80-Bus News

  

March–April 1983, Volume 2, Issue 2











Page 20 of 55











20 HIGH SPEED CASSETTE INTERFACE John R. Hunt

Il was interested in W Van Malderen*s article in Val.1l Iesue.3: it _omfirmed what I had thought for some time, that with appropriate ci cuitry data rates significantly higher than Nascom’s standard 1200 and 2400 Baud are possible.

I would like toa point out a possible problem with the Biphase (Manchester II) coding used, namely that the recorded data for a stream of ones and a stream of zeros are ambiguous, as the following sketch shows.

\ . o _ All bibs 1 ’ i | LPL LE LDL, Coded date : __! Alt bits O P TLICLIOLS LI – Cored dota, identical to Is, -ofher tren phose shift

Mernating 1 ond O

o —

STITT LO TL _t

{

Coded data

enon ene

Thus. when the tape is replayed, the "blank? sectian before the data starts can be decoded randomly as mark or space, and in the latter case the first data character will be missed. This does not matter for tapes recorded in Nas-Sys format as a block of mulls is autput before the data, and these will guarantee synchronisation af the decoder. For other file structures (eg Hisoft Fascal 47) reliable data recovery will not be achieved.

There is a related Biphase coding method, Biphase-M, which does not suffer from this deficiency, and is also unaffected by possible signal inversion in the recorder. tl refer you to an article in the Feb i968? issue of Wireless World for a discussion of this coding method along with circuits for coder and decoder. Apart fram the addition of a phase-locked loop for clock recovery, these are aniy a little more camplex than W Van Malderen*s circuit. T intend tea experiment with this when time permits.

The article in Wireless Worid also points aut the desirability of phase equilisatian of the signal oan playback for reliable operation. Based om the Wireless World circiut I have built an equiliser which I have installed permanently between the ouput of my portable recorder and Nascom. The result has been a great improvement in the reliability of reading data tapes, particularly for tapes recorded on other machines. Most significantly, error free reading can naw be achieved over a very wide range of volume control settings. The circuit is powered from the 12V supply available on the serial


This is an OCR’d version of the scanned page and likely contains recognition errors.











Page 20 of 55