What is a Microcontroller (Paralax, v2.2, student guide, 2004)

Chapter #8: Frequency and Sound · Page 223

DEBUG "Robot reply...", CR

PAUSE 100

FREQOUT 9, 100, 2800

FREQOUT 9, 200, 2400

FREQOUT 9, 140, 4200

FREQOUT 9, 30, 2000

PAUSE 500

DEBUG "Hyperspace...", CR

PAUSE 100

FOR duration = 15 TO 1 STEP 1

FOR frequency = 2000 TO 2500 STEP 20

FREQOUT 9, duration, frequency

NEXT

NEXT

DEBUG "Done", CR

END

How ActionTones.bs2 Works

The “Alarm” routine sounds like an alarm clock. This routine plays tones at a fixed frequency of 1.5 kHz for a duration of 0.5 s with a fixed delay between tones of 0.5 s. The “Robot reply” routine uses various frequencies for brief durations.

The “Hyperspace” routine uses no delay, but it varies both the duration and frequency. By using FOR…NEXT loops to rapidly change the frequency and duration, you can get some interesting sound effects. When one FOR…NEXT loop executes inside another one, it is called a nested loop. Here is how the nested FOR…NEXT loop shown below works. The

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duration variable starts at 15, then the frequency loop takes over and sends frequencies of 2000, then 2020, then 2040, and so on, up through 2500 to the piezo speaker. When the frequency loop is finished, the duration loop has only repeated one of its 15 passes. So it subtracts one from the value of duration and repeats the frequency loop all over again.

FOR duration = 15 TO 1

FOR frequency = 2000 TO 2500 STEP 15

FREQOUT 9, duration, frequency

NEXT

NEXT

Example Program: NestedLoops.bs2

To better understand how nested FOR…NEXT loops work, NestedLoops.bs2 uses the DEBUG command to show the value of a much less complicated version of the nested loop used in ActionTones.bs2.

√Enter and run NestedLoops.bs2.

√Examine the Debug Terminal output and verify how the duration and frequency arguments change each time through the loop.

'What's a Microcontroller - ActionTones.bs2

'Demonstrate how different combinations of pause, duration, and frequency

'can be used to make sound effects.

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

DEBUG "Alarm...", CR

PAUSE 100

FREQOUT 9, 500, 1500

PAUSE 500

FREQOUT 9, 500, 1500

PAUSE 500

FREQOUT 9, 500, 1500

PAUSE 500

FREQOUT 9, 500, 1500

PAUSE 500

DEBUG "Robot reply...", CR

PAUSE 100

FREQOUT 9, 100, 2800

Chapter #8: Frequency and Sound · Page 225

FREQOUT 9, 200, 2400

FREQOUT 9, 140, 4200

FREQOUT 9, 30, 2000

PAUSE 500

DEBUG "Hyperspace...", CR

PAUSE 100

FOR duration = 15 TO 1 STEP 1

FOR frequency = 2000 TO 2500 STEP 20

FREQOUT 9, duration, frequency

NEXT

NEXT

DEBUG "Done", CR

END

Your Turn – More Sound Effects

There is pretty much an endless number of ways to modify ActionTones.bs2 to get different sound combinations. Here is just one modification to the “Hyperspace” routine:

DEBUG "Hyperspace jump...", CR

FOR duration = 15 TO 1 STEP 3

FOR frequency = 2000 TO 2500 STEP 15

FREQOUT 9, duration, frequency

NEXT

NEXT

FOR duration = 1 TO 36 STEP 3

FOR frequency = 2500 TO 2000 STEP 15

FREQOUT 9, duration, frequency

NEXT

NEXT

√Save your example program under the name ActionTonesYourTurn.bs2.

√Have fun with this and other modifications of your own invention.

Two Frequencies at Once

You can send two frequencies at the same time. In audio, this is called “mixing”. Remember the FREQOUT command’s syntax from Activity #1:

FREQOUT Pin, Duration, Freq1 {, Freq2}

You can use the optional Freq2 argument to mix two frequencies using the FREQOUT command. For example, you can mix 2 and 3 kHz together like this:

FREQOUT 9, 1000, 2000, 3000

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Each touchtone keypad tone is also an example of two frequencies mixed together. In telecommunications, that is called DTMF (Dual Tone Multi Frequency). There is also a PBASIC command called DTMFOUT that is designed just for sending phone tones. For examples of projects where phone numbers are dialed, see the DTMFOUT command in the BASIC Stamp Manual.

