Inside the Electron is a sound synthesiser which you can program to generate virtually any sound you like. The synthesiser is controlled by two BASIC commands: SOUND and ENVELOPE, and each command requires you to type in a series of numbers (or parameters) after it. These parameters determine the type of sound you will hear from the Electron's internal loudspeaker, i.e. the type of sound, pitch, duration, and so on.

There is a vast range of parameter values which give you an almost unlimited range of possibilities when writing programs for the synthesiser. However, you don't need to be a wizard in order to use the sound system on a relatively simple level, and this chapter will serve to get you started. Once you've had some practice, read the last section in this chapter which goes into the SOUND commands in more detail.

The SOUND command must be followed by four parameters which we will call Q, A, P and D. So the SOUND command takes the form:

SOUND Q, A, P, D

Press SHIFT and type the following:

This will produce a single tone of fairly long duration. Type this line in again and increase the P parameter (the third number). You'll notice that the pitch increases.

There are four values of Q, and each one selects the SOUND channel number.

You may wonder why there are three SOUND channels which all select tone as opposed to noise. The only reason for this is to make the sound system as compatible with the BBC Microcomputer as possible. From now on, we will ignore channels 2 and 3.

This parameter does three things, depending on the value you give it:

When A is a negative number, the amplitude is at maximum, i.e. 'on'. When A is zero, the amplitude is at minimum, i.e. 'silence'. When A is between 1 and 16 inclusive, an ENVELOPE command of the same number is selected. Forget the ENVELOPE command for the moment; you will meet it later in this chapter.

Note that for the sake of compatibility with the BBC Microcomputer, use A=-15 for 'sound on' and A=0 for 'silence'.

The value of this parameter controls the pitch of the generated sound. The range of values are from 0 to 255, and each consecutive value will give a quarter-semitone pitch change. The lowest note (P=0) is the B one octave and a semitone below middle C, and the highest accurate note is the B above middle C (P=100). Although values above P=100 are not accurate to the even-tempered scale, they can still be put to good use for making sound effects. The table below is a quick reference guide to help you find the pitches you want.

When the noise channel (channel 0) is selected, the P parameter has the following effect:

The value of this parameter sets the duration of the tone in steps of 50mS. So if D=100, then the duration will be (100 × 50)/1000, or five seconds. The maximum value of D is 254 (12.75 seconds).

If D=-1, then the tone will carry on until it is turned off.

Before using the SOUND command in a program, try following through the example below, in order to produce a single sound.

First of all, decide which channel to use: channel 0 will give noise, so use one of the three tone producing channels, say channel 1. So the first number is 1 - this is the value of Q.

Now choose the value for the second number - the A parameter. At the moment we aren't using any ENVELOPEs, and we want to hear the sound, so the next number is -15 - this is the value of A.

The third number - the P parameter - determines the pitch. If you look at the pitch table, the C below middle C is the value 4. So P=4.

How long shall the sound last? The fourth number - the P parameter - gives the duration in 50mS (five hundredths of a second). So for a duration of five seconds, P=100.

Now type the following SOUND command with the number above:

This produces a tone which lasts for five seconds, and is the C below middle C.

Remembering that the pitch value is in quarter semitone steps, then to produce the sound corresponding to C (one semitone higher than the previous one), we must increase the pitch value by four.

To play the next note up the scale, increase P parameter by four again, and so on.

Instead of using numbers in the SOUND command, you can incorporate the SOUND command into a program, and use real or integer variables (see chapter 11). For example, let's produce a chromatic scale over an octave (equivalent to playing every note in sequence on a piano keyboard over an octave).

Instead of having to type in13 SOUND commands to get the 13 different notes in the scale, we can use a variable for the P parameter, and change the variable value 13 times by using a FOR...NEXT loop. Try the program below which demonstrates this.

Lines 10 and 30 set up a loop with variable X%, whose values are passed to the pitch parameter in line 20. Line 20 contains the SOUND command: channel 1 is selected, the sound is 'on' (A=-15), the pitch is determined by the value of X%, and each sound plays for 5 × 50mS, or 0.25 seconds.

As you can hear, each new note only starts after the one before has finished. If you want to put a pause between each note, insert another SOUND command between lines 20 and 30, but it 'off' by giving the A parameter value 0. The length of the pause will of course be decided by the value you give the last parameter. Try this line:

(Obviously the pitch value here can be anything you like). Now list the program:

When you RUN this program, you will hear a pause between each note. Try changing the values of X% and the duration of the tones and pauses to get different effects. If you want to make a crude sequencer, put the complete program into a REPEAT...UNTIL FALSE loop.

Each SOUND command, as we have seen, allows you to generate a single tone, whose pitch and duration is defined by you within the SOUND command. The ENVELOPE command allows you to change the single tone into something far more complex. Try typing the following:

If you run this, you will hear a continuous tone which lasts for five seconds. Now add this line:

When you run this program, you'll hear a sound rather like a police siren. Ignore all the numbers after the ENVELOPE command, except for the very first one, which defines the ENVELOPE number, in this case 2. The second parameter in the SOUND command selects ENVELOPE number 2; (ENVELOPE 2, ...); the third parameter selects the starting pitch and the fourth parameter selects a duration of five seconds.

