> true and for the data I gave the 125kbits/sec rate is too low. As it's > minima was 250kbits/s is twice that! Part of the recording scheme is

I've seen plenty of controllers and data separators that put limits on the maximum number of consecutive pulses and gaps. That's why you need clock pulses in MFM recording, and why Apple had the 5-3 and 6-2 encoder tables. I have _never_ seen a drive (and I've read OEM and service manuals for all sorts of drives) that specify any restrictions on the user data using the standard encoding schemes At normal 5.25" data rates (125kbps (user bits) FM, 250kbps MFM) : Repeated MFM 0's looks like repeated MFM 1's looks like repeated FM 1's, and consists of pulses every 4 us. Repeated MFM 1010... looks like repeated FM 0's, and consists of pulses every 8us. Now, all drives support user sectors of 1024 bytes (8192 bits), MFM at least (and I don't think this is a real limit of the _drive_ either). That means you could have either of the above patterns for 8192 bits -- the user bytes 'touch' each other with nothing between, and there's nothing to stop you having a sector of 0's, a sector of FF's or a sector of 55s if you want it. A disk drive that couldn't store said data would not be useful. I've got a Sony 3.5" drive on the bench at the moment. Now this drive rotates at 600rpm, so you would double the above data rates. Some of the tests involve recording pulses every 2us (corresponding to the first case above) and 4us (corresponding to the second case above) continuously for one revolution and then playing them back. The service manual for the Teac FD235 gives tests involving the recording and reproduction of 250kHz (pulse every 4us) and 125kHz (pulse every 8us) waveforms. So it would certainly appear that these 2 drives could correctly handle FM recording at half the user data rate of the standard MFM encoding. In other words that Teac (720K) drive would handle FM encoding at 125kbps. -tony