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Video Cable Converter FAQ

Note that this information on Video Cable Converter and descramblers has been collected from magazines, various posts over the past several years, etc. I do not have any personal experience with this stuff (which is why it may seem so vague in places where I’ve only heard bits and pieces about it) but at least it’s a start. All standard disclaimers apply.

Important Notice

The ownership of a signal descrambler does NOT give the owner the right to decode or view any scrambled signals without authorization from the proper company or individual. Use of such a device without permission may be in violation of state and/or federal laws. The information contained herein is intended to serve as a technical aid to those person seeking information on various scrambling techniques. No liability by myself or my employer is assumed for the (mis)use of this information.

{Note: Someone recommended a book called “Satellite TV Descrambling” by Sheets/Graf. I’ve never seen it, but it may be of interest to some.}

Scrambling Technologies

No Scrambling (Traps/Addressable Taps)

A cable system may not be scrambled at all. Some older systems (and many apartment complexes) use “traps” or “filters” which actually REMOVE the signals you aren’t paying for from your cable. (These are negative traps because they remove the WHOLE signal.) These systems are relatively secure because the traps are often located in locked boxes, and once a service technician finds out they’re missing or have been tampered with (by pushing a pin through a coax trap it to change its frequency, for example), it’s a pretty solid piece of evidence for prosecution. Another method is where the head-end ADDS an extraneous signal about 2.5 MHz above the normal visual carrier which causes a tuner to think its receiving a very strong signal–the tuner then adjust the automatic gain control and buries the real signal. If you pay for the service, the cable company adds a “positive trap” which then REMOVES the extraneous injected signal so it becomes viewable. (This system is very easy to circumvent by building your own notch filter, so it is not very commonly used.) Advantages to a cable system with this technology is that you don’t need a cable box–all your cable-ready TVs, VCRs, etc. will all work beautifully. The disadvantage is that pay-per-view events are not possible, and that every time someone requests a change in service, a technician has to be dispatched to add/remove the traps.

Becoming more and more popular, not only because of the Cable Act of 1992 but also in an effort to stop “pirates” are addressable taps. Many cable companies WILL be moving to this technology in the near future. These are devices located at the pole, where your individual cable feed is tapped from the head-end. Similar to addressable converters, they each have a unique ID number and can be turned on/off by a computer at the head-end. Any stations which you are not paying for are filtered out by electronicly switchable traps in the units. (Including the whole signal if you haven’t paid your bill or had the service disconnected.) {Several patents have already been issued for various methods of making SURE you don’t see a channel you don’t pay for.} Again, these almost GUARANTEE an end to piracy and don’t have any of the disadvantages of the manual traps. Plus, they provide a superior signal to those customers paying for service because they no longer need complicated cable boxes or A/B switches — and they can finally use all of the “cable-ready” capabilites of the VCR, TV, etc. About the only known attack on this type of system is to splice into a neighbors cable, which again provides plenty of physical evidence for prosecution.


Early Oak (and some very early Pioneer boxes) employed a sine-wave sync suppresion system. In this system, the picture would remain vertically stable, but wiggling black bars with white on either side would run down the center of the screen. The lines were caused by a 15,750 Hz sine-wave being injected with the original signal, causing the sync separator in the TV to be unable to detect and separate the sync pulses. Later, Oak came out with a “Vari-Sync” model, which also removed a 31,500 Hz sine-wave added to the signal. Oak was one of the first to use extra signals (“tags”) as a counter-measure for pirate boxes — in the normal mode, a short burst of a 100 KHz sine-wave (the tag signal) would be sent during the VBI, along with the AM sine-wave reference on the audio carrier and scrambled video. They would then put the AM sine-wave reference signal onto the audio carrier, leave the video alone, and NOT send the tag. Any box which simply looked for the AM sine-wave reference would effectively scramble the video by adding a sine-wave to the unscrambled video! Real decoders looked for the tag signal and still worked correctly. Other combinations of tag/no tag, scrambled/unscrambled video were also possible.

