Sync: Bugfix for MTC.Encoder.locate(to:) full-frame erratic generation conditions

Author: orchetect

Sources/MIDIKitSync/MTC/Generator/MTC Encoder.swift

@@ -137,23 +137,32 @@ extension MIDI.MTC {
 				setLocalFrameRate(frameRate)
 			}
 
+			// Step 1: set Encoder's internal MTC components
+			
 			let scaledFrames = localFrameRate
 				.scaledFrames(fromTimecodeFrames: Double(components.f))
 
 			var newComponents = components
 			newComponents.f = scaledFrames.rawMTCFrames
+			
 			// sanitize: clear subframes since we're working at 1-frame resolution with timecode display values
 			newComponents.sf = 0
 
 			setMTCComponents(mtc: newComponents)
 			mtcQuarterFrame = scaledFrames.rawMTCQuarterFrames
 			mtcQuarterFrameStreamHasStartedSinceLastLocate = false
 
-			// tell handler to transmit MIDI message
+			// Step 2: tell handler to transmit full-frame message if applicable
+			
 			switch transmitFullFrame {
 			case .always:
 				sendFullFrameMIDIMessage()
 			case .ifDifferent:
+				newComponents = MIDI.MTC.convertToFullFrameComponents(
+					mtcComponents: newComponents,
+					mtcQuarterFrames: scaledFrames.rawMTCQuarterFrames
+				)
+				
 				let newFullFrame = (mtcComponents: newComponents,
 									mtcFrameRate: mtcFrameRate)
 				if !mtcIsEqual(lastTransmitFullFrame, newFullFrame) {
@@ -250,8 +259,10 @@ extension MIDI.MTC {
 			// rr == 10: 29.97d frames/s (SMPTE drop-frame timecode)
 			// rr == 11: 30 frames/s
 
-			var newComponents = mtcComponents
-			newComponents.f += ((25 * Int(mtcQuarterFrame)) / 100)
+			let newComponents = convertToFullFrameComponents(
+				mtcComponents: mtcComponents,
+				mtcQuarterFrames: mtcQuarterFrame
+			)
 
 			let midiMessage: [Byte] = [
 				0xF0,

Sources/MIDIKitSync/MTC/MTC Utilities.swift

@@ -35,4 +35,17 @@ extension MIDI.MTC {
 
 	}
 
+	/// Internal: Converts MTC components and quarter frames to full-frame components
+	internal static func convertToFullFrameComponents(
+		mtcComponents: Timecode.Components,
+		mtcQuarterFrames: UInt8
+	) -> Timecode.Components {
+		
+		var newComponents = mtcComponents
+		newComponents.f += ((25 * Int(mtcQuarterFrames)) / 100)
+		
+		return newComponents
+		
+	}
+	
 }

