of the PEDs/NEDs to the sync and to the code. The code speeds up the oscillator or slows it down depending on where it sees the NED. How does this affect the NED? It doesn't! The NED will happen at the falling edge of the sync regardless of how fast the oscillator is running. What changes with oscillator speed is WHEN we look for the NED. If the oscillator is running too fast, then we are looking for the NED too early and if the oscillator is running too slow then we are looking for the NED too late. The NED itself does not change! Magicboxes says that the NED should be locked to the transition from a to b or directly in front of the section b. Let's look at the code again and re-comment it.

   ; section a
        movf    PORTB, 0             ; samples for negative edge of hsync
        iorwf   PORTB, 0
        iorwf   PORTB, 0              ;
        xorlw   NED                       ; If a NED was detected, then B2=0, if not then B2=1 because of XOR

        ; section b
        iorwf   PORTB, 0        ; There should be no neg here if we are on time...
        iorwf   PORTB, 0        ; There is some delay until edge detect
        iorwf   PORTB, 0        ;   will return back to normal.
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        movwf   OscDecision    ; If a NED was detected in a (B2=0) and was not detected in b, then B2 remains 0

; If a NED was detected in a (B2=0) and was also detected in b, then B2=1, 0 or 1=1

; If a NED was NOT detected in a (B2=1) and was not detected in b, then B2=1

; If a NED was NOT detected in a (B2=1) and was detected in b, then B2=1, 1 or 1 =1

        movlw   MAKEBURST    ; time to make the burst
        btfss      VIDEOKAY        ; IF video is not locked up THEN
        movlw   PASSVIDEO    ; just pass the video
        movwf   PORTB           ; Sync up, let burst pass by

        btfsc   OscDecision, NED        ; If B2=1(see conditions above) then
        bsf     PORTA, 0                        ; speed up the osc 2khz(not even close to 50 Khz)
        btfss   OscDecision, NED        ; ELSE (negative edge found in section a but not b, where it should be)
        bcf     PORTA, 0                        ; slow down oscillator 2Khz
        btfsc   OscDecision, PED        ; IF we heard a positive edge THEN
        bcf     PORTA, 0                        ;   slow down oscillator 50Khz (were okay)

Magicboxes assumes that if we see a NED in both a and b that the oscillator is going too slow and should be speeded up. If we speed it up, what will happen to the relationship between the NED and the a and b sections of the code? Remember, the NED will stay the same and WHEN we sample (the a and b sections) will shift to the left because the code executes faster and the a and b sections will happen earlier. Eventually, the a and b sections will shift to the left far enough that the NED is only in section b. This will still give us a "1" at OscDecision and will keep the oscillator speeded up. Eventually, the a and b section will shift to the left until the NED is not in either section, which STILL keeps the oscillator in the fast mode. At this point, we have lost lock and the a and b sections will shift left until a new sync comes along. Eventually, the a and b sections will find the next sync and the a section will move into the NED. Aha, at this point, in a and not in b, the OscDecision will switch to "0" and slow the oscillator down. The a and b sections will now shift to the right(code is executing slower) until the a section (and b section)is no longer in the NED. The OscDecision will switch back to "1" and speed up the oscillator. This is the point that the 3chip actually locks at. The trailing edge of the NED is locked to the START of the A section, not the a/b junction. If the oscillator locks to the a section alone, then what purpose does the b section serve. NONE!! The b section of code does nothing and merely confuses an already confusing piece of code. Compare this section of code with NTSC rev7t1a or PAL code and you will see that they all do the same thing, lock on the beginning of the a section. Some of Jens' PAL code actually locks on the PED at the end of the sync instead of the NED at the start of the sync.

ActiveLine

        movlw     PASSVIDEO        ; If invert/black level fails then pass normal video
        btfsc       VIDEOINVERT     ; IF video invert flag (in mode) is set THEN
        movlw     PASSINVERT      ; pass inverted video
        btfsc       VIDEOBLACK      ; IF video blackout flag (in mode) is set THEN
        movlw     PASSBLACK       ; pass nothing
        btfss       VIDEOKAY            ; IF the 3-chip is NOT locked onto the signal then
        movlw     PASSVIDEO        ; only pass the video
        movwf     PORTB           ; Set up the active line with config from above

