diff --git a/docs/guide/packets-and-samples.md b/docs/guide/packets-and-samples.md
index aa5c295..8b37d3c 100644
--- a/docs/guide/packets-and-samples.md
+++ b/docs/guide/packets-and-samples.md
@@ -176,7 +176,7 @@ Negative sequence numbers mean the packet's ordering is undefined. When creating
 
 ### Cloning packets
 
-Use the `clone` method to create a new packet from an existing packet. While doing so, you can change its timestamp and duration.
+Use the `clone` method to create a new packet from an existing packet. While doing so, you can partially change its data.
 ```ts
 // Creates a clone identical to the original:
 packet.clone();
diff --git a/examples/file-compression/file-compression.ts b/examples/file-compression/file-compression.ts
index 08b821d..0992dca 100644
--- a/examples/file-compression/file-compression.ts
+++ b/examples/file-compression/file-compression.ts
@@ -75,7 +75,7 @@ const compressFile = async (resource: File | string) => {
 		let progress = 0;
 		currentConversion.onProgress = newProgress => progress = newProgress;
 
-		const fileDuration = await input.computeDuration();
+		const fileDuration = (await input.computeDuration()) - (await input.getFirstTimestamp());
 		const startTime = performance.now();
 
 		const updateProgress = () => {
diff --git a/src/conversion.ts b/src/conversion.ts
index db0f55a..a303c58 100644
--- a/src/conversion.ts
+++ b/src/conversion.ts
@@ -84,7 +84,7 @@ export type ConversionOptions = {
 	trim?: {
 		/**
 		 * The time in the input file in seconds at which the output file should start. Must be less than `end`.
-		 * Defaults to 0 when omitted.
+		 * When omitted, defaults to the start timestamp of the input or to 0, whichever is higher.
 		 */
 		start?: number;
 		/**
@@ -462,9 +462,9 @@ export class Conversion {
 	/** @internal */
 	_options: ConversionOptions;
 	/** @internal */
-	_startTimestamp: number;
+	_startTimestamp!: number;
 	/** @internal */
-	_endTimestamp: number;
+	_endTimestamp!: number;
 
 	/** @internal */
 	_addedCounts: Record<TrackType, number> = {
@@ -592,9 +592,6 @@ export class Conversion {
 		this.input = options.input;
 		this.output = options.output;
 
-		this._startTimestamp = options.trim?.start ?? 0;
-		this._endTimestamp = options.trim?.end ?? Infinity;
-
 		const { promise: started, resolve: start } = promiseWithResolvers();
 		this._started = started;
 		this._start = start;
@@ -602,6 +599,14 @@ export class Conversion {
 
 	/** @internal */
 	async _init() {
+		this._startTimestamp = this._options.trim?.start ?? Math.max(
+			await this.input.getFirstTimestamp(),
+			// Samples can also have negative timestamps, but the meaning typically is "don't present me", so let's cut
+			// those out by default.
+			0,
+		);
+		this._endTimestamp = this._options.trim?.end ?? Infinity;
+
 		const inputTracks = await this.input.getTracks();
 		const outputTrackCounts = this.output.format.getSupportedTrackCounts();
 
@@ -900,8 +905,7 @@ export class Conversion {
 
 		const firstTimestamp = await track.getFirstTimestamp();
 		const needsTranscode = !!trackOptions.forceTranscode
-			|| this._startTimestamp > 0
-			|| firstTimestamp < 0
+			|| firstTimestamp < this._startTimestamp
 			|| !!trackOptions.frameRate
 			|| trackOptions.keyFrameInterval !== undefined
 			|| trackOptions.process !== undefined;
@@ -943,17 +947,19 @@ export class Conversion {
 						return;
 					}
 
-					if (alpha === 'discard') {
-						// Feels hacky given that the rest of the packet is readonly. But, works for now.
