diff --git a/internal/source/source.go b/internal/source/source.go
index e5186f0..7bed226 100644
--- a/internal/source/source.go
+++ b/internal/source/source.go
@@ -69,7 +69,7 @@ func Open(domain, flowID string) (*Source, error) {
 	if err := json.Unmarshal([]byte(def), &fd); err != nil {
 		r.Close()
 		inst.Close()
-		return nil, wrapError("parse flow definition JSON", ErrorKindInvalidConfig, err)
+		return nil, wrapError("parse video flow definition JSON", ErrorKindInvalidConfig, err)
 	}
 	if fd.FrameWidth == 0 || fd.FrameHeight == 0 {
 		r.Close()
@@ -311,19 +311,19 @@ type SyncSource struct {
 func OpenSameDomainSync(domain, videoFlow, audioFlow string) (*SyncSource, error) {
 	inst, err := mxl.NewInstance(domain, "")
 	if err != nil {
-		return nil, fmt.Errorf("NewInstance: %w", err)
+		return nil, wrapError("new MXL instance", ErrorKindUnavailable, err)
 	}
 
 	vr, err := inst.NewReader(videoFlow)
 	if err != nil {
 		inst.Close()
-		return nil, fmt.Errorf("NewReader(video): %w", err)
+		return nil, wrapError("open video reader", ErrorKindUnavailable, err)
 	}
 	ar, err := inst.NewReader(audioFlow)
 	if err != nil {
 		vr.Close()
 		inst.Close()
-		return nil, fmt.Errorf("NewReader(audio): %w", err)
+		return nil, wrapError("open audio reader", ErrorKindUnavailable, err)
 	}
 
 	vInfo, err := vr.Info()
@@ -331,26 +331,56 @@ func OpenSameDomainSync(domain, videoFlow, audioFlow string) (*SyncSource, error
 		ar.Close()
 		vr.Close()
 		inst.Close()
-		return nil, fmt.Errorf("Info(video): %w", err)
+		return nil, wrapError("get video info", ErrorKindUnavailable, err)
 	}
 	if !vInfo.Config.Common.Format.IsDiscrete() {
 		ar.Close()
 		vr.Close()
 		inst.Close()
-		return nil, fmt.Errorf("video flow is not discrete")
+		return nil, wrapError(
+			"validate video flow",
+			ErrorKindInvalidConfig,
+			errors.New("video flow is continuous"),
+		)
 	}
 	aInfo, err := ar.Info()
 	if err != nil {
 		ar.Close()
 		vr.Close()
 		inst.Close()
-		return nil, fmt.Errorf("Info(audio): %w", err)
+		return nil, wrapError("get audio info", ErrorKindUnavailable, err)
 	}
 	if aInfo.Config.Common.Format.IsDiscrete() {
 		ar.Close()
 		vr.Close()
 		inst.Close()
-		return nil, fmt.Errorf("audio flow is not continuous")
+		return nil, wrapError(
+			"validate audio flow",
+			ErrorKindInvalidConfig,
+			errors.New("audio flow is discrete"),
+		)
+	}
+	channels := uint64(aInfo.Config.Continuous.ChannelCount)
+	if channels == 0 {
+		ar.Close()
+		vr.Close()
+		inst.Close()
+		return nil, wrapError(
+			"validate audio flow",
+			ErrorKindInvalidConfig,
+			errors.New("audio flow has no channels"),
+		)
+	}
+	aRate := aInfo.Config.Common.GrainRate
+	if aRate.Num <= 0 || aRate.Den <= 0 {
+		ar.Close()
+		vr.Close()
+		inst.Close()
+		return nil, wrapError(
+			"validate audio flow",
+			ErrorKindInvalidConfig,
+			fmt.Errorf("invalid audio rate: %d/%d", aRate.Num, aRate.Den),
+		)
 	}
 
 	def, err := inst.FlowDef(videoFlow)
@@ -358,14 +388,38 @@ func OpenSameDomainSync(domain, videoFlow, audioFlow string) (*SyncSource, error
 		ar.Close()
 		vr.Close()
 		inst.Close()
-		return nil, fmt.Errorf("FlowDef: %w", err)
+		return nil, wrapError("read video flow definition", ErrorKindUnavailable, err)
 	}
 	var fd flowDef
 	if err := json.Unmarshal([]byte(def), &fd); err != nil {
 		ar.Close()
 		vr.Close()
 		inst.Close()
-		return nil, fmt.Errorf("parse flow def: %w", err)
+		return nil, wrapError("parse video flow definition JSON", ErrorKindInvalidConfig, err)
+	}
+	if fd.FrameWidth <= 0 || fd.FrameHeight <= 0 {
+		ar.Close()
+		vr.Close()
+		inst.Close()
+		return nil, wrapError(
+			"validate video flow",
+			ErrorKindInvalidConfig,
+			fmt.Errorf(
+				"invalid video dimensions: %dx%d",
+				fd.FrameWidth,
+				fd.FrameHeight,
+			),
+		)
+	}
+	if len(vInfo.Config.Discrete.SliceSizes) == 0 {
+		ar.Close()
+		vr.Close()
+		inst.Close()
+		return nil, wrapError(
+			"validate video flow",
+			ErrorKindInvalidConfig,
+			errors.New("video flow has no slice sizes"),
+		)
 	}
 
 	vRate := vInfo.Config.Common.GrainRate
@@ -374,7 +428,11 @@ func OpenSameDomainSync(domain, videoFlow, audioFlow string) (*SyncSource, error
 		ar.Close()
 		vr.Close()
 		inst.Close()
-		return nil, fmt.Errorf("invalid video rate %d/%d", vRate.Num, vRate.Den)
+		return nil, wrapError(
+			"validate video flow",
+			ErrorKindInvalidConfig,
+			fmt.Errorf("invalid grain rate: %d/%d", vRate.Num, vRate.Den),
+		)
 	}
 
 	group, err := inst.NewSyncGroup()
@@ -382,21 +440,33 @@ func OpenSameDomainSync(domain, videoFlow, audioFlow string) (*SyncSource, error
 		ar.Close()
 		vr.Close()
 		inst.Close()
-		return nil, fmt.Errorf("NewSyncGroup: %w", err)
+		return nil, wrapError(
+			"create native sync group",
+			ErrorKindUnavailable,
+			err,
+		)
 	}
 	if err := group.AddReader(vr); err != nil {
 		group.Close()
 		ar.Close()
 		vr.Close()
 		inst.Close()
-		return nil, fmt.Errorf("AddReader(video): %w", err)
+		return nil, wrapError(
+			"add video reader to native sync group",
+			ErrorKindUnavailable,
+			err,
+		)
 	}
 	if err := group.AddReader(ar); err != nil {
 		group.Close()
 		ar.Close()
 		vr.Close()
 		inst.Close()
-		return nil, fmt.Errorf("AddReader(audio): %w", err)
+		return nil, wrapError(
+			"add audio reader to native sync group",
+			ErrorKindUnavailable,
+			err,
+		)
 	}
 
 	return &SyncSource{
@@ -405,8 +475,8 @@ func OpenSameDomainSync(domain, videoFlow, audioFlow string) (*SyncSource, error
 		ar:     ar,
 		group:  group,
 		rate:   vRate,
-		aRate:  aInfo.Config.Common.GrainRate,
-		chans:  uint64(aInfo.Config.Continuous.ChannelCount),
+		aRate:  aRate,
+		chans:  channels,
 		idx:    idx,
 		width:  uint32(fd.FrameWidth),
 		height: uint32(fd.FrameHeight),