6 dB In-Band Sync Suppression

Early Jerrold boxes used in-band gated sync suppression. The horizontal blanking interval was suppressed by 6 dB. A 15.734, 31.468 or 94.404 KHz reference signal (conveniently all even multiples of the horizontal sync frequency) was modulated on the sound carrier of the signal, and used to reconstruct the sync pulse. An article in February 1984 issue of Radio-Electronics explains this somewhat-old technique. Converters which have been known to use this system include the Scientific-Atlanta 8500-321/421, a number of Jerrold systems [see numbering chart], Jerrold SB-#, SB-#-200, SB-#A, RCA KSR53DA, Sylvania 4040 and Magnavox Magna 6400.

Tri-mode In-Band Sync Suppression

A modification to the 6dB sync suppresion system, dubbed “tri-mode”, allows for 0, 6 and 10 dB suppression of the horizontal sync pulse. The three sync levels can be varied at random (as fast as once per field), and the data necessary to decode the signal is contained in unused lines during the VBI (along with other information in the cable data stream.) See the February 1987 issue of Radio-Electronics for a good article (both theory and schematics) on the tri-mode system. Converters which have been known to use this system include a number of Jerrold systems [see numbering chart], Jerrold SBD-#A, SBD-#DIC, Jerrold Starcom VI (DP5/DPV models), Regency, Scientific- Atlanta 8550-321 {anyone know any others for sure?} and early Pioneer systems {anyone know for sure which ones?}.

Out-Band Sync Suppression

Out-band gated sync systems also exist, such as in early Hamlin converters. In this system, the reference signal is located on an unused channel, usually towards the higher end (channels in the 40’s and 50’s are common, but never in the low 30’s due to potential false signalling.) The signal is comprised of only sync pulse information without any video. Tuning in such a channel will show nothing but a white screen and will usually have no audio.


SSAVI is an acronym for Synchronization Suppression and Active Video Inversion and is most commonly found on Zenith converters. ZTAC is an acronym for Zenith Tiered Addressable Converter. Besides suppressing sync pulses in gated-sync fashion, video inversion is used to yield four scrambling modes (suppressed sync, normal video; suppressed sync, inverted video; normal sync, inverted video; and normal sync, normal video). The mode of scrambling can be changed as fast as once per field. Their is no “reference signal” per-se, but the horizontal sync pulses during the VBI are not suppressed, allowing a phased-lock loop to be used to generate the missing sync pulses. Information on whether the video is inverted or not is contained in the latter-half of one of the lines of video, usually line 20 or 21. The Drawing Board column of Radio-Electronics starting in August ’92 and going through early ’93 described the system and provided several circuits for use on an SSAVI system. Audio in the system can be “scrambled” – usually by burying it on a subcarrier that’s related mathematically to the IF component of the signal. Addressable data for Zentih systems is sent in the VBI, lines 10-13, with 26 bits of data per line.

Tocom systems

The Tocom system is similar to the Zenith system since it provides three levels of addressable baseband scrambling: partial video inversion, random dynamic sync suppression and random dynamic video inversion. Data necessary to recover the signal is encrypted and sent during lines 17 and 18 of the VBI (along with head-end supplied teletext data for on-screen display). The control signal contains 92 bits, and is a 53 ms burst sent just after the color burst. Up to 32 tiers of scrambling can be controlled from the head-end. Audio is not scrambled.

New Pioneer systems

The newer 6000-series converters from Pioneer supposedly offer one of the most secure CATV scrambling technologies from a “major” CATV equipment supplier. From the very limited information available on the system, it appears that false keys, pseudo-keys and both in-band and out-band signals are used in various combinations for a secure system. From U.S. patent abstract #5,113,441 which was issued to Pioneer in May ’92 (and may or may not be used in the 6000-series converters, but could be), “An audio signal is used on which a key signal containing compression information and informaton concerning the position of a vertical blanking interval is superimposed on a portion of the audio signal corresponding to a horizontal blanking interval. In addition, a pseudo-key signal is superimposed…so that the vertical blanking interval cannot be detected through the detection of the audio signal… Descrambling can be performed by detecting the vertical blanking interval based on the information…in the key signal, and decoding the information for the position which is transmitted in the form of out-band data. Compression information can then be extracted from the key signal based on the detected vertical blanking interval, and an expansion signal for expanding the signal in the horizontal and vertical blanking periods can be generated.” {If anyone has any better information on the 6000-series scrambling technique, please send mail!} Note that Pioneer boxes are “booby-trapped” and opening the unit will release a spring-mechanism which positively indicates access was gained to the interior (and sends a signal to the head-end on a two-way system, and may disable the box completely.) {See U.S. patent #4,149,158 for details.} The mechanism cannot be reset without a special device.

