Scope: Control of Parametric EQ (Gain, Frequency) on Allen & Heath CQ-20B via proprietary TCP protocol.

1. Transport Layer

• Protocol: TCP/IP
• Port: 51326 (Distinct from MIDI/NRPN Port 51325)
• Keep-Alive: Mandatory. Connection drops after ~15s inactivity.
  • Requirement: Send any valid packet or dummy heartbeat every <10s.

2. Packet Structure

Format: F7 08 <Family> <Function> <Channel> <Band> <Val_Fine> <Val_Coarse>

3. Parameter Mappings

3.1 EQ Gain (Function 0x0C)

Controls amplitude of selected band.

• Center (0 dB): 0x80 (128)
• Range: +/- 15 steps (approx. 1dB/step)
  • -15 dB: 0x71 (113)
  • +15 dB: 0x8F (143)

Hex Examples:

• CH1 Band 1 @ 0dB: F7 08 0F 0C 00 00 00 80
• CH1 Band 1 @ +15dB: F7 08 0F 0C 00 00 00 8F

3.2 EQ Frequency (Function 0x0D)

Controls center frequency.

• Range: Logarithmic scale 20Hz - 20kHz.
  • 20 Hz: 0x29 (41)
  • 20 kHz: 0xDD (221)

Hex Examples:

• CH1 Band 1 @ 20Hz: F7 08 0F 0D 00 00 00 29
• CH1 Band 1 @ 20kHz: F7 08 0F 0D 00 00 00 DD

4. Implementation Snippets

JavaScript (Node.js / Companion)

function createPeqGainPacket(channel, band, dbValue) {
    // Channel: 1-16 -> 0-15
    // Band: 1-4 -> 0-3
    // dbValue: -15 to +15
    
    const chIdx = channel - 1;
    const bandIdx = band - 1;
    const clampedDb = Math.max(-15, Math.min(15, dbValue));
    
    // Map 0dB to 0x80 (128)
    const coarseVal = 0x80 + Math.round(clampedDb);
    
    return Buffer.from([
        0xF7, 0x08, 0x0F, 0x0C,
        chIdx, bandIdx, 0x00, coarseVal
    ]);
}

Python (Raw Socket)

import socket

def send_peq_gain(ip, ch_idx, band_idx, coarse_val):
    # ch_idx: 0-15
    # band_idx: 0-3
    # coarse_val: 0x71 (-15dB) .. 0x8F (+15dB)
    
    packet = bytearray([0xF7, 0x08, 0x0F, 0x0C, ch_idx, band_idx, 0x00, coarse_val])
    
    try:
        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        s.settimeout(2.0)
        s.connect((ip, 51326))
        s.sendall(packet)
        s.close()
    except Exception as e:
        print(f"TX Error: {e}")

5. Notes

• Stereo Links: Sending to the odd channel (Left) of a linked pair typically updates both.
• Feedback: The mixer echoes state changes on Port 51326 using the same binary format.

if interested, maps will follow

cheers