COPYBIT ELIMINATOR REVISITED
Our February 1994 issue carried anarticle describing an inexpensive andstraightforward circuit for eliminating thecopybit from a digital S/PDIF* audio signalto enable users to copy (digitally) their ownmusical work many times without degradationby the SCMS**. The present article describesan updated version of that circuit,which can be used with the latest DAT, DCCand MD players.
A comparisonThe integrated circuits used in digitalrecorders fulfil more and more functions.Even the until recently discrete S/PDIFbuffer/amplifier which converts the S/PDIFsignal to TTL level is integrated in modernequipment. This is a good thing, of course,since fewer ICs bring the cost, and thus theprice to the consumer, down.The original eliminator needed a TTLsignal at its input, but the updated versioncontains a separate S/PDIF buffer amplifier.However, the design allows S/PDIF signalsalready at TTL level to be processed withoutany difficulty.The ICs used in modern recorders arefaster and run at a higher master-clock frequencythan those produced only a fewyears ago. Nowadays, a 256fs master clockis quite normal, and even 384fs and 512fsmodels are in production. Since the eliminatorneeds a 128fs clock, a binary scaler isprovided in the updated version. If a 128fsclock is available in the recorder and thiscan be used, it should be preferred: experienceshows that this enables some recordersto lock on more readily.
The present version provides a choice ofusing either a LOCK or an UNLOCK signaland the facility of inverting the clock if required.These options may be useful formodifying types of DAT, DCC or MDrecorder that have not been considered bythe designer.The copybit indication outputs, COPYINand COPYOU, can not only drive LEDs directly,but may be used, if a copybit is present,as a block (=192 frames) trigger. Thisallows the S/PDIF signal to be inspected onan oscilloscope at or near the timing-slotposition of the copybit frame.The circuitThe circuit of the original version has beencomplemented with an IC (by using aMACH210 in the IC1 position), a couple ofresistors, some jumpers, and a capacitor—see F i g . 1. The MACH210 is equivalentto two MACH110s (used in the earlier version),These chips are identical as far ashousing is concerned.The operation of the updated eliminatoris identical to that of the earlier version: thereader is therefore referred to the earlierarticle for background information, blockschematic and timing diagram.