With the music industry become more digitally obsessed and studios equipped with digital delay lines, echo chambers and pitch-shifters, it seems all too likely that you may want your micro to follow into the world of digitally recorded sound.

Into this environment comes the Barry Box from BML Electronics in Bletchley. With a pair of fast analogue to digital (A-D) and digital to analogue (D-A) converters, it turns a sound into a set of numbers. The numbers can be stored and changed in your BBC micro and turn turned back to sound again.

The quality of a digital recording of a sound depends on how faithfully the sound can be represented by numbers. This boils down to two things: how fast the sound is sampled and the precision of the numbers storing the sound. The electrical signal from a microphone should be sampled at least twice the highest frequency in the signal. The numbers generated, representing the signal voltage at each sampling point, should have as many levels as possible. Professional digitial audio samples at around 44kHz and generates 16-bit numbers (i.e. 65, 536 different levels). The Barry Box samples at a user-defined rate up to 40kHz and has 8-bit precision with 256 levels.

In terms of quality of recording this is pretty good, much better than the internal speaker on the micro will reproduce.

It must be said though that the Barry Box is not designed to be a digital sound recorder. In any case, a 40kHz sampling rate only records a second of sound for every 40K of memory used (i.e. you don't store much on your BBC micro!) and for useful recording times of a few seconds you have to sample more slowly. This still produces reasonable quality and enables you to experiment with the sound stored digitally in your micro.

The box is 105 x 85 x 50mm and plugs into the 1MHz bus for control and the power outlet in the micro for power (5 volts). Here lies the only serious problem. The power comes out from the BBC Micro on a jack plug. It is generally bad practice to put volts on a male connector, especially if there is a risk of shorting the power supply. In this case it is impossible to plug the jack into the box without shorting your power supply rail to chassis (through the 1MHz bus connector). BML warn you not to connect the box with the micro powered up, but even a very brief short (such as caused by accidentally pulling the power jack out quickly) will crash the micro at the very least. The jack plug arrangement should be replaced and BML said it would look into this possibility.

BML thoughtfully provides a basic microphone and besides the micrphone input (a miniature jack socket) and the 1MHz bus cable there is a second jack for output as well as two volume controls. One of these controls input levels, and is for some reason a linear law potentiometer, and the other logarithmically controls output levels. (As the car's response to sound levels is logarithmic it is usual to use log potentiometers for volume controls.). The user guide has no information on input and output levels and impedances, although BML does recommend that a low impedance microphone is used. For better sound equality BML suggests using an external amplifier although you should keep the output level on the box turned low.

The Barry Box software comes in an EPROM and operates on two levels. First, in direct mode you operate the box by pushing function keys. Second, by using * commands and setting parameters in zero page (between &70 and &8F) you can operate the box from your own programs. Some examples of such applications are in the user guide. All the functions in direct mode have an equivalent in program mode.

There are two modes of recording. One is 'triggered' where the recording starts when a sound is detected, but this tends to chip the front off non-percussive sounds. The second simply starts recording when the key is pressed. The length of recording depends on your sampling rate (set as time between samples) and the length of the buffer into which the sound is written. As the Box software will only function in the micro and not a second processor, buffer space is limited and Mode 4 is used by the Barry Box direct mode.

To play the sound back, you simply push a button and out it comes. When you first hear your voice saying "Hello Mum" coming out of your micro speaker it is quite exciting - it makes a change from Kenneth Kendall! Another key combintaion plays it backwards.

Other keys allow you to look at the wave form (like a storage oscilloscope trace), look at its amplitude envelope and at its frequency spectrum. The first is fairly routine (although storage scopes usually cost a small fortune), the second is dubious (what you actually get looks more like a full-wave rectifier output than a true envelope), but the third is magic. It takes a bit of fiddling with parameters to get the best out of the spectrum display and in general it is not big enough on the screen - but to have a spectrum analyser, however rudimentary, for less than £100 is amazingly good value, never mind the other things you can do. A pure tone gives a sharp peak in the display whereas a percussive, noisy sound (like a hand clap) shows a broad distribution of frequencies.

Since the box stores the sound as a series of numbers in the buffer representing the waveform, it is possible to write a sequence of numbers into the buffer, describing a waveshape, and have the box play the resulting sound. Other functions include a fast Epson screen dump, some rudimentary clean-up for the recorded sounds, and saving the buffer contents to Sideways RAM or to disc.

At under £100 the Barry Box presents a fascinating tool for exploring digitised sound. The problem with the jack plug power lead should not detract from the value, both for education and as a tool or toy in the home.

The Barry Box, £79.95
BML Electronics, Unit 24, Larch Grove, Bletchley, Buckinghamshire MK2 2LL. Tel: (0908) 640805