                                       ; setup the oscillator speed control
        movlw   11111010b     ; Enable the Oscillator speed control to work
        tris    PORTA

        bsf     troubleshoot
        bcf     troubleshoot    ; pulse 2 on TRACE 2

; we have A LONG WAY TO GO!
; next up, Finding our place in the field, find data, and inverted video....

        nop

        movlw   17
        movwf   temp
        movf    PORTB, 0        ; If this is line 6 then no transitions, if data both PED+NED are set!

sampleloop
         iorwf   PORTB, 0        ; check for data for awhile.
        decfsz temp,1
        goto    sampleloop

        movwf   Data                ; place to store the status of potential data found

                                                    ; Where are we in the FRAME?
    ; You had better appreciate this image as it only happens once! (a real pain).
    ; Below you will see an image which will only every happen ONCE when you flip a channel:

; notice the dead video before pulse 3, In line 6 there should be nothing there as above.
; The following statement makes sure that

xorlw NED+PED ; (both this time)
movwf Line6 ; if we are on line 6 then line6 = NED+PED (remember this for later!)

        movf    PORTB, 0        ; Negative edge in line when field is not inverted
        iorwf   PORTB, 0        ;    These inclusive ors give us a healthy window
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        andwf   isInverted, 1    ; isInverted.2 = 1 when not inverted, 0 when inverted

        ; Look below at the figure. You are looking at Line 20 which controls invert. As you can see the third
        ; pulse window falls during the Invert level transition. If the pulse falls as below, then the video is not inverted
        ; and NED becomes 1 and isInverted.2=1 . If the video IS inverted then this pulse does not change and
        ; then NED would have been 0, then isInverted.2 = 0

        movf    PORTB, 0        ; This detects line 6 rising (look at pulse 4 above)
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        iorwf   PORTB, 0

xorlw NED ; This will make W= NED +PED if line 6 is rising here (which it is)
andwf Line6, 1 ; This will keep LINE6 = NED+PED if we are on line 6.

; this routine is normally run throught this procedure as a NOP in all cases. It simply reads the switch
; and sets an invert flag if so.

btfss MANUALINVERT ; IF RA3 is 0 (button pushed) THEN
bsf WANTSINVERT ; set inversion flag

        movf    PORTB, 0        ; Line 6 should have NO W bits set!
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        iorwf   PORTB, 0

xorlw PED+NED ; this will make PED+NED if on line 6
andwf Line6, 1 ; line6=PED + NED if it is still line 6

        movf    PORTB, 0        ; Check for falling edge of line 6 (look at pulse 5 as above!)
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        iorwf   PORTB, 0

xorlw PED ; you guessed it !, W = PED +NED if on line 6
andwf Line6, 1 ; and yes, line6 = PED+NED if still finding we are on line 6

        btfss   MANUALINVERT ; just bullshit as above, nops got boring.., checking the switch again.. fun..
        bsf     WANTSINVERT ;
        btfss   MANUALINVERT ;
        bsf     WANTSINVERT ;

        movf    PORTB, 0        ; Check for rising edge again! (look at the last pulse, #6)
        iorwf   PORTB, 0
        iorwf   PORTB, 0
        iorwf   PORTB, 0

        xorlw   NED            ; w = NED +PED if on line 6 (BTW if you now think NED stands for Ned Flanders then please
                                        ; move away from your computer...)
        andwf   Line6, 1    ; line6 = NED+PED if on 6

        movlw   26
        movwf   temp
        movf    PORTB, 0        ; Line 6 is quiet (if we are on line 6 - look above after pulse 6)

sampleloop2
         iorwf   PORTB, 0
        decfsz temp, 1
        goto   sampleloop2

xorlw PED+NED ; you know...
andwf Line6, 1 ; finally, if we are on line 6 then line6=NED+PED

rlf Line6, 0 ; Move NED to PED position in W
andwf Line6, 1 ; If PED in here is 1 then we are on line 6!!!!!!!!!!!!!!!!!

rlf Data, 0 ; Move NED to PED position in W
andwf Data, 1 ; If PED in Data is 1 then we found data on this line!!!!

movf FindLine6, 0 ; Are we searching for line6??????? (PED =1 if so)
andwf Line6, 1 ; Line6 (PED bit) is 1 if we have to now make up lines 7-10

btfsc FindLine6, PED ; IF we are looking for line 6 THEN
incf makelines, 1 ; keep searching (add an extra line to create until we find it!)