Pioneer systems transmit their addressing data on 110.0 MHz.

New Scientific-Atlanta Systems

Some of the early S-A boxes used 6 dB only sync suppression (some of the 8500 models), and some of the 8550 boxes are tri-mode systems. The three digit number after the model (such as 321) is a code which indicates the make of the descrambler in the unit. Apparently some of the newer S-A boxes use a technique called “dropfield”. {If anyone has more information on any of the 85xx-series or the 8600^x boxes, or an explanation of “dropfield”, send mail…}

Scientific-Atlanta systems transmit their addressing data on 106.2 or 108.2 MHz.

Oak “Sigma” Systems

This a secure system which replaces the horizontal sync of each line of video with a three-byte digital word. Video is switched from inverted to non-inverted between scene changes, and the colorburst frequency is shifted “up”. This is a standard “suppressed” sync video scrambling method and is relatively simple to defeat with the appropriate circuitry. HOWEVER, the three-byte digital word in the area where the sync normally is contains audio and sync information. The first two bytes contain a digitized versions of the audio, the third byte contains sync information (and perhaps addressing data?) The two bytes of digitized audio are encrypted; a separate carrier signal contains the decryption keys for the digital audio datastream.

Jerrold Baseband

No information on techniques used by Jerrold “baseband” converters. {If anyone has information on other Jerrold scrambling methods other than those mentioned above, send mail.}


The research and development division of Fundy Cable Ltd., NCA Microelectronics, has a systemd dubbed “Chameleon”. They claim it is a cost-effective solution that prevents pay TV theft by digitally encrypting the video timing information of sync suppression systems. The company claims the technology has been proven to be effective against pirate and tampered boxes. Supposedly, existing decoders can be upgraded to Chameleon technology with a low-cost add-in circuit, and that the card’s sealed custom IC, developed by NCA, is copy-proof.


The VideoCipher system is now owned by General Instrument and is used primarily for satellite signals at this time. VideoCipher I is the “commercial” version which uses DES (Data Encryption Standard)-encrypted audio AND video. A VCI descrambler is not available for “home” owners. VideoCipher II is the now-obsolete system which used a relatively simple video encryption method with DES-encrypted audio. (Specifically, the audio is 15 bit PCM, sampled at ~44.1 KHz. It is mu-law companded to 10 bits before transmission.) This has recently been replaced by the VideoCipher II+, which has been incorporated as the ‘default’ encryption method used by VideoCipher IIRS (a smart-card based, upgradeable system). Supposedly, coded data relating to the digitized, encrypted audio is sent in the area normally occupied by the horizontal sync pulse in the VCII system. (The Oak Sigma CATV system uses a similar technology.) Several methods existed for pirating the VCII based system, and some SUPPOSEDLY exist for the new VCII+ format, although this has never been verified. See the rec.video.satellite FAQ list for more information.


DigiCipher is an “upcoming” technology being developed by General Instrument for use in both NTSC and HDTV environments. The DigiCipher format is for use on satellites, and the DigiCable variation will address CATV needs. It provides compression algorithms with forward error correction modulation techniques to allow up to 10 “entertainment quality” NTSC channels in the space normally occupied by one channel. It provides true video encryption (as opposed to the VCII-series which only DES encrypts the audio). In a Multiple Channel Per Carrier (MCPC) application, the data rate is ~27 MB/second via offset QPSK modulation. Audio is CD-quality through Dolby AC-2 technology, allowing up to four audio channels per video channel. The system uses renewable security cards (like the VCIIRS), has 256 bits of “tier” information, copy protection capability to prevent events from being recorded, commercial insertion capability for CATV companies, and more. The multichannel NTSC satellite version of DigiCipher started testing in July of 1992, and went into production several months later.