-						delete packet.sideData.alpha;
-						delete packet.sideData.alphaByteLength;
-					}
+					const modifiedPacket = packet.clone({
+						timestamp: packet.timestamp - this._startTimestamp,
+						sideData: alpha === 'discard'
+							? {} // Remove alpha side data
+							: packet.sideData,
+					});
+					assert(modifiedPacket.timestamp >= 0);
 
-					this._reportProgress(track.id, packet.timestamp);
-					await source.add(packet, meta);
+					this._reportProgress(track.id, modifiedPacket.timestamp);
+					await source.add(modifiedPacket, meta);
 
-					if (this._synchronizer.shouldWait(track.id, packet.timestamp)) {
-						await this._synchronizer.wait(packet.timestamp);
+					if (this._synchronizer.shouldWait(track.id, modifiedPacket.timestamp)) {
+						await this._synchronizer.wait(modifiedPacket.timestamp);
 					}
 				}
 
@@ -1314,8 +1320,7 @@ export class Conversion {
 		let sampleRate = trackOptions.sampleRate ?? originalSampleRate;
 		let needsResample = numberOfChannels !== originalNumberOfChannels
 			|| sampleRate !== originalSampleRate
-			|| this._startTimestamp > 0
-			|| firstTimestamp < 0;
+			|| firstTimestamp < this._startTimestamp;
 
 		let audioCodecs = this.output.format.getSupportedAudioCodecs();
 		if (
@@ -1346,11 +1351,16 @@ export class Conversion {
 						return;
 					}
 
-					this._reportProgress(track.id, packet.timestamp);
-					await source.add(packet, meta);
+					const modifiedPacket = packet.clone({
+						timestamp: packet.timestamp - this._startTimestamp,
+					});
+					assert(modifiedPacket.timestamp >= 0);
 
-					if (this._synchronizer.shouldWait(track.id, packet.timestamp)) {
-						await this._synchronizer.wait(packet.timestamp);
+					this._reportProgress(track.id, modifiedPacket.timestamp);
+					await source.add(modifiedPacket, meta);
+
+					if (this._synchronizer.shouldWait(track.id, modifiedPacket.timestamp)) {
+						await this._synchronizer.wait(modifiedPacket.timestamp);
 					}
 				}
 
@@ -1449,6 +1459,9 @@ export class Conversion {
 							return;
 						}
 
+						// Offset the timestamp as needed
+						sample.setTimestamp(sample.timestamp - this._startTimestamp);
+
 						await this._registerAudioSample(track, trackOptions, source, sample);
 						sample.close();
 					}
diff --git a/src/mpeg-ts/mpeg-ts-demuxer.ts b/src/mpeg-ts/mpeg-ts-demuxer.ts
index 4a253ce..f5272c4 100644
--- a/src/mpeg-ts/mpeg-ts-demuxer.ts
+++ b/src/mpeg-ts/mpeg-ts-demuxer.ts
@@ -228,20 +228,13 @@ export class MpegTsDemuxer extends Demuxer {
 
 						switch (streamType) {
 							case MpegTsStreamType.MP3_MPEG1:
-							case MpegTsStreamType.MP3_MPEG2: {
-								info = {
-									type: 'audio',
-									codec: 'mp3',
-									aacCodecInfo: null,
-									numberOfChannels: -1,
-									sampleRate: -1,
-								};
-							}; break;
-
+							case MpegTsStreamType.MP3_MPEG2:
 							case MpegTsStreamType.AAC: {
+								const codec = streamType === MpegTsStreamType.AAC ? 'aac' : 'mp3';
+
 								info = {
 									type: 'audio',
-									codec: 'aac',
+									codec,
 									aacCodecInfo: null,
 									numberOfChannels: -1,
 									sampleRate: -1,
@@ -268,6 +261,11 @@ export class MpegTsDemuxer extends Demuxer {
 									reorderSize: -1,
 								};
 							}; break;
+
+							default: {
+								// If we don't recognize the codec, we don't surface the track at all. This is because
+								// we can't determine its metadata and also have no idea how to packetize its data.
+							}
 						}
 
 						if (info) {
diff --git a/src/packet.ts b/src/packet.ts
index fd0e93d..2fe7269 100644
--- a/src/packet.ts
+++ b/src/packet.ts
@@ -234,31 +234,51 @@ export class EncodedPacket {
 		);
 	}
 
-	/** Clones this packet while optionally updating timing information. */
+	/** Clones this packet while optionally modifying the new packet's data. */
 	clone(options?: {
+		/** The data of the cloned packet. */
+		data?: Uint8Array;
+		/** The type of the cloned packet. */
+		type?: PacketType;
 		/** The timestamp of the cloned packet in seconds. */
 		timestamp?: number;
 		/** The duration of the cloned packet in seconds. */
 		duration?: number;
+		/** The sequence number of the cloned packet. */
+		sequenceNumber?: number;
+		/** The side data of the cloned packet. */
+		sideData?: EncodedPacketSideData;
 	}): EncodedPacket {
 		if (options !== undefined && (typeof options !== 'object' || options === null)) {
 			throw new TypeError('options, when provided, must be an object.');
 		}
+		if (options?.data !== undefined && !(options.data instanceof Uint8Array)) {
+			throw new TypeError('options.data, when provided, must be a Uint8Array.');
+		}
+		if (options?.type !== undefined && options.type !== 'key' && options.type !== 'delta') {
+			throw new TypeError('options.type, when provided, must be either "key" or "delta".');
+		}
 		if (options?.timestamp !== undefined && !Number.isFinite(options.timestamp)) {
 			throw new TypeError('options.timestamp, when provided, must be a number.');
 		}
 		if (options?.duration !== undefined && !Number.isFinite(options.duration)) {
 			throw new TypeError('options.duration, when provided, must be a number.');
 		}
+		if (options?.sequenceNumber !== undefined && !Number.isFinite(options.sequenceNumber)) {
+			throw new TypeError('options.sequenceNumber, when provided, must be a number.');
+		}
+		if (options?.sideData !== undefined && (typeof options.sideData !== 'object' || options.sideData === null)) {
+			throw new TypeError('options.sideData, when provided, must be an object.');
+		}
 
 		return new EncodedPacket(
-			this.data,
-			this.type,
+			options?.data ?? this.data,
+			options?.type ?? this.type,
 			options?.timestamp ?? this.timestamp,
 			options?.duration ?? this.duration,
-			this.sequenceNumber,
+			options?.sequenceNumber ?? this.sequenceNumber,
 			this.byteLength,
-			this.sideData,
+			options?.sideData ?? this.sideData,
 		);
 	}
 }
diff --git a/todo.txt b/todo.txt
deleted file mode 100644
index bad55fa..0000000
--- a/todo.txt
+++ /dev/null
@@ -1 +0,0 @@
-- Conversion should offset the start, no?
\ No newline at end of file