        movlw       PASSBLACK
        btfss         VIDEOKAY        ; IF 3-chip not locked on THEN
        movlw       PASSVIDEO    ; pass normal video
        movwf       PORTB           ; create front porch of next sync.

        decf        makelines, 1
        btfsc        Z               ; IF no more hlines to create THEN
        retlw        1               ;   return to main pgm

        nop
        btfsc         Line6, PED      ; IF Line6 found THEN
        clrf            FindLine6            ;   Stop searching for it.

        movlw       4
        btfsc          Line6, PED      ; Create 4 more lines (7-10) for main pgm
        movwf       makelines        ;

        goto          MAKEHLINE

        btfsc   Error, PED              ; IF there was an error from MAKEVSYNC THEN
        incf     ErrorCnt, 1              ;   add 1 to error counter
        btfsc   ErrorCnt, 2             ; IF 4 errors THEN
        bcf      VIDEOKAY             ; stop descrambling
        movf    Error, 0
        btfsc   ErrorCnt, 2        ; IF we have had 4 errors in a row THEN
        movwf   FindLine6        ;   look for line6

btfsc FindLine6, PED ; IF we are looking for line 6 THEN
clrf ErrorCnt ; restart counter

btfss Error, PED ; IF there was not an error THEN
incf VideokayCnt, 1 ; increment the good video counter

        btfsc   WANTSINVERT    ; IF the user pressed the invert switch THEN
        bsf     DataLine12             ; say we saw data on line 12
        btfsc   WANTSINVERT     ; IF the user pressed the invert switch THEN
        bsf     DataLine13            ; say we saw data on line 13

        btfss   DataLine12            ; IF no data on line 12 detected THEN
        clrf    VideokayCnt            ; clear the video okay counter
        btfss   DataLine13
        clrf    VideokayCnt

        btfsc   VideokayCnt, 1    ; IF 2 good frames have been heard THEN
        bsf     VIDEOKAY            ; then allow DECODING
        btfsc   VideokayCnt,1    ; IF 2 good frames have been heard THEN
        clrf      VideokayCnt      ; Clear this counter

        movlw   41
        movwf   temp

modeloop1
        decfsz temp, 1         ; waste some time..
        goto    modeloop1

        movlw   MAKESYNC     ; time to make the sync
        btfss     VIDEOKAY        ; IF 3-chip not locked on THEN
        movlw   PASSVIDEO    ; just pass video
        movwf   PORTB             ; change mode.
        nop

        movlw   5
        movwf   temp

makehalflineloop1
        decfsz temp, 1
        goto    makehalflineloop1        ; waste time..

        nop

        movlw   MAKEBURST    ; time to make the burst
        btfss     VIDEOKAY    ; IF 3-chip is not locked on THEN
        movlw   PASSVIDEO ; pass video
        movwf   PORTB           ; change mode

        movlw   5
        movwf   temp

makehalflineloop2
        decfsz temp, 1
        goto    makehalflineloop2

        movlw   PASSBLACK        ; time to send black level
        btfss   VIDEOKAY                ; is the 3-chip bla bla bla not locked on then
        movlw   PASSVIDEO            ; pass video
        movwf   PORTB                     ; change mode

       goto    makehalfl1                ; this is called tuning at 3AM in the morning.
makehalfl1                                   ; I hated some writting some of this fricking code...
     goto    makehalfl2
makehalfl2
    goto    makehalfl3
makehalfl3
   goto    makehalfl4

        movlw   PASSBLACK    ; This is called bad tuning at 3am, this code does nothing but
        btfss   VIDEOKAY           ; was used until I looked at my screwup here on a scope.
        movlw   PASSVIDEO     ; kept this useless code cuz I was too lazy to remove it...
        movwf   PORTB           ; Useless

        movlw   28
        movwf   temp

makehalfloop2
       decfsz temp, 1
        goto    makehalfloop2    ;waste some time

        movlw   PASSBLACK ; this was WORSE alignment than the above (probably about 2am)
        btfss     VIDEOKAY        ; kept as I got fed up with recalculating the above loop
        movlw   PASSVIDEO
        movwf   PORTB           ; Useless

        nop
        nop

; Generates the vertical blanking interval
; makes sure we are actually writing this vsync when the real one is happening

        movlw   MAKESYNC        ; time to make sync level
        btfss    VIDEOKAY            ; IF 3-chip not locked THEN
        movlw   PASSVIDEO        ; just pass video
        movwf   PORTB           ; change mode