MAC is an acronym for Mixed Analog Components. It refers to placing TV sound into the horizontal-blanking interval, and then separating the color and luminance portions of the picture signal for periods of 20 to 40 microseconds each. In the process, luminance and chrominance are compressed during transmission and expanded during reception, enlarging their bandwidths considerably. Transmitted as FM, this system, when used in satellite transmission, provides considerably better TV definition and resoluton. Its present parameters are within the existing NTSC format, but is mostly used in Europe at this time. {Does anyone know if the D2-MAC system is just a variation of this, or is it completely different? What’s new in the D2-MAC system?}

Miscellaneous Information

Two-Piece vs. One-Piece

There are both advantages and disadvantages to the one-piece and two-piece descramblers often advertised in the back of electronics magazines. The “one-piece units” are real cable converters, just like you’d get if you rented one from the cable company. It has the advantages of “real” descrambling circuitry and the ability to “fit-in” well when neighbors come over (avoids those “my box doesn’t look like that…or get all these channels!” conversations 🙂 A disadvantage is that if you move or the cable company installs new hardware, you may now have a worthless box — most one-piece units only work on the specific system they were designed for. Another disadvantage is that if the box has not been modified, it can be very easy for the head-end to disable the unit completely. (See Market Codes & Bullets, below.)

A “two-piece unit” (“combo”) usually consists of an any-brand cable TV tuner with a third-party “descrambler” (often referred to as a “pan”) which is designed to work with a specific scrambling technology. The descrambler typically connects to the channel 3 output of the tuner, and has a channel 3 output which connects to your TV. (Although some tuners have a “decoder loop” for such devices.) They have the advantage that if you move or your system is upgraded, you can try to purchase a new descrambler — which is much cheaper than a whole new set-up. You also can select the cable TV tuner with the features you want (remote, volume control, parental lockout, baseband video output, etc.) Two-piece units typically cannot be disabled by the data stream on your cable. (Note however that there ARE add-on “pans” manufactured by the same companies who make the one-piece units that DO pay attention to the data stream and can be disabled similarly!) The main disadvantage is that a third-party descrambler MAY not provide as high of quality descrambling as “the real thing”, and it may arrouse “suspicion” if someone notices your “cable thing” is different from theirs.

Jerrold Numbering System

To decode older Jerrold converters, the following chart may be helpful. (Note that some spaces may be blank.) {Send along any additions or other numbering systems you know of!}

How to Decode Jerrold converters

Also note that some Jerrold converters (particularly the DP5 series and maybe others) have a tamper-switch, and that opening the box will clear the contents of a RAM chip in the converter. This may or may not be corrected by letting the unit get “refreshed” by the head-end data stream.

Most Jerrold systems transmit their addressing data near 106.5 MHz.
Scientific-Atlanta Suppressed Sync Boxes

Model 8600

Scientific-Atlanta Suppressed Sync Boxes - Model 8600

The 8600 has 240 character on-screen display, multimode scrambling, 8 event 14 day timer, and is “expandable”…

Model 859

Scientific-Atlanta Suppressed Sync Boxes  - Model 859

The 8590s feature volume control, multimode scrambling, 8 event 14 day timer…

Model 858
Model 858 Cable Converter and Descramblers

The 8580s use dynamic sync suppression, 8 event 14 day timer, and built-in pre-amp.

The 8570 is similar to the 8580.

Model 8550

Model 8550 Sync Box

The 8550 is not a current model; it can be replaced with an 8580-321.

Non-addressable products include the 8511, 8536, and 8540.

{If anyone has more details/corrections, please send them along.}

Market Codes

Note that almost every addressable decoder in use today has a unique “serial number” programmed into the unit — either in a PROM, non-volatile RAM, EAROM, etc. This allows the head-end to send commands specifically to a certain unit (to authorize a pay-per-view events, for example.) Part of this “serial number” is what is commonly called a “market code”, which can be used to uniquely identify a certain cable company. This prevents an addressable decoder destined for use in Chicago from being used in Houston. In most cases, when a box receives a signal with a different market code, it will enter an “error mode” and become unusable. This is just a friendly little note to anyone who might consider purchasing a unit from the back of a magazine — if the unit has not been “modified” in any way to prevent such behavior, you could end up with an expensive paper weight… (see next section)

Test Chips

So-called “test chips” are used to place single-piece converters (that is, units with both a tuner and a descrambler) into full service. There are a number of ways to accomplish this, but in most cases, the serial number/market code for the unit is set to a known “universal” case (RARE THESE DAYS) or, better yet, the comparison checks to determine which channels to enable/disable are bypassed by replacing an IC in the unit. Hence, the “descrambler” will always be active, no matter what. This latter type of chip is superior because it cannot be disabled and is said to be “bullet proof”, even if the cable company finds out about a “universal” serial number. (When the cable company finds out about a universal serial number, it is easy for them to disable the converter with a variation on the “bullet” described below.)


