diff --git a/src/output/config.rs b/src/output/config.rs
index c765fea..35c0278 100644
--- a/src/output/config.rs
+++ b/src/output/config.rs
@@ -14,6 +14,7 @@ use crate::output::writer::{self, BufferWriter, FileWriter, StdoutWriter, Writer
 
 /// The configuration repository for the TestRun.
 pub struct Config {
+    // All fields are readable for any impl inside the crate.
     pub(crate) timestamp_provider: Box<dyn TimestampProvider + Send + Sync + 'static>,
     pub(crate) writer: WriterType,
 }
@@ -46,6 +47,7 @@ impl ConfigBuilder {
         }
     }
 
+    // TODO: docs for all these
     pub fn timezone(mut self, timezone: chrono_tz::Tz) -> Self {
         self.timestamp_provider = Box::new(ConfiguredTzProvider { tz: timezone });
         self
@@ -103,17 +105,3 @@ impl TimestampProvider for ConfiguredTzProvider {
         chrono::Local::now().with_timezone(&self.tz)
     }
 }
-
-pub struct NullTimestampProvider {}
-
-impl NullTimestampProvider {
-    // warn: linter is wrong here, this is used in a serde_json::json! block
-    #[allow(dead_code)]
-    pub const FORMATTED: &str = "1970-01-01T00:00:00.000Z";
-}
-
-impl TimestampProvider for NullTimestampProvider {
-    fn now(&self) -> chrono::DateTime<chrono_tz::Tz> {
-        chrono::DateTime::from_timestamp_nanos(0).with_timezone(&chrono_tz::UTC)
-    }
-}
diff --git a/src/output/emitter.rs b/src/output/emitter.rs
index 21a2fa7..0d42aba 100644
--- a/src/output/emitter.rs
+++ b/src/output/emitter.rs
@@ -17,8 +17,7 @@ use crate::output::{
 use crate::spec;
 
 pub struct JsonEmitter {
-    // HACK: public for tests, but this should come from config directly to where needed
-    pub(crate) timestamp_provider: Box<dyn config::TimestampProvider + Send + Sync + 'static>,
+    timestamp_provider: Box<dyn config::TimestampProvider + Send + Sync + 'static>,
     writer: writer::WriterType,
     seqno: Arc<atomic::AtomicU64>,
 }
@@ -35,21 +34,26 @@ impl JsonEmitter {
         }
     }
 
-    fn serialize_artifact(&self, object: &spec::RootImpl) -> serde_json::Value {
+    fn incr_seqno(&self) -> u64 {
+        self.seqno.fetch_add(1, Ordering::AcqRel)
+    }
+
+    fn serialize_artifact(&self, object: &spec::RootImpl) -> String {
         let root = spec::Root {
             artifact: object.clone(),
             timestamp: self.timestamp_provider.now(),
             seqno: self.incr_seqno(),
         };
-        serde_json::json!(root)
+
+        serde_json::json!(root).to_string()
     }
 
-    fn incr_seqno(&self) -> u64 {
-        self.seqno.fetch_add(1, Ordering::AcqRel)
+    pub fn timestamp_provider(&self) -> &(dyn config::TimestampProvider + Send + Sync + 'static) {
+        &*self.timestamp_provider
     }
 
     pub async fn emit(&self, object: &spec::RootImpl) -> Result<(), io::Error> {
-        let s = self.serialize_artifact(object).to_string();
+        let s = self.serialize_artifact(object);
 
         match &self.writer {
             WriterType::File(file) => file.write(&s).await?,
@@ -72,6 +76,20 @@ mod tests {
 
     use super::*;
 
