From: wdp@gagme.chi.il.us (Bill Pfeiffer)
Subject: Re: Stupid question 
Originator: wdp@gagme
Organization: Gagme Public Access UNIX, Chicago, Illinois.
Distribution: na
Date: Thu, 19 Nov 1992 21:44:41 GMT

In article <4431@vidiot.UUCP> brown@vidiot.UUCP (Vidiot) writes:
>In article <1992Nov11.230818.10229@gagme.chi.il.us> wdp@gagme.chi.il.us (Bill Pfeiffer) writes:
><
><Scrambled channels in the Jerrold system, are transmittted with a 'bit
><stream' enoded in the Vertical Blanking Interval (the black horizontal
><line you see when you turn the vertical hold control).  Premium
><channels each have their own bit-stream, and all basic channels share
><a bit-stream.    The decodr is addressable to one of several 'tiers'
><of service.  
>
>Gee, this is news to me.  I've never heard of a scramble system that uses bits
>in the video for scrambling.

I did NOT say that the bits are used for scrambling (or descrambling).

The poster I was responding to, said that the cable box was somehow
'hardwired'(not his term but ...) to only turn the decode function on
for certain channel *numbers*, i.e. channel 17 for HBO etc.  The
person HE was replying to suggested that a second converter box be
used to down-convert ALL channels to channel three, where the system
had a scrambled 'basic' channel, thereby passing the encoded premium
channel through a channel that was 'authorized' for decoding.  His
logic was that if channel three was already authorized (as a scrambled basic
channel) that passing encoded RF from a premiun channel through a
simple converter through the legit decoder would then decode the
premium channel.

This will not work, because while it is true that 'addressing' signals
are sent on a seperate carrier (near 106 mhz), this signal does not
authorize the reception of a single 'channel', but authorizes the
reception (decoding) of a specific 'signal', regardless of which
channel it appears at.  This determination is made by superimposing a
binary code which represents a specific service, then telling the
decoder to 'read' that code, compare it to it's authorization and
react accordingly.  This is why the jerrold 450 takes a split second
to either decode, or switch off, after a specific channel is tuned to.

If this was not true, all someone would have to do is shift the
frequency of a desired premium channel to an authorized scrambled
basic channel, and the premium service would be decoded... provided
that this frequency shift was done by simple superhetrodyne methods,
and *NOT* by demodulating and re-modulating, as a VCR does.  Jerrold
is not that stupid :-).  


>The Jerrold systems that I have seen use in-band gated sync.  This means that
>the horizontal sync level is raised to push the horizontal sync into the same
>video level as the picture, effectively removing the sync for the TV to use.
>That is part of the gated part.  The other part is that the signal used to
>restore the sync position is an AM modulated sine wave on the TV audio.

Actually, what you are describing here is the Oak Industries 'sine wave' 
scheme of encoding/decoding.  In 'in-band gated' a pulse is trained
along with the horizontal sync, out of phase with the real sync, 
to 'suppress', not amplify, the sync pulse below the readable level.,
thereby causing the TV's sync seperator to 'read' video instead of
sync.  The Jerrold 450 uses a tri-mode scheme where the sync is
suppressed either 6 or 10 db (or both).  Jerrold also makes a single
mode decoder (CALLED THE STARBASE III or SB-3) which only uses the 6
db level of supression.   The sinewave system is disgustingly easy do
defeat, in fact many tv's can be fine tuned to virtually ignore
sinewave, especially on 'light' scenes.  We once had a uhf pay-tv
station here that used sinewave.  They showed a lot of spoorting games
(like hockey and such)  the bright backgrounds made it possible to
veiw those programs with no decoder at all.

>
>The addressing part of the system is normally a separate carrier on the cable
>system that carries a digitally modulated signal.  Each box listens to that
>carrier, looking for its address.  Within that stream are the bits that tell
>it which channels it is allowed to decode.

True, but as I stated above, it is not which 'channel' (as in a
channel number) but which 'service' it is allowed to decode.  

>If you pass the cable signal through a VCR and remodulate it on channel 3/4,
>you lose the addressing signal (the box will quit after a period of time) and
>you will lose the AM modulated restoration timing signal.

If you pass it through a VCR, the VCR's own internal IF and video detector 
react to the suppressed sync and deliver a procssed product of encoded
video. As you said in your original post, all decoding is done at IF
level, not at demodulated video level (although demodulation IS
required to get the information needed to decode)  once the encoded
signal is demodulated, then re-modulated, the re-modulator assumes
that what it is seeing is what you want to have modulated.  In essence,
the modulator in the VCR is sending you a perfect representation of
encoded video, NOT raw signal.  Cable converters usually have no IF
and demod/remod circuitry, they are exactly what they are called,
converters, they merely hetrodyne the cable signal to a single IF
frequency, usually channel 3 or 4 VHF tv. 

>There are also systems that are known as out-band gated sync.  In these cases,
>the restoration signal is modulated on a different carrier on the
>cable system. 

True, but that's another story :-).