Example Program: MixingTones.bs2

This example program demonstrates the difference in tone that you get when you mix 2 and 3 kHz together. It also demonstrates an interesting phenomenon that occurs when you mix two sound waves that are very close in frequency. When you mix 2000 Hz with 2001 Hz, the tone will fade in and out once every second (at a frequency of 1 Hz). If you mix 2000 Hz with 2002 Hz, it will fade in and out twice a second (2 Hz), and so on.

Beat is when two tones very close in frequency are played together causing the tone you hear to fade in and out. The frequency of that fading in and out is the difference between the two frequencies. If the difference is 1 Hz, the tone will fade in and out at 1 Hz. If the difference is 2 Hz, the tone will fade in and out at 2 Hz.

The variations in air pressure made by the piezoelectric speaker are called sound waves. When the tone is loudest, the variations in air pressure caused by the two frequencies are adding to each other (called superposition). When the tone is at its quietest, the variations in air pressure are canceling each other out (called interference).

√Enter and run MixingTones.bs2.

√Keep an eye on the Debug Terminal as the tones play, and note the different effects that come from mixing the different tones.

'What's a Microcontroller - MixingTones.bs2

'Demonstrate some of the things that happen when you mix two tones.

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

DEBUG "Frequency = 2000", CR

FREQOUT 9, 4000, 2000

DEBUG "Frequency = 3000", CR

FREQOUT 9, 4000, 3000

DEBUG "Frequency = 2000 + 3000", CR

FREQOUT 9, 4000, 2000, 3000

DEBUG "Frequency = 2000 + 2001", CR

Chapter #8: Frequency and Sound · Page 227

FREQOUT 9, 4000, 2000, 2001

DEBUG "Frequency = 2000 + 2002", CR

FREQOUT 9, 4000, 2000, 2002

DEBUG "Frequency = 2000 + 2003", CR

FREQOUT 9, 4000, 2000, 2003

DEBUG "Frequency = 2000 + 2005", CR

FREQOUT 9, 4000, 2000, 2005

DEBUG "Frequency = 2000 + 2010", CR

FREQOUT 9, 4000, 2000, 2010

DEBUG "Done", CR

END

Your Turn – Condensing the Code

MixingTones.bs2 was written to demonstrate some interesting things that can happen when you mix two different frequencies using the FREQOUT command’s optional Freq2 argument. However, it is extremely inefficient.

√Modify MixingTones.bs2 so that it cycles through the Freq2 arguments ranging from 2001 to 2005 using a word variable and a loop.

ACTIVITY #3: MUSICAL NOTES AND SIMPLE SONGS

Figure 8-3 shows the rightmost 25 keys of a piano keyboard. It also shows the frequencies at which each wire inside the piano vibrates when that piano key is struck. The keys and their corresponding notes are labeled C6 through C8. These keys are separated into groups of 12. Each group spans one octave, made up of 8 white keys and 4 black keys. The sequence of notes repeats itself every 12 keys. Notes of the same letter are related by frequency, doubling with each higher octave. For example, C7 is twice the frequency of C6, and C8 is twice the frequency of C7. Likewise, if you go one octave down, the frequency will be half the value; for example, A6 is half the frequency of A7.

Internet search for – “musical scale”: By using the words "musical scale" in a search engine like Google or Yahoo, you will find lots of fascinating information about the history, physics and psychology of the subject. The 12 note per octave scale is the main scale of western music. Other cultures use scales that contain 2 to 35 notes per octave.

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If you’ve ever heard a singer practice his/her notes by singing the Solfege, “Do Re Mi Fa Sol La Ti Do”, the singer is attempting to match the notes that you get from striking the white keys on a piano keyboard. These white keys are called natural keys. A black key on a piano can either be called sharp or flat. For example, the black key between the C and D keys is either called C-sharp (C#) or D-flat (Db). Whether a key is called sharp or flat depends on the particular piece being played, and the rules for that are better left to the music classes.

Figure 8-3: Rightmost Piano Keys and Their Frequencies

Chapter #8: Frequency and Sound · Page 229

Tuning Method: The keyboard in Figure 8-3 uses a method of tuning called equal

temperament. The frequencies are determined using a reference note, then multiplying it by 2(n/12) for values of n = 1, 2, 3, etc. For example, you can take the frequency for A6, and multiply by 2(1/12) to get the frequency for A6#. Multiply it by 2(2/12) to get the frequency for

B6, and so on. Here is an example of calculating the frequency for B6 using A6 as a reference frequency:

The frequency of A6 is 1760 2(2/12) = 1.1224

1760 X 1.224 = 1975.5

1975.5 is the frequency of B6

Programming Musical Notes

The FREQOUT command is also useful for musical notes. Programming the BASIC Stamp to play music using a piezospeaker involves following a variety of rules used in playing music using any other musical instrument. These rules apply to the same elements that were used to make sound effects, frequency, duration, and pause. This next example program plays some of the musical note frequencies on the piezospeaker, each with a duration of half a second.