Tests/MIDIKitSyncTests/MTC/Generator/MTC Encoder Tests.swift

@@ -933,6 +933,143 @@ final class MTC_Generator_Encoder_Tests: XCTestCase {
 
 	}
 
+	func testMTC_Encoder_LocateBehavior() {
+		
+		var mtcEnc: MIDI.MTC.Encoder
+		
+		var message: [Byte]?
+		
+		func initNewEnc() -> MIDI.MTC.Encoder {
+			return MIDI.MTC.Encoder { midiMessage in
+				message = midiMessage
+			}
+		}
+		
+		mtcEnc = initNewEnc()
+		mtcEnc.locate(to: Timecode(at: ._24))
+		
+		XCTAssertEqual(mtcEnc.generateFullFrameMIDIMessage().components,
+					   TCC(h: 0, m: 00, s: 00, f: 00))
+		XCTAssertEqual(message,
+					   [
+						0xF0, 0x7F, 0x7F, 0x01, 0x01,
+						0b0000_0000, // 0rrh_hhhh
+						0x00, // M
+						0x00, // S
+						0x00, // F
+						0xF7
+					   ])
+		
+		mtcEnc = initNewEnc()
+		mtcEnc.locate(to: Timecode(at: ._25))
+		
+		XCTAssertEqual(mtcEnc.generateFullFrameMIDIMessage().components,
+					   TCC(h: 0, m: 00, s: 00, f: 00))
+		XCTAssertEqual(message,
+					   [
+						0xF0, 0x7F, 0x7F, 0x01, 0x01,
+						0b0010_0000, // 0rrh_hhhh
+						0x00, // M
+						0x00, // S
+						0x00, // F
+						0xF7
+					   ])
+		
+		mtcEnc = initNewEnc()
+		mtcEnc.locate(to: TCC(h: 1, m: 02, s: 03, f: 04).toTimecode(at: ._29_97_drop)!)
+		
+		XCTAssertEqual(mtcEnc.generateFullFrameMIDIMessage().components,
+					   TCC(h: 1, m: 02, s: 03, f: 04))
+		XCTAssertEqual(message,
+					   [
+						0xF0, 0x7F, 0x7F, 0x01, 0x01,
+						0b0100_0001, // 0rrh_hhhh
+						0x02, // M
+						0x03, // S
+						0x04, // F
+						0xF7
+					   ])
+		
+		mtcEnc.locate(to: TCC(h: 1, m: 02, s: 03, f: 05).toTimecode(at: ._29_97_drop)!)
+		
+		XCTAssertEqual(mtcEnc.generateFullFrameMIDIMessage().components,
+					   TCC(h: 1, m: 02, s: 03, f: 05))
+		XCTAssertEqual(message,
+					   [
+						0xF0, 0x7F, 0x7F, 0x01, 0x01,
+						0b0100_0001, // 0rrh_hhhh
+						0x02, // M
+						0x03, // S
+						0x05, // F
+						0xF7
+					   ])
+		
+		mtcEnc = initNewEnc()
+		mtcEnc.locate(to: TCC(h: 2, m: 04, s: 06, f: 08).toTimecode(at: ._30)!)
+		
+		XCTAssertEqual(mtcEnc.generateFullFrameMIDIMessage().components,
+					   TCC(h: 2, m: 04, s: 06, f: 08))
+		XCTAssertEqual(message,
+					   [
+						0xF0, 0x7F, 0x7F, 0x01, 0x01,
+						0b0110_0010, // 0rrh_hhhh
+						0x04, // M
+						0x06, // S
+						0x08, // F
+						0xF7
+					   ])
+		
+		// scaling frame rates
+		
+		mtcEnc = initNewEnc()
+		
+		// scales to MTC-24 fps
+		mtcEnc.locate(to: TCC(h: 2, m: 04, s: 06, f: 08).toTimecode(at: ._48)!)
+		
+		XCTAssertEqual(mtcEnc.generateFullFrameMIDIMessage().components,
+					   TCC(h: 2, m: 04, s: 06, f: 04))
+		XCTAssertEqual(message,
+					   [
+						0xF0, 0x7F, 0x7F, 0x01, 0x01,
+						0b0000_0010, // 0rrh_hhhh
+						0x04, // M
+						0x06, // S
+						0x04, // F
+						0xF7
+					   ])
+		
+		// scales to MTC-24 fps
+		mtcEnc.locate(to: TCC(h: 2, m: 04, s: 06, f: 09).toTimecode(at: ._48)!)
+		
+		XCTAssertEqual(mtcEnc.generateFullFrameMIDIMessage().components,
+					   TCC(h: 2, m: 04, s: 06, f: 04))
+		XCTAssertEqual(message,
+					   [
+						0xF0, 0x7F, 0x7F, 0x01, 0x01,
+						0b0000_0010, // 0rrh_hhhh
+						0x04, // M
+						0x06, // S
+						0x04, // F -- rounds down to 4 from 4.5 from scaling
+						0xF7
+					   ])
+		
+		// scales to MTC-24 fps
+		mtcEnc.locate(to: TCC(h: 2, m: 04, s: 06, f: 10).toTimecode(at: ._48)!)
+		
+		XCTAssertEqual(mtcEnc.generateFullFrameMIDIMessage().components,
+					   TCC(h: 2, m: 04, s: 06, f: 05))
+		XCTAssertEqual(message,
+					   [
+						0xF0, 0x7F, 0x7F, 0x01, 0x01,
+						0b0000_0010, // 0rrh_hhhh
+						0x04, // M
+						0x06, // S
+						0x05, // F
+						0xF7
+					   ])
+		
+	}
+	
 	func testMTC_Encoder_QFMIDIMessage() {
 
 		let mtcEnc = MIDI.MTC.Encoder()