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From: agn@unh.cs.cmu.edu (Andreas Nowatzyk)
Newsgroups: sci.electronics
Subject: Re: 3800 Hz notch filter [will] be used on CD masters
Message-ID: <1007@unh.cs.cmu.edu>
Date: Fri, 26-Jun-87 01:51:22 EDT
Article-I.D.: unh.1007
Posted: Fri Jun 26 01:51:22 1987
Date-Received: Sun, 5-Jul-87 19:53:01 EDT
References: <598@neoucom.UUCP> <67@sdeggo.UUCP> <1597@ihuxi.ATT.COM>
Organization: Carnegie-Mellon University, CS/RI
Lines: 49
Keywords: with possible icky results!

> Sorry, but the protection circuit will be buried deep in one of the LSI's.
> There will be no way to remove it and even if you could, the notch will
> still be there on the source CD.  That is what has everyone so up in arms.

It seems very unlikely that the copy-protection circuitry will be burried
deep inside. Consider the nature of that chip: several filters, various
integrators, gain-control, etc. (The block diagram was recently published in
an audio magazin). This can be implemented either as an switched-capacitor
device (a) or as a completely digital circuit (a).

The process for (a) is incompatible with a controller micro used to control
the entire DAT. (b) is fairly complex, requires a more difficult interface
to get the digital data stream (that isn't available on the controller:  the
extra data-path may not live long... :-) and needs a substantial ammount of
fast logic.  Again, this is different from the usual low speed, low power
process that is used for the controller.  Besides, both approaches require
significant silicon real estate and extra design efford to integrate this
gadget in other circuitry (mostly standardized stuff). The larger chip has a
lower yield, longer design time and is more difficult to test. The DAT
controller (user interface, display, tape counter, remote controll,
lots of other boring stuff - but no audio data handling) is a unlikely
place for the CP stuff.

Similar arguments can be made against placing the CP circuitry in parts
of the recorder data-path. Note that there are at least 3 chips involved
here: 1) ADC (perhaps with an integrated digital filter), 2) Digital data
path (fifo, generation of EDC codes, adding control info, formatting)
and 3) Driver (converts the digital data into an analog signal to drive
the write head, various servo-loops, etc.)
The technology for these essential parts are quite different (1: precision
CMOS with some analog capabilities, isolated from the noisy digital power;
2: high performance CMOS; 3: bipolar).

In all likelyhood, the CP chips will be extra that listens to the analog
input. Either the input or the output are begging to be cut off. Note that
Sony (and probably other sources) provide really nice service-manuals with
circuit diagrams, PC-board layouts and anything you need to know.

For more fun, the control processor may exchange encrypted messages with
the CP-chip. But even this exercise leaves the input to it vulnerable.

Now for the 10c question: why should a DAT company spend a lot of resources
on the CP part to do a good job here? After all, they are fighting this
restriction.

I think that the time it takes a person with EE background to defeat this
circuit is best measured in minutes.

  --  Andreas