+    pub struct NullTimestampProvider {}
+
+    impl NullTimestampProvider {
+        // warn: linter is wrong here, this is used in a serde_json::json! block
+        #[allow(dead_code)]
+        pub const FORMATTED: &str = "1970-01-01T00:00:00.000Z";
+    }
+
+    impl config::TimestampProvider for NullTimestampProvider {
+        fn now(&self) -> chrono::DateTime<chrono_tz::Tz> {
+            chrono::DateTime::from_timestamp_nanos(0).with_timezone(&chrono_tz::UTC)
+        }
+    }
+
     #[tokio::test]
     async fn test_emit_using_buffer_writer() -> Result<()> {
         let expected = json!({
@@ -80,13 +98,13 @@ mod tests {
                 "minor": spec::SPEC_VERSION.1,
             },
             "sequenceNumber": 0,
-            "timestamp": config::NullTimestampProvider::FORMATTED,
+            "timestamp": NullTimestampProvider::FORMATTED,
         });
 
         let buffer = Arc::new(Mutex::new(vec![]));
         let writer = writer::BufferWriter::new(buffer.clone());
         let emitter = JsonEmitter::new(
-            Box::new(config::NullTimestampProvider {}),
+            Box::new(NullTimestampProvider {}),
             writer::WriterType::Buffer(writer),
         );
 
@@ -112,7 +130,7 @@ mod tests {
                 "minor": spec::SPEC_VERSION.1,
             },
             "sequenceNumber": 0,
-            "timestamp": config::NullTimestampProvider::FORMATTED,
+            "timestamp": NullTimestampProvider::FORMATTED,
         });
         let expected_2 = json!({
             "schemaVersion": {
@@ -120,13 +138,13 @@ mod tests {
                 "minor": spec::SPEC_VERSION.1,
             },
             "sequenceNumber": 1,
-            "timestamp": config::NullTimestampProvider::FORMATTED,
+            "timestamp": NullTimestampProvider::FORMATTED,
         });
 
         let buffer = Arc::new(Mutex::new(vec![]));
         let writer = writer::BufferWriter::new(buffer.clone());
         let emitter = JsonEmitter::new(
-            Box::new(config::NullTimestampProvider {}),
+            Box::new(NullTimestampProvider {}),
             writer::WriterType::Buffer(writer),
         );
 
diff --git a/src/output/step.rs b/src/output/step.rs
index c02577e..9455077 100644
--- a/src/output/step.rs
+++ b/src/output/step.rs
@@ -9,10 +9,9 @@ use std::sync::atomic::{self, Ordering};
 use std::sync::Arc;
 
 use crate::output as tv;
-use crate::spec::TestStepStart;
-use crate::spec::{self, TestStepArtifactImpl};
+use crate::spec::{self, TestStepArtifactImpl, TestStepStart};
 use tv::measure::MeasurementSeries;
-use tv::{emitter, error, log, measure};
+use tv::{config, emitter, error, log, measure};
 
 use super::OcptvError;
 
@@ -31,7 +30,7 @@ impl TestStep {
             name: name.to_owned(),
             emitter: Arc::new(StepEmitter {
                 step_id: id.to_owned(),
-                run_emitter,
+                emitter: run_emitter,
             }),
         }
     }
@@ -528,7 +527,8 @@ impl StartedTestStep {
 
 pub struct StepEmitter {
     step_id: String,
-    run_emitter: Arc<emitter::JsonEmitter>,
+    // root emitter
+    emitter: Arc<emitter::JsonEmitter>,
 }
 
 impl StepEmitter {
@@ -538,13 +538,12 @@ impl StepEmitter {
             // TODO: can these copies be avoided?
             artifact: object.clone(),
         });
-        self.run_emitter.emit(&root).await?;
+        self.emitter.emit(&root).await?;
 
         Ok(())
     }
 
-    // HACK:
-    pub fn timestamp_provider(&self) -> &dyn tv::config::TimestampProvider {
-        &*self.run_emitter.timestamp_provider
+    pub fn timestamp_provider(&self) -> &(dyn config::TimestampProvider + Send + Sync + 'static) {
+        self.emitter.timestamp_provider()
     }
 }