A relatively new “test device” has been advertised in magazines such as Electronics Now (formerly Radio-Electronics) and Nuts & Volts. It’s called a “cube” and it SIMULATES the addressing data signal for a cable box. You plug the cable into one side, where it filters out the real data signal, and out the other side comes a normal signal, with a new data stream. This new data signal tells whatever boxes are connected after it to go into “full-service” mode (including any cable company-provided boxes). It is usually a non-destructive signal, and if the the “cube” is removed from the line, the real data signal gets sent to the converter which then goes back to normal operating mode. I say “usually non-destructive” because there are some cubes that re-program the electronic serial number in a box to a new value. (This has the advantage that it will work with ANY converter the unit was designed for.) The “non-destructive” versions of the “cube” usually require that you provide the serial number from the bottom of the converter you’re interested in “testing”. That way a custom IC can be programmed to address that converter with the necessary codes. (Otherwise the converter would ignore the information, since the serial number the cube was sending and the one in converter wouldn’t match.)


First and foremost, THE “BULLET” IS NOTHING MORE THAN THE NORMAL CABLE DATA STREAM WITH THE APPROPRIATE “CODE” TO DISABLE A CONVERTER WHICH HAS NOT BEEN ACKNOWLEDGED BY THE CABLE COMPANY. For instance, the head end could send a code to all converters which says “unless you’ve been told otherwise in the last 12 hours, shut down.” All legitimate boxes were individually sent a code to ignore this shut down code, but the pirate decoders didn’t get such a code because the cable company doesn’t have their serial number. So they shut down when the see the “bullet” code. The “bullet” is NOT a harmful high-voltage signal or something as the cable companies would like you to believe — if it was, it would damage anyone with a cable-ready TV or VCR connected to the cable (not something the cable company wants to deal with!) The only way to get “caught” by such a signal is to contact the cable company and tell them your illegal descrambler just quit working for some reason. 🙂 Not a smart thing to do, but you’d be surprised (especially if it’s someone else in the house who calls, like a spouse, child, babysitter, etc.) While we’re on the subject, it’s also not a good idea to have cable service personnel come into your residence and find an unauthorized decoder. If you have one, use common sense and tell anyone you live with to call YOU and NOT the cable company if something goes wrong. Just some friendly advice

Time Domain Reflectometry / Leak Detection

The cable company can use a technique called “Time Domain Reflectometry” (TDR) to try and determine how many devices are connected to your cable. In simple terms, a tiny, short test signal is sent into your residence and the time domain reflectometer determines the number of connections by the various “echoes” returned down the cable (since each device is at a different point along the cable, they can be counted.) Each splitter, filter, etc. will affect this count. A simple way to avoid being “probed” is to install an amplifier just inside your premises before any connections. This isolates the other side of the cable from the outside, and a TDR will only show one connection (the amplifier).

The cable company also has various ways of detecting signal “leaks” in their cable. The FCC REQUIRES them to allow only so much signal to be radiated from their cables. You may see a suspicious looking van driving around your neighborhood with odd-looking antennas on the roof. These are connected inside to field strength meters which help locate where the leaks are coming from so they can be fixed (to prevent a fine from the FCC!) If you’ve tampered with a connection at the pole (say, to hook up a cable that had been disconnected) and didn’t do a good job, chances are the connection will “leak” and be easily found by such a device. This can also happen INSIDE your residence if you use cheap splitters/amplifiers or have poorly-shielded connections. The cable company will ask to come inside, and bring with them a portable field strength meter to help them locate the problem. Often they will totally remove anything causing the leak, and may go further (e.g., legal action) if they feel you’re in violation of your contract with them (which you agree to by paying your bill.) Obviously it’s a bad idea to let cable service personnel into your house if you ARE doing something you shouldn’t (which you shouldn’t be in the first place), but if you DON’T let them in (as is your right), it will definitely arouse suspicion. Eventually you will have to let them in to fix the “leak”, or they will disconnect your cable to stop the leak altogether. (After all, it’s a service, not a right, to receive cable!)

