Bit Masking and Bit Shifting in MIDI

MIDI data is transmitted as bytes, but many MIDI parameters are larger than a single byte or need to be split to fit MIDI’s rules. Bit masking and bit shifting are techniques used to pack and unpack values into bytes safely and efficiently.

Why Bit Operations Are Needed in MIDI

MIDI has important constraints:

• Data bytes must be 7-bit (0–127)
• Status bytes must have the MSB set
• SysEx data must not look like status bytes
• Parameters may be larger than 7 bits

Because of this, values often need to be:

• split across multiple bytes
• packed into bit fields
• reconstructed on reception

Bit masking and shifting make this possible.

What Is Bit Shifting?

Bit shifting moves bits left or right inside a number.

Left Shift (<<)

Shifting left multiplies by powers of two.

value << 1  → value × 2
value << 7  → value × 128

Example:

00000101 (5)
<< 1
00001010 (10)

Right Shift (>>)

Shifting right divides by powers of two.

value >> 1 → value ÷ 2

Example:

00001010 (10)
>> 1
00000101 (5)

What Is Bit Masking?

A bit mask is used to isolate specific bits using the AND operation. Example mask:

00001111

This keeps the lower 4 bits and clears the rest. Example:

10101101
AND 00001111
=   00001101

Masking is used to:

• extract parts of a value
• clear unwanted bits
• enforce 7-bit limits

Splitting a Value into 7-bit MIDI Bytes (Sending)

Problem

You want to send a value larger than 127 in a SysEx message. Example:

value = 1000

Binary

1111101000

This does not fit in one MIDI data byte.

Solution: Split into Multiple 7-bit Bytes

msb = (value >> 7) & 0x7F
lsb = value & 0x7F

This produces:

• MSB: upper bits
• LSB: lower 7 bits

Why Mask with 0x7F?

0x7F = 01111111

This guarantees:

• MSB of the byte is 0
• Byte is valid MIDI data

Example SysEx Payload

F0 <vendor> <param> <msb> <lsb> F7

Reconstructing a Value from MIDI Bytes (Receiving)

Reconstructing a Value from MIDI Bytes (Receiving)

value = (msb << 7) | lsb

Example:

msb = 7
lsb = 104

value = (7 << 7) + 104
value = 1000

Why Parameters Are Split Across Multiple Bytes

1. MIDI Data Byte Limit
   • MIDI data bytes must be 0–127.
   • Any larger value must be split.
2. Avoiding Status Byte Conflicts If a byte has MSB = 1, it looks like a status byte. Splitting prevents this.

Packing Multiple Small Values into One Byte

Sometimes multiple parameters are packed into one byte. Example:

• upper 4 bits → parameter A
• lower 4 bits → parameter B Sending

byte = (a << 4) | b

Receiving

a = (byte >> 4) & 0x0F
b = byte & 0x0F

This is common in compact SysEx formats.

Summary

Bit masking and bit shifting are fundamental techniques for working with MIDI data. They allow values to be split into safe 7-bit bytes for transmission and reassembled on reception. These techniques are essential when composing or parsing SysEx messages, where parameters may span multiple bytes to avoid status byte conflicts, support higher resolution, and maintain compatibility. Mastery of bit operations makes MIDI protocols understandable, predictable, and robust.