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System Design Masterclass · Web Content Delivery
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Audio is often an afterthought in system design discussions, but it is the glue that holds video content together. Users will tolerate a slightly blurry video, but choppy or distorted audio makes content unwatchable. Studies by Qualcomm Research show that audio quality affects perceived video quality more than actual video resolution. Your brain fills in visual gaps. It cannot fill in missing audio.
Uncompressed CD-quality audio (44.1 kHz, 16-bit, stereo PCM) uses about 1.4 Mbps. That is a trivial fraction of a video stream, but it adds up across millions of concurrent listeners. Spotify serves over 600 million users. At their scale, the difference between 128 Kbps MP3 and 96 Kbps Opus (at the same perceived quality) saves terabytes of bandwidth per day. The codec choice directly hits the bill.
For real-time communication (voice calls, video conferencing), the codec choice determines . Zoom, Discord, and Google Meet all use Opus because it was designed from the ground up for low-latency interactive audio. Using AAC for a voice call adds 100+ ms of algorithmic delay that Opus avoids.
MP3 (MPEG-1 Audio Layer III). Released in 1993. The codec that started digital music. Every device on earth plays MP3. Patent expired in 2017, now royalty-free. Quality at low bitrates is poor compared to modern codecs. Spotify still offers MP3 for compatibility with older devices, but newer streams use Ogg Vorbis or AAC.
AAC (Advanced Audio Coding). The successor to MP3, standardized in 1997 but refined through multiple profiles (LC, HE-AAC, HE-AAC v2). Better quality than MP3 at the same bitrate, roughly 30% more efficient. Apple's default codec for iTunes, Apple Music, and all Apple devices. YouTube uses AAC for audio in MP4 containers. The streaming industry standard for video-on-demand audio tracks.
Opus. Developed by Xiph.org and IETF, released in 2012. Royalty-free. Handles everything from low-bitrate speech (6 Kbps) to high-fidelity music (510 Kbps) in a single codec. Supports frame sizes from 2.5 ms to 60 ms, making it ideal for real-time communication where low matters. Discord uses Opus for voice chat. WebRTC mandates Opus support. Listening tests consistently show Opus beats AAC and MP3 at every bitrate.
FLAC (Free Lossless Audio Codec). Lossless compression: the decompressed output is bit-for-bit identical to the original. File sizes are about 50-60% of uncompressed WAV. Used for archival, audiophile streaming (Tidal HiFi, Amazon Music HD), and as a source format before lossy encoding. You never encode from MP3 to AAC, that is lossy-to-lossy and destroys quality. Always keep a lossless source.