Some Common Ways Pirates Get Caught

There are many ways for a “pirate” to get caught. Since stealing cable is illegal in the U.S., you can be fined and sent to jail for theft of service. Cable companies claim to lose millions of dollars in revenue every year because of pirates, so they are serious in their pursuit of ridding them from their system.

  • First, a pirate will often show-off the fact they can get every channel to their friends. Pretty soon lots of people know about it, and then the cable company offers a “Turn In A Pirate And Get $100” program. A friend needs the money and turns the pirate in. Busted.
  • Second, a pirate (or unsuspecting housemate of a pirate who knows nothing about whats going on) calls the cable company to report a problem with the equipment or signal. The cable company makes a service call and finds illegal equipment connected to the cable. Busted.
  • Third, during a pay-per-view event such as a fight, the cable company offers a free T-shirt to all viewers. Little does the pirate know that just before that message appeared on the screen, legitimate viewer’s boxes were told to switch to another channel WHILE STILL DISPLAYING THE ORIGINAL CHANNEL NUMBER (yes, cable boxes can do this.) So now the legitimate subscriber continues to see the “original” signal (without the T-shirt offer), while the pirate gets an 800 number plastered on the screen. The pirate calls, and the cable company gets a list of all pirates. Busted.
  • Fourth, a big cable descrambler business gets busted. The authorities confiscate their UPS shipping records and now have a list of “customers” who most likely ordered descramblers for illegitimate use. Busted.

And this is only the beginning. Unconfirmed reports of the cable company driving around with special equipment allowing them to determine what you’re watching on your TV (like HBO, which you don’t pay for) have also been mentioned.

The Universal Descrambler

In May of 1990, Radio-Electronics magazine published an article on building a “universal descrambler” for decoding scrambled TV signals. There has been much talk on the net about the device, and many have found it to be lacking in a number of respects. Several modifications, hoping to fix some of the problems have also been posted, with limited success. The Universal Descrambler relies on the presence of the colorburst for its reference signal. In a normal line of NTSC video, the colorburst is 8 to 11 cycles of a 3.579545 MHz clock (that comes out to 2.31 microseconds) which follows the 4.71 microsecond horizontal sync during the horizontal blanking interval. {Whew!} Since a large number of scrambling systems depend on messing with the horizontal sync pulse to scramble the picture, the Universal Descrambler attempts to use the colorburst signal to help it replace the tainted sync pulse. Unfortunately, random video inversion is still a problem, as are color shifts which occur from distorted or clamped colorburst signals, etc. Most people have not had very good results from the system, even after incorporating some modifications.

Glossary of Related Terms

{Suggestions or contributions to the glossary are welcome!}


Acronym for Community Antenna TeleVision. Originally cable TV came about as a way to avoid having everyone in a community have to spend a lot of money on a fancy antenna just to get good TV reception. Really all you need is one very good antenna and then just feed the output to everyone. It was called Community Antenna Television (CATV). Of course, it has grown quite a bit since then and everyone now just calls it cable TV. The old acronym still sort-of works.


A device, sometimes issued by the cable company, to “convert” many TV channels to one specific channel (usually channel 3). Used early-on when VHF & UHF channels were on different dials (and before remote controls) to provide “convenience” to cable customers. Now mostly considered a nuisance, thanks to the advent of cable-ready video equipment, they are mainly used as descramblers.

An “addresable” converter is one that has a unique serial number and can be told (individually) by the head-end to act in a certain manner (such as enabling channel x, but not channel y). Addressable converters nearly always contain descramblers for decoding premium services subscribed to by the customer.


Approximately 8 to 10 cycles of a 3.579545 MHz clock sent during the HBI. This signal is used as a reference to determine both hue and saturation of the colors. A separate colorburst signal is sent for each line of video, and are all exactly in phase (to prevent color shifts).