Note that once an ENVELOPE statement has been executed, it will stay in the computer until it is either re-defined, or until the computer is switched off. So even if you delete line 20 in the program above, when you run the program ENVELOPE number 2 will still be selected and used.

The ENVELOPE command is followed by 14 parameters, each separated by a comma. To make life easier, only the first eight actually do anything on the Electron, but the rest must be included for the sake of compatibility with the BBC Microcomputer. These eight parameters are shown below, and are followed by six zeros to make up the 14 parameters necessary.

If you want to run Electron programs containing ENVELOPE statements on the BBC Microcomputer, then you should get into the habit of always using the following numbers for the last six parameters:

This will ensure that the resulting sound will be the same on both computers. For those of you who are not concerned with compatibility, use any numbers you like. In the rest of this section, 'zeros' have been used to save initially confusing you with too many numbers!

The duration of the ENVELOPE effect varies, depending on the values in the ENVELOPE command. However, as long as the duration is less than that of its associated SOUND command, it will repeat itself until the SOUND command finishes. For example, if you set the duration of a SOUND command to five seconds and select an ENVELOPE which makes a 'WOW' sound lasting for one second, then the result will be 'WOWOWOWOWOW'. With the ENVELOPE command, you can:

• Alter the SOUND pitch parameter by a specified increment
• Specify how many increments you want
• Set the length of time each increment 'plays'

The length of each increment can only be specified once in the ENVELOPE command, but the increment value itself, and the number of increments can both be specified three times!

Have a look at the graph below, which represents pitch plotted against time for an ENVELOPE command. As you can see, the resulting envelope contains three stages. In the first stage, the pitch increases by an increment of 1, then by -2 in the second stage, then 1 in the third stage. In the first stage there are seven pitch increments, in the second stage five, and in the third stage 12. (Notice that the duration of every pitch increment is the same).

IMAGE MISSING

The first eight parameters of the ENVELOPE command are as follows.

This is the ENVELOPE number, and is selected by the second parameter of the SOUND command. Values are from 1 to 16.

This gives the duration of every pitch change in the ENVELOPE command, (i.e. all three stages), and each value corresponds to 10mS. Values are from 0 to 255. Values 1 to 127 give durations of 1 × 10mS to 127 × 10mS, and when the ENVELOPE is finished, it repeats itself until the duration of the associated SOUND command has run out. Values 128 to 255 are equivalent to 1 to 127, except that at the end of the ENVELOPE, the final pitch is held until the associated SOUND command has run out.

This gives the value of every pitch increment during the first stage. Values are from -128 to127.

This gives the value of every pitch increment during the second stage. Values are from -128 to 127.

This gives the value of every pitch increment during the third stage. Values are from -128 to 127.

This gives the number of pitch increments in the first stage. Values are from 1 to 255.

This gives the number of pitch increments in the second stage. Values are from 1 to 255.

This gives the number of pitch increments in the third stage. Values are from 1 to 255.

These parameters must be put into the ENVELOPE command, but their values will have no effect on the effect produced by the Electron's ENVELOPE command. In order to keep the command compatible with the BBC Microcomputer, these values should be 126,0,0,-126,126,126.

Type in the following program:

If you run this, you will hear a whirring sound. The sound is being transmitted on channel 1, ENVELOPE 2 is selected, the pitch value is 4, and the sound lasts for 50 × 50mS or 2.5 seconds.

The graph below shows the effect of the ENVELOPE on the sound. This graph is exactly the same as the one at the beginning of this section on the ENVELOPE command, and was drawn using the parameter values in the ENVELOPE statement above. The parameters are as follows:

IMAGE MISSING

The total time take for this ENVELOPE command is 10mS × total number of pitch changes, or 24 × 0.01 seconds, = 0.24 seconds. So the ENVELOPE is repeated over and over again, until the SOUND command finishes.

The first parameters of the SOUND command can be given extra values which provide you with some more facilities. To use these values, you must enter the first parameter as a four digit hexadecimal number (which must be preceded by a & sign, to tell the computer that a hexadecimal number follows). The four digits in this number can be represented by HOFC, and here is a description of each digit.

The program below turns the Electron into a keyboard instrument by using the 12345 and QWERTY keys.

This next program takes a string of note names, and plays the notes. The range of notes you can play is from the C below middle C, to the B above middle C. To produce the full range in sequence enter the following: cdefgabCDEFGAB (RETURN). This will give you two octaves in the key of C major.

Laser Zap! This program uses an ENVELOPE with a downwards pitch sweep. To fire the laser, press any key.

This last program uses a repeated ENVELOPE with an upwards pitch sweep to produce a spaceship take-off sound.