Standard Serial MIDI Communication Baud Rate

Standard serial MIDI (Musical Instrument Digital Interface) communication operates at a fixed transfer speed of 31,250 bits per second, commonly referred to as 31.25 kbaud. Established with the MIDI 1.0 specification in 1983, this asynchronous serial protocol allows synthesizers, sequencers, computers, and other musical hardware to reliably transmit performance data in real time. This article covers the exact specifications of the MIDI baud rate, the engineering reasons behind its selection, data framing details, and how it impacts latency.

The Standard Baud Rate: 31,250 Baud

The official baud rate for traditional 5-pin DIN serial MIDI is precisely 31,250 baud (±1%). At this rate, the interface transmits 31,250 bits per second over a current loop cable system.

Unlike modern computer interfaces such as USB, standard serial MIDI does not negotiate transfer speeds. Every traditional MIDI 1.0 compliant transmitter and receiver must operate at this exact rate to maintain synchronization and prevent communication errors.

Why 31,250 Was Chosen

The rate of 31,250 baud was chosen primarily due to the hardware limitations and microprocessors widely used in the early 1980s:

• Direct Clock Division: Many standard microprocessors of that era operated using internal clocks derived from common crystal frequencies. Dividing a 1 MHz clock signal by 32 results exactly in 31,250.
• Cost Efficiency: Using a standard binary divisor of 32 allowed manufacturers to implement the UART (Universal Asynchronous Receiver/Transmitter) logic using simple, inexpensive digital divider circuits rather than dedicated, costly baud rate generators.
• Optocoupler Performance: MIDI employs optoisolators to prevent ground loops between audio gear. In 1983, low-cost optocouplers could reliably handle signals at around 31.25 kHz without introducing excessive rise and fall time distortion.

Framing and Transmission Speed

Standard MIDI uses asynchronous serial communication without hardware flow control. Each byte of data is packaged into a 10-bit serial frame:

• 1 Start Bit (logic low / 0)
• 8 Data Bits (transmitted least significant bit first)
• 1 Stop Bit (logic high / 1)
• No Parity Bit

Because each byte requires 10 bits of transmission time:

\[\frac{31,250 \text{ bits/second}}{10 \text{ bits/byte}} = 3,125 \text{ bytes/second}\]

The interface can transmit a maximum of 3,125 bytes per second. Transmission time for an individual byte is exactly 320 microseconds (0.32 ms).

Throughput and Latency Considerations

Most standard MIDI channel voice messages—such as "Note On" or "Note Off"—consist of three bytes (a status byte followed by two data bytes representing pitch and velocity):

• 3-Byte Message: 30 bits total.
• Transmission Time: 30 bits / 31,250 bits/second = 0.96 milliseconds.

Under normal conditions, playing a single note incurs less than one millisecond of transmission latency, which is virtually imperceptible to the human ear. However, sending complex sequences, polyphonic chords, or continuous controller data (such as pitch bend or modulation wheel sweeps) can saturate the 3,125 byte/second limit, leading to noticeable timing delays known as MIDI jitter. To help mitigate this, the specification includes "Running Status," a protocol feature that omits the status byte for consecutive messages of the same type, reducing transmission requirements to two bytes per message.