# Lossy Codec Preview Compare Hear and measure what streaming does to your master: encode to MP3, AAC or Opus at real streaming bitrates, decode back, and compare loudness shift, true-peak overshoot and headroom against the original — the Apple Digital Masters round-trip audition as a report plus downloadable preview files. > Canonical page: https://elysiatools.com/en/tools/audio-lossy-preview-compare - **Category:** Media - **Keywords:** lossy preview, codec compare, mp3, aac, opus, round trip, true peak overshoot, dbtp, apple digital masters, mastered for itunes, afclip, streaming, mastering, loudness shift, codec damage, encoding check ## Overview Lossy encoders rebuild the waveform from a frequency-domain approximation, so the decoded signal can peak above the original — levels that show no overs on the PCM master can still clip after encoding. That is why Apple’s Digital Masters workflow (afclip / AURoundTripAAC) auditions every master through a 256 kbps AAC round-trip before release, and why AES TD1008 recommends keeping the true peak at or below -1 dBTP at the codec input: overshoot grows as the bitrate drops (an AAC 128 encode can add roughly +0.4 dB; lower bitrates more). This tool runs that round-trip for the codecs your listeners actually get: MP3 (LAME) at 320 and 128 kbps, AAC at 256 kbps (Apple Music tier) and 128 kbps (web-player tier), Opus at 160 kbps (YouTube Music high) and 96 kbps. Each selected codec encodes your master, the encode is decoded back to PCM, and both sides are measured with the BS.1770-4 oversampled meter: integrated loudness shift, true-peak overshoot (decoded peak minus master peak), headroom against 0 dBFS and against the -1 dBTP codec-input recommendation, plus file size. Each preview is graded: “clips after decode” (decoded true peak above 0 dBTP), “tight headroom” (above -1 dBTP) or “safe”. The encoded preview files and a comparison.json are packaged as a ZIP so you can A/B the actual artefacts in your DAW or player. Encoders differ per platform (Apple’s own AAC encoder is not the one used here), so treat the numbers as a revealing preview, not the exact stream. ## Inputs - **Master audio** (file) - **Preview codecs** (select) ## When to use - Auditioning a finished PCM master against streaming platform encoding formats like AAC, MP3, and Opus prior to digital distribution. - Checking whether inter-sample true peaks or lossy reconstruction filters will cause clipping distortion above 0 dBTP upon decoding. - Evaluating whether your audio meets the recommended -1 dBTP true-peak ceiling across low and high streaming bitrates. ## How it works - Upload your uncompressed master audio file (up to 100 MB) and select your target lossy preview codecs and bitrates. - The tool encodes the master to the selected formats (such as AAC 256 kbps, MP3 320 kbps, or Opus 160 kbps) and decodes them back to PCM. - A BS.1770-4 oversampled meter evaluates integrated loudness shift, true-peak overshoot, and headroom margins against 0 dBFS and -1 dBTP. - Each format receives a safety grade (safe, tight headroom, or clips after decode), and you receive a ZIP archive containing all decoded audio files and a comparison report. ## Use cases - Mastering engineers verifying true-peak headroom compliance before delivering final tracks to streaming aggregators. - Music producers performing A/B listening tests to detect codec-induced high-frequency smearing or transient degradation. - Podcast editors checking voice master files against lower-bitrate mobile streaming tiers like AAC 128 kbps and Opus 96 kbps. ## Frequently asked questions ### Why do audio files clip after lossy encoding even if the master peaked below 0 dBFS? Lossy codecs reconstruct audio using frequency approximations, which alter waveform peaks and frequently cause inter-sample true-peak overshoot beyond original PCM levels. ### What codecs and bitrates can I audition? You can select MP3 at 320 and 128 kbps, AAC at 256 and 128 kbps, and Opus at 160 and 96 kbps. ### What does the 'tight headroom' safety status mean? It indicates the decoded audio peaks between -1 dBTP and 0 dBTP, which avoids hard clipping but violates the AES TD1008 recommended safety margin. ### What is included in the downloadable output ZIP? The ZIP package contains the lossy-encoded audio preview files and a comparison.json report containing loudness and peak measurements. ### Does this tool use Apple's proprietary AAC encoder? No, it utilizes standard open-source encoding libraries to approximate streaming behavior, serving as a reliable benchmark rather than a platform-identical clone. ## Related tools - [Codec Artifact Detector](https://elysiatools.com/en/tools/audio-codec-artifact-detector): Compare an original master with its lossy encode and flag pre-echo, transient smearing, bandwidth loss and stereo damage, with per-event timestamps. - [Batch Audio Converter](https://elysiatools.com/en/tools/audio-batch-converter): Convert multiple audio files between different formats like MP3, WAV, FLAC, AAC, OGG, Opus with quality settings and batch processing - [Audio Codec Swap](https://elysiatools.com/en/tools/audio-codec-swap): Change the codec of an audio file within a chosen output format - [Audio to Multitrack MIDI (Draft)](https://elysiatools.com/en/tools/audio-to-multitrack-midi): Split a full mix into stems (drums/bass/other/vocals) and transcribe each to MIDI — a multi-track starting point for transcription - [WAV Compress](https://elysiatools.com/en/tools/wav-compress): Compress a WAV file by converting to MP3, AAC, OGG, or Opus - [Audio Melody Contour Extractor](https://elysiatools.com/en/tools/audio-melody-contour-extractor): Extract a dominant melody path from audio and export MIDI, note events, a pitch contour, SVG, and JSON in one ZIP. - [Audio Set CBR](https://elysiatools.com/en/tools/audio-set-cbr): Re-encode audio using Constant Bitrate (CBR) - [Audio Profile Setter](https://elysiatools.com/en/tools/audio-profile-setter): Set a specific encoding profile (e.g., AAC-LC, HE-AAC, Opus voice/audio) ## Samples - [Copyright-Free FLAC Audio Samples](https://elysiatools.com/en/samples/flac-samples): Lossless FLAC audio samples for testing and development, mirrored from MP3 set with nature sounds and meditation music - [Copyright-Free MP3 Audio Samples](https://elysiatools.com/en/samples/mp3-samples): Collection of royalty-free audio samples for testing and development purposes including nature sounds, meditation music, and ambient audio - [Copyright-Free WAV Audio Samples](https://elysiatools.com/en/samples/wav-samples): Uncompressed PCM WAV audio samples for testing and development, mirrored from MP3 set with nature sounds and meditation music - [Copyright-Free M4A Audio Samples](https://elysiatools.com/en/samples/m4a-samples): Apple iTunes compatible M4A audio samples with AAC encoding, optimized for quality and file size balance ## Related content - [Audio Master Lossy Codec Preflight and Delivery Checks](https://elysiatools.com/en/hubs/audio-master-lossy-codec-preflight-and-delivery): Preview a finished master through lossy encoding, inspect decoded peak and loudness changes, adjust only when necessary, and verify the exact final MP3, AAC or Opus delivery file.