        bsf    troubleshoot
        nop
        bcf    troubleshoot

        movlw   2
        movwf   temp
makevbiloop1
       decfsz temp,1
        makevbiloop1        ; waste time

        movlw   MAKEBLACK        ; you know, pass black if okay, pass video if not...
        btfss    VIDEOKAY
        movlw   PASSVIDEO
        movwf   PORTB           ; change mode

        movlw   45
        movwf   temp
makevbiloop2
       decfsz temp,1
        goto   makevbiloop2

        movlw   6               ; for the vsyncs next
        movwf OscDecision         ; confusing but i recycled the variables
        decfsz makelines, 1       ; have we made 6 yet???

        goto     MAKEVBI

; Now make 6 serrated vertical sync pulses - Lines 4-6
; This is where we check for errors!

        bsf     MODE,PED ; this is set for anding the result later (a vsync pos det. mask)
makesync

        movlw   MAKESYNC        ; make vsync now...
        btfss     VIDEOKAY
        movlw   PASSVIDEO
        movwf   PORTB           ; change mode

        movlw   28
        movwf   temp

        goto    makevbi1
makevb1
     goto    makevbi2
makevbi2
     goto    makevbi3
makevbi3
     goto    makevbi4
makevbi4
     bsf        troubleshoot            ;nop

        clrw

        nop
lookforpossync
        iorwf   PORTB, W        ; sample for vsync pulses
       decfsz temp, 1        ;
        goto   lookforpossync             ; pos sync detected during sample means we are not lined up...

        movwf   temp           ; crap
        andwf   Mode    ; if we got an error then the PED bit will be 1
        movwf   temp        ; save the result
        movlw   2               ; old code crap
        btfsc   temp, PED      ; IF positive edge was detected in vsync THEN
        bcf     Mode, PED ;   clear the mask bit we set earlier
        btfsc   temp,PED
        bsf     Error, PED   ;   set the error bit, let SETMODE clean things up

        movlw   PASSBLACK ; sync is over... pass black
        btfss   VIDEOKAY
        movlw   PASSVIDEO
        movwf   PORTB           ; make mode change

        movlw   4
        movwf   temp

makevbiloop3
       decfsz temp,1
        goto    makevbiloop3        ; waste time

        movlw   6
        movwf   makelines
        decfsz OscDecision, 1        ; sorry for the confusion, a recycled variable ...

        goto    makevsync

        bsf    troubleshoot             ;nop

makepost
        movlw   MAKESYNC
        btfss   VIDEOKAY
        movlw   PASSVIDEO
        movwf   PORTB           ; change mode to sync again

        clrwdt

        movlw   2
        movwf   temp

makevbiloop6
       decfsz temp,1
        goto    makevbiloop6        ; waste time

        movlw   PASSBLACK
        btfss    VIDEOKAY
        movlw   PASSVIDEO
        movwf   PORTB           ; change mode to black if video okay

        movlw   43
        movwf   temp

makevbiloop7
        decfsz temp,1
        goto    makevbiloop7

        decf    makelines,1
        btfsc   Z       ;IF we made all 6 THEN

        retlw   1               ; RETURN TO MAIN

        goto    makevbi10
makevbi10
     goto    makevbi11
makevbi11
     goto    makevbi12
makevbi12
     goto    makepost

call MAKEVBI ; Make FIELD 1 - LINES 1-9

clrf DataLine ; this clears data heard in video lines (DataLine10 - DataLine15)

        btfss   WANTSINVERT ; IF user does not want inverted video THEN
        bsf     VIDEOINVERT ;   enable inverted video

       movlw   1
        movwf makelines

        call    MAKEHLINE        ; Line 10

        btfsc   Data, PED            ; IF we heard data on line 10 THEN
        bsf     DataLine10            ; save that in DataLine10 bit

        nop
        movlw   1
        movwf   makelines

        call    MAKEHLINE        ; Line 11

        btfsc   Data,PED            ; IF we heard data on line 11 THEN
        bsf     DataLine11            ; save it in bit dataline11

btfsc Data, PED ; IF we heard data on line 12 THEN
bsf DataLines12 ; save it in bit dataline12

btfsc Data,PED ; IF we heard data on line 13 THEN
bsf DataLine13 ; save it in bit dataline13

btfsc Data,PED ; IF we heard data on line 14 THEN
bsf DataLine14 ; remember that...

