Whats A Microcontroller v3

Frequency and Sound · Page 261

'What's a Microcontroller - NotesAndDurations.bs2

'Play the first few notes from Frere Jacques.

'{$STAMP BS2} '{$PBASIC 2.5}

DEBUG "Program Running!"

FREQOUT 9, noteDuration, noteFreq

index = index + 1

LOOP

END

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How NotesAndDurations.bs2 Works

The Notes and Durations DATA directives were discussed before the program. These directives combined with the WholeNote constant are used to store all the musical data used by the program.

The declarations for the five variables used in the program are shown below. Even though a FOR...NEXT loop is no longer used to access the data, there still has to be a variable (index) that keeps track of which DATA entry is being read in Notes and Durations. The offset variable is used in the LOOKDOWN and LOOKUP commands to select a particular value. The noteLetter variable stores a character accessed by the READ command. LOOKUP and LOOKDOWN commands are used to convert this character into a frequency value. This value is stored in the noteFreq variable and used as the FREQOUT command’s Freq1 argument. The noteDuration variable is used in a READ command to receive a value from the Durations DATA. It is also used to calculate the Duration argument for the FREQOUT command.

The main loop keeps executing until the letter “Q” is read from the Notes DATA.

DO UNTIL noteLetter = "Q"

A READ command gets a character from the Notes DATA, and stores it in the noteLetter variable. The noteLetter variable is then used in a LOOKDOWN command to set the value of the offset variable. Remember that offset stores a 1 if “b” is detected, a 2 if “B” is detected, a 3 if “C” is detected, and so on. This offset value is then used in a LOOKUP command to figure out what the value of the noteFreq variable should be. If offset is 1, noteFreq will be 1865, if offset is 2, noteFreq will be 1976, if offset is 3, noteFreq is 2093, and so on.

READ Notes + index, noteLetter

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The note’s frequency has been determined, but the duration still has to be figured out. The READ command uses the value of index to place a value from the Durations DATA

into noteDuration.

READ Durations + index, noteDuration

Then, noteDuration is set equal to the WholeNote constant divided by the noteDuration. If noteDuration starts out as 4 from a READ command, it becomes 2000 ÷ 4 = 500. If noteDuration is 8, it becomes 2000 ÷ 8 = 250.

noteDuration = WholeNote / noteDuration

Now that noteDuration and noteFreq are determined, the FREQOUT command plays the note.

FREQOUT 9, noteDuration, noteFreq

Each time through the main loop, the index value must be increased by one. When the main loop gets back to the beginning, the first thing the program does is read the next note, using the index variable.

index = index + 1

LOOP

Your Turn – Experimenting with Tempo and a Different Tune

The length of time that each note lasts is related to the tempo. You can change the tempo by adjusting the WholeNote constant. If you increase it to 2250, the tempo will decrease, and the song will play slower. If you decrease it to 1750, the tempo will increase and the song will play more quickly.

9Save NotesAndDurations.bs2 under the name NotesAndDurationsYourTurn.bs2.

9Modify the tempo of NotesAndDurationsYourTurn.bs2 by adjusting the value of WholeNote. Try values of 1500, 1750, 2000, and 2250.

9Re-run the program after each modification, and decide which one sounds best.

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Entering musical data is much easier when all you have to do is record notes and durations. Here are the first eight notes from Beethoven’s Fifth Symphony.

Durations DATA 8, 8, 8, 2, 8, 8, 8, 8, 2

WholeNote CON 2000

9Save your modified program as Beethoven’sFifth.bs2.

9Replace the Notes and Durations DATA directives and the WholeNote constant declaration with the code above.

9Run the program. Does it sound familiar?

