diff --git a/src/codec/packet/packet.rs b/src/codec/packet/packet.rs
index 3981a9b..f12c99c 100644
--- a/src/codec/packet/packet.rs
+++ b/src/codec/packet/packet.rs
@@ -147,13 +147,16 @@ impl Packet {
 	}
 
 	#[inline]
-	pub fn duration(&self) -> i64 {
-		self.0.duration as i64
+	pub fn duration(&self) -> Option<i64> {
+		match self.0.duration as i64 {
+			AV_NOPTS_VALUE => None,
+			value => Some(value),
+		}
 	}
 
 	#[inline]
-	pub fn set_duration(&mut self, value: i64) {
-		self.0.duration = value;
+	pub fn set_duration(&mut self, value: Option<i64>) {
+		self.0.duration = value.unwrap_or(AV_NOPTS_VALUE);
 	}
 
 	#[inline]
diff --git a/src/codec/parameters.rs b/src/codec/parameters.rs
index a50c3cf..aa59a8d 100644
--- a/src/codec/parameters.rs
+++ b/src/codec/parameters.rs
@@ -38,9 +38,34 @@ impl Parameters {
 		unsafe { media::Type::from((*self.as_ptr()).codec_type) }
 	}
 
+	pub fn set_medium(&mut self, value: media::Type) -> &mut Self {
+		unsafe {
+			(*self.as_mut_ptr()).codec_type = value.into();
+		}
+		self
+	}
+
 	pub fn id(&self) -> Id {
 		unsafe { Id::from((*self.as_ptr()).codec_id) }
 	}
+
+	pub fn set_id(&mut self, value: Id) -> &mut Self {
+		unsafe {
+			(*self.as_mut_ptr()).codec_id = value.into();
+		}
+		self
+	}
+
+	pub fn tag(&self) -> u32 {
+		unsafe { (*self.as_ptr()).codec_tag }
+	}
+
+	pub fn set_tag(&mut self, value: u32) -> &mut Self {
+		unsafe {
+			(*self.as_mut_ptr()).codec_tag = value;
+		}
+		self
+	}
 }
 
 impl Default for Parameters {
diff --git a/src/format/context/common.rs b/src/format/context/common.rs
index a221c50..70c8587 100644
--- a/src/format/context/common.rs
+++ b/src/format/context/common.rs
@@ -79,8 +79,13 @@ impl Context {
 		unsafe { (*self.as_ptr()).bit_rate }
 	}
 
-	pub fn duration(&self) -> i64 {
-		unsafe { (*self.as_ptr()).duration }
+	pub fn duration(&self) -> Option<i64> {
+		unsafe {
+			match (*self.as_ptr()).duration {
+				AV_NOPTS_VALUE => None,
+				value => Some(value),
+			}
+		}
 	}
 
 	#[inline]
diff --git a/src/format/stream/stream.rs b/src/format/stream/stream.rs
index f3548e0..dd3824f 100644
--- a/src/format/stream/stream.rs
+++ b/src/format/stream/stream.rs
@@ -54,12 +54,22 @@ impl<'a> Stream<'a> {
 		unsafe { Rational::from((*self.as_ptr()).time_base) }
 	}
 
-	pub fn start_time(&self) -> i64 {
-		unsafe { (*self.as_ptr()).start_time }
+	pub fn start_time(&self) -> Option<i64> {
+		unsafe {
+			match (*self.as_ptr()).start_time {
+				AV_NOPTS_VALUE => None,
+				value => Some(value),
+			}
+		}
 	}
 
-	pub fn duration(&self) -> i64 {
-		unsafe { (*self.as_ptr()).duration }
+	pub fn duration(&self) -> Option<i64> {
+		unsafe {
+			match (*self.as_ptr()).duration {
+				AV_NOPTS_VALUE => None,
+				value => Some(value),
+			}
+		}
 	}
 
 	pub fn frames(&self) -> i64 {