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The required property "SHDR" holds an SScoreHeader as defined in these C
declarations and following documentation.  An SHDR specifies global
information for the score.  It must appear before the TRAKs in a FORM SMUS.

    #define ID_SMUS MakeID('S', 'M', 'U', 'S')
    #define ID_SHDR MakeID('S', 'H', 'D', 'R')

    typedef struct {
	UWORD tempo;	/* tempo, 128ths quarter note/minute */
	UBYTE volume;	/* overall playback volume 0 through 127 */
	UBYTE ctTrack;	/* count of tracks in the score */
	} SScoreHeader;

[Implementation details.  In the C struct definitions in this memo, fields
are filed in the order shown.  A UBYTE field is packed into an 8-bit byte.
Programs should set all "pad" fields to 0.  MakeID is a C macro defined in
the main IFF document and in the source file IFF.h.]

The field tempo gives the nominal tempo for all tracks in the score.  It
is expressed in 128ths of a quarter note per minute, i.e., 1 represents 1
quarter note per 128 minutes while 12800 represents 100 quarter notes per
minute.  You may think of this as a fixed point fraction with a 9-bit
integer part and a 7-bit fractional part (to the right of the point).  A
coarse-tempoed program may simply shift tempo right by 7 bits to get a
whole number of quarter notes per minute.  The tempo field can store tempi
in the range 0 up to 512.  The playback program may adjust this tempo,
perhaps under user control.

Actually, this global tempo could actually be just an initial tempo if
there are any "set tempo" SEvents inside the score (see TRAK, below).  Or
the global tempo could be scaled by "scale tempo" SEvents inside the
score.  These are potential extensions that can safely be ignored by
current programs.  [See More SEvents To Be Defined, below.]

The field volume gives an overall nominal playback volume for all tracks
in the score.  The range of volume values 0 through 127 is like a MIDI key
velocity value.  The playback program may adjust this volume, perhaps
under direction of a user "volume control".

Actually, this global volume level could be scaled by dynamic-mark SEvents
inside the score (see TRAK, below).

The field ctTrack holds the count of tracks, i.e., the number of TRAK
chunks in the FORM SMUS (see below).  This information helps the reader
prepare for the following data.

A playback program will typically load the score and call a driver routine
PlayScore(tracks, tempo, volume), supplying the tempo and volume from the
SHDR  chunk.

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