Example Program: DoReMiFaSolLaTiDo.bs2

√Enter and run DoReMiFaSolLaTiDo.bs2

'What's a Microcontroller - DoReMiFaSolLaTiDo.bs2

'Send an octave of half second tones using a piezoelectric speaker.

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

Your Turn – Sharp/Flat Notes

√Use the frequencies shown in Figure 8-3 to add the five sharp/flat notes to DoReMiFaSolLaTiDo.bs2

√Modify your program so that it plays the next octave up. Hint: Save yourself

some typing and just use the * 2 operation after each Freq1 argument. For example, FREQOUT 9, 500, 1175 * 2 will give you D7, the D note in the 7th octave.

Storing and Retrieving Sequences of Musical Notes

A good way of saving musical notes is to store them using the BASIC Stamp module’s EEPROM. Although you could use many WRITE commands to do this, a better way is to use the DATA directive. This is the syntax for the DATA directive:

{Symbol} DATA {Word} DataItem {, {Word} DataItem, … }

Here is an example of how to use the DATA directive to store the characters that correspond to musical notes.

Notes DATA "C","C","G","G","A","A","G"

You can use the READ command to access these characters. The letter ‘C’ is located at address Notes + 0, and a second letter ‘C’ is located at Notes + 1. Then, there’s a letter ‘G’ at Notes + 2, and so on. For example, if you want to load the last letter ‘G’ into a byte variable called noteLetter, use the command:

READ Notes + 6, noteLetter

You can also store lists of numbers using the DATA directive. Frequency and duration values that the BASIC Stamp uses for musical notes need to be stored in word variables because they are usually greater than 255. Here is how to do that with a DATA directive.

Frequencies DATA Word 2093, Word 2093, Word 3136, Word 3136, Word 3520, Word 3520, Word 3136

Chapter #8: Frequency and Sound · Page 231

Because each of these values occupies two bytes, accessing them with the read command is different from accessing characters. The first 2093 is at Frequencies + 0, but the second 2093 is located at Frequencies + 2. The first 3136 is located at Frequencies + 4, and the second 3136 is located at Frequencies + 6.

The values in the Frequencies DATA directive correspond with the musical notes in the Notes DATA directive.

Here is a FOR…NEXT loop that places the Notes DATA into a variable named noteLetter, then it places the Frequencies DATA into a variable named noteFreq.

FOR index = 0 to 6

READ Notes + index, noteLetter

READ Frequencies + (index * 2), Word noteFreq

DEBUG noteLetter, " ", DEC noteFreq, CR

NEXT

What does the (index * 2) do? Each value stored in the Frequencies DATA directive takes a word (two bytes), while each character in the Notes DATA directive only takes one byte. The value of index increases by one each time through the FOR…NEXT loop. That’s fine for accessing the note characters using the command READ Notes + index, noteLetter. The problem is that for every one byte in Notes, the index variable needs to point twice as far down the Frequencies list. The command READ Frequencies + (index * 2), Word noteFreq, takes care of this.

The next example program stores notes and durations using DATA, and it uses the FREQOUT command to play each note frequency for a specific duration. The result is the first few notes from the children’s song Twinkle Twinkle Little Star.

The Alphabet Song used by children to memorize their “ABCs” uses the same notes as Twinkle Twinkle Little Star.

Example Program: TwinkleTwinkle.bs2

This example program demonstrates how to use the DATA directive to store lists and how to use the READ command to access the values in the lists.

√Enter and run TwinkleTwinkle.bs2

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√Verify that the notes sound like the song Twinkle Twinkle Little Star.

√Use the Debug Terminal to verify that it works as expected by accessing and displaying values from the three DATA directives.

'What's a Microcontroller - TwinkleTwinkle.bs2

'Play the first seven notes from Twinkle Twinkle Little Star.

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

FOR index = 0 TO 6

READ Notes + index, noteLetter

DEBUG " ", noteLetter

READ Durations + (index * 2), Word noteDuration

DEBUG " ", DEC4 noteDuration

READ Frequencies + (index * 2), Word noteFreq

DEBUG " ", DEC4 noteFreq, CR

FREQOUT 9, noteDuration, noteFreq

NEXT

END

Your Turn – Adding and Playing More Notes

This program played the first seven notes from Twinkle Twinkle Little Star. The words go “Twin-kle twin-kle lit-tle star”. The next phrase from the song goes “How I won-der