Adding Musical Features

The example program you just finished introduced notes, durations, and rests. It also used the duration of a whole note to determine tempo. Here are three more features we can add to a music-playing program:

•Play “dotted” notes

•Determine the whole note duration from the tempo

•Play notes from more than one octave

The term dotted refers to a dot used in sheet music to indicate that a note should be played 1 ½ times as long as its normal duration. For example, a dotted quarter note should last for the duration of a quarter note, plus an eighth note. A dotted half note lasts for a half plus a quarter note’s duration. You can add a data table that stores whether or not a note is dotted. In this example, a zero means there is no dot while a 1 means there is a dot:

Music-playing programs typically express the tempo for a song in beats per minute. This is the same as saying quarter notes per minute.

BeatsPerMin CON 200

Figure 8-4 is a repeat of Figure 8-3 from page 253. It shows the 6th and 7th octaves on the piano keyboard. These are the two octaves that sound the clearest when played by the

Frequency and Sound · Page 265

piezospeaker. Here is an example of a DATA directive you will use in the Your Turn section to play notes from more than one octave using the Notes DATA directive.

Figure 8-4: Rightmost Piano Keys and Their Frequencies

Example Program: MusicWithMoreFeatures.bs2

This example program plays the first few notes from “For He’s a Jolly Good Fellow.” All the notes come from the same (7th) octave, but some of the notes are dotted. In the Your Turn section, you will try an example that uses notes from more than one octave, and dotted notes.

9Enter and run MusicWithMoreFeatures.bs2.

9Count notes and see if you can hear the dotted (1 ½ duration) notes.

9Also listen for notes in octave 7. Try changing one of those notes to octave 6. The change in the way the music sounds is pretty drastic.

Page 266 · What’s a Microcontroller?

'What's a Microcontroller - MusicWithMoreFeatures.bs2

'Play the beginning of For He's a Jolly Good Fellow. '{$STAMP BS2}

'{$PBASIC 2.5}

DEBUG "Program Running!"

READ Octaves + index, noteOctave noteOctave = 8 - noteOctave

noteFreq = noteFreq / (DCD noteOctave)

READ Durations + index, noteDuration noteDuration = WholeNote / noteDuration

READ Dots + index, noteDot

IF noteDot = 1 THEN noteDuration = noteDuration * 3 / 2

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FREQOUT 9, noteDuration, noteFreq

index = index + 1

LOOP

END

How MusicWithMoreFeatures.bs2 Works

Below is the musical data for the entire song. For each note in the Notes DATA directive, there is a corresponding entry in the Octaves, Durations, and Dots DATA directives. For example, the first note is a C note in the 7th octave; it’s a quarter note and it’s not dotted. Here is another example: the second from the last note (not including “Q”) is an E note, in the 7th octave. It’s a half note, and it is dotted. There is also a BeatsPerMin constant that sets the tempo for the song.

In the previous example program, WholeNote was a constant. This time, it’s a variable that will hold the duration of a whole note in ms. After this value is calculated, WholeNote will be used to determine all the other note durations, just like in the previous program. The index, offset, noteLetter, and noteDuration variables are also used in the same manner they were in the previous program. The noteFreq variable is handled a little differently since now it has to be adjusted depending on the octave the note is played in. The noteOctave and noteDot variables have been added to handle the octave and dot features.

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The wholeNote variable is calculated using BeatsPerMin. The tempo of the song is defined in beats per minute, and the program has to divide BeatsPerMin into 60000 ms, then multiply by 4. The result is the correct value for a whole note.

wholeNote = 60000 / BeatsPerMin * 4

Math executes from left to right. In the calculation wholeNote = 60000 / beatsPerMin * 4, the BASIC Stamp first calculates 60000 / beatsPerMin. Then, it multiplies that result by 4.

Parentheses can be used to group operations. If you want to divide 4 into beatsPerMin first, you can do this: wholeNote = 60000 / (beatsPerMin * 4).