Control Signal:

The first 11.1 microseconds of a line of NTSC video. The signal area from 0 to 0.3 volts (-40 to 0 IRE units) is reserved for control signals, the rest for picture information. If the signal is at 0.3 volts (or 0 IRE) the picture will be black. See IRE Units; Set-up Level.


One half of a full video frame. The first field contains the odd numbered lines, the second field contains the even numbered lines. Each field takes 1/60th of a second to transmit. Note that both fields contain a complete vertical-blanking interval and they both (should) have the same information during that interval. Since the NTSC standard is 525 lines, each field contains 262.5 lines–therefore it’s the half-line that allows the two fields of a frame to be distinguished from one another. See Frame; Line. Frame:

An NTSC video signal which contains both fields. A frame lasts 1/30th of a second. See Field; Line.


The main cable distribution facility where your CATV signal originates from. (Easily identifed by several large satellite dishes, some smaller ones, and usually an antenna tower.)


Acronym for Horizontal Blanking Interval. The first 11.1 microseconds of a line of video. It contains the front porch, the 4.71 microsecond horizontal sync pulse, the 2.31 microseconds of colorburst, and the back porch. The horizontal sync pulse directs the beam back to left side of the screen. Almost every scrambling method in use today mutataes this part of the signal in some way to prevent unauthorized viewing. See Colorburst.


Term used to describe the dual-field approach used in the NTSC standard. By drawing every other line, screen flicker is increased–but if all the lines were painted sequentially, the top would begin to fade before the screen was completely “painted”. (Computer monitors, which do “paint” from top to bottom, do not have the problem due to higher refresh rates.)


Impulse Pay-Per-View. A method whereby a viewer can order a pay-per-view event “on impulse” by just pushing an “Order” (or similar) button on a remote control or cable converter keypad. A customer’s purchases are sent back to the head-end via a standard telephone connection (the converter dials into the cable co. computer and uploads the data) or via radio frequency (RF) if the cable supports two-way communication (most don’t). A pre-set maximum number of events can be ordered before the box requires the data to be sent to the head-end for billing purposes.

IRE Units:

IRE is an acronym for Institure of Radio Engineers. The NTSC standard calls for a peak-to-peak signal voltage of 1 volt. Instead of referring to the video level in volts, IRE units are used instead. The IRE scale divides the 1- volt range into 140 parts, with zero-IRE corresponding to about 0.3V. The full scale goes from -40 IRE to +100 IRE. This is convenient scale to make a distinction between control signals (< 0 IRE) and picture signals (> 0 IRE). See Control Signal.


A video signal is a series of repeated horizontal lines, consisting of control and picture information. The color NTSC standard allows a total time of 63.56 microseconds for each line, and each frame is composed of 525 lines of video information. The first 11.1 microseconds make up the horizontal blanking interval, or control signal, the following 52.46 microseconds make up the picture signal. See HBI; VBI.


Acronym for National Television Standards Committee (or Never The Same Color, if you prefer 🙂

Picture Signal:

The 52.46 microseconds of signal following the control signal. Information in this area is between 0 and 100 IRE units. See IRE Units.


Acronym for Pay-Per-View. A revenue-enhancing system where customer’s pay to watch a movie or event on a “per view” basis. Cusomers usually place a phone call to a special number and order the event of their choice; some systems provide Impulse PPV. The presence of a PPV movie channel or your system guarantees you have addressable converters. See IPPV.

Set-up Level:

Picture information technically has slightly less than 100 IRE units available. That’s because picture information starts at 7.5 IRE units rather than at 0 IRE units. The area from 0 to 7.5 IRE units are reserved for what is commonly called the “set-up level”. Having a small buffer area between the control signal information and the picture information is a “fudge factor” to compensate for the fact that real-life things that don’t always work as nicely as they do on paper. 🙂 See IRE Units.


Acronym for Vertical-Blanking Interval. The first 26 lines of an NTSC video signal. This signal is used to direct the beam back to the upper-left corner of the screen to start the next frame. In order for the horizontal sync to continue operating, the vertical pulse is serrated into small segments which keep the horizontal circuits active. Both actions can then take place simultaneously. The VBI is the most common place for “extra” information to be sent, such as various test signals, and in some cable systems, a data stream.

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