From: cohen@GOMEZ.phys.virginia.edu
Subject: TI sound chip (reprinted)
Reply-To: cohen@GOMEZ.phys.virginia.edu
Organization: Dept. of Physics, University of Virginia
Date: Mon, 12 Oct 1992 16:17:26 GMT

    Article 44 of freenet.sci.comp.ti.news:
    Newsgroups: freenet.sci.comp.ti.news
    Subject: TI Sound Chip--tech data
    Orig-Date: Thu, 17 Dec 87 12:14:24 -0400
    Date: Mon, 31 Jul 89 03:32:33 GMT
    Lines:      201
    
    HOW ACCURATE IS THE 99/4A'S SOUND CHIP?
    
    By Charles Good - Lima 99/4A - November
    87
    
         Some of the musical programs in our
    user group's library sound just a bit
    off, with an occasional sour note.  This
    is particularly true of songs played as
    single notes rather than chords.  You
    don't have to be a music expert to
    detect these sour notes.  Either the
    programmer programs in the wrong
    frequency, or perhaps the sound chip is
    less than totally accurate.
    
         Reading about the CALL SOUND
    statement in the Users Reference Guide
    makes one believe that the sound chip is
    very accurate.  The frequency portion of
    a CALL SOUND statement can have a
    frequency between 110 and 44733 hertz
    (cycles per second) suggesting that
    there are 44623 separate sounds
    possible! Of course, many of these
    frequencies are beyond the ability of
    most monitor speakers or exceed the
    limits of human hearing, but these
    numbers suggest the possibility of great
    accuracy.
    
         The User's Reference Guide on page
    III7 gives the frequencies for four
    octaves of commonly used musical notes.
    In order to check the accuracy of the
    musical frequencies listed on page III7,
    I wrote the following program:
         100 PRINT "INPUT FREQUENCY"
         110 INPUT F
         120 CALL SOUND (4000,F,0)
         130 GOTO 100
         I entered various frequencies,
    including those on page II7 and then
    used an ARION HUB 8000 Micron Chromatic
    Tuner to measure the resulting sounds as
    they came out of the speaker on my TI
    10-inch monitor.  This chromatic tuner
    is used by musicians to tune stringed
    instruments, including pianos, and is
    quite accurate.  I played around with
    different CALL SOUND frequency values
    and found those that, according to my
    Chromatic Tuner, were closest to being
    perfectly "in tune" for all of the notes
    listed on page III7. The results are
    shown in the table below.  These data
    were identical on each of the two 99/4A
    consoles I checked.
    
         Not all frequencies listed on page
    III7 actually produce the desired note.
    These are noted in the COMMENTS section
    of the table.  The inaccurately
    published frequencies are in fact the
    theoretically correct values, but they
    are one hertz too large to give the
    correct sound from the TI sound chip.
    Music programmers take note! This may be
    the source of some of the "off" sounds
    in some of the songs in our group's
    software library.  It is possible that
    these frequency errors are due to the
    speaker used in the test rather than the
    TI sound chip.  Maybe a high-quality
    speaker would make these errors
    disappear, but I doubt it.
    
         For some notes, it is not possible
    to get a sound that is perfectly in
    tune.  This is particularly true of the
    higher notes.  Values shown in the table
    show which CALL SOUND frequency gives
    the closest approximation of a note, and
    the percent error from perfect "in
    tune". Again, a better quality speaker
    might make these apparent errors
    disappear, but I doubt it.  It is
    possible, however, that the magnitude of
    the high frequency errors (10-15% off
    true "in tune") would be reduced with a
    better speaker.
    
         Beginning with E above middle C
    (330 hertz) more than on adjacent
    frequency number produces EXACTLY the
    same sound. Thus the apparent tremendous
    accuracy of the TI sound chip with its
    44623 different sounds is a mith.  The
    number of possible sounds is much less.
    This particular characteristic of the TI
    sound chip would not be affected one way
    or another by a higher quality speaker.
    For A above high C (theoretically 880
    hertz ) any frequency number between 878
    and 884 in a CALL SOUND statement
    produces the EXACT SAME in tune sound.
    If you go one higher to 885, you get a
    distinctly higher sound.  You don't need
    a sound meter to detect this change of
    sound between 884 and 885. The
    difference can easily be heard with
    normal hearing. The above small bASIC
    program lets you input a new frequency
    before the old one quits playing so you
    can listen to frequencies back to back
    with only an instantaneous pause in
    between.  Try it. Type in the program
    and RUN it.  Input frequencies 878, 879,
    etc up to 884 and they all sound EXACTLY
    the same.  Then try 885 and note the
    difference.
    
         Hopefully the data in this article
    will help music programmers appreciate
    some of the limitations built into the
    TI sound chip and correct some of the
    sour notes published on page III7 of the
    Reference Guide.
    
    NOTE|CALL SOUND   |Actual Measured| CO
    
         |frequency,   |CALL SOUND     |
         |as reported. |frequencies    |
         |on page III7 |that generate  |
         |of Users Ref.|this note in   |
         |Guide        |tune.          |
    --------------------------------------
    A...110....NA.....Volume too soft
                       to meter
    A#.117.....NA.....  " " "
    C(low C).131............131
    C#....139...138.....Off by 1Hz,
                         also 4% flat
    D..147....147
    D#.156....156.....5% sharp
    E..165....165
    F..175....175.....5% sharp
    F#.185....185
    G..196....196
    G#.208....207.....Ref Guide high
                       by 1hz
    A..220....220.....5% sharp
    A#.233....233
    B..247....247
    C(mid C).262...261..Ref Guide
                        high by 1HZ
    C#.277....277
    D..294....293.....Ref Guide
                      high by 1HZ
    D#.311....311
    E..330....329-330
    F..349....349-350
    F#.370....369.....Ref Guide
                      high by 1HZ
    G..392....392-393
    G#.415....415-416
    A..440....440-441
    A#.466....466
    B..494....491-491...Ref Guide
                     high by >=1HZ
    C(high C)523..522-523
    C#.554....553-555
    D..587....584-587
    D#.622...? (Missprint in orig.
                  article
    E..659....656-659
    F..698....697-701
    F#.740....738-743
    G..784....780-784
    G#.831....826-831
    A..880....878-884
    A#.932....929-935
    B..988....986-994
    C.1047....1041-1050
    C#1109....1102-1112
    D.1175....1172-1183..5% sharp
    D#1245....1235-1249
    E.1319....1309-1323
    F.1397....1389-1406
    F#1480....1462-1481..10% flat
    G.1568....1565-1586..5% sharp
    G#1661....1657-1681..10% sharp
    A.1760....1734-1761..15% flat
    ---------------------------------------
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