This is all the same as the previous program:

DO UNTIL noteLetter = "Q"

READ Notes + index, noteLetter

Now that octaves are in the mix, the part of the code that figures out the note frequency has changed. The LOOKUP command’s table of values contains note frequencies from the 8th octave. These values can be divided by 1 if you want to play notes in the 8th octave, by 2 if you want to play notes in the 7th octave, by 4 if you want to play notes in the 6th octave and by 8 if you want to play notes in the 5th octave. The division happens next. All this LOOKUP command does is place a note from the 8th octave into the noteFreq variable.

Here is how the noteFreq variable is adjusted for the correct octave. First, the READ command grabs the octave value stored in the Octaves DATA. This could be a value between 5 and 8.

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READ Octaves + index, noteOctave

Depending on the octave, we want to divide noteFreq by either 1, 2, 4, or 8. That means that the goal is really to divide by 20 = 1, 21 = 2, 22 = 4, or 23 = 8. The statement below takes the value of noteOctave, which could be a value between 5 and 8, and subtracts that value from 8. If noteOctave was 8, now it’s 0. If noteOctave was 7, now it’s 1. If noteOctave was 6, now it’s 2, and if noteOctave was 5, now it’s 3.

noteOctave = 8 - noteOctave

Now, noteOctave is a value that can be used as an exponent of 2, but how do you raise 2 to a power in PBASIC? One answer is to use the DCD operator. DCD 0 is 1, DCD 1 is 2, DCD 2 is 4, and DCD 3 is 8. Dividing noteFreq by DCD noteOctave means you are dividing by 1, 2, 4, or 8, which divides noteFreq down by the correct value. The end result is that noteFreq is set to the correct octave. You will use the Debug Terminal in the Your Turn section to take a closer look at how this works.

noteFreq = noteFreq / (DCD noteOctave)

How am I supposed to know to use the DCD operator? Keep learning and practicing. Every time you see a new command, operator, or any other keyword used in an example, look it up in the BASIC Stamp manual. Read about it, and try using it in a program of your own design.

Get in the habit of periodically reading the BASIC Stamp Manual and trying the short example programs. That’s the best way to get familiar with the various commands and operators and how they work. By doing these things, you will develop a habit of always adding to the list of programming tools you can use to solve problems.

The first two lines of code for determining the note duration are about the same as the code from the previous example program. Now, however, any note could be dotted, which means the duration might have to be multiplied by 1.5. A READ command is used to access values stored in EEPROM by the Dots DATA directive. An IF...THEN statement is used to multiply by 3 and divide by 2 whenever the value of the noteDot variable is 1.

READ Durations + index, noteDuration noteDuration = WholeNote / noteDuration

READ Dots + index, noteDot

IF noteDot = 1 THEN noteDuration = noteDuration * 3 / 2

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Integer math The BASIC Stamp does not automatically process a number like 1.5. When performing math, it only works with integers: …, -5, -4, -3, -2, -1, 0, 1, 2, 3, … The best solution for multiplying by 1.5 is to multiply by 3/2. First, multiply by 3, and then divide by 2.

There are many ways to program the BASIC Stamp to handle fractional values. You can program the BASIC Stamp to use integers to figure out the fractional portion of a number. This is introduced in the Basic Analog and Digital Student Guide. There are also two operators that make fractional values easier to work with, and they are: ** and */. These are explained in detail in the Applied Sensors Student Guide and in the BASIC Stamp Manual.

The remainder of this example program works the same way that it did in the previous example program:

FREQOUT 9, noteDuration, noteFreq index = index + 1

LOOP

END

Your Turn – Playing a Tune with More than One Octave

MusicWithMoreFeatures.bs2 made use of rests, but it stayed in one octave. The tune “Take Me Out to the Ball Game” shown below plays most of its notes in the 6th octave. There are two notes in the 7th octave, and they make a big difference to the way it sounds.

9Save a copy of the program as MusicWithMoreFeaturesYourTurn.bs2.

9Modify the program by replacing the four data directives and one constant declaration with these:

9 Run the program and verify that it sounds right.