btfsc Data,PED ; IF we heard data on line 15 THEN
bsf DataLine15 ; remember that...

        nop
        movlw   4
        movwf   makelines        ; make 4 lines now

btfsc DataLine10 ; IF line 10 contains data THEN
bsf isInverted, NED ; look at the inversion flag on line 20

btfsc isInverted, NED ; IF negative edge found THEN
bcf VIDEOINVERT ; Normal video (remember the default is inverted from above)

        movlw   1
        movwf   makelines
        call    MAKEHLINE         ; Line 21

        clrf    isInverted
        btfss   DataLine11         ; IF no data at line 11 THEN
        bsf     isInverted, NED    ;   look for inversion flag on line 22

        movlw   1
        movwf   makelines
        call    MAKEHLINE         ; Line 22   (some systems moved invert flag to line 22)

btfsc isInverted, NED ; IF no inversion flag detected THEN
bcf VIDEOINVERT ; set normal video

        call    MAKEHLINE         ; Lines 23-24
        bcf     VIDEOBLACK      ; Clearing this flag allows normal video/inverted video
        nop
        nop

call MAKEHLINE ; Lines 25-259
bsf VIDEOBLACK ; No video allowed to pass, only black..

        call    MAKEHLINE         ; Lines 260-262

        nop
        nop
        nop
        movlw   1
        movwf   makelines         ; junk

        call    MAKEHALFLINE        ; Line 263.5

        nop
        nop
        nop
        movlw   6
        movwf   makelines

        call    MAKEVBI                     ; make the VBI (make the vsync)

        nop
        nop

        movlw   1
        movwf   makelines

        call    SETMODE            ; is the descrambler locked on properly?

        btfss   WANTSINVERT     ; IF user does not want invert THEN
        bsf     VIDEOINVERT      ;   enable inverted video
        bcf     WANTSINVERT     ; Clear user flag

        movlw   1
        movwf   makelines

        call    MAKEHLINE        ; Line 10
        nop                     ;
        nop                     ;
        nop

        clrf    isInverted
        btfsc   DataLine10         ; IF data on line 10 THEN
        bsf     isInverted, NED    ;   look for inversion flag.

btfsc isInverted, NED ; IF inversion not detected THEN
bcf VIDEOINVERTED ; Normal video

btfss DataLine11 ; IF line 11 has no data THEN
bsf isInverted, NED ; check line 22

        nop
        btfsc   isInverted,NED    ; IF no inversion detected THEN
        bcf     VIDEOINVERTED ;   normal video

        bsf     VIDEOBLACK ; DO not pass video during these lines...
        nop
        nop
        movlw   3
        movwf   makelines

        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP

BSF FastCnt,7 ; lets lock on to the video real quick as the board has just turned on.

1.    This version above of the 3-chip program dose not support Jerrold Baseband (invert sync pedistal)
2.    This version is not properly timed in field 2 so the horizontal sync dose not lock up until about line 15.
3.    Slight video brightness levels can be noticed during high-speed mode changes due to the failed lockup.

3-chip Rev 5 (2200.src / .obj)
The new 2200 3-chip file solves all those problems and MORE!
1.    Perfect locks on EVERY LINE
2.    Perfect VBI with errors
3.    Traps for split-sync invert pedistal inversion and clamps on it
4.    The finest work I have done yet, I cannot notice ANY change in video on ANY high speed mode on both
       Jerrold and Zenith using my ZTAC headend, GI - Jerrold MVP-I or MVP-II.

Two things actually, I will be documenting/adding commented code for both the 2200 version of code (rev 5) and PAL code. Further, as I now have the active TOCOM invert code done that will be put online as well.
I will start on the PAL code now so I don't get linched!

1. More commented source code with pictures (still got some good vbi pictures yet)
2. PAL commented source code with pictures (until its all done) [both PAL and PAL-M]
3. New Rev 5 commented source code with pictures
4. TOCOM active invert source code with pictures and comments
5. Newest 3-chip family member (the 2-chip) with GI active invert, multi-mode source code and board
6. Sound Privacy

Well, the code and explanations following took a lot of work as my Tek scope dose not like to lock up on PAL video as nicely as NTSC.

3-Chip Descrambler Theory of Operation

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Welcome to massmind.org!