=====Using SOUND with true frequencies=====

//by Richard Russell, May 2014//\\ \\  The [[http://www.bbcbasic.co.uk/bbcwin/manual/bbcwin7.html#sound|SOUND]] statement's third parameter normally specifies the required **pitch** in units of quarter-semitones. For example the value 100 corresponds to **middle C** and the value 136 corresponds to the **A above middle C** (440 Hz, the standard musical pitch).\\ \\  Steps of ¼ semitone will normally be sufficiently fine, even for //glissando// or //vibrato// effects, but occasionally it may be useful to specify the pitch more precisely, in terms of a frequency in **Hertz**. For example if one prefers to use [[http://en.wikipedia.org/wiki/Just_intonation|just intonation]] rather than an equally-tempered scale it would be preferable to approximate the frequencies as accurately as possible (in practice BB4W provides a resolution of approximately one-third Hz).\\ \\  The function **FNfreqout** listed below makes this possible. It takes as a parameter the required frequency in Hz (the maximum being around 10 kHz, but at frequencies that high aliasing effects can be excessive) and returns a value which should be specified as the third parameter of the SOUND statement.\\ \\  Here is an example program illustrating its use:

<code bb4w> 
        FOR freq = 500 TO 1000
          SOUND 1, -15, FNfreqout(freq), 1
        NEXT
        END

        DEF FNfreqout(f)
        LOCAL I% : PRIVATE ftab%, indx&
        IF ftab% = 0 THEN
          LOCAL base%
          SYS "GetModuleHandle", 0 TO base%
          FOR I% = base% TO base% + 65534 STEP 2
            IF !I% = &03550354 IF I%!4 = &06060606 EXIT FOR
          NEXT
          IF I% > base% + 65534 ERROR 100, "Cannot locate frequency table"
          ftab% = I% + 8
          SYS "VirtualProtect", ftab% AND -&1000, &2000, &40, ^I% TO I%
          IF I% = 0 ERROR 100, "Cannot make memory image writable"
        ENDIF
        indx& += 1 : IF indx& = 0 indx& = 1
        I% = 2 * indx&
        ftab%!I% = ftab%!I% AND &FFFF0000 OR INT(f * &10000 / 22050 + 0.5)
        = indx&
</code>

A limitation of this technique is that a maximum of **255** different frequencies can be 'active' at once. If **FNfreqout** is called more than that number of times the oldest frequencies will be overwritten. If an **ENVELOPE** statement is used it should not contain a pitch component (at least, not unless special measures are taken); amplitude envelopes work as expected.\\ \\  Note that **FNfreqout** works by dynamically rewriting BBC BASIC's frequency table, so if used in a program run from the BB4W **IDE** subsequent programs which use the SOUND statement may be affected. If in doubt quit //BBC BASIC for Windows// and restart it. This issue does not affect its use in a 'compiled' program.