Density – Frequency Protections

Changing the density level was a common trick for putting more data on a track, as well as preventing copiers from clocking in the data properly. – Jim Drew

Clock Bits Protection

This is a protection where invalid GCR was used. Most commonly known as ‘weak bit’ protection. This was the most difficult protection to detect unless you were using a 1541-II or 1571 disk drive. Neither of these drives would ‘float’ invalid bits like a 1541 does. So, you can easily spot these and correct the data before being re-written. – Jim Drew

I do not know what this is, but some copy protection software mentioned this in their ad.
Anyone have information on this one?

Sync Bytes Copy Protection

Sometimes you would find protections counting the number of sync bits in a row or timing the duration of series of sync bytes to see if they matched. These were difficult to duplicate because we really didn’t have much control of the data separator. We had to write routines that calculated the bit length for every sequence and handle sync bits a little differently. – Jim Drew

Checksums – Copy Protections
Typically error 23 was used. Later on, when the job queue was used for reading errors without rattling the head, people figured out you could get valid data even though the checksum was wrong. – Jim Drew

GAPS – Copy Protections

The space between sectors or between the track end and start. Gaps were not used very often in protection schemes, especially when hardware copiers came out. – Jim Drew