Review of Cottis-Blandford Cassette Interface.
By J.R. Keneally.
This note describes some experiences in using the cassette interface
supplied by Newbear Ltd. (Kit form, current price 12.50), and may be of
use to anyone wishing to upgrade the speed and reliability of the
cassette interface on their Nascom 1.
1. Uses CUTS type encoding (one – 2400Hz, zero – 1200Hz).
2. Capable of running at 300 to 2400 bits per second.
3. Fairly easy to interface to the Nascom, though the method suggested
by Newbear can be improved upon (eg: see below).
Value for money ?
The kit seems reasonable value for money, but there were several
pitfalls for the unwary. The two main ones were:
1. The printed circuit board lay-out is poor, and must be very
carefully checked for bridges across tracks. Also, it is very easy to
cause bridges when soldering certain connections.
2. The documentation is wrong in places, and vague in others. For
example, two of the board terminal points were labelled incorrectly on
the board lay-out diagram, and it was not stated anywhere that all the
output terminal points had to be wired to go to the outside world via
spare inverter-buffers in IC3.
The moral is to cross-check everything against the various diagrams
Does it work ?
On first switching on the finished kit, the transmitter section worked
properly, and the tapes could be read back on the lowest speed of 300
bits/sec, though with low reliability. Clearly, there were no wiring
faults. The problem was traced to two causes:
1. The amplitudes of the high & low frequency tones on replay were
different by a factor of about three times. This was due partly to the
low-pass filtering used on the transmitter output circuitry, and
possibly also due to poor tape recorder frequency response. When the
transmitter output circuit was set up so that roughly equal amplitudes
were obtained on replay, a big improvement in reliability was obtained.
This required a high-pass RC filter, just the opposite of that
provided. Obviously, adjustments are required to obtain good results
with particular recorders.
2. The input circuitry from the replay has no means of adjusting the
D.C. bias level, so as to give an even mark/space ratio on pin 10 of
IC10 (a). This ratio seems to be very critical for high reliability,
and the required bias can easily be obtained using a potentiometer
across the 5V supply. The trick seems to be to record a section of tape
having no data on it, i.e just a continuous high frequency tone at 2400